Wide field monitoring in soft X-rays with deep sensitivity
To greatly increase the rate of GRB detection at high redshifts and detect large numbers of other transients while simultaneously providing accurate localisations requires the provision of a large field-of-view soft X-ray imaging instrument. We have performed simulations (Ghirlanda et al. 2015) which show the scientific goals can be met with an instrument field of view of 0.5sr, a sensitivity in 1000 seconds of 10-10 erg cm-2 s-1 (0.3-5 keV) and imaging capability sufficient to provide 1-2 arcmin localisations (this requires a PSF FWHM 5.5 arcmin). Multiple timescale software triggers are required to find the range of flux versus duration transient events. Using such an instrument (the THESEUS SXI) and taking into account the soft X-ray background, the Figure shows the expected annual rate of GRBs as a function of redshift. Also plotted is the rate of GRBs found by Swift (where the redshift distribution has been linearly scaled up based on those with redshift determinations - only approximately one third of Swift discovered GRBs have redshifts, all determined from the ground). The predicted annual rate of GRB detections by THESEUS SXI is ~500 per year, with a very high (x50) increased rate relative to Swift at the highest redshifts. As discussed below in the section on IR follow-up, imaging and photometric redshifts will be obtained on-board for the highest redshift GRBs and spectroscopic redshifts for the majority. For those GRBs detected on board but without spectroscopy triggers sent to ground telescopes can be used to obtain spectra – giving priority to those with photometric indication of high redshift. THESEUS alone will obtain more spectroscopic redshifts on board in a year than Swift has provided in a decade. The search for high-z GRBs is part of a more general unprecedentedly deep monitoring of the X-ray transient Universe.
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| Figure 1. Observer frame peak energy versus bolometric flux of GRBs with well constrained redshifts and spectra detected by various missions with a cloud of points from GRB population simulations (Ghrirlanda et al. 2015). Yellow points at those at z>5 The use of a softer X-ray band permits the detection of GRBs with lower fluence and hence enhances the detection of higher redshift objects. | Figure 2. The annual rate of GRBs predicted for THESEUS SXI (red) compared to Swift (blue). The upper scale shows the age of the Universe. For Swift the actual number of known redshifts is approximately one third that plotted and none were determined on board (the blue curve has been linearly scaled upwards to match the total Swift trigger rate). For THESEUS the red region uses the simulations from Ghirlanda et al. (2015) and adopts the instrument sensitivity for the SXI. |
The predicted rate of detection of electromagnetic counterparts of GW signals and of other transient and variable source types during the survey is shown in Table 2. The very large detection rate of other transient types is due to the high sensitivity of THESEUS. This is illustrated in Figure 4, where the source detection sensitivity of the proposed SXI and XGIS instruments are plotted verses integration time and overlaid are various sources types.
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| Table 2. Theseus detection rates for different astrophysical transients and variables |