TITLE:   GCN CIRCULAR
NUMBER:  30272
SUBJECT: GRB 210619B: Redshift from OSIRIS/GTC
DATE:    21/06/20 11:28:59 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. A. Kann, C. Thoene, M. Blazek, J.F. Agui Fernandez, (all HETH/IAA-CSIC), L. Izzo (DARK/NBI), N.R. Tanvir (U. Leicester), J.P.U. Fynbo (DAWN/NBI), A. M. Garcia Rodriguez, and G. Gomez (GTC) report:

We observed the afterglow of GRB 210619B (D’Avanzo et al. GCN 30261; Lipunov et al. GCN 30262; Jelinek et al. GCN 30263; Zhao et al. GCN 30264; Kong GCN 30265; Beardmore et al. GCN 30267; Pellegrin et al. GCN 30268; Axelsson et al., GCN 30270; Perley GCN 30271) with OSIRIS on the 10.4 m GTC telescope at Roque de los Muchachos Observatory, in the Island of La Palma. The observation started at 2:27 UT (2.45 hr after the burst) and consisted of 3x600 s with grism R1000B, covering the spectral range between 3700 and 7800 AA.

In a preliminary reduction with old calibrations we detect a strong continuum with a plethora of absorption features superposed to it. We identify features that include SiII, SiIV, OI, CI, CII, CIV, FeII, AlII, AlIII, NiII, CrII ZnII, as well as fine structure lines of SiII, FeII, NiII at a common redshift of 1.937, which we identify as the redshift of the GRB. We also detect an intervening system with MgII and MgI at z=1.095.

Additionally, the flux calibrated spectrum shows a strong broad absorption feature at ~4500AA, resembling a dust bump. This could be consistent with a 2175 AA dust feature due to the intervening absorber.

We note that at a redshift of 1.937, the initial prompt emission spike is extremely luminous, and we expect the afterglow to be among the most luminous prompt flashes ever detected. Using the fluence reported on the automatic Swift/BAT analysis page in the 15-350 keV band, we derive an E_iso ~ 1.7e54 erg, without any k-correction.  This implies GRB 210619B will likely be among the most energetic GRBs ever detected once a k-correction to the bolometric band is applied.

Further analysis is ongoing. We encourage further observations.