{
  "bibcode": "2016GCN.20222....1T",
  "body": "E. Troja (UMD/GSFC), N. Tanvir (U. Leicester), S. B. Cenko (NASA/GSFC),\nA. Levan (U. Warwick), J. Barnes (U. Berkeley), A. Castro-Tirado (IAA-\nCSIC), A. S. Fruchter (STScI), N. Gehrels (NASA/GSFC), J. Greiner (MPE),\nN. Kawai (Tokyo Tech), R. Hounsell (UCSC), J. Hjorth (DARK/NBI), A. Lien\n(NASA/GSFC), B. Metzger (Columbia),  D. Perley  (DARK/NBI), S. Rosswog\n(U. Stockholm), T. Sakamoto (AGU), C. Thoene (IAA-CSIC), A. de Ugarte\nPostigo (IAA-CSIC), and D. Watson (DARK/NBI) report:\n\nWe monitored the location of the short GRB 160821B (Siegel et al. GCN\n19833; Xu et al. GCN 19834) with the Hubble Space Telescope under our\napproved guest observer programs (GO14237 PI: Tanvir; GO14087 PI: Troja).\nObservations were carried out with the Wide Field Camera (WFC3) in three\nfilters, F606W, F110W and F160W, at epochs 3.6, 10.4 and 23.2 days post-\nburst. The GRB counterpart is clearly detected in all filters during the\nfirst two epochs, and fades from a magnitude of F606W~25.8 (AB) in the\nfirst epoch to become undetectable in the third epoch.\n\nAssuming a redshift of z=0.162 from the nearby galaxy identified as the\nlikely host (Levan et al. GCN 19846), our observations rule out the\npresence of an emerging supernova comparable to SN1998bw or to other SNe\nassociated to long GRBs. The observed fluxes constrain the contribution\nof any r-process kilonova/macronova component to be at least a\nfactor ~5 fainter in the IR than that seen in GRB 130603B. The lack of\na bright supernova and the moderate-to-low ejecta mass implied by our\nobservations are consistent with this event being produced by the merger\nof two neutron stars.\n\nHowever, the current dataset cannot firmly exclude the presence of an\nunderlying, higher redshift host galaxy. Deeper HST observations aimed\nat placing better constraints on the GRB redshift are on-going.\n\nWe thank the STScI staff, in particular Tricia Royle, for assistance\nwith rapidly scheduling our observations.",
  "circularId": 20222,
  "createdOn": 1480559797000,
  "email": "eleonora.troja@nasa.gov",
  "subject": "GRB 160821B: HST detection of the optical and IR counterpart",
  "submitter": "Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>",
  "eventId": "GRB 160821B"
}