A primary goal of the LIGO and Virgo observatories is the direct detection of gravitational waves from compact binary mergers. Coalescing binaries comprised of neutron stars and/or black holes are expected to have event rates measurable by the advanced LIGO-Virgo network (as many as 50/yr). If at least one neutron star is in the binary, there may also be an electromagnetic counterpart to the gravitational-wave emission. For instance, it is thought likely that short, hard gamma-ray bursts (which are known to emit electromagnetic radiation across the spectrum) are the result of binary neutron star or neutron star - black hole mergers. Coincident observation of electromagnetic radiation and gravitational waves would provide a wealth of information about the coalescing binary source, including the sky location, distance, component masses, spins and more.
Gravitational-wave detectors have an advantage over more traditional telescopes in that the whole sky is scanned over a long period of time. Traditional telescopes are more likely to miss a faint afterglow or delayed emission from a particular sky position if a gamma-ray burst is beamed away from the earth and hence no gamma-rays catch astronomers' attention. When gravitational waves are detected from a compact binary merger, gravitational-wave detectors may be able to tell astronomers quickly where to look.
Ten partnerships have been formed so that electromagnetic followups can be performed on coincident triggers found in LIGO and Virgo data. If a coincident trigger from the LIGO-Virgo network passes a number of quality requirements and its sky position is localized, then an alert is sent to participating observatories. These incude the Liverpool telescope, LOFAR, the Palomar Transient Factory, Pi of the Sky, QUEST, ROTSE III, SkyMapper, Swift, TAROT and the Zadko telescope. The first test of these partnerships was performed between September 19 and October 20, 2010, when the LIGO and Virgo detectors collected data during their S6 and VSR3 science runs and analyzed it with a low-latency pipeline. Most decisions about sending an alert were issued between 20 and 40 minutes after the LIGO-Virgo trigger time.
A total of 42 triple coincident triggers were identified by the software within minutes of each one's trigger time. Online data quality flags removed 5 of the coincident triggers from the analysis. Sky localization techniques determined positions for 23 of the triggers, however only 13 triggers met the requirement of having false alarm rates less than 0.25 events per day. Of those, only 3 had at least one neutron star involved in the merger. Human monitors performed quality checks and made the decision whether or not to alert the optical telescopes about a possible gravitational-wave candidate.
The three coincident triggers that passed all requirements necessary to send an alert to the partner observatories were established within 14, 39, and 16 minutes from the trigger time. The process was slowed by human intervention, which accounts for most of that time. The first trigger occurred during a test period, and the observatories were not notified. An alert for the second trigger, on September 19, resulted in EM followup observations performed by Quest, ROTSE, SkyMapper, TAROT and Zadko. Analysis of this data will be published separately. The third trigger indicated a position that was too close to the sun for optical observations to be performed.
This exciting analysis proves that gravitational-wave triggers can be established within minutes of their trigger time in the LIGO-Virgo network. As the advanced detector era begins, it is hoped that even lower latency can be achieved. Future searches will rely on partnerships like these to establish the link between electromagnetic and gravitational-wave astronomy.