Support ATel At Patreon

[ Previous | Next | ADS ]

AT2018cow: Fermi/GBM Data Search

ATel #11793; T. Dal Canton (GSFC/USRA), M. S. Briggs (UAH), E. Burns (GSFC/USRA), N. Christensen (OCA), A. Goldstein (USRA), P. Shawhan (UMD), C. Wilson-Hodge (MSFC), P. Veres (UAH), D. Kocevski (MSFC)
on 28 Jun 2018; 20:29 UT
Credential Certification: Daniel Kocevski (dankocevski@gmail.com)

Subjects: Gamma Ray, Transient

We report the search results of gamma-ray emission from AT2018cow in the Fermi GBM data. Using the position (RA, Dec) = (244.00090, 22.26800) from Smartt et al. (ATel #11727) we searched for impulsive events from the last non-detection on June 13th (ATeL #11738) until the end of June 16th. During this time period the sky position was observed by GBM 54% of the time. GBM was not observing the source at the times of the spatially consistent IceCube neutrinos (ATel #11785). During this time range there were 5 on-board triggers, 3 of which were due to GRBs and the blind untargeted search of GBM data identified 4 additional GRB candidates in the time range. None of these transients are consistent with the source position of AT2018cow. We also performed dedicated sub-threshold searches. Both a custom run of the blind untargeted search and the GBM targeted search (traditionally run around gravitational wave triggers) identified a number of transients that are either before the 13th or statistically insignificant. We list those that are spatially consistent (with large uncertainties) here for completeness. 1. The untargeted search identified a 64ms signal at 2018-06-12 06:40:53.5. This is about a day before the last optical non-detection. 2. The untargeted search identified a 0.7 s long signal at 2018-06-16 00:04:45.4 but this may be due to background fluctuations. 3. The targeted search identified a ~2s long soft signal at 2018-06-12 14:30:22.5, before the last optical non-detection. 4. The targeted search identified a ~8s long, soft signal at 2018-06-14 23:06:13.3. 5. The targeted search identified a ~2s long, soft signal at 2018-06-15 at 02:10:47.8. All events identified by the targeted search are close in time to Earth occultations of Sco X-1, and their sky localizations are also consistent with the position of Sco X-1. For this reason, and considering their weakeness compared to the GBM background, these events do not constitute strong evidence for activity related to AT2018cow. If there was an impulsive gamma-ray signal at event time it is either one of these candidate transients, or it occurred while GBM was not observing the position of AT2018cow. Otherwise, we set impulsive upper limits of ~few x 10^-7 erg/s/cm^2 in the 10-1000 keV energy range for the times we were observing the event. At a distance of 60 Mpc and assuming a soft spectrum represented by a Band function with (alpha, beta, Epeak) of (-1.9, -3.7, 70 keV) this corresponds to an upper limit of ~few x 10^49 erg/s in the 1 keV - 10 MeV energy range. We also searched for longer-term gamma-ray emission using the Earth Occultation Technique. There are a few spurious detections likely caused by confusion with Sco X-1, but nothing consistent with the flux expected from the spectrum measured by NuSTAR (ATel #11775) or the possible detection in Swift/BAT (ATel #11782). We will continue to perform searches of the GBM data for this transient. We acknowledge the Max Planck Gesellschaft and the Atlas cluster computing team at AEI Hannover for part of the computations required by this analysis.