{
  "bibcode": "2016GCN.19423....1L",
  "body": "D. Lennarz (Michigan State University), I. Taboada (Georgia Tech) report\non behalf of the HAWC collaboration\n(http://www.hawc-observatory.org/collaboration/):\n\nWe used data from the HAWC detector to perform a search for VHE emission\nin temporal coincidence with GRB 160509A (F.Longo et al., GCN 19403). At\nthe time of the LAT trigger, the elevation of the burst in HAWC's field\nof view was only 27.98 degrees (it was rising, but culminated at an\nelevation of 33 degrees). The sensitivity of HAWC at this elevation is\nmore than 2 orders of magnitude poorer than near the zenith.\nFurthermore, the energy threshold towards the horizon is much higher.\nCombined with the moderate redshift of z=1.17 (N. R. Tanvir et al., GCN\n19419) it makes a detection by HAWC unlikely.\n\nWe used four search windows with respect to the LAT trigger time: one in\nthe range -5 s to 45 s, which covers the main GBM emission episode (O.J.\nRoberts et al., GCN 19411) and appears to be correlated with the >100\nMeV soft emission observed by the LAT (F.Longo et al., GCN 19413), a\nwindow from -5 s to 375 s, which extends slightly beyond the T90\nobserved by GBM and a time window from 45 s to 375 s, where the LAT data\nis fit with a power-law of index -2.0 +/- 0.1. We also searched -20 s to\n20 s around the time of the highest-energy LAT photon (52 GeV) 77\nseconds after the GBM trigger. A 2 degree angular bin is defined around\nthe position of the Swift-XRT afterglow position (J. A. Kennea et al.,\nGCN 19408) and the number of background events is estimated using an\nON/OFF method. We find the counts in the search bin to deviate by 1.9 /\n0.9 / 0.2 / -1.4 sigma from the background expectation. Our observations\nare consistent with background only.\n\nThe search was conducted using the main data acquisition that\nreconstructs the incident direction of showers. It uses data\nreconstructed at the HAWC site, not applying gamma-hadron separation.\nThe implications of this non-detection with respect to the VHE fluence\nof this GRB will be reported elsewhere.\n\nHAWC is a very-high-energy gamma-ray observatory operating in Central\nMexico at a latitude of 19 deg north. HAWC has an instantaneous field of\nview of 2 sr and surveys 2/3 of the sky every day. A detailed\ndescription of the sensitivity of HAWC to GRBs can be found in A.U.\nAbeysekara et al., Astroparticle Physics 35, 641-650 (2012).",
  "circularId": 19423,
  "createdOn": 1462987657000,
  "email": "dirk.lennarz@gatech.edu",
  "subject": "GRB 160509A: non-observation of VHE emission with HAWC",
  "submitter": "Dirk Lennarz at HAWC  <dirk.lennarz@gatech.edu>",
  "eventId": "GRB 160509A"
}