Session D10: Gravitational Waves and Gamma Ray Bursts

Live

The combined observation of GRB170817A and GW170817 marked the first multimessenger success of gravitational wave astronomy. Although no further coincident observations were found in the second observing run of Advanced LIGO, we are continuing this search in the third observing run, which is planned to conclude on April 30, 2020. This talk will present the current status of the effort to conduct template-based searches for gravitational waves in Advanced LIGO data, triggered by GRBs disseminated by the Gamma-ray Coordinates Network (GCN). This talk will discuss both the high-latency, full analysis and efforts to bring a lightweight, rapid-followup analysis into production.   

Live

The coincident detection of GW170817 and GRB170817A was a shining start to the era of multi-messenger astronomy informed by gravitational waves. Despite the successes surrounding GW170817, it is intriguing to wonder what remains to be discovered in the $\sim$11 hours between gamma ray observations and those at other wavelengths. Prompt observations at other bands will further inform our understanding of r-process nucleosynthesis, shock-heated ejecta, and the nature of the remnant immediately following merger. We aim to facilitate follow up of electromagnetically bright candidates by providing coalescence time estimates and sky maps up to one minute before merger. We present efforts to realize early warning detections in Advanced LIGO’s third observing run.   

Live

Gravitational waves produced from the merger of binary neutron stars (BNSs) are accompanied by electromagnetic counterparts, making it possible to identify the associated host galaxy. We explore how properties of the host galaxies relate to the astrophysical processes leading to the mergers. It is thought that the BNS merger rate within a galaxy at a given epoch depends primarily on the galaxy's star-formation history as well as the underlying merger time-delay distribution of the binary systems. We find that different time-delay distributions predict different properties of the associated host galaxies, including the distributions of stellar mass, star-formation rate, halo mass, and local and large-scale clustering of hosts. BNSs that merge today with short delay times prefer to be in hosts that have high star-formation rates, while those with long delay times live in dense regions within massive halos that have low star formation. We show that with ${\mathcal O}(10)$ events from current gravitational-wave detector networks, it is possible to make preliminary distinctions between formation channels which trace stellar mass, halo mass, or star-formation rate.   

Live

On 14 August 2019, the LIGO and Virgo Collaborations announced the detection of gravitational waves from a neutron star - black hole merger, the first event of its kind. In search of an optical counterpart, The Dark Energy Survey (DES) Gravitational Wave Search and Discovery Team targeted 99 percent of the localization area with Blanco/DECam 1,2,3,4,6 and 16 nights after the merger. Objects with varying brightness were detected by the DES Difference Imaging Pipeline, and we systematically reduced the list of candidates through catalog matching, light curve properties, host-galaxy photometric redshifts, SOAR spectroscopic follow-up, and machine-learning-based photometric classification. All candidates were ruled out. We also applied our selection criteria to simulations of supernovae and kilonovae as they would appear in the DECam observations. We find that if a kilonova occurred during this merger, the ejected matter must have been less than 0.006 solar masses, had a lanthanide abundance of more than $10^{-3}$, and had a velocity of less than $0.25c$ if the lanthanide abundance was greater than $10^{-1}$. Furthermore, we find that without reducing the background to smaller than one object per follow-up, the properties of a kilonova are not constrainable.   

Live

We present the results from the search for electromagnetic counterpart of the LIGO/Virgo event S190510g using the Dark Energy Camera. S190510g is a binary neutron star (BNS) event of moderate significance detected at a distance of 227 \textpm 92 Mpc and localized within an area of 31 (1166) square degrees at 50{\%} (90{\%}) confidence. We use the DES GW search and discovery pipeline for analysis that identified 11 candidates all of which appear consistent with supernovae following offline analysis and spectroscopy by other instruments. However, one candidate, desgw-190510h, does ooks promising enough given the properties of it's host galaxy, that we suggest follow up from radio telescopes. In this talk, I will discuss how we implement our candidate selection procedure on real candidates as well as simulated kilonovae and supernovae under DECam observing conditions. Using this, we can inform future observing strategies for similar events. Additionally, we show that given $\sim $27 identical events observed with this strategy would be needed to be able to detect a GW170817-like counterpart at the 3$\sigma $ confidence level. We conclude that follow up of just the highest probability region of many high significance events with poor localization is the most efficient way to find KN counterparts.   

LIGO Sub-threshold Alerts for the Swift Observatory

Under many circumstances, advanced LIGO and Virgo are not able to localize neutron star mergers to sufficient accuracy to be useful for electromagnetic (EM) follow-up observations. However, temporal coincidence with a sub-threshold short gamma-ray burst (GRB) may provide compelling evidence of a joint origin and an indirect way to obtain accurate positions. Swift is a large field-of-view gamma-ray burst (GRB) observatory that can localize GRBs to arcminute precision. The rapid follow-up of LIGO triggers by Swift can result in dramatically improved sky localizations through temporal coincidence. This will allow the rest of the EM follow-up community to efficiently search for counterparts in the hours to days following the GW signal. Unfortunately, not all sub-threshold Swift data is saved or archived. We have developed a sub-threshold trigger pipeline which has enabled Swift to recover data around LIGO events that otherwise would have been lost. This could potentially increase the number of EM counterparts to BNS events found by LIGO and Virgo. We will describe this project and what has been achieved by the pipeline so far.   

Counting on Short Gamma-Ray Bursts: Gravitational-Wave Constraints of Jet Geometry

The detection of GW170817 in gravitational waves and gamma rays revealed that short gamma-ray bursts are associated with neutron-star mergers. Gamma rays are thought to result from the formation of collimated jets, but the details of this process continue to elude us. One fundamental observable is the emission profile of the jet as a function of viewing angle. We present two methods to measure the effective angular width of short gamma-ray burst jets using gravitational wave and gamma-ray data. The first is a counting experiment, where we combine the known detection thresholds of the LIGO/Virgo and Fermi Gamma Ray Burst Monitor detectors to infer parameters of systems that are detected in gravitational waves. This method requires minimal knowledge about each event, beyond whether or not they were detected in gamma-rays. The second method uses additional information from the gravitational-wave and electromagnetic data to estimate parameters of the source, and thereby improve constraints on jet properties. In the limit of many detections, the second method achieves marginal improvements; we conclude that the majority of the information about jet structure comes from the relative sensitivities of gravitational-wave and gamma-ray detectors as encoded in simple counting experiments.   

Optical followup of gravitational wave events by the DESGW group during LIGO/VIRGO O3

The Dark Energy Camera on the CTIO Blanco telescope has been used to follow up LIGO/Virgo gravitational wave events since the first such event in 2015. CTIO is evolving the way it is being used towards being a facility for gravitational wave source electromagnetic counterpart identification. At present, three different groups share target of opportunity data from events triggered for followup by one group or another. Our group, DESGW, consists of members of the Dark Energy Survey, LIGO, and the larger astronomical community. In this talk we present our analyses of the GW events for which the Blanco/DECam triggered during the third LIGO/Virgo observing season (O3; beginning April 2019). We will discuss our observing strategies, the search and discovery pipeline, and candidate identification processes. We describe the spectroscopic followup of compelling candidates with other instruments in order to better classify them as potential real counterparts or false positives. In O3 we followed up binary black holes, a binary neutron star merger, and a neutron star-black hole merger.   

Not Participating

As the era of multi-messenger astronomy continues there will be an increasing number of gravitational wave candidate events in need of a coordinated electromagnetic followup campaign. Fast and accurate identification of signals that are most interesting or most likely to produce an electromagnetic counterpart will be of primary importance in order to make best use of valuable telescope time and capture the most complete picture of astrophysical events. We demonstrate a method to produce accurate parameter estimates from binary neutron star and neutron star-black hole signals in very low latency by extending the method of [Zackay et al, 2018]. We generate a population of simulated binary coalescence signals and measure their source properties using this method, and we discuss implications for understanding of neutron stars and binary systems.