{
  "bibcode": "2021GCN.30392....1D",
  "body": "A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, \nC. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC),\nJ. P. U Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), N. R. Tanvir (U. Leicester), \nG. Lombardi, A. Marante (both GTC) report:\n\nWe observed the afterglow of GRB 210704A (BALROG localization: \nKunzweiler et al., GCN #30369; Fermi/LAT detection: Berretta et al., \nGCN #30375; Fermi/GBM detection: Malacaria & Meegan, GCN #30382; \nAGILE/MCAL detection: Ursi et al., GCN #30372; AstroSat/CZTI detection: \nPrasad et al., GCN #30378; Konus-Wind detection: Ridnaia et al., GCN \n#30388; Swift/XRT afterglow detection: D'Ai et al., GCN #30379; optical \nafterglow: Kim et al., GCN #30384, D'Avanzo et al., GCN #30385, Kann et \nal. GCN #30391) with OSIRIS, mounted on the 10.4 m GTC telescope, at \nthe Roque de los Muchachos observatory (La Palma, Spain). The \nobservation consisted of 2x60s acquisition images in r-band followed by \n4x600s spectra with grism R1000B, covering the spectral range between \n3700 and 7800 AA. The observation started at 21:48:57 UT (26.26 hr after \nthe burst) and the combined spectrum has an average epoch of 22:18:25 \nUT (26.75 hr after the burst).\n\nThe afterglow is well detected in the acquisition image at an r-band \nmagnitude of 22.40 +/- 0.07 as compared to several SDSS field stars. The \nspectrum shows continuum above 3800 AA with a typical SNR of 5 above \n5000 AA. \n\nThe detection of continuum down to 3800 AA implies an upper limit on the \nredshift of z < 3.15.\n\nWe do not detect any absorption or emission features at the redshift of the \nfield galaxy (z = 0.0817) or field cluster (z = 0.203) reported in by Levan et \nal. in GCN #30381. However, a featureless spectrum would be expected \nif this event were a short GRB in an intracluster environment.\n\nWe note that there is a tentative detection of a broad dip in the continuum at \n~ 4050 AA, that could be identified as due to Ly-alpha. However, we caution \nthat the SNR in this part of the spectrum is very low. This would imply a \nredshift of z = 2.34, and could be sustained by further low-significance \ndetections of OI, SiII and CII. However, we also note that at this redshift \nthere is no detection of CIV, nor SiIV, which is uncommon but not \nunprecedented. \n\nThe spectrum is publicly available for inspection at GRBSpec.eu.",
  "circularId": 30392,
  "createdOn": 1625581446000,
  "email": "deugarte@iaa.es",
  "subject": "GRB 210704A: OSIRIS/GTC spectroscopy",
  "submitter": "Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>",
  "eventId": "GRB 210704A"
}