{
  "bibcode": "2021GCN.30436....1W",
  "body": "Alan M. Watson (UNAM), Eleonora Troja (GSFC/UMD), Rosa L. Becerra (UNAM), Rub��n\nS��nchez (INAF),  Aishwarya Thakur (INAF), Simone Dichiara (GSFC/UMD), Nat Butler\n(ASU), William H. Lee (UNAM), Oc��lotl Lopez (UNAM), Margarita Pereyra (UNAM),\nand Srihari Ravi (ASU) report:\n\nWe observed the field of GRB 210704A (Berretta et al., GCN Circ. 30375; D'Ai et\nal., GCN Circ. 30379; Malacaria & Meegan, GCN Circ. 30380; Kim et al., GCN Circ.\n30384) with the OSIRIS instrument on the 10.4 meter GTC telescope.\n\nWe observed at two epochs. The first epoch was at 2.10 days after burst and the\nsecond at 5.10 days after burst.  At both epochs, we detect a source at the\nposition of the candidate optical afterglow (Kim et al., GCN Circ. 30384) at a\nmagnitude consistent with the TNG observations of r~23.3 at 4.1 days (D'Avanzo\net al., GCN Circ. 30432). We also resolve a second fainter (r~24.5 AB mag)\nsource (S2) located 0.7 arcsec E and 2.4 arcsec S of the OT position.\n\nOur observations indicate only a marginal fading of about 0.2 magnitudes in the\nr-band between our two epochs, which is unusually shallow for a GRB afterglow or\nfor a kilonova.\n\nArchival pre-explosion images of the field reveal the presence of an underlying\nfaint source (S1), which might be the host galaxy. Based on a deep exposure\nobtained in 2012 with the MegaPrime/MegaCam, we estimate a magnitude of r~25.3\n+/- 0.2 AB mag for S1. The source S2 is also detected in archival images at a\nmagnitude consistent with our GTC observations.\n\nAssuming no intrinsic variability, S1 would only partially contribute to the\nobserved optical light (~25%) and this by itself is not sufficient to explain\nthe slow fading of the optical emission. Given the ambiguous classification of\nthe gamma-ray emission, whose duration lies at the intersection between short\nand long GRBs, we cannot exclude that the observed flattening marks the onset of\nan associated supernova.\n\nFurther observations to monitor the evolution of this source are encouraged.\n\nWe thank David Garcia and Antonio Cabrera for assistance with these\nobservations.",
  "circularId": 30436,
  "createdOn": 1625901825000,
  "email": "alan@astro.unam.mx",
  "subject": "GRB 210704A: OSIRIS/GTC Observations and Archival Detection of the Possible Host Galaxy",
  "submitter": "Alan M Watson at UNAM  <alan@astro.unam.mx>",
  "eventId": "GRB 210704A"
}