TITLE:   GCN CIRCULAR
NUMBER:  17832
SUBJECT: GRB 150518A: VLT/X-shooter redshift of the likely host galaxy
DATE:    15/05/20 07:44:28 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Xu (NAOC/CAS), A. J. Levan (Univ. Warwick), A. de Ugarte Postigo 
(IAA/CSIC), N. R. Tanvir (Univ. Leicester), D. Malesani (DARK/NBI), J. 
P. U. Fynbo (DARK/NBI), report on behalf of the X-shooter GRB collaboration.

We observed the optical counterpart (Xu et al., GCN 17829; Mao et al., 
GCN 17830; Littlejohns et al., GCN 17831) of the MAXI GRB 150518A 
(Kawamuro et al., GCN 17825; Sbarufatti et al., GCN 17827) using the 
X-shooter spectrograph at the ESO Very Large Telescope (VLT).

A total of 40 min integration was secured starting on 2015 May 20.184 UT 
(1.28 days after the GRB) covering the wavelength range 3000-18000 AA. 
The counterpart, which may include light from both host galaxy and 
afterglow, is clearly detected, and a number of prominent emission lines 
are visible. Among others, we identify the Balmer lines, [O II] 3727, [O 
III] 4957,5007, [N II], [S II], all at a common redshift of z = 0.256.

At this redshift, any supernova similar to other associated with GRBs 
would peak at R ~ 21. Although extinction along the line of the sight to 
the burst is unknown at present, the low redshift makes GRB 150518A an 
ideal opportunity for supernova characterisation.

We acknowledge excellent support from the observing staff in Paranal, in 
particular Dimitri Gadotti, Marcelo Lopez, and John Pritchard.