THESEUS: A key space mission concept for Multi-Messenger Astrophysics
Introduction
With the first detection in 2015 of gravitational waves (GWs) from black hole binary systems during their coalescing phase (Abbott et al., 2016a, Abbott et al., 2016b), a new observational window on the Universe has been opened. Stellar-mass black hole coalescences, together with binary neutron star (NS-NS), NS-black hole (BH) mergers, burst sources as core-collapsing massive stars and possibly NS instability episodes, are among the main targets of ground-based GW detectors.1 Some of these sources are also expected to produce neutrinos and electromagnetic (EM) signals over the entire spectrum, from radio to gamma-rays.
These expectations were astonishingly satisfied for the first time on August 17th, 2017, when a GW signal consistent with a binary neutron star merger system (Abbott et al., 2017e) was found shortly preceding the short gamma-ray burst GRB170817A (Abbott et al., 2017d). The GW170817 90% confidence sky area obtained with the Advanced LIGO (Harry and LIGO Scientific Collaboration, 2010) and Advanced Virgo (Acernese et al., 2015) network was fully contained within the GRB error box. In addition, a “kilonova” (or “macronova”) emission (AT2017gfo), theoretically predicted from such systems (e.g. Li and Paczyński, 1998a), has been found within the GW-GRB error-box and positionally consistent with NGC4993, a lenticular galaxy at a distance compatible with the GW signal (Abbott et al., 2017f, Smartt et al., 2017, Tanvir et al., 2017, Pian et al., 2017, Coulter et al., 2017).
By the end of the twenties, the sky will be routinely monitored by the second-generation GW detector network, composed by the two Advanced LIGO (aLIGO) detectors in the US, Advanced Virgo in Italy, ILIGO in India (e.g. Abbott et al., 2016c) and KAGRA in Japan (Somiya, 2012). Then, around 2030, more sensitive third generation ground-based GW interferometers, such as the Einstein Telescope (ET, e.g. Punturo et al., 2010) and LIGO Cosmic Explorer (LIGO-CE, e.g. Abbott et al., 2017b), are planned to be operational and to provide an increase of roughly one order of magnitude in sensitivity. In parallel to these advancements, IceCube and KM3nNeT and the advent of 10 km3 detectors (e.g. IceCube-Gen2, IceCube-Gen2 Collaboration et al., 2014, and references therein) will enable to gain high-statistics samples of astrophysical neutrinos. The 2030 will therefore coincide with a golden era of multi-messenger astrophysics (MMA, Fig. 1).
By that time, the ESA M5 approved missions for space-based astronomy will be launched. THESEUS (Transient High Energy Sky and Early Universe Surveyor2 Amati et al., 2017) is a space mission concept developed by a large International collaboration currently accepted by ESA for phase A study within the selection process for next M5 mission of the Cosmic Vision Programme. If selected, the launch of THESEUS (2032) will provide a very strong contribution to MMA. In the following sections, after a short review of the main characteristics (Section 2; see Amati et al., 2017, for a more exhaustive description of the mission concept), we describe the role of THESEUS in the MMA and the most promising GW (Section 4) and neutrino (Section 5) sources that THESEUS will observe. We also provide the expected joint GW + EM detection rates for the most promising GW + EM sources (e.g. NS-NS) taking into account the facilities planned to be operational by the end of the twenties and beyond.
Section snippets
The THESEUS mission
The THESEUS mission aims at exploiting Gamma-Ray Bursts (GRBs) for investigating the early Universe and at providing a substantial advancement in multi-messenger and time-domain astrophysics (see Amati et al. (2017), for a detailed review).
The instrumentation foreseen on board, illustrated in Fig. 2, includes:
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Soft X-ray Imager (SXI, 0.3–6 keV): a set of 4 lobster-eye telescopes units, covering a total FoV of 1 sr with source location accuracy arcmin;
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X-Gamma ray Imaging Spectrometer (XGIS,
The role of THESEUS in the Multi-Messenger Astronomy
The detection of EM counterparts of GW and neutrino signals will enable a multitude of science programmes (see, e.g., Bloom et al., 2009, Phinney, 2009) by allowing for parameter constraints that the GW or neutrino observations alone cannot fully provide. GW detectors have relatively poor sky localisation capabilities, mainly based on triangulation methods, that on average will not be better than few dozens of square degrees (Abbott et al., 2016c). For GW sources at distances larger than the
NS-NS/ NS-BH mergers: collimated emission from Short GRBs
Compact binary coalescences (CBCs) involving neutron stars (NS) and stellar mass black holes (BH) are among the sources of GWs that will be likely detected in spades in the next decade. These systems 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 , and gravitational waveforms well predicted by General Relativity (see, e.g., Baiotti and Rezzolla (2017) for a review). From the merger of two
Neutrino sources
Several gamma-ray and X-ray sources that THESEUS will observe as GRBs, CCSNe and AGNs, are also expected to originate neutrinos. Due to their low interaction cross-section, neutrinos can probe the innermost regions similarly to gravitational waves but, in addition, neutrino detectors can provide a more refined sky localisation than GW interferometers, with an uncertainty that goes from few degrees down to a fraction of a degree. Current neutrino deep-water-based detectors include DUMAND, Lake
Summary
The first detection of the electromagnetic counterparts of a GW source has confirmed a number of theoretical expectations and boosted the nascent multi-messenger astronomy. In this review we have discussed several classes of sources, including compact binary coalescences, core-collapsing massive stars, and instability episodes on NSs that are expected to originate simultaneously high-frequency GWs, neutrinos and EM emission across the entire EM spectrum, including in particular high energy
Acknowledgements
Giulia Stratta acknowledges EGO support through a VESF fellowship (EGO-DIR-133-2015). Maria Giovanna Dainotti acknowledges funding from the European Union through the Marie Curie Action FP7-PEOPLE-2013-IOF, under grant agreement No. 626267 (”Cosmological Candles”). Sergio Colafrancesco is supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa (Grant no. 77948).
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