CTAO Science
Gamma-ray bursts and multi-messenger transients
High-energy transient sources, like Gamma-Ray Bursts (GRBs), appear in the sky only for short periods of about a few to thousands of seconds. Their study requires coordinated campaigns synchronised and communicating telescopes in which CTAO will be integrated. Recently, it was proved that transient sources can emit electro-magnetic radiation and gravitational waves. CTAO will address the questions: Which is the origin of GRBs? Do these explosions leave a long-lived CR accelerator or are they purely leptonic? What is the nature and variety of other repeating or not transients? What powers particle acceleration into them?
Powerful particle accelerators in astronomical objects require a huge power to maintain electric and magnetic fields. The necessary power may be available only for short moments, as, for example, when a tidal disruption event (TDE) occurs with an SMBH engulfing matter, or during the gravitational collapse of a massive star or a merger of binary neutron stars (BNS). It is commonly believed that gravitational collapses of massive stars are mostly at the origin of long GRBs with their “prompt” gamma-ray emission lasting about 103 s and that short GRBs of the order of 1 s originate from BNS mergers, as observed for the gravitational wave event GW 170817. The BNS merger event occurring on Aug. 17, 2017, triangulated by the LIGO and Virgo interferometers was followed by the prompt gamma-ray emission detected 1.7 s after by Fermi-GBM and INTEGRAL ACS, followed by an optical/IR/UV afterglow emission that allowed the identification of the host galaxy NGC 4993 at about 40 Mpc distance.
This event is a milestone of Multi-Messenger astrophysics, which informed us on items concerning gravitation and cosmology, particle and nuclear physics and astrophysics. GRBs were the first transients discovered back in the 60s. Only now we are beginning to understand their origin, thanks to the new Multi-Messenger approach but the details of their origin and their relation to cosmic rays remain largely not understood.
The most credited model for GRBs, the fireball model, concerns the hot relativistic fireball generated by an SN collapse or a BNS merger. The fireball forms after the catastrophic event and expands isotropically in the jet frame. After its adiabatic expansion, the fireball becomes optically thin to prompt gamma-ray emission of keV to MeV photons lasting from ms to 1000s, followed by internal shock collisions between faster and slower blobs in the jet. When the fireball is decelerated by the surrounding medium, the afterglow emission happens in all bands.
The current generation of IACTs has revealed the afterglow at TeV energies and possibly of a short GRB. These last are particularly hard to detect as current IACTs need about minutes to point to alerts from the network of astronomical observatories for gamma-ray transients. Only several GRBs have been detected in the VHE band so far. The small size of the GRB sample seen in gamma-rays from the ground is also due to the small FoV of IACTs. CTAO telescopes are specially designed for a large FoV (5°-9°) and to be able to re-point in a few tens of seconds. The 100-ton heavy body of the LST can turn in the direction of a GRB within about 30s. This capability, combined with the high sensitivity of the cameras, will reveal a large population of VHE GRBs by automatically repointing in their direction right at the moment of their appearance in the sky and following up the sources deep into the afterglow phase. It is currently not clear what is the mechanism of the highest energy gamma-ray emission from GRBs. Recent detections with MAGIC and H.E.S.S. telescopes reveal hard spectra that are at odds with the possibility that gamma-rays are produced as the result of the scattering of low-energy photons by the accelerated electrons. An alternative hypothesis of synchrotron origin would require peculiar source parameters, such as unreasonably fast relativistic motions.
Particularly puzzling is the recent detection of gamma- rays with energies higher than 10 TeV from GRB 221009A by the LHAASO gamma-ray telescope. It indicates that the intrinsic spectrum of GRBs may be hard also in the 10 TeV band, making the problem of their theoretical understanding even more severe. The better quality of CTAO data and the larger source sample of observed GRBs will help solving the puzzle of their VHE emission mechanism. New types of transients will also be the object of Multi-Messenger campaigns, such as GW mergers and fast radio bursts, which at least in part may originate from earthquakes in highly magnetised stars (magnetars). The imminent start of operations of the massive sky surveys of the Vera Rubin Observatory in the visible band and SKAO in the radio, will result in an "avalanche" of transient detections. The variable and transient activity of astronomical sources will be discovered automatically in the survey data every day and reports on the discoveries will be broadcasted to CTAO and other telescopes for rapid follow-up observations. This will certainly lead to discoveries of new types or unexpected transient activities. A proper understanding of the physics of these diverse transients will require the availability of data at all energies, including the gamma-ray range.
CTAO will provide such data and help to uncover the nature of the new rich variety of transient phenomena in the sky. It is not clear a priori which of the Multi-Messenger transient sources are active at high-energy. For example, the BNS merger events that are sources of short GRBs and also of GWs certainly accelerate particles, but the highest energy gamma-rays produced by these particles may not be able to escape from the dense radiation environment of the source. CTAO detection or non-detection of different types of transients will help clarifying the nature of the physical processes taking place during the brief period of source appearance. Another example is given by “dormant” SMBHs in the centres of galaxies, that may suddenly become active during short TDEs. It cannot be predicted a priori if the transient AGN emerging from the TDE would function as a particle accelerator. We need first to look at TDEs and detect (or not) the gamma-ray sources associated with them. Transient phenomena may occur not only in "one-time" violent transformations of astronomical sources but also due to cataclysmic events in persistent sources, such as AGNs, flaring simultaneously in multiple energy bands, with subtle relations between their variability patterns. Blazar monitoring programs of CTAO will shed light on inter-band connection, evidence for quasi-periodicity, jet emission morphology and geometry.