Electromagnetic Counterparts to Multi-messenger events
For hundreds of years telescopes have been used to study our universe across the electromagnetic spectrum; from space-based Gamma-ray satellites to ground-based radio telescope arrays. However, it is now possible to study astronomical sources outside of the electromagnetic spectrum. Non-electromagnetic messengers for transient detection, such as gravitational waves and neutrinos, are arguably one of the most exciting discoveries of the 21st Century. The campaigns around the detection of gravitational wave emission from the GW170817 neutron star (NS) merger (Abbott+ 2017, PhRvL, 119, 161101) and the neutrino detection from the blazar TXS 0506+056 (IceCube Collaboration+ 2018, Science, 361, 147) highlight the importance of electromagnetic (EM) follow-up campaigns for verification and elucidation of these transient events.
The difficulty of counterpart detection is not just the transient discovery itself, but distinguishing the true counterpart from the large numbers of unrelated candidates in a sky region of many square degrees. Early observations of GW170817 demonstrated the importance of identifying the counterpart rapidly, since the first 12 hours showed a very rapid spectral and photometric evolution (e.g. Pian+ 2017, Nature, 551, 67; Smartt+ 2017, Nature, 551, 75; Tanvir+ 2017, ApJ, 848, 27).
Detection of new events and characterisation of the early evolution of these sources is of the highest priority to the time domain community: open questions include whether all neutron star-neutron star mergers produce short GRBs, whether outflows from neutron star-neutron star and neutron star-black hole mergers are similar, and the properties and structure of the jet (Lamb+ 2017, MNRAS, 472, 495; Kasliwal+ 2017, Science, 358, 1559). Gravitational wave events also offer an independent means of measuring the Hubble constant (Schutz 1986, Nature, 323, 310; Abbott+ 2017, Nature, 551, 85), but an EM counterpart must be identified to do this with precision.
The masses of neutron stars and black holes measured through gravitational waves and electromagnetic observations. The yellow and purple markers represent the electromagnetic measurements of neutron stars and black holes, respectively, while the orange and blue markers are the corresponding measurements using gravitational waves. (Click here for the full article on the LIGO-Virgo website).
The most recent LIGO/Virgo run discovered over a dozen bright EM events, which were pursued by the community, however, no new counterparts were positively identified. This is a new and emerging field, but based on the evidence to date it seems likely that the GW170817 counterpart was exceptional and future detections will be significantly fainter. While the 2m LT continued to be one of the most active follow-up facilities during the third science run (Kasliwal+ 2020, arXiv:2006.11306) the majority of the data taken was photometry, compared to the earlier runs where most observations were spectroscopic classifications. This highlights two points: the important role for rapidly reacting robotic telescopes in these science programmes, but also the need for a larger aperture in science runs to come. NRT will complement new discovery facilities such as the UK-led GOTO project on La Palma. The 4-metre NRT will be able to undertake follow-up spectroscopy of any transient discovered by GOTO, and automated cooperation between these two robotic facilities provides the potential for colours and spectroscopic classifications within minutes of candidate discovery.
The next decade will see the exploration of the time-domain sky in different electromagnetic regimes via facilities such as the Cherenkov Telescope Array (CTA); the northern component of which will be co-located with the NRT on La Palma, eRoSITA and LOw-Frequency ARray (LOFAR). The NRT will continue to lead the rapid response of ground-based facilities by being on target and taking data within 30 seconds of trigger, catching faint, rapidly fading sources before larger facilities.
One particularly interesting field that is rapidly developing is that of Fast Radio Bursts (FRBs). Although a relatively small number of Fast Radio Bursts have been published, at least a thousand have been discovered to date and they form a key part of our prospective science cases. FRBs are millisecond-long bursts of radio emission detected by dedicated facilities such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project, ASKAP, Molonglo and the Parkes Telescope. Their origin, in at least some cases, might be attributed to radio emission from young magnetars; highly magnetised young neutron stars that occasionally flare in X-rays and gamma-rays. There is also the possibility that a subset of them are associated with a cataclysmic event such as the merger of neutron stars or the formation of a black hole. With the improvement in localisation of FRBs and the ongoing searches for afterglows or prompt emission at wavelengths other than radio, we will keep monitoring the progression of the FRB science case and in general, the engineering developments made for the GRB science case are likely to work well as we move forward with further discoveries in the field of FRBs.
Future science cases: FRBs
One possible mechanism for creating FRBs is magnetar flares colliding with surrounding material. The collision generates a shock-front containing huge magnetic fields. Electrons circling around these magnetic field lines create a burst of radio emission. The shock waves also heats electrons, which causes them to emit X-rays.
Caption and Image taken from Nature.