D. Malesani (DARK/NBI), T. Kruehler (ESO), D. Xu (NAOC/CAS), G. Pugliese (API/Uva), D. Watson, J. P. U. Fynbo, B. Milvang-Jensen (DARK/NBI), A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), N. R. Tanvir (U. Leicester), V. D'Elia (ASI/ASDC and INAF/Roma), K. Wiersema (U. Leicester), J. Greiner(MPE Garching), J. Japelj (U. Ljubljana) report on behalf of a larger collaboration:
We observed the optical afterglow (Varela et al., GCNs 17729, 17732, Perley, GCN 17733, Littlejohns et al. GCN 17736, Kann et al. GCN 17738, Fong & Milne GCN 17741) of the Swift short-GRB 150423A (Pagani et al. GCN 17728) with the VLT equipped with the X-shooter spectrograph in the rapid response mode (RRM).
A series of individual spectra with a total exposure time of around 5000 s were obtained starting on 2015-04-23 06:50 UT, which is 22 minutes after the BAT trigger.
A preliminary analysis of the spectrum reveals a faint and blue continuum detected down to 3200 AA. This sets a robust upper limit of z < 2.5 to the redshift of the GRB. Similarly to Perley (GCNs 17733, 17744), we do not detect obvious absorption or emission features in our spectra. There is, however, a tentative detection of a doublet in absorption at around 6900 AA. If real, the lines match the Mg II doublet at a redshift of z = 1.394.
We acknowledge the excellent support provided by Paranal staff, and in particular Fernando Selman and Emanuela Pompei.