TITLE: GCN CIRCULAR NUMBER: 19377 SUBJECT: VERITAS rapid follow-up observations of IceCube event 160427A DATE: 16/05/03 00:39:16 GMT FROM: Reshmi Mukherjee at Columbia U/VERITAS Title: GCN CIRCULAR Subject: VERITAS rapid follow-up observations of IceCube event 160427A From: VERITAS Collaboration On April 27th, 2016, the IceCube collaboration reported the detection of a high-energy neutrino of potential astrophysical origin (GCN #19363). The neutrino event (run ID: 127853, event ID: 67093193) was detected at 05:52:32 UTC and follow-up observatories, VERITAS among them, were notified at 05:53:53 through a GCN/AMON notice. See http://gcn.gsfc.nasa.gov/notices_amon/67093193_127853.amon for details. VERITAS performed follow-up observations of the alert position (RA: 239.6639°, Dec: 6.8528°, in J2000 coordinates) between 05:55:45 UTC, 193 s after the neutrino detection, and 07:39:36 UTC in normal ‘wobble’ mode, where the pointing direction of the telescope is offset from the source position to allow for simultaneous measurement of the background. These observations resulted in a total exposure of 71 min, of which 60 min were taken in bright moonlight conditions. No gamma-ray source was detected within the 50% error circle of the neutrino position (1.6° radius) or anywhere else in the VERITAS field of view which has a diameter of 3.5°. A second revision of the alert was circulated by IceCube on 04/27/16 23:24:24 UTC with a refined event position given by more advanced reconstruction algorithms. The new alert position (RA: 240.5683°, Dec: 9.3417°, in J2000 coordinates) was outside the field covered by the first VERITAS observations. New observations of the revised position were performed on April 28th between 07:41:32 and 09:47:15 UTC under bright moonlight conditions, resulting in a total exposure of 118 min. No gamma-ray source was detected within the 0.6°-radius 90% error circle of the new neutrino position or anywhere else in the VERITAS field of view. Differential flux upper limits for a gamma-ray point source were calculated at the revised neutrino position (RA: 240.5683°, Dec: 9.3417°, in J2000 coordinates) and are listed below for four energy bins, as follows: ---------------------------------------------------- Emin [TeV] Emax [TeV] dN/dE UL [cm^-2 s^-1 TeV^-1] ---------------------------------------------------- 0.2 0.282 5.7e-11 0.282 0.398 1.5e-11 0.398 0.562 2.6e-12 0.562 0.794 3.0e-12 VERITAS (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.