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VERITAS follow-up observations of IceCube neutrino event 170922A

ATel #10833; Reshmi Mukherjee
on 9 Oct 2017; 22:32 UT
Credential Certification: Reshmi Mukherjee (muk@astro.columbia.edu)

Subjects: Gamma Ray, TeV, VHE, Neutrinos, AGN

Referred to by ATel #: 10838, 10844, 10845, 10861

The VERITAS gamma-ray telescope array was used to perform follow-up observations of the high-energy neutrino event detected by the IceCube collaboration on September 22nd, 2017 20:54:30 UTC (GCN Circular #21916). 


VERITAS observed the location around the initial position reported by IceCube in the GCN/AMON Notice dated Fri 22 Sep 17 20:55:13 UTC (RA = 77.29 deg, Dec = 5.75 deg in J2000 coordinates) under partial cloud coverage for one hour. Observations started on September 23rd, 2017 09:06 UTC, 12.2 hours after the IceCube detection. No gamma-ray source was detected at the neutrino position or anywhere else in the 3.5-degree VERITAS field of view.


Additional VERITAS observations were collected following the report by the Fermi LAT collaboration (ATel #10791) of the detection of a strong, hard GeV flare from the blazar TXS 0506+056 (RA = 77.358 deg, Dec = 5.693 deg in J2000 coordinates) located within the neutrino error region. 


A total of five hours of additional observations centered on the blazar location were collected between September 28th 08:57 UTC and September 30th 11:04 UTC. A preliminary analysis of the data optimized for soft-spectrum sources shows no evidence of gamma-ray emission at the blazar location. The integral gamma-ray flux upper limit derived from these observations at the TXS 0506+056 position is 6.80 x 10^-12 cm^-2 s^-1 at 99% CL above an energy threshold of 160 GeV for an assumed spectral index of -2.7. Null VHE gamma-ray observations were also reported by the H.E.S.S. (ATel #10787) and HAWC collaborations (ATel #10802), while the MAGIC collaboration reports the detection of a gamma-ray source coincident with the blazar position above a 100 GeV energy threshold in 12 hours of observations taken between September 28 and October 3 (ATel #10817).


Questions regarding the VERITAS observations should be directed to Reshmi Mukherjee (rmukherj@nevis.columbia.edu). VERITAS (Very Energetic Radiation Imaging Telescope Array System) is located at the Fred Lawrence Whipple Observatory in southern Arizona, USA, and is most sensitive to gamma rays between ~85 GeV and ~30 TeV (http://veritas.sao.arizona.edu).