Have optical and radio counterparts to first observed merger of a Neutron Star and Black Hole by LIGO (S190814bv) finally been detected?

Greetings from Palmia Observatory

Well ever since the first reported merger of a neutron star and a black hole, LIGO gravitational wave event (S190814bv), was reported on August 14, 2019, we have been waiting for confirmation by all the other observatories that slewed to possible locations of the event.  Well,

it seems that maybe now after almost 2 weeks of waiting, the possible detection of both optical and radio emissions have been reported.

So, let's go over what we now are seeing for this LIGO detected event.  For those of you who have installed the GW Events application on your smart phone, your first indication of a possible detection came as your phone sounded off with the characteristic chirp signal associated with merger events.  The first set of sky maps, showing possible arcs of locations on the sky where the merger could have occurred is shown below.  With only three LIGO detectors online, the location of the source can only be located to within a certain number of square degrees on the sky.

The GW Events screenshot below shows three sky maps.  The top one was the first predicted location of the possible merger.  The bottom two maps show how the location had been refined and improved within a day of the first detection.  Narrowing the sky location helps in the search for electromagnetic counterparts because all of the observatories that start to look at those regions of the sky have a smaller area to cover.

GW Event screenshot showing improvement of arcs of possible sky locations for S190814bv as of August 15, 2019 (Source: gracedb.ligo.org)

The GW Event screenshot of next importance was the one identifying the False Alarm Rate (FAR) and likelihood of possible types of mergers.  Note that the probability of a neutron star - Black Hole merger is estimated at 0.99.  Hmm, pretty good data so far.  Next we see the FAR is reported one out of some fantastically huge number of times that the event could have been the result of just the right amount and type of noise occurring in all three LIGO detectors at the time.  Some of the chirp announcements only report FAR, say 1/100 or 1/1000, or some such number, so this detection is much more likely to be a real merger signal.  Yep, it seems like it is not just some random noise!

GW Event screenshot showing LVC Update Skymap for S190814bv as of August 15, 2019 (Source: LVC Update Skymap)

So, after these initial announcements that went out to the general astronomical community on August 15, we were all waiting to see if any of the observable locations had any optical transients.  My phone was chirping many times a day during this two week period, but each report just said that no optical counterparts were observed in that observatory's search.  There was no reported increase from neutrino observatories and no transient reported by gamma ray observatories.  So, I kept hoping that one of the upcoming chirps would report some great observation.  Each report of non-detection is still helpful.  One published report of non-detection is shown in the screenshot below from a paper by S. Gomez, et al, who in their article, "A Galaxy-Targeted Search for Optical Counterparts of the Candidate NS-BH Merger S190814bv with Magellan", looked at images of the most probable sky location and found no net increase in optical emissions.

No early counterparts detected in this Magellan search (Source: S Gomez, et al, arXiv: 1908.08913v1, 23 Aug 2019)

The paper goes into all of the details of the search and how they were capable of measuring to faint transients at magnitude 22.  They also reported some other details of the sources involved in the merger and said that one of the pair was less than 3 solar masses and the other was greater than 5 solar masses, hence the binary pair was considered a neutron star and a black hole.  The distance was quite a quite further away than the recent NS-NS merger and at about 900 million light years the signals were quite weak.

But, finally, a chirp report had some good news. On August 27, the ASKAP observatory reported a detection (if you don't recognize ASKAP, check out our recent post on ASKAP at: http://www.palmiaobservatory.com/2019/08/planning-for-dark-skies-and-glacier-in.html).

GW Event screenshot showing ASKAP counterpart detection for S190814bv (Source: GCN Circular 25487)

In addition, we finally got another GCN Circular identifying an optical transient in the region of interest.  Again the transient was very dim with reported magnitude of about 22.

GW Event screenshot showing optical counterpart detection for S190814bv (Source: GCN Circular 25488)

So, it seems that counterpart detections are finally showing up.  It is not because of a different propagation speed for gravity waves and electromagnetic waves, but more likely just due to the time it takes for the merger effects to result in a large enough signal to be detected here.  Any way we will have to stay tuned.  So, if you want to get the latest results be sure to get the free GW Events app on your smartphone and hear the chirp signal 24 hours per day!

Until next time,

Resident Astronomer George

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