The THESEUS mission (Chair Enrico Bozzo)

THESEUS is a space mission concept, currently under Phase A study by ESA as candidate M5 mission, aiming at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. In addition to fully exploiting high-redshift GRBs for cosmology (pop-III stars, cosmic re-ionization, SFR and metallicity evolution up to the "cosmic dawn"), THESEUS will allow the identification and study of the electromagnetic counterparts to sources of gravitational waves which will be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/aVirgo, LISA, KAGRA, and Einstein Telescope (ET), as well as of most classes of X/gamma-ray transient sources, thus providing an ideal sinergy with the large e.m. facilities of the near future like, e.g., LSST, ELT, TMT, SKA, CTA, ATHENA. These breakthrough scientific objectives will be achieved by an unprecedented combination of X/gamma-ray monitors, providing the capabilities of detecting and accurately localize and kind of GRBs and may classes of transient in an energy band as large as 0.1 keV - 10 MeV, with an on-board NIR telescope providing detection, localization (arcsec) and redshift measurement of the NIR counterpart. A Guest Observer programme, further improving the scientific return and community involvement is also envisaged. We summarize the main scientific requirements of the mission and provide an overview of the updated concept, design (instruments and spacecraft) and mission profile.

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The ultimate driver of the THESEUS mission design are its scientific requirements. They describe in an holistic way the raison d'être of the mission, flowing down in a seamless tapestry from the highest-level ("Level 0") science goals to the individual requirements describing key performance of the spacecraft, payload and ground segment ("Level 2"). In this talk I will describe the pattern and the fabric of this tapestry as embedded in the THESEUS Science Requirement Document and in the THESEUS Yellow Book, the key contributions of the THESEUS Science Study Team to the selection process.

We are entering a new era for high energy astrophysics with the use of new technology to increase our ability to both survey and monitor the sky. I will discuss the Soft X-ray Instrument (SXI) on THESEUS, which is under Phase A study by ESA for its M5 mission. THESEUS will carry two large area monitors utilising Lobster-eye (SXI) and coded-mask (XGIS) technologies and an optical-IR telescope (IRT) to provide redshift estimates using imaging and spectroscopy. Incorporating wide-field of view microchannel plate optics and CMOS detectors, the SXI will locate and monitor hundreds of soft X-ray transients per year. SXI data will facilitate: (1) an exploration of the earliest phase of star formation; (2) a search for electromagnetic counterparts of multi-messenger astronomy has begun to provide a new window on the universe; and (3) monitor activity in classes of X-ray transients, which can then be studied by other large facilities.

XGIS is a large Field of View instrument that together with the Soft X-Ray Imager instrument (SXI) will allow THESEUS to detect and localize GRB events. The spacecraft will be capable of performing fast repointing of the onboard InfraRed Telescope (IRT) to the error region provided by the two monitor systems, thus allowing it to localize the transient sources down to a few arcsec accuracy, for immediate identification and redshift determination. XGIS is an a hard X-ray spectroscopic instrument operating in the 2 keV – 10 MeV energy range, with imaging capabilities in lower part of the spectra up to 150 keV. His prime goal will be to detect transient sources, with monitoring timescales down to milliseconds, both independently of, or following up SXI detections, identify the sources performing localisation at

The THESEUS mission (Chair Lorenzo Amati)

We will present the current design and performance of the Infra-Red Telescope (IRT), to be flown on board the THESEUS mission. The main goal of the IRT is to identify, localize and measure the distance of the THESEUS Gamma-Ray Bursts afterglows in near real time, on board the satellite. In order to achieve this goal the IRT instrument will implement an imaging mode in five filters over the 0.7 to 1.8 microns range, enabling the measure of the photometric redshift (z) for GRBs with z larger than 5.5. In addition the IRT will provide a spectroscopic mode (R~400), which will allow for further characterization of the brightest afterglows, as well as for the measure of the spectroscopic redshift. As a complement to the mission main goals, IRT will be used as a flexible NIR observatory in space, thanks to the guest observer and Target of Opportunity observing programmes of THESEUS.

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Recent studies of the prompt emission of GRBs highlighted the presence of a low energy break between ~5 and ~150 keV bringing new support to the synchrotron interpretation. However, combining Swift and Fermi data, this new feature could be identified in the brightest long bursts. A thermal emission component has also been observed around few tens of keV. These features reflect the nature of the prompt emission process and of the jet energy content, yet their study is hampered by the instrumental bandpass and sensitivity. The large energy range covered by THESEUS with the combination of SXI (0.3-5 keV) and XGIS (2keV-10 MeV) and its unprecedented large effective area will provide large count statistics for a large fraction of GRBs thus allowing us to study the prompt emission spectrum, its temporal evolution and short timescale variability and the transition to the early steep X—ray decay phase. With a detection rate of ~ 300 yr^-1, THESEUS will provide us a representative burst sample to understand the nature of the dissipation mechanism in both short and long GRBs, the origin of the prompt emission and the jet structure.

The presentation aims at providing an overview and objectives of the THESEUS Mission Observation Simulator MOS implemented by ESA during the Phase A. The introduction of such a simulator, normally reserved for later phases of the mission design, was deemed necessary since early THESEUS study phases (ESA CDF) due to many relevant aspects of THESEUS that can have an impact on the scientific return and system design that is difficult to estimate realistically with simple approximations. With the inputs from the THESEUS Payload Consortium and Phase A industrial partners, the MOS and its results represented during the Phase A, in a simulation environment, the status of the mission observation efficiency, the latter being the connection between the implementation of the THESEUS concept of operations and the top level Science performance requirements validation. The MOS ultimate goal was then to provide inputs to the THESEUS Science Study Team and consolidate, as much as possible at this early phase, the Mission Requirements Document MRD (system and spacecraft) and EID-A (instruments) performance requirements.

In this talk will be presented the THESEUS ground segment as it is currently foreseen.

Investigating the early Universe & cosmic evolution via GRBs (Chair Diego Götz)

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GRBs are luminous sources that provide information on the chemical content of their host galaxies out to the largest distances in the Universe. Absorption lines seen in GRB afterglow spectra have to date allowed us to obtain a few very accurate measurements of metallicities of their host galaxies at z~6, which is otherwise inaccessible from emission-studies of high-redshift galaxies. With its unique sensitivity to high-redshift GRBs, THESEUS will enable us to clearly identify afterglows of high-redshift events ( z>6 ) via drop-out detections and multi-band photometry with the on-board near-infrared telescope (IRT). Rapidly followed by IRT spectroscopy, or by follow-up observations from ground-based telescopes, THESEUS will give us unprecedented details of the chemical enrichment of young galaxies. We will gain fundamental new knowledge on the explosion sites of some of the earliest generations of massive stars, reveal details of population III enrichments and the nature of their progenitors and environments. With a large sample of high-z GRB detections, THESEUS data allow us to constrain the star-formation-rate histories of galaxies through metal-enrichment generated by the stellar explosions, and provide information on the low-luminosity end of the galaxy luminosity function at the highest redshifts.

In this talk I will show some of the uses of LGRBs as powerful tools to explore the high-redshift universe. In particular, I will show that we can use them to probe cosmic star formation at high redshift, and that they can be unique tools to probe faint galaxies belonging to the bulk of the very high-redshift galaxy population. I will present some of the studies we carried / are carrying out on both topics, and put all that in the THESEUS perspective.

The IRT on-board THESEUS will play a key role to localize GRB counterparts and enable their immediate spectro-imaging follow-up at NIR wavelengths. I will discuss the IRT capabilities in autonomously providing accurate photometric redshifts of GRBs at 5.5<z<11 on timescale <1h after trigger. This will allow unprecedented estimates of the GRB N(z) beyond reionization, and will lead to new constraints on the rate of massive star formation in the early Universe. In relation with this topic, I will report on-going works carried out to further improve our understanding of the connection between long GRBs and star-forming environments.

Optical/NIR emission can be seen in 90% of X-ray afterglows if deep observations are taken within 4 hours after the burst onset. Amongst them, about 1/4 would be considered “dark bursts” with very faint optical counterparts, due to a combination of two contributing factors: (i) moderate intrinsic extinction at moderate redshifts, and (ii) bursts z >5. Reverse shocks are not easy to observe as they require prompt measurements by robotic facilities. Regarding forwards shocks, although the majority of optical/nIR afterglows are consistent with the standard model a significant fraction deviate from this behavior, implying additional causes (i.e., energy injection, variation in the ambient matter, etc.). Host galaxies have been detected only for z < 5. The host galaxies optical/nIR colours for long-duration GRBs imply they are mostly blue objects (as expected from star forming systems) whereas for dark GRBs redder hosts are normally seen (EROs in a few cases). I will summarize the general properties of optical/nIR emission of long-duration GRB afterglows and host galaxies observed so far.

Investigating the early Universe & cosmic evolution via GRBs (Chair Nial Tanvir)

Since its launch, Swift has detected a small sample of Gamma-ray Bursts at z>6, showing that GRBs are a new, fundamental tool to explore the early epochs of the Universe. In this talk, I will highlight a few research areas in which GRBs will play a major role in enriching our knowledge of the first galaxies, the first stars and their environment.

One of the key mission aims of THESEUS is to provide triggers of transient events such as Gamma-Ray Bursts (GRBs) to next generation ground- and space-based facilities. These events will be subject to multi-wavelength follow-up on-board. Some of these sources will be short GRBs associated with gravitational wave events. Redshift measurement, on-board or on the ground, will help to generate a sample of these bursts to constrain Hubble’s constant. The THESEUS Mission Observation Simulator has shown that ~25% of short Gamma-Ray Bursts (sGRBs) detected by the Soft X-ray Imager (SXI) and X-Gamma-ray Imaging Spectrometer (XGIS) will be detected and localised by the InfraRed Telescope (IRT) with arcsecond precision. The remaining ~75% of sGRBs detected will have larger error regions (within 15 arcmin) containing many galaxies, hindering the identification of the host galaxy. This talk will present the prospects for detecting the optical afterglows of on- and off-axis short GRBs triggered on THESEUS with ground based facilities available in the THESEUS era. Results from simulations of synthetic short GRB light curves will be compared with the capabilities of ground-based facilities.

Population synthesis of metal-poor massive stars – How to use THESEUS' high-redshift GRB data to constrain the physics of Pop-II and Pop-III progenitors The progenitors of long-duration GRBs are fast-rotating, metal-poor massive stars. They follow a stellar evolutionary path called 'chemically homogeneous evolution', and can be either a single star, or born and evolve as a member of a binary system. This evolutionary path can, furthermore, explain not only L-GRBs but several other phenomena such as the strong ionization radiation in dwarf galaxies and even gravitational wave progenitors. The existence of such chemically homogeneously evolving stars is not confirmed observationally, however, and it is hard to imagine how it could be. For if they indeed exist, they are (i) embedded in their forming clouds (as in dwarf galaxies) and/or (ii) too far away from us to resolve spectroscopically (as in the high-redshift universe). There is a way to prove their existence however – and we need THESEUS for this. THESEUS will observe L-GRBs together with their high-redshift host galaxies in a large enough number that we can do statistically meaningful comparisons to theoretical event rate predictions based on binary population synthesis. To be able to do this, a population synthesis tool is needed where detailed information on the nature of the stellar progenitor is included (that is, the structure of the collapsing core of the chemically-homogeneously evolving star, in particular its specific angular momentum). By synthesising a population of massive stars and binaries with this particular piece of information included, we will be able to test stellar evolutionary predictions on the nature of L-GRB progenitors against THESEUS' observational data. In short, we can constrain the physics of Pop-II and Pop-III stellar progenitors of GRBs. In this talk, I present this research plan in detail. I will explain the theory behind, and show preliminary results from my new tool FINFAT (developed to identify L-GRB progenitors in stellar evolution models, which in the future will be implemented into population synthesis simulations). This is a unique way to study Pop-II and Pop-III stars – a way that relies on THESEUS' collecting data on L-GRBs and their high-redshift hosts. Therefore, I would like to add these research plans to the Yellow Book as an explicit science goal with THESEUS.

Gamma Ray Bursts provide one of the best unbiased way of detecting high and ultra high redshift galaxies. However, to understand these galaxies, we need to collect ancillary data over the electromagnetic spectrum, including deep optical data (e.g. VRO's LSST) but also far-infrared/sub-millimeter data (e.g. ALMA, IRAM-NOEMA) and X-ray data (Athena). The question we will try to address (probably non exhaustively) are: What are the main objectives at stake, galaxy wise? What are the main synergies to set up?

In this talk, we briefly review the status of SNe-Ia and Gamma-ray bursts as cosmic rulers. Both distance indicators converge to Omega_M ~ 0.3. The situation for the cosmological evolution of dark energy appears much more uncertain. Current data will not exclude that w(z) might vary with the redshift, possibly to values w 1.5.

So far large and different data sets revealed the accelerated expansion rate of the Universe, which is usually explained in terms of dark energy. The nature of dark energy is not yet known, and several models have been introduced: a non zero cosmological constant, a potential energy of some scalar field, effects related to the non homogeneous distribution of matter, or effects due to alternative theories of gravity. Recently a tension with the flat ΛCDM model has been discovered using a high-redshift Hubble diagram of supernovae, quasars, and gamma-ray bursts. Here we use Union2 type Ia supernovae and Gamma Ray Bursts Hubble diagram, and a set of direct measurements of the Hubble parameter to explore different dark energy models. We use the Chevallier-Polarski- Linder parametrization of the dark energy equation of state, a minimally coupled quintessence scalar field, and, finally, we consider models with dark energy at early times. We perform a statistical analysis based on the Markov chain Monte Carlo method, and explore the probability distributions of the cosmological parameters for each of the competing models. We use the Akaike Information Criterion to compare these models: it turns out that an evolving dark energy, described by a scalar field with exponential potential seems to be favoured by observational data.

Multi-messenger astrophysics (Chair Carlo Ferrigno)

Multi-messenger astrophysics is becoming a major avenue to explore the Universe. During the 2030s, gravitational wave and neutrino sources will be observed with unprecedented sensitivity by next generation detectors and the ability to catch their electromagnetic counterparts will be of the utmost importance. In such a challenge, THESEUS will play a prime role due to its unique capabilities to independently detect and pinpoint the most important high-energy counterparts expected from these sources, and to characterize (and further localize) them via the combination of gamma-ray, X-ray, and IR observations. In addition, the excellent sky localization of THESEUS will allow for the activation of dedicated follow-up campaigns with ground-based facilities such as ELT or SKA (among others) as well as space-based high-energy telescopes such as ATHENA, which will further boost the scientific return of the observations. In this talk, I will introduce the THESEUS science case on multi-messenger astrophysics as it results after the recent investigation accompanying the 3-year phase 0/A study of the mission.

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I will argue that if black holes represent one the most fascinating implications of Einstein's theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the inspiral and merger of a binary system of neutron stars is more than a strong source of gravitational waves. Indeed, while the gravitational signal can provide tight constraints on the equation of state for matter at nuclear densities, the formation of a black-hole--torus system can explain much of the phenomenology of short gamma-ray bursts, while the the ejection of matter during the merger can shed light on the chemical enrichment of the universe.

A fraction of binary neutron star mergers will result in the immediate formation of a long-lived neutron star. I will talk about how we can infer the presence of these objects from gamma-ray burst observations and discuss the prospect of detecting gravitational waves. I will also discuss what the population of these objects inferred from observations of all short gamma-ray bursts tells us about the nuclear equation of state and the dynamics of these neutron stars.

Multi-messenger astrophysics (Chair Giulia Stratta)

The first detection of gravitational waves in 2015 provided not only spectacular confirmation of the predictions of general relativity, but opened up an entirely new window on the Universe. Since current generation interferometers can only localize gravitational waves to at best a few tens of square degrees, searching for their faint counterparts at optical wavelengths remains exceptionally challenging. At only 40 Mpc distance, the nearby GW170817 provided the first (and so far only) spectroscopically confirmed kilonova and off-axis GRB associated with a gravitational wave. In 2018, the ENGRAVE collaboration was formed, bringing together over 250 researchers with a goal of finding more counterparts to nearby gravitational wave sources. I will give an overview of our observational campaigns for probable NS-NS and NS-BH mergers, including GW190814. In addition, I will discuss the experience of ENGRAVE in light of upcoming facilities, as well as future plans.

Neutrino telescopes are under-water/ice huge detectors (order of 1 km^3 instrumented volume) able to observe, with almost 100% duty cycle, secondary particles induced by neutrino interactions. The Earth is almost transparent for neutrinos with energy below 100 TeV: upgoing events produced by neutrino interactions are background-free and can be measured with angular resolution reaching 0.1 degrees. Above 100 TeV, the Earth opacity must be considered, and neutrino candidates can be selected within a wide solid angle and with angular resolution that depend on the detector. The reconstruction (direction, energy, probability to be an event of cosmic origin) of neutrino candidates is performed in real time and the information can be disseminated within few seconds. Thus, neutrino telescopes are ideal instruments for time-domain and multi-messenger studies of objects in which hadron acceleration mechanisms take place. In the presentation, we review the status of neutrino telescopes and the long-term projections, the synergies (present and future) with other detectors and a summary of the observations.

Multimessenger observations may hold the key to learn about the most energetic sources in the universe. The recent construction of large scale observatories opened new possibilities in testing non thermal cosmic processes with alternative probes, such as high energy neutrinos and gravitational waves. We propose to combine information from gravitational wave detections, neutrino observations and electromagnetic signals to obtain a comprehensive picture of some of the most extreme cosmic processes. Gravitational waves are indicative of source dynamics, such as the formation, evolution and interaction of compact objects. These compact objects can play an important role in astrophysical particle acceleration, and are interesting candidates for neutrino and in general high-energy astroparticle studies. In particular we will concentrate on the most promising gravitational wave emitter sources: compact stellar remnants. The merger of binary black holes, binary neutron stars or black hole-neutron star binaries are abundant gravitational wave sources and will likely make up the majority of detections. However, stellar core collapse with rapidly rotating core may also be significant gravitational wave emitter, while slower rotating cores may be detectable only at closer distances. The joint detection of gravitational waves and neutrinos from these sources will probe the physics of the sources and will be a smoking gun of the presence of hadrons in these objects which is still an open question. Conversely, the non-detection of neutrinos or gravitational waves from these sources will be fundamental to constrain the hadronic content.

At the time of defining the science objectives of the INTernational Gamma-Ray Astrophysics Laboratory (INTEGRAL), such a rapid and spectacular development of multi-messenger astronomy could not be predicted, with new impulsive phenomena becoming accessible through different channels. Neutrino telescopes have routinely detected energetic neutrino events coming from unknown cosmic sources since 2013. Gravitational wave detectors opened a novel window on the sky in 2015 with the detection of the merging of two black holes and in 2017 with the merging of two neutron stars, followed by signals in the full electromagnetic range. Finally, since 2007, radio telescopes detected extremely intense and short burst of radio waves, known as Fast Radio Bursts (FRBs) whose origin is for most cases extra-galactic, but enigmatic. The exceptionally robust and versatile design of the INTEGRAL mission allowed the researchers to exploit data collected not only with the pointed instruments, but also with the active cosmic-ray shields of the main instruments to detect impulses of gamma rays in coincidence with unpredictable phenomena. The full-sky coverage, mostly unocculted by the Earth, the large effective area, the stable background, and the high duty cycle (85privileged position to give a major contribution to multi-messenger astronomy. We will report on the outcome of the serendipitous and follow-up observations of multimessenger events with INTEGRAL with a focus on improved algorithms and automatized methods which could be useful also for future missions.

"New perspectives for testing physics at fundamental level with multimessenger astronomy." One of the most important issues of modern physics is the validation of basic theories like general theory of gravity or standard model of elementary particles. Such theories are based on different assumptions and use different mathematical formalisms making it extremely difficult to unify them e.g. within the so-called quantum gravity. However, both of them have been built on the ground of a few key principles which in particular may be modified in a particular manner. This creates new opportunities for testing: some models of quantum gravity predict exotic, non-standard effects like the existence of massive gravitons, running nature of fundamental constants or violation of some essential principles underlying our understanding of nature (e.g. Lorentz Invariance Violation). In particular, standard relativistic dispersion relation may be modified leading to changes in travel time of signals emitted from a distant astrophysical objects. This, in turn, creates possibillity of quantum gravity testing by means of the so called time-of-flight measurements. Strong gravitational lensing of signals emitted from a carefully selected class of extra-galactic sources like gamma-ray bursts or compact object binaries is predicted to play an important role in this context. Such tests seem to be promising especially in the time of successful operating runs of LIGO/Virgo gravitational wave detectors (recently joined by KAGRA observatory) resulting in numerous records of GW signals from coalescing compact object binaries, mostly from the BH mergers. However, the first evidence of coalescing binary neutron star system (assigned as GW170817) has also been announced along with identification of its electromagnetic counterpart at different wavelengths. In this talk I will discuss in details the new perspectives for quantum gravity testing via gravitational lensing available in the era of multimessenger astronomy after the GW window is open.

Time-domain astrophysics (Chair Giancarlo Ghirlanda)

The main objectives of THESEUS are to investigate the early Universe through the observation of gamma-ray bursts and to identify the electromagnetic counterparts of gravitational waves and neutrino events. On the other hand, its monitoring instruments, comprising a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focussing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal to carry out unprecedented studies in time-domain astrophysics for a variety of source classes. I will present the THESEUS pointing strategy and discuss its capabilities for studying the time variability of galactic and extragalactic sources on different timescales.

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Title: Magnetars and the transient sky: from Galactic X-ray outbursts to extra-Galactic FRBs New bursting activities detected in X-rays and radio from a few magnetars in the past couple of years have added important information on the mechanism triggering these transient events. Furthermore, these strengthened the proposed connection between magnetars and Fast Radio Bursts. In this talk I will present new results on magnetar outbursts, discussing their rates, and connection with FRBs.

Magnetars are young (≲ 10 kyr) and highly magnetised (B ~ 10^(13-15) G) neutron stars powered by magnetic energy. Identified through transient flaring activity and persistent emission of X-/soft γ-ray radiation, 30 of these sources are currently known within the Milky Way and local galaxies. On rare occasions, some magnetars emit extremely energetic and hard-spectrum flares, known as Giant Flares (GFs). The radiation released during the initial spikes that characterise these events is energetic enough to be observed to ~Mpc distances, allowing for the study of an extragalactic magnetar population. Although the current generation of high-energy instruments are technically capable of observing such sources, they are limited by the energy range used for triggering, resulting in the identification of only four extragalactic GF candidates to date. This talk will present the results of a study assessing THESEUS’s capabilities to increase the population of known extragalactic magnetar GFs and to differentiate their emission from that of short Gamma-ray Bursts.

Millisecond pulsar binaries (MSPs) represent a class of X-ray binaries harbouring fast spinning weakly magnetised neutron stars. They display a wide range of variabilities encompassing accretion outbursts (the AMXPs), lasting tens of days to months and reaching X-ray luminosities up to 10^36-10^38 erg/s and returning to quiescence down to 10^31-10^34 erg/s, as well as an intriguing sub-luminous (10^33-10^34 erg/s) X-ray state with unusual low, high and flaring mode variability. A few of them, dubbed transitional MSPs, have been surprisingly found to display transitions from accretion-powered to rotation-powered states and viceversa over timescales of a few days-weeks and lingering in one or the other state for months-years. They are believed to be the missing link between LMXBs and radio MSP binaries, where the NS has been spun-up to msec periods during a Gyr-long accretion phase. The discovery of an increasing number of AMXPs in outburst and of MSP binaries emitting at GeV energies, either in the rotation or accretion-powered sub-luminous state, now challenge our understanding of the complex interplay between accretion and magnetic field rotation power loss. We here present the key role that the THESEUS mission will play for these binaries in discovering state changes from accretion outbursts to quiescence and from/to sub-luminous and rotation-powered states.

Understanding tidal disruption events with Theseus Tidal disruption events (TDEs) are associated with the catastrophic disruptions of stars passing too close to the supermassive black holes in the centres of their host galaxies. At the tidal radius of the black hole, the tidal forces exceed the binding energy of the star, causing it to be torn apart. About half of the stellar mass is expected to be captured and accreted by the black hole on a timescale of about one year, making previously faint black holes very bright. Observing TDEs could be an efficient way to identify a population of the elusive intermediate mass black holes, that are thought to be the building blocks of supermassive black holes and they can help probe the galactic environment in distant galaxies. Currently around 90 TDEs are known, but Theseus will be able to find approximately the same number on a yearly basis. Discovering such a large population and following them with Theseus will allow us to address questions such as why do we observe TDEs with very different outburst durations? Why are some seen in X-rays but not in other electromagnetic domains and vice versa? What is the physics behind the accretion regimes observed? During this talk I will describe in detail the open questions surrounding TDEs and show how Theseus will help us answer them.

Time-domain astrophysics and GRB science (Chair Sandro Mereghetti)

Following the amazing discovery of GW 170817 and the associated unusual short GRB 170817, I review our understanding of this event. I consider further multi-messenger discoveries of similar events in the Theseus era. In particular I explore the potential of additional observational channels that can shed new light on mergers and resolves some of the puzzles that the interpretation of GW 170817 has left. I also discuss the potential implications of detection of future mergers on the resolution of the H0 tension.

The shock breakout emission represents the first flash of radiation that is emitted in the core-collapse of a massive star, ending as a supernova. The observation and the analysis of the shock break-out phenomenology bears a lot of information on the physical structure of the SN progenitors, as well as on their explosion mechanisms. In this talk, I will discuss the role of THESEUS in detecting SBO emission, in particular from energetic broad-line SNe associated, or not, with gamma-ray bursts, where the jet component plays an important role.

Metal abundances in GRB host galaxies, out to high-z GRB afterglows are powerful cosmic beacons that unveil the characteristics of their elusive host galaxies, out to high-z. In particular, the metal abundances that can be derived from absorption-line spectroscopy of GRB afterglows can reveal: 1) the (dust-corrected) metallicity in the host galaxy ISM and its evolution with z; 2) the dust content from the study of dust depletion and dust-to-metal ratio; 3) peculiar metal enrichment in the ISM due to stellar nucleosynthesis, such as alpha-element enhancement, or potential signatures of supermassive stars (like for GRB 130606A at z=6) or PopIII. THESEUS will be key to constrain these properties out to z~6 and beyond.

THESEUS and the star formation near and far Do we expect the star formation beyond z=6 to be similar to what we witness in the Local Universe? While we have nice theories on why and how the conditions for star formation changed in the Universe with time, there aren’t much direct observational evidence to verify these at high redshifts. I will address the importance of long (t>3s) gamma-ray bursts (LGRBs) as possible tracers of star formation based on recent studies. The observed cosmic cosmic variation of star forming rate density, and the evolution of galaxies will be compared to the parameters of the galactic environment of LGRBs. At last, the importance of THESEUS in exploring the intrinsic galactic environment of LGRBs in the post-reionization Universe will be discussed.

The future of the redshift estimation of GRBs Gamma-ray bursts may be tracers to star formation conditions that occur in the distant Universe, thus knowing the distance of GRBs is an essential issue because they modify many measured physical parameters of the bursts. It can be shown that the presence of Swift UVOT optical detections can trigger ground-based redshift measurements. Examining the number of GRB redshift measurements taken in the last decades, we have found that these measurements decrease year by year. Machine learning may provide an estimate of the distance of bursts as well as the distance distribution of these objects.

Long duration gamma-ray ray bursts (LGRBs) are thought to be associated with the collapse of massive, rapidly spinning stars and the formation of compact objects. Developments in asteroseismology point towards efficient angular momentum transport inside stars, which implies that the core of massive stars slows down after they expand to become super-giants and lose any significant initial angular momentum via stellar winds or binary mass transfer. On the other hand, tidal interactions in close binary systems can maintain a star tidally locked to short orbital periods leading to the formation of spinning black holes. I will discuss how the formation of fast spinning binary black hole (BBH) mergers, originating from the evolution of isolated binaries and involving a common envelope phase or chemically homogeneous evolution, are associated with LGRBs. Using population synthesis studies that employ detailed stellar structure and binary evolution calculations, we find that these BBH formation pathways, which well reproduce all the GW observable properties of the so far detected BBH sample, can also account for up-to the majority of the observed luminous LGRBs. The two sets of observations together can be used to not only constrain the cosmic SFR, but also the evolution of metallicity as a function of time.

Next-generation facilities and their synergies with THESEUS (Chair Paul O'Brien)

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The LISA gravitational wave observatory will be launched in 2037. Among the numerous types of sources that LISA will detect, those involving supermassive black holes are particularly interesting, because they may be accompanied by electromagnetic radiation, presumably in the X-rays, from the accreted gas. While there should be significant overlap between the LISA and the Athena X-ray observatory missions, there could also be some overlap with THESEUS if the THESEUS mission is extended. Under this assumption, I will discuss the possible role of THESEUS in providing early detection of the electromagnetic counterparts of such LISA gravitational-wave events.

Thanks to a unique combination of X-ray/gamma-ray monitors, an on-board NIR telescope and an automated fast slewing capabilities, THESEUS will be a formidable machine for the detection, the characterization and the measurement of the redshift of any type of GRB and many classes of X-ray transients. This information will be provided in real time to the largest ground-based facilities thanks to a ground-based VHF antenna array. Theseus, by being operational during the next decade, will be concomitant with the largest optical ground-based telescopes such as the ELT, LSST and TMT. During this presentation, the synergy between Theseus and the ELT, the TMT and the large optical/NIR facilities will be highlighted and discussed. In particular, examples of the scientific impact of these joint observations will be given.

Theseus and SKA will be two major facilities to study the epoch of the formation of the first stars and cosmic reionization. These are major open issues in present day galaxy formation field. The synergy between the two instruments is key to build a coherent scenario linking early star formation and the intergalactic medium. I will sketch a few scientific ideas that uniquely benefit of this promising scenario.

Title: Highlights of the H.E.S.S gamma-ray burst observation program Authors: F. Schüssler on behalf of the H.E.S.S. Collaboration Abstract: The emission of very-high energy (VHE, >100 GeV) radiation from gamma-ray bursts (GRBs) - the most luminous explosions in the universe - remained elusive for a long time. After more than a decade of efforts by current Imaging Atmospheric Cherenkov Telescopes (IACTs), within the last two years, the detection of three GRBs at VHEs has been confirmed. In this contribution we present the H.E.S.S. GRB observation program and highlight recent results including the detections of VHE emission from GRB 180720B and GRB 190829A, remarkably achieved deep into the afterglow phase. We will put the VHE detections in context with the multi wavelength data, discuss possible emission mechanisms, underline their implications for future GRB detections as well as for synergies and opportunities with THESEUS.

Time-domain astrophysics and GRB science (Chair Maria Caballero-Garcia)

Despite more than 5,000 detected Gamma-Ray Bursts (GRBs), the nature of the prompt emission and the physical mechanisms powering the GRB relativistic jets are still strongly debated. During the past years, several studies showed that the gamma-ray prompt emission spectra are more complex than the smoothly broken power-law traditionally used. New models emerged, and among them, the three-component model that we propose provides an excellent description of the broadband time-resolved prompt emission of both short and long GRBs from optical to high-energy gamma-rays: (i) a quasi-thermal component interpreted as emission from the jet photosphere, (ii) a non-thermal component interpreted as synchrotron radiation from accelerated electrons within the jet; and (iii) a second non-thermal component, which may be related to magnetic reconnections downstream the jet. Moreover, this model enables a new luminosity/hardness relation suggesting that GRBs may be standard candles; this relation may reveal the underlying physics behind the famous Amati, Ghirlanda, and Yonetoku relations. I will present this three-component model using GRBs detected with Fermi, CGRO/BATSE, Swift+Suzaku/WAM, and Wind/Konus. I will discuss the striking similarities of all GRBs using this model and the possible universality of the derived luminosity/hardness relation.

Gamma-ray bursts (GRBs) are the most energetic phenomena in the Universe; thought to be the result of the collapse and eventual eruption of massive stars. Although it has a profound effect on our understanding of their nature and selection biases, the dust properties of galaxies hosting GRBs are little understood.  GRB host galaxies are mostly distant galaxies with star formation, with a considerable fraction of their UV and optical emission is reprocessed by dust. We present the largest compilation of Herschel FIR observations of 45 GRB host galaxies. Measuring the FIR emission of the dust allows us to estimate parameters like star-forming rate (SFR) and the mass of stars and interstellar medium (ISM). Herschel PACS and SPIRE flux densities and upper limits were derived using various methods providing reliable values with error bars. Galaxy parameters, such as total stellar mass, star-forming rate and dust mass were estimated by fitting their spectral energy distributions (SEDs) modelled via CIGALE code. The ISM and star formation parameters of GRB host galaxies were compared to other galaxies at the same redshifts indicating an increased star-forming activity relative to main-sequence galaxies.

A new type of changing look AGN with extreme X-ray properties Accreting supermassive black holes (SMBHs) are known to show variable optical, ultraviolet and X-ray emission. One of the most intriguing aspects of this behaviour is associated with “changing- look’ sources, in which the optical/ultraviolet broad emission lines, produced by rapidly-moving material surrounding the SMBH, appear or disappear. In my talk I will discuss the drastic transformation of the X-ray properties of a nearby active galactic nucleus (AGN), following a changing-look event. After the optical/UV outburst the power-law component, produced in the X-ray corona, completely disappeared, and the spectrum instead became dominated by blackbody-like emission. This implies that the X-ray corona, ubiquitously found in AGN, was destroyed in the event. In my talk I will discuss in detail the results of our 450 days X-ray monitoring campaign on this source, and provide possible explanations for this phenomenon.

What X-rays and OIR tell us about accretion and outflows in MAXI J1820+070 Based on NICER and Swift monitoring observations we investigated short-term variability of the newly discovered transient X-ray binary MAXI J1820+070 in the soft (0.2-12 keV) X-ray band. X-ray monitoring revealed that this source is one of the brightest black hole X-ray binaries ever observed. We found that the centroid frequency of a certain variability feature (type-C quasi-periodic oscillation) is observed at rather low values (≲ 1 Hz) and does not show much evolution during the hard state. This behaviour is exceptional and implies that the source remains at a state of low efficient accretion during large parts of its outburst. This finding is consistent with the presence of a strong low ionisation disc wind observed in optical and infrared data taken during the hard state of this source. Based on this showcase, we will elaborate on the unique opportunities offered by strictly simultaneous X-ray and IR observations with THESEUS of X-ray binaries.

We present a numerical model for computing emission spectra and light curves from the top surface of relativistic jet, which is based on the single-pulse approximation. The parameters of the model are the jet half-opening angle and radius, the relativistic Gamma factor, and the normalization of the comoving-frame emissivity law. The latter can be a broken-powerlaw, a cut-off powerlaw or a blackbody. The model is of particular interest as it allows to investigate the effects of the observation angle in terms of received flux and peak energy, and may be suitable to study science cases for GRB-related missions such as Theseus. It has been developed for the pyXspec package and made available to the scientific community for data analysis of the GRB spectra.

Title: Outflows from neutron-star mergers: the case of GW170817/GRB170817A Abstract: The observations of GW170817/GRB170817A were extremely rich in electromagnetic counterparts and confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst.

Current and next-generation facilities related to Theseus science (Chair Andrea Santangelo)

We present the SVOM mission that the Chinese National Space Agency and the French Space Agency have decided to jointly implement for a launch target mid-2022.In the line of Swift, SVOM has been designed to detect, characterize and quickly localize gamma-ray bursts (GRBs) and other types of high-energy transients. For this task, the spacecraft will carry two wide field high-energy instruments: ECLAIRs, a hard X-ray imager, and the Gamma-Ray Monitor, a broadband spectrometer. Upon localizing a transient, SVOM will quickly slew towards the source and start deep follow-up observations with two narrow-field telescopes: the Micro-channel X-ray Telescope in X-rays and the Visible Telescope in the visible. The space instrumentation is completed by a ground instrumental ensemble i.e. a Wide Angle Camera and two dedicated ground robotic telescopes. The nearly anti-solar pointing of SVOM combined with the fast transmission of GRB positions to the ground thanks to a VHF antenna network will facilitate the observations of SVOM transients by the largest ground based telescopes. Finally, the development of the SVOM mission will enable the development of concepts and objects that will be reused by the Theseus mission (MPOs, VHF network,…) With a SVOM launch date planned for mid-2022, all these concepts /objects will have reached a great maturity at the time of the Theseus mission adoption.

The Einstein Probe is a space mission dedicated to X-ray time-domain astrophysics, with a broad range of scientific goals. It will carry two instruments, one wide-field X-ray telescope (WXT) to monitor the soft X-ray sky in 0.5-4keV with a 3600 square-degree field-of-view, and one narrow-field X-ray telescope (FXT) for deep follow-up observations in 0.3-10keV and precise source locating. The WXT is an imaging telescope making use of novel X-ray focusing technology of lobster-eye micro-pore optics, which enables the detection of fainter X-ray sources than those by conventional non-focusing techniques. The Einstein Probe is a CAS-led mission with participation of ESA and MPE. In this presentation I will introduce the mission concept and the current status, and its main science goals.

HERMES-TP/SP is a constellation of six 3U nano-satellites hosting simple but innovative X-ray detectors for the monitoring of Cosmic High Energy transients such as Gamma Ray Bursts and the electromagnetic counterparts of Gravitational Wave Events, and for the determination of their position. The projects are funded by the Italian Space Agency and by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 821896. HERMES-TP/SP is an in orbit demonstration, that should be tested in orbit starting from 2022. It is intrinsically a modular experiment that can be naturally expanded to provide a global all sky monitor for high energy transients. In this respect HERMES can be considered also as a precursor of more ambitious missions like Theseus. On behalf of the HERMES-TP and HERMES-SP collaborations I will present the main scientific goals of HERMES-TP/SP, as well as a progress report on the project developments.

High-z gamma-ray bursts unraveling the dark ages and extreme space-time mission - HiZ-GUNDAM Japanese GRB community is now promoting a future GRB satellite mission, HiZ-GUNDAM, whose mission concept has been successfully selected as one of possible candidates for Japanese competitive M-class mission by ISAS/JAXA. The launch schedule will be at the end of 2020s in the earliest case. The aim of HiZ-GUNDAM is to progress a time domain astronomy, especially, (1) probing the early universe with high redshift GRBs at z > 7 (especially z > 9), and (2) progress the multi-messenger astronomy. A base-line architecture of mission payloads is wide field X-ray monitors with micro-pore (Lobster-eye) optics and CMOS or pnCCD imaging detectors on the focal plane, and an optical-near infrared telescope (from 0.5 to 2.5 micron) with aperture size of 30 cm in diameter. HiZ-GUNDAM automatically performs follow-up observation with simultaneous four-band-photometry, and efficiently selects high-redshift GRB and kilonova candidates. Then, quickly performing spectroscopic observation for the candidates by large area telescopes, we obtain physical information such as neutral fraction and metallicity of inter-galactic medium with high-z GRBs, and diversity of kilonova with gravitational wave sources, and so on. We will introduce the focusing sciences and mission overview of HiZ-GUNDAM.

eROSITA on SRG - The New X-ray All-sky Survey The next generation of wide-area, sensitive X-ray surveys designed to map the hot and energetic Universe has arrived, thanks to eROSITA (extended ROentgen Survey with an Imaging Telescope Array), the core instrument on the Russian-German Spektrum-Roentgen-Gamma (SRG) mission. eROSITA's high sensitivity, large field of view, high spatial resolution and survey efficiency is bound to revolutionize X-ray astronomy and deliver large legacy samples for many classes of astronomical objects in the energy range 0.2-8 keV. I will present an overview of the instrument capabilities, the current status of the mission, a few selected early science results and the expectations for the survey program, which has completed last December the second of its eight planned charts of the whole sky.

Current and next-generation facilities related to Theseus science (Chair Piero Rosati)

Gamma Ray Bursts (GRBs) are a unique probe that can be used to address key questions about the high redshift Universe: When and how did the intergalactic medium become re-ionized? What processes governed early chemical enrichment? How does massive star formation evolve at high redshift? A Long GRB signals when a massive star collapses to form a black hole. The GRB afterglow is for a few days a bright light that far outshines the brightest supernovae and quasars. It can be used to observe the spectral fingerprints of the host galaxy and intergalactic medium out to the highest redshift z where star formation is active. Observations of GRB afterglows by large telescopes can directly determine the period of reionization and metal enrichment. GRBs can be used to test models for massive star formation and offer the tantalizing possibility to directly observe the death of the first population III stars. Out of the hundreds of GRBs per year, only a few percent are from high redshift. Speed is of the essence. The proposed Gamow Explorer is optimized to identify high-z GRBs to enable rapid follow up by JWST and >8m ground-based telescopes. Its sensitivity to high-z GRBs will be twenty times the Niel Gehrels Swift Observatory. An onboard 5 channel infrared telescope (0.6 to 2.5 micron) will use the Lyman alpha drop out to find high redshift (z > 6) LGRBs and alert ground based telescopes within 500s of the trigger. An L2 orbit provides continuous observations of the sky that will be optimized for follow up by JWST and large ground based observatories. A second key science goal for Gamow Explorer is to enable rapid identification of kilonova associated with the merger of binary neutron stars. These merger events are associated with gravitational wave signals that have large uncertainties in celestial location. Gamow Explorer is timed to coincide with the A+ era of advanced gravitational wave detectors and will provide < 1000s follow up wide field of view X-ray observations of the associated short gamma ray burst to rapidly identify electromagnetic counterparts. Gamow Explorer will be proposed to the 2021 NASA Explorer MIDEX opportunity and if approved will be launched in 2028 and have a three year prime mission life.

The enhanced X-ray Timing and Polarimetry mission (eXTP) is a flagship observatory for X-ray timing, spectroscopy and polarimetry developed by an International Consortium led by the Chinese Academy of Science, with a large participation of European institutions. Thanks to its very large collecting area, good spectral resolution and unprecedented polarimetry capabilities, eXTP will explore the properties of matter and the propagation of light in the most extreme conditions found in the Universe. eXTP will investigate three fundamental science areas: the equation of state of ultra-dense matter, the effects of strong-field gravity, the astrophysics and physics of very strong magnetic fields. eXTP will, in addition, be a powerful, wide-ranging X-ray observatory. The mission will continuously monitor the X-ray sky, characterizing the active X-ray Universe on a large range of time scales, and will enable multi-wavelength and multi-messenger studies for gravitational waves and neutrinos sources. Specific onboard capabilities are devoted to the detection, localization and autonomous follow-up of GRB and transient sources, also envisaging a fast communication channel to distribute event coordinates in nearly realtime. The mission is currently undergoing its phase B study, targeting a launch in 2027. In this paper I will present the mission and its current status of development.

Gamma-ray bursts (GRBs) are the brightest explosions in the Universe. Some of them, the short GRBs, are important sources of the recently detected gravitational waves and they originate in mergers of neutron stars or possibly also in neutron stars with black holes. More observations of short GRBs are needed for their better understanding. I will present our proposed fleet of nano­satellites to perform an all sky monitoring and timing based localisation of GRBs called Cubesats Applied for MEasuring and LOcalising Transients (CAMELOT). The fleet of nine CubeSats equipped with CsI(Tl) scintillator detectors and readout readout by the Si-PM photon counters will measure the time difference between the arrival of the signal at different satellites (synchronized by GPS) and determine the location of bright GRBs in the sky by triangulation. The satellites will downlink the data about the detected transient within minutes, enabling rapid follow-up observations at other wavelengths. We have built the prototypes of our detector system, which will be launched on two precursor missions, GRBAlpha and VZLUSAT-2, in next few months.

Following the discoveries by the LIGO-VIRGO observatories of gravitational waves (GW) from merging black holes and neutron stars, gamma-ray astronomers aim to observe the entire sky with increasing sensitivity to increase the sample of joint detections of gravitational and electromagnetic waves. The ability to localize these transients is crucial for their followup at longer wavebands. We present a novel gamma-ray detector configuration for best sensitivity and angular localization. The proposed detection unit is a three-dimensional non-uniform pattern of small scintillators with SiPM light-sensors. The non-uniform pattern utilizes the occultation between detectors to provide angular sensitivity to reconstruct the transient's position. The localization accuracy achievable with such a configuration is considerably better than existing scintillator configurations while maintaining a field of view of the entire sky. This new design can be scaled to apply to small satellites, as well as for large missions, such as Theseus.

Several types of extragalactic high-energy transients have been discovered, which include high-luminosity and low-luminosity long-duration gamma-ray bursts (GRBs), short-duration GRBs, supernova shock breakouts, and tidal disruption events without or with an associated relativistic jet. In this talk, we introduce our study on the event rate density of these transients. We applied a unified method to systematically study the redshift-dependent event rate densities and the global luminosity functions (GLF,). Long GRBs have a large enough sample to reveal features in the GLF, which is best charaterized as a triple power law (PL). All the other transients are consistent with having a single PL LF. The total event rate density depends on the minimum luminosity. We will introduce each of them in details. The rate study is crucial to the future prospects for transient detection with wide-field telescopes like THESEUS.

After almost 14 years activity, the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite continues its exploration of the high-energy sky. The AGILE payload houses a suite of detectors operating in the 20–60 keV, 400 keV–100 MeV, 50 MeV–50 GeV energy ranges, allowing broadband observations of Galactic and extragalactic sources, with excellent energy resolution and unique timing capabilities. Among the main scientific targets, Gamma-Ray Bursts (GRBs) represent one of the most interesting subject of study. Up to now, AGILE detected about 500 bursts, providing insights on the high-energy component of the GRB spectrum, and investigating these events on the shortest timescales. AGILE is part of the 3rd Inter-Planetary Network (IPN), delivering prompt automatically-generated Gamma-ray Coordinates Network / Transient Astronomy Network (GCN/TAN) notices on the detected events, and contributing to GRB localization by means of triangulation with other space instruments. We present a review of the most important results on the GRBs detected by AGILE between 2007 and 2020, providing an update on the AGILE detection capabilities of GRBs and other short-duration high-energy transients.

Observatory and additional science (Chair Stephane Basa)

I will describe some of the opportunities that a sensitive, space-borne IR spectrograph and imager will offer the community in the Euclid, JWST, Roman and Rubin era.

In this contribution a review of the recent detections of GRBs by current generation Imaging Atmospheric Cherenkov Telescopes will be provided. The main features of Gamma Ray Bursts emitting at Very High Energy (E>100 GeV) that could be addressed by the Cherenkov Telescope Array in the era of Theseus will be addressed.

Accreting black holes emit in X-rays at the wave-band in which THESEUS will be observing (0.3 keV-20 MeV) due to their extreme physical conditions. The softer energy range is devoted to thermal emission from the accretion disc and the harder is due to the existence of a hard X-ray emitting corona (with undefined geometry so far). The importance of one component versus the other gives rise to the diverse state classification of accreting black holes. This accretion state can be guessed through the X-ray spectral and timing properties that will be routinely measured by THESEUS. There is also the global contribution of the X-ray fluorescence of the accretion disc by the irradiating photons from the corona, which is a powerful tool to unveil its geometry. This component gives rise to the relativistically broadened Fe lines observed in the 0.3-10 keV X-ray spectra and the time-lags of the soft 0.3-1keV energy emission with respect to the main coronal (1-4 keV) emission. The latter require huge detector areas as those present in the current and future XMM-Newton and ATHENA satellites. Nevertheless the former could be achievable (after merging pointing exposures) for the brightest sources (usually located at kpc distances). I will discuss the results obtained with realistic simulations of these features with THESEUS.

GrailQuest (Gamma-ray Astronomy International Laboratory for Quantum Exploration of Space-Time) is an ambitious astrophysical mission concept that uses a fleet of small satellites whose main objective is to search for a dispersion law for light propagation in vacuo. Within Quantum Gravity theories, different models for space-time quantization predict relative discrepancies of the speed of photons w.r.t. the speed of light that depend on the ratio of the photon energy to the Planck energy. This ratio is as small as 10^{−23} for photons in the Gamma-ray band (100 keV). Therefore, to detect this effect, light must propagate over enormous distances and the experiment must have extraordinary sensitivity. Gamma-Ray Bursts, occurring at cosmological distances, could be used to detect this tiny signature of space-time granularity. This can be obtained by coherently combine a huge number of small instruments distributed in space to act as a single detector of unprecedented effective area. This is the first example of high-energy distributed astronomy: a new concept of modular observatory of huge overall collecting area consisting in a fleet of small satellites in low orbits, with sub-microsecond time resolution and wide energy band (keV-MeV). The enormous number of collected photons will allow to effectively search these energy dependent delays. Moreover, GrailQuest will allow to perform temporal triangulation of impulsive events with arc-second positional accuracies: an extraordinary sensitive X-ray/Gamma all-sky monitor crucial for hunting the elusive electromagnetic counterparts of Gravitational Waves, that will play a paramount role in the future of Multi-messenger Astronomy. A pathfinder of GrailQuest is already under development through the HERMES (High Energy Rapid Modular Ensemble of Satellites) project: a fleet of six 3U cube-sats to be launched by the end of 2022.

Accretion luminosities of X-ray binaries (XRBs) span over five orders of magnitude and vary on timescales from seconds to years. High sensitivity and cadence provided by Theseus on long timescales will provide a new perspective on this variability, allowing for the first time to study variability in faint XRBs, monitor sources with low duty cycles such as supergiant fast X-ray transients (SFXTs), and probe spectral variations in bright objects. In the talk I will provide an overview of science cases mentioned above and discuss what Theseus can do in this context.

Observatory and additional science (Chair Filippo Frontera)

In this work, we focus on the ability of IRT-Theseus Telescope to observe Quasars and Lensed by the foreground galaxies Quasars. Our analysis is based on the recent proposals for the Quasar Luminosity Function in infrared as well as on the mass, luminosity distribution function of galaxies and their velocity dispersion, on the z-distribution of quasars and galaxies and on the sensibility of IRT-Theseus.

While the community has studied accretion flows around persistently-accreting AGN for decades, recently-discovered transient AGN phenomena hint at additional complexity and diversity in accretion flows around supermassive black holes. Dozens of Changing-State AGN (CSAGN) indicate cases where the global accretion supply was either cut-off or re-established. Meanwhile, Quasi-Periodic Eruptions (QPEs) may hint at unstable disk modes. Such events are our first observations clues into typical AGN duty cycles and into how rapidly the disk, X-ray corona, and BLR can evolve in response to changes in the global accretion supply. Such events occur very rarely on a per-object basis. However, THESEUS will provide large-area X-ray monitoring of a large sample of AGN and galaxies on timescales of days--months, and thus detect new X-ray-transient CSAGN and QPE events "in the act." These new detections will yield statistically meaningful samples of QPE/CSAGN discoveries, and can be used to trigger multi-wavelength follow-ups to explore how various AGN components interact with each other during extreme-accretion events. Finally, THESEUS can monitor persistently-accreting AGN for new X-ray occultation events by discrete clumpy structures that transit the line of sight, provide much-needed statistical constraints for clumpy-torus models, help determine the distribution and origin of clouds, and explore their links to other AGN structures.

Magnetars are slowly-rotating neutron stars with extremely strong magnetic fields (10^13-15 G), episodically emitting 100 ms long X-ray bursts with energies of about 10^40-41 erg. Rarely, they produce extremely bright, energetic giant flares that begin with a short (200 ms) intense flash, followed by fainter emission lasting several minutes that is modulated by the magnetar spin period (typically 2-12 s), thus confirming their origin. Over the last 40 years, only three such flares have been observed within our local galactic group, which all suffered from instrumental saturation due to their extreme intensity. It has been proposed that extragalactic giant flares likely constitute a subset of short gamma-ray bursts, noting that the sensitivity of current instrumentation prevents us from detecting the pulsating tail to distances slightly beyond the Magellanic clouds. However, their initial bright flash is readily observable out to distances of less than 60 Mpc. Here, we report X- and gamma-ray observations of GRB 200415A, which exhibits a rapid onset, very fast time variability, flat spectra and significant sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extra-galactic magnetar, which was directionally associated with the Sculptor galaxy (NGC 253), 3.5 Mpc away. We discuss the implications of this result and how future observations with THESEUS can elucidate the mechanism responsible for generating these "repeating" short GRBs.

Title:Mrk 110 multiwavelength variability Abstract: I present the first intensive continuum reverberation mapping study of the high accretion-rate Seyfert galaxy Mrk 110. The source was monitored almost daily for more than 200 days with the Swift X-ray and UV/optical telescopes, supported by optical/Near-IR ground-based observations from Las Cumbres Observatory, the Liverpool Telescope, and the Zowada Observatory, thus extending the wavelength coverage to 9100Ang., Mrk 110 was found to be significantly variable at all wavebands. Analysis of the intraband lags reveals two different behaviours, depending on the timescale. On timescales shorter than 10 days the lags, relative to the shortest UV waveband (~1928Ang.), increase with increasing wavelength up to a maximum of ~2 d lag for the longest waveband ~9100Ang. consistent with the expectation from disk reverberation. On longer timescales, however, the z band lags the x-rays of more than 15 days, indicating the presence of a complex interplay between the emission from the accretion disk and diffuse continuum radiation from the broad line region. Finally I will discuss how the flexible simultaneous X-ray/IR coverage of THESEUS can play a crucial role for these kind of studies.