The last prediction of Einstein’s general theory of relativity was the observation of gravitational waves emitted by two coalescing black holes, and yet again Einstein was proven to be right. Thanks to LIGO/Virgo instruments, we can now test General Relativity in the strong field regime using the gravitational-wave signals from merging black holes.
The first-ever direct detection of the gravitational wave (GW) signal, GW 150914, was made by Advanced-LIGO in 2015 September from a binary black hole (BBH) merger. This discovery entered us into the GW era, and was dramatically followed by the first observation of a binary neutron star merger, GW 170817, associated with a short gamma-ray burst (sGRB), GRB 170817A, inaugurating the era of multi-messenger astronomy.
DiRAC researches — Dr. Golkhou, Prof. Bellm, and Dr. Graham — are engaged in finding and characterizing optical counterparts to neutron star mergers (double neutron star or neutron star-black hole) discovered by the LIGO/Virgo Collaboration using the Zwicky Transient Facility (ZTF). ZTF is a new optical time-domain survey that uses a powerful new camera with a 47-square-degree field of view to find exotic and fast-evolving transients. ZTF and other facilities around the world are conducting Target of Opportunity observations of LIGO detections under the coordination of the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. ZTF plans to trigger on all LIGO/Virgo triggers with a significant probability of having at least one neutron star in the system and a localization that is at least partially accessible from the Palomar night sky. In addition to ZTF, as a new member of the GROWTH project we have now access to more than 18 telescopes around the world which all have ongoing multi-wavelength follow-up programs and are actively searching for candidate EM counterparts to LIGO/Virgo sources. In order to fulfill our duty to the project, we brought the unique imaging and spectroscopic capabilities of the 3.5m telescope at Apache Point Observatory, New Mexico into the GW follow-up efforts.
The goal is to discover compact binary coalescence (CBC) events — powerful engines for the production of GW, EM, and neutrino radiation. Rapid follow-up observations of such events are required to provide us a more accurate measurement of basic astrophysical properties such as the luminosity and energetics of this strong-field gravity event. Joint EM-GW detections constrain fundamental physical properties of CBC events including the distance scale, luminosity, and host galaxy environment. However, identifying a counterpart is remarkably challenging due to the LIGO/Vigo inherently weak localization of GW events (∼ a few hundred deg2). Nonetheless, the scientific returns of such discovery — and equally, event characterization — justify many efforts taken, even a small step forward.
Finally, after a long-period upgrading the LIGO (L1 and H1) and Virgo instruments, the third observing run (O3) for detection of gravitational waves was began on April 1st, 2019. This new search would reach deeper into the universe (LIGO: up to ~170Mpc and Virgo: up to ~85Mpc for binary neutron-stars; BNS) than the previous two searches with the advanced detectors.
At the time of writing we have received thirteen triggers from LIGO/Virgo collaboration; the first one is S190408an and the last one S190521r. Three out of these thirteen triggers (S190425z, S190426c, and S190510g) are estimated to be associated with coalescence of BNS with a very high probability. Nine of these triggers are associated with BBH merger candidates and the remaining one could be the highly awaited NS-BH merger event, which has not been observed yet.
DiRAC astronomers as part of the bigger GROWTH collaboration team were actively engaged in the follow-up of candidates localized with the ZTF and DECam searches and suspected to be sources of BNS-merger signals detected with the LIGO/Virgo detectors. Though, no EM counterpart to those LIGO/Virgo BNS events has been identified yet. DIRAC Researcher and Moore-Sloan Postdoctoral Fellow, V. Zach Golkhou, is a coauthor on a recent paper regarding follow-up of a distant BNS merger candidate associated with S190510g. (https://arxiv.org/abs/1906.00806)
Figure 1 shows the localization of the GW (from the LIGO/Virgo 3-detector global network), gamma-ray (by the Fermi and INTEGRAL satellites) and optical (the Swope discovery image) signals from the transient event detected on the 17th of August, 2017 (image credit: Abbott et al. 2017, ApJL, 848, 12A). An artist’s impression of two stars orbiting each other and progressing (from left to right) to merger with resulting gravitational waves is also illustrated in the figure (image credit: NASA). It includes examples of the optical-infrared photometric and spectroscopic follow-up of GW 170817 from the GROWTH network, and other APO-class telescopes, as an example of the data that our program aims to obtain (images credit: Kasliwal et al. 2017, Science, 358, 6370 and Smartt et al. 2017, Nature, 551, 75).
So far, LIGO/Virgo have identified, on average, one BNS candidate every two weeks (two CBC events per week). This rate would exceed the expected value of LIGO/Virgo’s BNS identifications over the course of O3; 10 BNS/year. At this rate, many Astronomers including DiRAC-GROWTH team will be On Call all the time and probably are not going to get much sleep for a year while O3 is on!