TITLE:   GCN CIRCULAR
NUMBER:  30194
SUBJECT: GRB 210610B: Redshift confirmation from GTC
DATE:    21/06/11 08:38:27 GMT
FROM:    Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>

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

We observed the afterglow of GRB 210610B (Page et al., GCN 30170; Kumar et al., GCN 30174; Rumyantsev et al., GCN 30175; Hu et al., GCN 30177; de Wet et al., GCN 30180; Romanov, GCN 30181; Fynbo et al GCN 30182; Moskvitin et al. GCN 30187; Becerra et al. GCN 30190; Mong et al. GCN 30193) with OSIRIS at the 10.4m GTC telescope, located at Roque de los Muchachos Observatory, in La Palma (Spain). Observation consisted of 3 x 900 s exposures with grism R1000B, covering the range between 3700 and 7800 AA. The first spectrum started at 01:36:20 UT (5.75 hr after the burst).

The spectrum shows a very strong continuum with very weak superposed lines. We detect lines corresponding to FeII, MgII and MgI at a common redshift of z = 1.1345, in agreement with the measurement from the NOT (Fynbo et al. GCN 30182). Additionally, the high SNR allows us to detect a weak intervening system through the identification of MgII at a redshift of z = 0.557. Although the redshift 1.1345 is, strictly speaking, a lower limit, the lack of any further features at higher redshift, especially considering the high SNR of the spectrum, allows us to consider this as the redshift of the GRB.

We note that the features detected at the redshift of the GRB are very weak. We can compare their strength to a sample of long GRB afterglow features using the line strength parameter, following the method of de Ugarte Postigo et al. (2012, A&A 548, A11). For this spectrum we measure LSP = -2.1 +/- 0.8, which implies that the lines are weaker than 99.8% of the sample.