TITLE:   GCN CIRCULAR
NUMBER:  23889
SUBJECT: GRB 180703A: VLT/MUSE host galaxy redshift measurement
DATE:    19/02/12 08:58:39 GMT
FROM:    Luca Izzo at IAA-CSIC  <luca.izzo@gmail.com>

L. Izzo (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), P. Schady (U. Bath), D. B. Malesani (DAWN/NBI and DARK/NBI), D. A. Kann (HETH/IAA-CSIC), C. Kouveliotou (GWU), V. D’Elia (SSDC) and N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:

We have analysed the data obtained using the MUSE instrument, mounted on the ESO VLT UT4 (Yepun) telescope in Paranal, of the Fermi GRB 180703A (Racusin et al., GCN #22883; Poolakkil &  Meegan, GCN #22896). MUSE is an integral-field-unit spectrograph which covers a field of view of up to ~1 arcmin**2. Our observations started on 2018 July 04 at 07:53:56 UT, 10.92 hr after the GRB detection, and consisted of four exposures of 250 s each.

When we integrate the data cube along the full MUSE wavelength range, we detect a source within the uncertainty of the Swift/XRT afterglow position (Gibson et al., GCN #22891), and consistent with the position of the reported optical counterpart (Guidorzi et al., GCN #22892, #22944). We extracted a spectrum of this source within the wavelength range 4750-9350 AA. The spectrum is noisy, and we cannot confirm continuum afterglow emission. However, we identify three strong emission lines which can be consistently interpreted as [O III] 4960,5008 AA and [O II] 3727/29 AA at a common redshift of z = 0.6678. Given the spatial coincidence with the optical counterpart, we propose that these features come from the GRB host galaxy and we conclude that this is the redshift of GRB 180703A.

We acknowledge support from the ESO observing staff in Paranal, in particular Andrea Mehner.