iPTF SEARCH FOR AN OPTICAL COUNTERPART TO GRAVITATIONAL-WAVE TRANSIENT GW150914

Abstract

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational-wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the northern night sky, due both to Sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 deg 2 , after extensive filtering, eight candidates were deemed worthy of additional follow-up. Within two hours, all eight were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational-wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves.

Detectability of compact binary merger macronovae

“…We can now relate our model predictions with ongoing surveys such as iPTF (Kasliwal et al 2016) and Pan-STARRS . These are typically conducted in the r band, reaching limiting magnitudes of ∼ 20 − 21.…”

“…However, note that the current LIGO configuration can detect NSNS mergers only out to 75 Mpc (Martynov et al 2016). For such a distance, the wind model would be easily detected by the search such as that by , and also the more optimistic dynamical ejecta models (e.g., N5 MNmodel2 DZ31) could be detected by ZTF (Kasliwal et al 2016) if nearby enough. With the rate of such events assumed above, we would expect about one such event per year, and not all may be observable by optical telescopes.…”

Localization and Broadband Follow-Up of the Gravitational-Wave Transient Gw150914

“…No GRB-like afterglow was found in X-rays with Swift XRT (Evans et al 2016) or MAXI (N. Kawai et al, in preparation), in UV/optical with Swift UVOT (Evans et al 2016), or at GeV energies with Fermi LAT (Fermi-LAT Collaboration 2016). Tiled observations with wide-field optical instruments listed in Table 1 found many transients, but spectroscopy with the instruments listed in Table 2 along with further photometry showed that none of them were associated with GW150914 (Kasliwal et al 2016;Smartt et al 2016;Soares-Santos et al 2016;Morokuma et al 2016). Annis et al (2016) used DECam to search for a missing supergiant in the LMC, which would have been evidence for the collapse of a massive star that could have produced GWs, but failed to produce a typical corecollapse SN.…”

“…Candidate classification, comparison of redshift with the GW distance, and use of source age are crucial constraints to rule candidates in and out. Detailed discussions of candidate selection, spectroscopic and broadband follow-up are presented in survey-specific publications about iPTF candidates (Kasliwal et al 2016) and about PESSTO follow-up of Pan-STARRS1 candidates (Smartt et al 2016). Notes.…”

Multi-Messenger Astronomy

“…Rana et al 27 developed an enhanced scheduling algorithm that increases the odds of discovering an EM counterpart from a ground-based survey. This algorithm is being used by the iPTF group for scheduling follow-up observations 28 , and also being adapted for a radio searchs using the Jansky Very Large Array.…”