Hubble and LIGON. Reid (inr[at]stsci.edu) for the HST-LIGO Working Group
Sometimes the Universe surprises us. Observational gravitational-wave astronomy only became a reality recently with the commissioning of the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) in 2015. The first event, a black hole/black hole merger, was detected soon after the first campaign started (September 14, 2015) with several similar events following over the next two years. Then, on August 17, 2017, LIGO not only detected a different type of event, a neutron star/neutron star merger, but an optical counterpart was quickly identified in the outskirts of the spiral galaxy NGC 4993. It would be fair to say that finding a bright optical counterpart so early in LIGO's career came as a surprise to the astronomical community. Plans for coordinated follow-up observations were not at a highly developed level. And to complicate matters, mid-August 2017 saw approximately 95% of US astronomers distributed across the country on a line stretching from Salem, Oregon through Columbia, South Carolina observing a total solar eclipse.
Follow-up observations were obtained of course, including photometry and low-resolution spectroscopy with Hubble. However, to better prepare for the future, the STScI Director convened a Working Group,* chaired by Professor Raffaella Margutti (Northwestern), to advise him on options for maximizing the scientific impact of future Hubble observations of optical counterparts to gravitational wave sources. The Working Group (WG) met through the late spring and summer of 2018 and their report was submitted to the Director and presented to the Space Telescope Users Committee in October 2018.
The recommendations from the WG fall under four broad categories: guidance for optimizing the observing strategy with Hubble; suggestions on associated policy, coordination among observatories; and maximizing the Hubble impact on time domain astronomy at large.
STScI is actively exploring options for implementing the recommendations.
Since the submission of the report, the Cycle 26 Telescope Allocation Committee met and a major ToO follow-up program has been accepted for execution (GO 15664: New insights from gravitational waves combined with electromagnetic light). This large non-proprietary program aims to obtain multi-epoch, multi-band imaging and spectra of gravitational-wave counterparts, including one ultra-rapid ToO for observations with the UV prism on Wide-Field Camera 3. LIGO's O3 observing campaign begins in April; we look forward to more surprises.
*The HST–LIGO Working Group members are Raffaella Margutti (Northwestern University – chair), Brad Cenko (GSFC), Ben Farr (University of Oregon), Ori Fox (STScI), Erik Kuulkers (ESA/ESTEC), Emily Levesque (University of Washington) and Danny Milisavljevic (Purdue). Further details, including the full report, are available here.