{
  "bibcode": "2015GCN.17333....1F",
  "body": "W. Fong (U. Arizona), E. Berger (Harvard), D. Fox (PSU) and B.J. Shappee\n(Carnegie) report:\n\n\"We re-observed the field of the short/soft GRB 150101B (Cummings, GCN\n17267) with the Gemini Multi-Object Spectrograph (GMOS) mounted on the\nGemini-South 8-m telescope starting on 2015 Jan 12.326 UT, 10.7 days after\nthe BAT trigger and 9.0 days after our initial Magellan observations (Fong\net al., GCN 17271). We obtained a total of 1710-sec of r-band imaging in\n0.58\" seeing. To assess fading of the candidate optical afterglow (Fong et\nal., GCN 17271), we performed digital image subtraction using the ISIS\npackage between the Gemini and Magellan observations. We find a residual\npoint source with a refined position of\n\nRA(J2000) = 12:32:05.08\nDec(J2000) = -10:56:03.16\nwith an uncertainty of 0.5\" in each coordinate.\n\nThis indicates that the optical source has faded between 1.7 and 10.7 days\npost-burst. Thus far, this is the only evidence for a fading source within\nthe BAT position. Fading is also supported by the lack of optical\ncounterpart in other filters from late-time VLT observations (Levan et al.,\nGCN 17321). We note that digital image subtraction between our two Magellan\nepochs at 1.7 days and 2.7 days post-burst revealed no residuals (Fong et\nal., GCN 17285), suggesting that the source had an initial period of\nshallow decay.\n\nThe optical source is ~3.1\" offset from the center of the galaxy 2MASX\nJ12320498-1056010. Given the separation and optical brightness (Cummings et\nal., GCN 17268), the galaxy has a low probability of chance coincidence of\n~2e-3. At the redshift of the galaxy, z=0.134 (Levan et al., GCN 17281),\nthe projected physical offset is ~7.4 kpc. In addition, the location of the\noptical source is consistent with the position of the faint Chandra source\n(SRCX #2 in Troja et al., GCN 17289).\n\nWe conclude that the fading of the optical source, spatial coincidence with\nan X-ray counterpart, and proximity to a galaxy with low probability of\nchance coincidence confirms the source as the optical afterglow, and 2MASX\nJ12320498-1056010 as the host galaxy of GRB 150101B. At z=0.134, this is\nthe among the lowest confirmed redshifts for a short GRB to date.\"",
  "circularId": 17333,
  "createdOn": 1421806392000,
  "email": "wfong@email.arizona.edu",
  "subject": "GRB 150101B: Confirmation of a fading optical counterpart",
  "submitter": "Wen-fai Fong at U of Arizona  <wfong@email.arizona.edu>",
  "eventId": "GRB 150101B"
}