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GCN Circular 21153

Subject
LIGO/Virgo G268556: VERITAS Very-High-Energy Gamma-Ray Observations
Date
2017-05-27T00:20:02Z (7 years ago)
From
David A. Williams at UC Santa Cruz <daw@ucsc.edu>
Reshmi Mukherjee (Barnard College and Columbia University) reports on behalf of the VERITAS Collaboration:

The VERITAS gamma-ray telescope array was used to perform follow-up observations of the localization region for the gravitational-wave candidate G268556 between 2017-01-05 07:14:53 and 11:39:39 UTC. During this period, the northern section of the 50% containment region for the event was observable at >50�� elevation from the VERITAS site and 39 consecutive exposures were performed to tile the localization region using the 3.5��-diameter field-of-view of the VERITAS telescopes. Each pointing was observed for 5 minutes, with an average spacing between pointings of 1.83�� to allow some overlap between neighboring fields. The observations cover approximately 27% of the containment probability of the event.   Unfortunately, the majority of the exposures were affected by the presence of clouds.  Furthermore, one of the four VERITAS telescopes was offline during the final two-thirds of the observations, decreasing the sensitivity by ~20%.  Preliminary results from the analysis of the VERITAS data show no significant evidence of gamma-ray emission.  Observations performed by VERITAS under clear skies using a similar observational strategy have been able to detect the Crab Nebula at high significance (>10 standard deviations) using similar exposures with all four telescopes.  Under ideal conditions, these observations of G268556 would have been sensitive to a gamma-ray source with a flux greater than ~50% of the Crab Nebula, i.e., greater than ~3 x 10^{-10} cm^{-2} s^{-1} above 100 GeV, during the time period and in the region of sky observed.  However, the sensitivity of VERITAS was reduced due to the poor weather conditions, and it is challenging to set specific limits at this time.  Questions regarding the VERITAS observations should be directed to Reshmi Mukherjee (muk@astro.columbia.edu). VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is a very-high-energy gamma-ray observatory located at the Fred Lawrence Whipple Observatory in southern Arizona, USA.
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