TITLE:   GCN CIRCULAR
NUMBER:  25653
SUBJECT: LIGO/Virgo S190901ap: GTC follow-up spectroscopic observations of ZTF19abvizsw
DATE:    19/09/04 11:44:44 GMT
FROM:    Peter Jonker at SRON/RU  <p.jonker@sron.nl>

A.J. Levan (Radboud), P.G. Jonker (SRON/Radboud), P. Rodriguez Gil, M.A.P.
Torres (IAC/ULL), K. Maguire (TCD), M. Fraser (UCD), D. Mata (JBCA, University
of Manchester) report on behalf of a larger collaboration:

We obtained 3x900 sec of spectroscopic observations of ZTF19abvizsw with the GTC on La Palma on 2
September 2019 over the time interval 20:37 - 22:05 UT  using the R1000R grating and the OSIRIS
instrument. The spectra cover the range 5100-10000 AA. A strong continuum is detected with
prominent absorption lines which we identify as FeII (2344, 2374, 2383, 2586, 2600 AA) and MgII
(2796, 2803 AA) and MgI (2852 AA) at a common redshift of z=1.258. This is in agreement with the
results of Burdge et al. (GCN 25639). Indeed the spectral features provide a good match to the
composite spectra of GRB afterglows (Christensen et al. 2011).  This offers further support for
the identification of ZTF19abvizsw as an untriggered GRB (Perley et al. GCN 25643). Searches in
existing gamma-ray observations around the rise time would be strongly merited. 


We thank the GTC staff and especially Peter Pessev for their assistance in
acquiring these observations.

Acknowledgements: PGJ acknowledges support from the European Research
Council under ERC Consolidator Grant agreement no 647208.