[vsnet-grb-info 17930] VERITAS rapid follow-up observations of IceCube event 160427A

[GCN Circulars]

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  <muk at astro.columbia.edu>

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.