Optical Follow-up of Gravitational-wave Events with Las Cumbres Observatory

Abstract

We present an implementation of the Gehrels et al. (2016) galaxy-targeted strategy for gravitationalwave (GW) follow-up using the Las Cumbres Observatory global network of telescopes. We use the Galaxy List for the Advanced Detector Era (GLADE) galaxy catalog, which we show is complete (with respect to a Schechter function) out to ∼ 300 Mpc for galaxies brighter than the median Schechter function galaxy luminosity. We use a prioritization algorithm to select the galaxies with the highest chance of containing the counterpart given their luminosity, their position, and their distance relative to a GW localization, and in which we are most likely to detect a counterpart given its expected brightness compared to the limiting magnitude of our telescopes. This algorithm can be easily adapted to any expected transient parameters and telescopes. We implemented this strategy during the second Advanced Detector Observing Run (O2) and followed the black hole merger GW170814 and the neutron star merger GW170817. For the latter, we identified an optical kilonova/macronova counterpart thanks to our algorithm selecting the correct host galaxy fifth in its ranked list among 182 galaxies we identified in the Laser Interferometer Gravitational-wave Observatory LIGO-Virgo localization. This also allowed us to obtain some of the earliest observations of the first optical transient ever triggered by a GW detection (as presented in a companion paper).

Optimizing gravitational waves follow-up using galaxies stellar mass

“…NGC 4993 host galaxy of the event is ranked in 27 th position in the standard approach. The (Arcavi et al 2017) grade ranks it at the 6 th position and our grade is even more successful by putting NGC 4993 in the 4 th position with both equations 3 and 4 expressions. Those results shows that the gain of our method for the follow up of gravitational waves event is even bigger in the case of wide skymap.…”

“…NGC 4993 host galaxy of GW170817 is ranked in 5 th position in the standard approach. The (Arcavi et al 2017) grade improved its rank to the 2 nd position and is ranked first with our method for both (Equation 3) and (Equation 4) expressions. These results show that if we had used our grade to monitor this event, there would have been a gain in the speed of observation of the host galaxy.…”

“…Those results shows that the gain of our method for the follow up of gravitational waves event is even bigger in the case of wide skymap. This example of wider skymap allows us to see that sometime the (Arcavi et al 2017) grade and our grades behave differently and even oppositely. For example the galaxy PGC043966 is ranked in 37 th position with the standard grade and, when the Arcavi et al (2017) grade upgrade its rank to the 30 th , both of our grades (equations 3) and (4) downgrade its rank to the 54 th and 49 th position respectively.…”

“…The conservative choice of 3σ on the distance constraint is motivated by the fact that galaxies with a low distance probability will be always penalised in the ranking process. Regarding the condition on the distance, we use the pixel by pixel information and not the mean distance estimation for the LIGO-Virgo candidate, as in the approach adopted by (Arcavi et al 2017). This is more efficient because the distance estimation can be very inhomogeneous in a given skymap.…”

“…Using the B band luminosity in those cases would have led to observe preferentially less massive galaxies. We illustrate the difference in the behavior between our grade and the one from (Arcavi et al 2017) in Figure 5, where we plot the stellar mass estimation using WISE1 band as a function of the B band luminosity for the same galaxies. We see that for a given B band luminosity there is an important scatter in stellar mass spanning a few order of magnitudes.…”