Article Index

Science with THESEUS

The Transient High Energy Sources and Early Universe Surveyor (THESEUS) will address multiple fundamental questions for modern cosmology and astrophysics. The primary scientific goals of the mission address the Early Universe ESA Cosmic Vision theme “How did the Universe originate and what is made of?” and, specifically, the sub-themes 4.1 Early Universe, 4.2 The Universe taking shape and 4.3 The evolving violent Universe. It will also have a relevant impact on the The Gravitaitonal Wave Universe (3.2) and “The Hot and Energetic Universe” themes. THESEUS will have unique capabilities to: a) Explore the Early Universe (the cosmic dawn and re-ionization era) by unveiling the Gamma-Ray Burst (GRB) population in the first billion years; and b) Perform an unprecedented deep monitoring of the soft X-ray transient Universe, thus providing a fundamental synergy with next-generation gravitational waves and neutrino detectors (multi-messenger astrophysics), as well as the large e.m. facilities of the next decade (ATHENA, E-ELT, SKA, CTA, LSST, etc.)

Because of their huge luminosities, mostly emitted in X- and gamma-rays, their redshift distribution, extending at least to z ~10 and their association with explosive death of massive stars, Gamma-Ray Bursts (GRBs) are unique and powerful tools for cosmology. In particular, GRBs represent a unique tool to study the early Universe up to the re-ionization era. To date there is no consensus on the dominant sources of re-ionization, and GRB progenitors and their hosts are very good representatives of the massive stars and star-forming galaxies that may have been responsible. A statistical sample of high–z GRBs (about 50 at z > 6) can provide fundamental information such as: measuring independently the cosmic star–formation rate, even beyond the limits of current and future galaxy surveys, the number density and properties of low-mass galaxies, the neutral hydrogen fraction, the escape fraction of UV photons from high-z galaxies. Even JWST and E-ELTs surveys, in the 2020s, will be not able to probe the faint end of the galaxy Luminosity Function at high redshifts (z>6-8). The first, metal–free stars (the so–called Pop III stars) can result in powerful GRBs (e.g. Meszaros+10). GRBs offer a powerful route to directly identify such elusive objects (even JWST will not be able to detect them directly) and study the galaxies in which they are hosted. Even indirectly, the role of Pop III stars in enriching the first galaxies with metals can be studied by looking to the absorption features of Pop II GRBs blowing out in a medium enriched by the first Pop III supernovae (Wang+12). More generally, high-z GRBs will allow the cosmic chemical evolution to be investigated at early times.

Besides high-redshift GRBs, THESEUS will detect and localize in the X-rays and promptly follow-up in the IR a large number of both transients and variable X-ray sources serendipitously during regular observations. These data will provide a wealth of unique science opportunities, by revealing the violent Universe as it occurs in real-time, through an all-sky X-ray smonitoring of extraordinary grasp and sensitivity carried out at high cadence. Here we emphasise the following primary objectives:

  1. Provide real time trigger and accurate (~1-2 arcmin within a few seconds; ~1 arcsec within a few minutes) location of (long/short) GRBs and high-energy transients for follow-up with next-generation optical-NIR (E-ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), TeV (CTA) or neutrino telescopes and identify electromagnetic counterpart of detections by next generation gravitational wave and neutrino detectors;

  1. Discover new high-energy transient sources over the whole sky, including supernova shock break-outs, black hole tidal disruption events, magnetar flares, and monitor known X-ray sources, with high cadence observations.

By finding huge numbers of GRBs the survey will also permit unprecedented insights in the physics and progenitors of GRBs and their connection with peculiar core-collapse SNe, and substantially increase the detection rate and characterization of sub-energetic GRBs and X-Ray Flashes. The provision of a high cadence soft X-ray monitoring in the 2020s together with a 0.7m IRT in orbit will enable a strong synergy with transient phenomena observed with the Large Synoptic Survey Telescope (LSST) and the other large facilities that will be operating in the e.m. domain (E-ELT, SKA, CTA, JWST, ATHENA, …).

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Figure 1: Left: THESEUS in context. Right: THESEUS will have the ideal combination of  instrumentation and mission profile for detecting all types of GRBs (long, short/hard, weak/soft, high-redshift), localizing them from a few arcmindown to arsecand measure the redshift for a large fraction of them.


Exploring the Early Universe

A major goal of contemporary astrophysics and cosmology is to achieve a broad understanding of the formation of the first collapsed structures (Pop III and early Pop II stars, black holes and galaxies) during the first billion years in the life of the universe. This is intimately connected to the reionization of the IGM and build up of global metallicity. The latter is very poorly constrained, and even in the JWST era will rely on crude emission line diagnostics for only the brightest galaxies. Regarding reionization, measurements of the Thomson scattering optical depth to the microwave background by the Planck satellite now suggest it substantially occurred in the redshift range z ~ 7.8 – 8.8 (e.g., Planck collaboration. 2016), whereas the observations of the Gunn-Peterson trough in the spectra of distant quasars and galaxies indicate it was largely finished by z ~ 6.5 (e.g., Schenker et al. 2014). Statistical measurements of the fluctuations in the redshifted 21 cm line of neutral hydrogen by experiments such as LOFAR and SKA are expected to soon provide further constraints on the time history (e.g, Patil et al. 2014). The central question, however, remains whether it was predominantly radiation from massive stars that both brought about and sustained this phase change, or whether more exotic mechanisms must be sought?

Solving this problem largely splits into two subsidiary issues: how much massive star formation was occurring as a function of redshift? And, on average, what proportion of the ionizing radiation produced by these massive stars escaped from the immediate environs of their host galaxies? The former can be extrapolated based on observed candidate z > 7 galaxies found in HST deep fields, but two very significant uncertainties are, firstly, the completeness and cleanness of the photometric redshift samples at z > 7, and, secondly, the poorly constrained form and faint-end behaviour of the galaxy luminosity function (at stellar masses <~ 108 M), especially since galaxies below the HST detection limit very likely dominate the star-formation budget. Even though some constraints on fainter galaxies can be obtained through observations of lensing clusters (e.g. Atek et al. 2015 ApJ 814 69), which will be improved further by JWST, simulations suggest star formation was likely occurring in considerably fainter systems still (Liu et al. 2016). The second problem, that of the Lyman continuum escape fraction, is even more difficult since it cannot be determined directly at these redshifts, and lower redshift studies have generally found rather low values of fesc of only a few percent (e.g., Nestor et al. 2013), likely insufficient to drive reionization unless fesc increases significantly at early times and/or for smaller galaxies (e.g., Robertson et al. 2013).

GRBs and their host galaxies provide several powerful alternative routes to answering these key questions, potentially probing star formation, metal enrichment and galaxy evolution even preceding reionization.

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Figure 2. Top left: redshift distribution of long GRBs as of 2020 (De Cia 2011, PhD Thesis). Top right: The cumulative distribution of GRBs with redshift determination as a function of the redshift for Swift (in 10 yr) and THESEUS (in 3 yr). We note that these predictions are conservative in so far as they reproduce the current GRB rate as a function of redshift. However, with our sensitivity, we can detect a GRB of  Eiso ~ 1053 erg (corresponding to the median of the GRB radiated energy distribution) up to redhsift 12. Indeed, our poor knowledge of the GRB rate - SFR connection does not preclude the existence of a sizeble number of GRBs at such high redshifts, in keeping with recent models of pop III stars. Bottom: the IR spectrum of the faint host of the GRB 050730 (Starling et al. 2005). 


Global star formation from GRB rate as a function of redshift

Long-duration GRBs are produced by massive stars, and so track star formation, and in particular the populations of UV-bright stars responsible for the bulk of ionizing radiation production. Although there is evidence at low redshift that GRBs are disfavoured in high metallicity environments, since high-z star formation is predominantly at low metallicity (e.g., Salvaterra et al. 2013; Perley et al. 2016) it is likely that the GRB rate to massive star formation rate is approximately constant beyond z ~ 3. Thus simply establishing the GRB N(z), and accounting for the instrumental selection function, provides a direct tracer of the global star formation rate density as a function of cosmic time. Analyses of this sort have consistently pointed to a higher SFR density at redshifts z > 6 than traditionally inferred from UV rest-frame galaxy studies (Figure 3 left), which rely on counting star-forming galaxies and attempting to account for galaxies below the detection threshold. Although this discrepancy has been alleviated by the growing realisation of the extremely steep faint-end slope of the galaxy LF at z > 6, it still appears that this steep slope must continue to very faint magnitudes, MAB ~ -11 in order to provide consistency with GRB counts and indeed to achieve reionization (something that can only be quantified via a full census of the GRB population).

The high-z galaxy luminosity function

As discussed above, this is a key issue for understanding of reionization since, to the depth achieved in the Hubble Ultra-deep Field (HUDF), it appears that the faint-end of the LF at z > 6 approaches a power-law of slope α = 2. Thus the value of the total luminosity integral depends sensitively on the choice of low-luminosity cut-off (and indeed the assumption of continued power-law form for the LF). By conducting deep searches for the hosts of GRBs at high-z we can directly estimate the ratio of star-formation occurring in detectable and undetectable galaxies, with the sole assumption that GRB-rate is proportional to star-formation rate (Figure 3 left). Although currently limited by small-number statistics, early application of this technique has confirmed that the majority of star formation at z > ~ 6 occurred in galaxies below the effective detection limit of HST (Tanvir et al. 2012; McGuire et al. 2016). Since the exact position and redshift of the galaxy is known via the GRB afterglow, follow-up observations are more efficient than equivalent deep field searches for Lyman-break galaxies.

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Figure 3. Left: Star Formation Rate density as a function of redshift as derived from rest-frame UV surveys (gray points and green hatched region for corresponding model) and from GRBs based on different assumptions (GRB rate to SFR ratio, GRB progenitor metallicity) (Robertson & Ellis 2012). Yellow points show corresponding estimates expected from THESEUS. Right: Absorption-line based metallicities [M/H] as a function of redshift of Damped Lyman-alpha absorbers, for GRB-DLAs (blue symbols) and emission line metallicities for GRB hosts (red symbols) [adapted from Sparre et al. 2014]. GRBs are essential to probe evolution of ISM metallicities in the first billion years of cosmic history.

The Lyman continuum escape fraction

Direct observations of escaping Lyman continuum radiation is essentially impossible at z > 6, especially for the small galaxies responsible for the bulk of star formation. GRBs provide a powerful alternative since high-S/N afterglow spectroscopy reveals the neutral hydrogen column along our line-of-sight to the GRB. Since the opacity of the medium to FUV photons depends on this column, a statistical sample of afterglows can be used to infer the average escape fraction over many lines of sight, specifically to the locations of massive stars dominating global ionizing radiation production. Useful constraints have so far only been possible at z = 2-4, indicating an upper limit of fesc< 7.5% (e.g., Fynbo et al. 2009), but future observations of z > 6 GRBs, particularly in the era of 30 m ground based telescopes, promise to provide a much more precise constraints during the epoch of reionization (see below and Figure 4).

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Figure 4: The UV luminosity density from stars at z~8 and average escape fraction <fesc> are insufficient to sustain reionization unless the galaxy luminosity function steepens to magnitudes fainter than MUV=-13 (grey hatched region), and/or <fesc> is much higher than that typically found at z~3 (grey shaded region). Even in the late 2020s, <fesc> at these redshifts will be largely unconstrained by direct observations. The green contours show the 1- 2-σ expectations for a sample of 25 GRBs at z~7-9 for which deep spectroscopy provides the host neutral column and deep imaging constrains the fraction of star formation occurring in hosts below the JWST limit (Robertson et al. 2013 ApJ 768 71). The input parameters were log10(ρUV)=26.44 and <fesc>=0.23, close to the (red) borderline for maintaining reionization by stars.

The build-up of metals

Bright GRB afterglows with their intrinsic power-law spectra provide ideal backlights for measuring not only the hydrogen column, but also obtaining exquisite abundances and gas kinematics probing to the hearts of their host galaxies (e.g., Hartoog et al. 2015). Further, the imprint of the local dust law, and in some cases observation of H2 molecular absorption, provides further detailed evidence of the state of the host ISM (e.g. Friis et al. 2015). Thus they can be used to monitor cosmic metal enrichment and chemical evolution to early times, and search for evidence of the nucleosynthetic products of even earlier generations of stars. In the late 2020s, taking advantage of the availability of 30m class ground-based telescopes (and possibly Athena), superb abundance determinations will be possible through simultaneous measurement metal absorption lines and modelling the red-wing of Lyman-alpha to determine host HI column density, potentially even many days post-burst (e.g. Figure 5). We emphasize that using the THESEUS on-board NIR spectroscopy capabilities, will provide the redshifts and luminosity measurements that are essential to optimising the time-critical follow-up observations using the highly expensive next-generation facilities, allowing us to select the highest priority targets and use the most appropriate telescope and instrument.

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Figure 5. Simulated E-ELT 30 min spectrum of a faint GRB afterglow observed after ~1 day. The S/N provides exquisite abundance determinations from metal absorption lines, while fitting the Ly-a damping wing simultaneously fixes the IGM neutral fraction and the host HI column density, as illustrated by the two extreme models, a pure 100% neutral IGM (green) and best-fit host absorption with a fully ionized IGM (red).

Topology of reionization

In practice, it is expected that reionization should proceed in a patchy way, with ionized bubbles being created first around the highest density peaks where the first galaxies occur, expanding and ultimately filling the whole IGM. The topology of the growing network of ionized regions reflects the character of the early structure formation and the ionizing radiation field. With high-S/N afterglow spectroscopy the Ly-alpha red damping wing can be decomposed into contributions due to the host galaxy and the IGM. The latter provides the hydrogen neutral fraction and so measures the progress of reionization local to the burst. With samples of several tens of GRBs at z > 7-8, we can begin to statistically investigate the average and variance of the reionization process as a function of redshift (e.g., McQuinn et al. 2008).

Population III stars

In addition to possible evidence for population III chemical enrichment, it has been argued that pop III stars may also produce collapsar-like jetted explosions, which are likely to be of longer duration than typical long-GRBs, and may be detected through surveys with longer dwell times (Meszaros & Rees 2010). To date no direct evidence of this connection has been observationally established. The multiwavelength properties of GRBs with a Pop-III progenitor are only predicted on the expected large masses, zero metallicity of these stars. Even the detection of a single GRB from a popIII progenitor would put fundamental constraints on the unknown properties of the first stars.

The role of THESEUS

Our detailed simulations indicate that THESEUS will detect between 30 and 80 GRBs at z > 6 over a three year mission, with between 10 and 25 of these at z > 8 (and several at z > 10). The on-board follow-up capability will mean that redshifts are estimated for the large majority of these, and powerful next generation ground- and space-based telescopes available in this era will lead to extremely deep host searches and high-S/N afterglow spectroscopy for many (e.g. using JWST, if still operating, E-ELT, ATHENA etc.). To illustrate the potential of such a sample, we simulate in Figure 4 the precision in constraining the product of the UV luminosity density and average escape fraction, ρ UV  fesc, that would be obtained with 25 GRBs at 7<z<9 having high-S/N afterglow spectroscopy and (3hr) JWST depth host searches (for definiteness the ρUV axis corresponds to z=8). This will provide a much clearer answer to the question of whether stars were the dominant contributors to reionization. In addition, this sample will allow us to map abundance patterns across the whole range of star forming galaxies in the early universe, providing multiple windows on the nature of the first generations of stars.

As an example of the great relevance of THESEUS in the context of the next generation large facilities (e.g., SKA, CTA, E-ELT, ATHENA), we highlight here the THESEUS synergy with Athena. Two of the primary science goals for Athena are: (1) Locate the missing baryons in the Universe by probing the Warm Hot Intergalactic Medium (the WHIM); this requires about 10 bright GRBs per year. (2) Probe the first generation of stars by finding high redshift GRB; this requires about 5 high-redshift GRBs per year. THESEUS will enable Athena to achieve these goals by greatly increasing the rate of GRBs found per year with good localisations and redshifts and the X-ray band of the THESEUS SXI will find a greater proportion of high-redshift GRBs than previous missions. Hence THESEUS will: (1) localise bright GRBs with sufficient accuracy using the SXI to enable a rapid repointing of the Athena X-IFU for X-ray spectroscopy of the WHIM, and (2) find high-redshift GRBs using the SXI and XGS and accurately localising them using the IRT for redshift determination on-board and on the ground to provide reliable high-redshift targets for Athena. Many of the other transients found by THESEUS, such as tidal disruption events and flaring binaries will also be high-value targetsfor Athena.

Gravitational-wave sources and multi-messenger time domain Astrophysics

The launch of THESEUS will coincide with a golden era of multi-messenger astronomy. With the first detection of gravitational waves (GWs) by Advanced detectors (Abbott et al. 2016a, Abbott et al. 2016b), a new window on the Universe has been disclosed. By the end of the present decade, the GW sky will be routinely monitored by a network of second generation GW detectors, an ensemble of Michelson-type interferometers composed by the two Advanced LIGO (ALIGO) detectors in the USA and by Advanced Virgo (AdVirgo) in Italy, plus ILIGO in India and KAGRA in Japan joining later within a few years. By 2030, more sensitive third generation GW detectors, such as the Einstein Telescope and Cosmic Explorer, are planned to be operative. In parallel to this advancement, IceCube and KM3nNeT and the advent of Megaton detectors will likely revolutionize neutrino astrophysics.

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Figure 6. Top left: by the end of the ‘20s, if ET will be a single detector, almost no directional information will be available for GW sources, and a GRB-localising satellite will be essential to discover EM counterparts. If multiple third-generation detectors will be available, the typical localization uncertainties will be comparable to the aLIGO ones. The plot shows the SXI field of view (~110x30 deg2) superimposed on the probability skymap of GW151226 observed by aLIGO. Top right: THESEUS will detect and localize down to 0.5-1 arcmin the soft X-ray short/long GRB afterglows, of NS-NS (BH) mergers and of many classes of galactic and extra-galactic transients. For several of these sources, THESEUS/IRT will provide detection and study of associated NIR emission, location within 1 arcsec and redshift. Middle: the detection, study and arcsecond localization of afterglow and kilonova emission  from short GRB/GW events will be possible with THESEUS/IRT. In this figure, we show in particular the light curve of the kilonova associated to the gravitational wave/short GRB event GW170817/GRB170817A in the IRT filters. The continuous and dashed red lines indicate the THESEUS/IRT limiting H magnitudes for imaging and prism spectroscopy, respectively, with 300s of exposure. Bottom: THESEUS will be able to promptly and accurately localize e.m. counterparts to GW events.


Several of the most powerful transient sources of GWs predicted by general relativity, e.g. binary neutron star (NS-NS) or NS-black hole (BH) mergers (with likely detection rate of ~50 per year by the LIGO-Virgo network at design sensitivity; Abadie et al. 2010), are expected to produce bright electromagnetic (EM) signals across the entire EM spectrum and in particular in the X-ray and gamma-ray energy bands, as well as neutrinos. GW detectors have relatively poor sky localization capabilities of ~10 sq. degrees (e.g. Klimenko et al. 2011). Neutrino detectors such as IceCube and KM3NeT can localize to an accuracy of better than a few sq. degrees but within a smaller volume of the Universe (e.g. Santander 2016 and references therein). In order to maximize the science return of the multi-messenger investigation it is essential to have an in-orbit trigger and search facility that can either detect an EM signal simultaneous with a GW/neutrino event or rapidly observe with good sensitivity the large error boxes provided by the GW and neutrino facilities following a trigger.

This combined requirements are uniquely fulfilled by THESEUS, which is able to trigger using XGIS or SXI and observe a very large fraction of the GW/neutrino error boxes within an orbit due to the large grasp of the SXI instrument (if compared for example to current generation X-ray facilities such as Swift, THESEUS/SXI has a grasp of ~40 times that of Swift/XRT). For events triggered on-board with XGIS or SXI, GW searches can also be carried on the resultant known sky locations with lower GW detector signal-to-noise thresholds and hence an increased search distance.

The detection of EM counterparts of GW (or possibly neutrino) signals will enable a multitude of science programmes (e.g. Bloom et al. 2009; Phinney 2009) by allowing for parameter constraints that the GW/neutrino observations alone cannot fully provide. For example, finding a GW/neutrino source EM counterpart in X-rays with THESEUS/SXI, will allow to localize the source with an accuracy good enough for optical follow-up and hence to possibly measure its redshift and luminosity. On the other hand, not finding an EM counterpart will constrain merger types (such as BH-BH mergers), emission mechanisms, astrophysical conditions at the time of merger, and total energetics.

Moreover, in 2020s synergies with the future facilities like James Webb Space Telescope (JWST), WFIRST, ATHENA, Einstein Probe, E-ELT, TMT, GMT, SKA, CTA zPTF and LSST telescopes would be desirable in order to be able to extend the multi-messenger observations across the whole EM spectrum.

Gravitational wave transient sources

NS-NS / NS-BH mergers: Collimated EM emission from Short GRBs

Compact binary coalescing (CBC) mergers are among the most promising sources of GWs that will be detected in the next decade. Indeed, these systems are expected to radiate GWs within the most sensitive frequency range of ground based GW detectors (1-2000 Hz), with large GW energy output, of the order of 10-2 Moc2, and gravitational waveforms well predicted by General Relativity (see Baiotti & Rezzolla 2016 for a review). The expected rate of NS-NS systems inferred from binary pulsar observations and population synthesis modeling, is taken to lie between 10 and 10000 Gpc-3 yr-1. To date, no NS-BH systems have been observed, but the rate can still be predicted through population synthesis, constrained by the observations of NS-NS, to be ~1-1000 Gpc-3 yr-1 (see Abadie et al. 2010 and references therein).

Mounting indirect evidence associates short GRB progenitors to CBC systems with at least one neutron star (e.g. Berger 2014) and provides possible hints that the merger of magnetized NS binaries will lead to the formation of a jet with an opening angle of ~30°-40° (Rezzolla et al. 2011). GW observations combined with multi-wavelength follow-up campaigns in the next years will likely confirm this scenario by simultaneous detections of CBC-GWs temporally and spatially consistent with short GRBs. With the present sensitivity distance range, of the second-generation GW detectors can detect CBC systems up to ~200-300 Mpc. Within such a volume the expected short GRB rate is rather low, of the order of ~1/yr or less. However, by the time of the launch of THESEUS, gravitational radiation from such systems will be easily detectable by third-generation detectors such as the Einstein Telescope (ET) up to redshifts z ~ 2 (e.g. Chassande-Mottin et al. 2011) implying that a large fraction of the short GRBs that THESEUS will detect, i.e., ~20 short GRBs per year, will have a detectable GW emission. The estimated rate of detected short GRBs is ~1-10 Gpc-3 yr-1 (Wanderman & Piran 2015 and reference therein, see also Ghirlanda et al. 2016).

Almost all short GRBs are accompanied by an X-ray afterglow that SXI will detect and monitor just after the burst emission. Once localized with SXI, about 40% of detected short GRBs are expected to have a detectable optical/IR counterpart. The IRT could point the SXI localized afterglow within few minutes from the trigger. If bright enough, spectroscopic observations could be performed onboard, thus providing redshift estimates and information on chemical composition of circumburst medium. In addition, precise sky coordinates will be disseminated to ground based telescopes to perform spectroscopic observations.

Distance measurements of a large sample of short GRBs, combined with the absolute source luminosity distance provided by the CBC-GW signals, can deliver precise measurements of the Hubble constant (Schutz 1986), helping to break the degeneracies in determining other cosmological parameters via CMB, SNIa and BAO surveys (for example Dalal et al. 2006). Using the predicted XGIS/SGI GRB rates, the XGIS field of view and assuming a conservative 20% beaming fraction, the predicted THESEUS/ALIGO/AdVirgo triggered coincidence number is 3-4 within the nominal mission life, but rises to 10 or more when including SXI follow-up observations of ALIGO/AdVirgo error boxes. With 10 GW+EM events, the Hubble constant could be constrained to 2-3%, thus providing a precise independent measure of this fundamental parameter (Dalal et al. 2006). In addition, each individual joint GW+EM observation would provide an enormous science return from THESEUS. For example, the determination of the GW polarization ratio would constrain the binary orbit inclination and hence, when combined with an EM signal, the jet geometry and source energetics. Likewise, a better understanding of the NS equation of state can follow from combined GW and EM signals (e.g., Lasky et al. 2014, Ciolfi & Siegel 2015a,b, Messenger et al. 2015).

NS-NS / NS-BH mergers: Optical/NIR and soft X-ray isotropic emissions

Nearly isotropic EM emission is expected from NS-NS / NS-BH mergers at minutes-days time scale from the merger. GW emission depends only weakly on the inclination angle of the inspiral orbit, and GW detectors will mostly trigger on off-axis mergers (i.e. for binary system with a non zero inclination angle). Re-pointing of THESEUS in response to a GW trigger will allow SXI to study off-axis X-ray emission. One expected off-axis X-ray emission is the late afterglows from the laterally spreading jet as soon as it decelerates (“orphan afterglows”, van Eerten et al. 2010). Peak brightness is expected at 1-10 days after the trigger, with peak fluxes equal or below ~10-12-10-13 erg cm-2 s-1 at ~200 Mpc (Kanner et al. 2012). Therefore, despite their low collimation, off-axis afterglows will be detected only for the most nearby CBC systems.

Another nearly-isotropic emitting component is expected if a massive millisecond magnetar is formed from two coalescing NSs. In this case, X-ray signals can be powered by the magnetar spindown emission reprocessed by the matter surrounding the merger site (isotropically ejected during and after merger), with luminosities in the range 1043-1048 ergs/s and time scales of minutes to days (Metzger & Piro 2014, Siegel and Ciolfi 2016a,b). Alternatively, X-ray emission may come from direct dissipation of magnetar winds (e.g. Zhang 2013, Rezzolla & Kumar 2015). Numerical simulations suggest that such emission is collimated but with large half-opening angles (30°-40°, beaming factor of ~0.2). As an additional channel in X-rays, the magnetar may power the isotropically expelled matter through wind pressure to relativistic speeds generating a shock with ISM (“confined winds”). Synchrotron radiation produced in the shock is emitted nearly isotropically, with an enhanced intensity near the equator. A beaming factor of ~0.8 is expected in this case (e.g. Gao et al. 2013). Overall, typical time scales for these X-ray signals are comparable to magnetar spin down time scales of ~103-105 s, and the predicted luminosities span a wide range that goes from~1041 erg/s to ~1048 erg/s. With THESEUS/SXI in combination with sensitive GW detectors as ET, several X-ray counterparts of GW events from NS-NS merging systems will be easily detected possibly up to large distances (depending on the largely uncertain intrinsic luminosity of such X-ray component), providing a unique contribution to probe and classify X-ray emission from BNS systems.

In the optical band, the expected EM component is represented by the so-called “macronova” (often named “kilonova”) emission. During NS-NS or NS-BH mergers, in fact, a certain amount of ejected mass is expected to become unbound. This matter has the unique conditions of high neutron density and temperature to initiate the r-process nucleosynthesis of very heavy elements. Days after merger, the radioactive decay of such elements heats up the ejected material producing a transient signal peaking in the optical/near-infrared (NIR) band and with luminosities of ~1041 erg/s (e.g. Li & Paczynski 1998, see also Baiotti & Rezzolla 2016 for a review). Today, macronovae are promising electromagnetic counterparts of binary mergers because (i) the emission is isotropic and therefore the number of observable mergers is not limited by beaming; (ii) the week-long emission period allows for sufficient time needed by follow-up observations. Once identified, source location can be accurately recovered, allowing for the identification of the host galaxy and the search for counterparts in other electromagnetic bands (e.g. radio). The detectability of macronovae is currently limited by the lack of sufficiently sensitive survey instruments in the optical/NIR band that can provide coverage over tens of square degrees, the typical area within which GW events will be localized by the Advanced LIGO-Virgo network (LIGO Scientific Collaboration et al. 2013). In the 2020s we expect several synergies at different wavelegths such as: 1) the space-based telescopes James Webb Space Telescope (JWST), ATHENA and WFIRST, 2) the ground-based telescope with large FOV like zPTF and LSST which will be able to select the GW candidates in order to follow-up them afterwards 3) the large multi-wavelengths telescopes such as the 30-m class telescopes GMT, TMT and E-ELT, which will all follow-up the optical/NIR counterparts like macronovae, and 4) the Square Kilometer Array (SKA) in the radio, which is well suited to detect, for example, the late-time (~weeks) signals produced by the interaction of the ejected matter with the interstellar medium. THESEUS/IRT will be perfectly integrated in this search, thanks to its photometric and spectroscopic capabilities and its spectral range coverage, allowing us to improve the study of such (until now) uncertain emission mechanisms. With IRT we will acquire not only the light curves of EM counterparts but also their spectra, thus having the opportunity to disentangle the different components associated to those events (e.g. disk wind + dynamical ejecta contributions).

Core collapse of massive stars: Long GRBs, Low Luminosity GRBs and Supernovae

The collapse of massive stars are expected to emit GWs since a certain degree of asymmetry in the explosion is inevitably present. Estimates of the GW amplitudes are still largely uncertain and the expected output in energy has enormous uncertainties, ranging from 10-8 to 10-2 Moc2. If the efficiency of the GW emission is effectively very low, then only the third-generation GW detectors such as ET, which will be operative at the same time as THESEUS, will be able to reveal the GW emission from these sources and thus obtain crucial insights on the innermost mass distribution, unaccessible via EM observations.

The collapsar scenario invoked for long GRBs (e.g. Woosley 1993, Paczynski 1998) requires a rapidly rotating stellar core, so that the disk is centrifugally supported and able to supply the jet. This rapid rotation may lead to non-axisymmetric instabilities, such as the fragmentation of the collapsing core or the development of clumps in the accretion disk. Inspiral-like GW signals are predicted with an amplitude that will be observable by ET to luminosity distances of order 1 Gpc (e.g. Davies et al. 2002). With a rate of observed long GRB of ~0.5 Gpc-3 yr-1 , THESEUS could provide a simultaneous EM monitoring of ~1 long GRBs per year. Off-axis X-ray afterglow detections (“orphan afterglows”) can potentially increase the simultaneous GW+EM detection rate by a factor that strongly depends on the jet opening angle and the observer viewing angle. The possible large number of low luminosity GRBs (LLGRBs) in the nearby Universe will provide clear signatures in the GW detectors because of their much smaller distances with respect to long GRBs.

GW/EM synergy is key in this investigation. Since the gamma-ray emission and afterglow are expected to be produced at large distances (i.e. ~1013 cm) from the central engine, they provide only indirect evidence for the nature of that engine. By contrast, gravitational waves will be produced in the immediate vicinity of the central engine, offering a direct probe of its physics.

Soft Gamma Repeaters

Fractures of the solid-crust on the surface of highly magnetized neutrons stars and dramatic magnetic-field readjustments represent the most widely accepted explanation to interpret X-ray sources such as giant flares and soft gamma repeaters (SGRs) (e.g. Thompson & Duncan 1995). NS crust fractures have also been suggested to excite non radial oscillation modes that may produce detectable GWs (e.g., Corsi & Owen 2011, Ciolfi et al. 2011). The most recent estimates for the energy reservoir available in a giant flare are between 1045 erg (about the same as the total EM emission) and 1047 erg. The efficiency of conversion of this energy to GWs has been estimated in a number of recent numerical simulations and has been found to be likely too small to be within the sensitivity range of present GW detectors (Ciolfi & Rezzolla 2012, Lasky et al. 2012). However, at the typical frequencies of f-mode oscillations in NSs frequencies, ET will be sensitive to GW emissions as low as 1042 – 1044 erg at 0.8 kpc, or about 0.01% to 1% of the energy content in the EM emission in a giant flare. In the region of 20 − 100 Hz, ET will be able to probe emissions as low as 1039 erg, i.e. as little as 10−7 of the total energy budget (e.g. Chassande-Mottin et al. 2010 and reference therein).

Neutrino sources

Several high-energy sources that THESEUS will monitor are also thought to be strong neutrino emitters, in particular SNe and GRBs. The shocks formed in the GRB ultra-relativistic jets are expected to accelerate protons to ultrarelativistic energies and that, after interacting with high energy photons, produce charged pions decaying as high energy neutrinos (>105 GeV, e.g. Waxman & Bachall 1997). Pulses of low energy neutrinos (<10 MeV) are expected to be released during core-collapse supernovae (CCSNe) with an energy release up to 1053 erg. Low energy neutrinos have been detected from SN1987A at 50 kpc distance and are expected in general from CCSNe. Still significant uncertainties are affecting supernova models. GW and neutrino emission provide important information from the innermost regions as the degree of asymmetry in the matter distribution, as well the rotation rate and the strength of the magnetic fields, that can be used as priors in numerical simulations (e.g. Chassande-Mottin et al. 2010 and reference therein).

Because of the very small cross-sections and low fluxes of the emitted neutrinos, neutrino detectors necessarily require huge amounts of water or liquid scintillator in neutrino detectors. Future Megatons detectors as Deep-TITAND that are expected to work during the 3rd generation GW detectors, will reach distances up to 8 Mpc, that would guarantee simultaneous GW/neutrino and EM detection of ~ 1 SN per year. Very promising for such multi-messenger studies are the LLGRBs, given their expected larger rate than for standard long GRBs (up to 1000 times more numerous) and their proximity. For long GRBs, in the context of CCSNe, joint THESEUS and GW/neutrino observations would constrain progenitor models, clarifying the fraction of energy channeled via dynamical instabilities (Fryer et al., 2002) and the relative neutrino/EM energy budgets. Neutrino observations would also constrain the composition of the GRB jet and the relation of GRBs to high-energy cosmic rays (Abbasi et al. 2012).

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).