Article Index

Observatory science

The suite of THESEUS instruments will allow the world-wide community to carry out innovative scientific investigations on many different classes of Cosmic sources. We discuss below just a few examples.

IRT observatory science

Based on our conservative estimate of GRB triggers, we expect that 30-40% of the IRT time will be available and open to the scientific community for additional GO science programs. On one hand, we expect that a wide range of extragalactic science cases will be optimally addressed with the Euclid survey (e.g. high-z obscured AGN and quasars, high-z clusters). Euclid (launch 2021) will cover 15,000 square degrees of the extragalactic sky, down to AB mag(YJH)=24, with R=380 slitless NIR spectroscopy on the same area; this is ~2 mag deeper than the IRT sensitivity in 1 hr exposure. However, Euclid will not have any ability to follow-up transients. In light of this, we will dedicate a fraction of the THESEUS observing time to fast Target of Opportunity (ToO) open to the community, in order to broaden our science cases. A spectro-photometric facility to follow-up transient sources provides a unique service to the community at large and is known to yield high-impact science. A hard lesson that we are learning from current surveys is that the number of detected transients is very large and only a minimal fraction of them can be followed-up, resulting in a severe loss of efficiency (as a matter of fact, a large fraction of newly-discovered transients remains unclassified). In the next decade, time-domain surveys (like CTA, LSST and SKA) and next generation multi-messenger facilities (see answer 1d) will revolutionise astronomy, providing insight in basically all areas but, at the same time, the need of follow-up facilities will dramatically increase. THESEUS will be a key facility to provide the multiwavelength partner to any kind of transient survey (and will itself work as an X-ray transient factory thanks to the SXI instrument). In recent years, at the dawn of time-domain astronomy, the experience of the Swift mission is enlightening. A large fraction of the Swift observing time is fully open to ToO observations from the community. Thanks to a flexible planning system, this open time has been optimally exploited, significantly contributing to the success of the Swift mission. However, as also mentioned in the answer 3b, no other similar space missions with flexible scheduling are planned to be in operation around 2030. More importantly, while the astronomical community has (and likely will have in the future) a relatively easy access to optical imaging facilities (mainly ground based) dedicated to the follow-up of transients, the access to facilities operating in the NIR and/or with spectroscopic capabilities will remain difficult. To this end, a multi-frequency space mission equipped with a NIR telescope with spectro-photometric capabilities represents an asset, enabling a fast and efficient precise (sub-arcsecond) localisation and/or classification of newly-discovered transient sources through multi-frequency SED and spectroscopic characterization with very limited constraints with respect to ground-based observatories (day/night, weather, background).

A flexible and efficient use of the non-GRB THESEUS time with GO and ToO programs will enable to tackle a wide range of studies of variable and transients sources. The near-infrared bands are critical for Solar-system object tracking and multi-epoch variability studies. Cool stars, whose photon fluxes peak in the near-IR, are ideal targets for the detection and characterization of exoplanets using the transit technique, either in surveys or for follow-up observations of individual sources (see, e.g., Clinton et al. 2012, PASP, 124, 700 and references therein). Simultaneous X-ray and NIR monitoring of samples of T-Tauri stars will shed light on the mechanisms responsible for the onset of the observed outbursts, and how the accretion rate of matter on these stars and the emission of jets can influence the formation of proto-planetary systems. Several open questions for low-mass X-ray binaries, hosting either neutron stars or stellar-mass black holes, require simultaneous IR and X-ray photometry (e.g. concerning the physics of jet emission from these sources; see, e.g., Migliari et al. 2010, ApJ, 710, 117; Russell et al. 2013, MNRAS, 429, 815). Recent studies have found that the peak luminosity of SNe Ia are genuine standard candles in the NIR (e.g. Krisciunas et al. 2004, ApJ 602 L81; Burns et al. 2014, ApJ 789 32). Considering also the reduced systematics in the NIR related to host-galaxy reddening, the IRT will represent an very efficient tool to construct a low-z sample of SNe Ia to be compared with the high-z samples that will be built by forthcoming IR facilities (e.g. JWST). In addition, gravitational time delays (a technique increasingly exploited in recent years for competitive H0 measurements) and AGN reverberation mapping variability projects will be particularly advantageous for THESEUS, where the cadence can be chosen more flexibly.

We remark that, so far, studies as the one listed above require the access to NIR ground-based facilities (usually subjected to a high pressure of requests from the community) and/or a rather complex organisation of coordinated observational campaigns involving ground and space-based facilities. The use of THESEUS as a single, flexible multi-frequency observatory will overcome all these problems, making it very attractive for the community.

GRB physics

The progresses in fitting the prompt emission spectra of gamma-ray bursts (GRBs) are boosting our understanding of the still elusive origin of this radiation and have been made possible thanks to a more and more detailed analysis of the low-energy part (< 100 keV) of the prompt spectrum. In this energy domain, the spectral shape is sometimes found to deviate from a simple power-law shape. The deviation is well described by a spectral break or, alternativelyby the addition of a thermal component. Spectral data extending down to less than 1 keV are extremely relevant for these studies, but presently they are available only for a small subsample of Swift GRBs observed by Swift/XRT (0.3-10 keV) during the prompt emission. The space mission THESEUS will allow a systematic study of prompt spectra from 0.3 keV to several MeV. We show that observations performed by THESEUS will allow us to discriminate between different models presently considered for GRB prompt studies, solving the long-standing open issue about the nature of the prompt radiation, with relevant consequences on the location of the emitting region, magnetic field strength and presence of thermal components.

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Figure 7. Left: contour plot (showing the 1,2,3 σ levels) for the population (adapted from Ghirlanda et al. 2015) of GRBs that will be detected by THESEUS (red solid curves) in the E peak -flux plane (the flux is integrated in the 10-1000 keV energy range). The subsample of events that will be detected by both SXI and XGIS for which a broad band spectral study will be possible is shown by the shaded contours. The solid yellow and cyan lines show the sensitivity limits of Fermi and BATSE, respectively, adapted from Nava et al. (2011). The entire GRB population simulated in Ghirlanda et al. (2015) is shown by the dashed contour lines. Right: simulation of the spectrum of the first 13 s of the prompt emission of GRB 990705 as would be measured by SXI (green) and XGIS (red and black). As can be seen, the transient absorption edge at ∼3.8 keV and the high absorption column detected by the BeppoSAX/WFC (Amati at al. 2000) would be revealed by THESEUS with very high significance.

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Figure 8. Left: distributions in the spectral peak energy (Ep) – Fluence plane of soft X-Ray Rich (XRR) GRBs and X-Ray Flashes (XRFs) compared to that of normal GRBs (Sakamoto et al. 2005). Right: Blast wave velocity and energy for massive star explosions (adapted from Soderberg et al. 2010). Soft/weak GRBs may constitute the bulk of the GRB population and the link with other explosive events associated to the death of massive stars.

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Figure 9. Left: SNe–Ia + GRB Hubble diagram obtained by exploiting the correlation between spectral peak energy (Ep) and radiated energy or luminosity (Eiso, Liso) in GRBs (Kodama et al. 2008). Right: determination of the dark energy equation of state parameters w0 and wa with the expected sample of GRBs form THESEUS by using the same technique (e.g., Amati & Della Valle 13).