HAWC TeV gamma-ray follow-up observation of the sky region of IceCube's multi-PeV neutrino-induced event

ATel #7868; Ignacio Taboada (GATECH) on behalf of the HAWC collaboration
on 3 Aug 2015; 13:25 UT
Credential Certification: Alberto Carramiñana (alberto@inaoep.mx)

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We have studied HAWC TeV gamma-ray archival data in the sky region corresponding to the multi-PeV neutrino-induced event reported by IceCube (ATEL # 7856). Putative cosmic ray sources that produce neutrinos are also expected to produce gamma-rays with approximately the same spectrum. If the source is transient, the neutrino light-curve is expected to be very similar to the gamma-ray light-curve. An observation by HAWC is possible, if, the neutrino spectrum extends from the observed PeV scale to the TeV energy scale to which HAWC is sensitive, and if source is not distant enough that the gamma-rays are attenuated by extragalactic background light. For TeV scale gamma-rays, sources can be observed if they are closer than approximately redshift 0.1. HAWC is a gamma-ray observatory, operating in central Mexico. It has an instantaneous field of view of about 2 sr. It can operate continuously, day or night and in any weather conditions. Over the course of a day, HAWC observes over half the sky in TeV gamma rays. HAWC, began full operations on March 20, 2015, but operated in partial configurations previously. Though the declination of the event is ideal for observations by HAWC, at the time, this region of the sky was at the occulted by Earth. Therefore, we could not perform a search in strict temporal coincidence. We conducted the follow up search using 5 different time periods. None of these searches found a gamma-ray source. In all five studies a region of 2 degrees by 2 degrees centered at the location reported by IceCube was searched. This matches the 1 degree uncertainty reported by IceCube for 99% containment of the PSF. The time periods studied are: 1) August 2, 2013 to May 6, 2015: Time integrated search 2) June 10, 2014: Sidereal day before the neutrino 3) June 11, 2014: Sidereal day after the neutrino 4) June 10-13, 2014: Four sidereal days around the time of the neutrino 5) June 6-15, 2014: Ten sidereal days around the time of the neutrino For the time integrated search, we used data from the partially built HAWC (aka HAWC-111, or 37% of final detector size and 283 days effective live-time) from August 2nd, 2013 to July 7th, 2014 and data with a second partially built HAWC (aka HAWC_250, 83% of final size and 150 days effective live-time) from November 26, 2014 to May 6th, 2015. The date of the neutrino event, June 11th, 2014, corresponds to the end of operation of HAWC-111. The preliminary 5 sigma discovery potential for this first search is (E^2 flux) 1.9e-12 TeV/cm^2/s at 100TeV assuming a power law flux with index of -2.3. For the other time periods the preliminary one day discovery potential was 3.9e-11 TeV/cm^2/s at 100TeV with a power-law index of -2.3. The discovery potential scales as the square root of time. The HAWC contact for this observation are Ignacio Taboada (Georgia Institute of Technology, itaboada@gatech.edu) and Michelle Hui (Michigan Technological University, cmhui@mtu.edu)