D. Xu (NAOC/CAS), N. R. Tanvir (U. Leicester), D. Malesani, J. Fynbo (DARK) report:
We observed the optical afterglow of GRB 151027B (Ukwatta et al. GCN 18499, Malesani et al. GCN 18501) with the VLT X-shooter spectrograph covering a spectral range 3400A to 18000A. Observations began at 2015 Oct 28 03:50 UT, approximately 5h09m post-burst.
The spectrum shows a clear continuum in the red, with a strong break corresponding to Ly-alpha at a redshift of z=4.063. This is supported by detection of absorption lines of Si II 1259/1260, Si II* 1265, CII 1335, CII* 1336, OI 1302, SiII 1304, S IV 1394/1403, CIV 1548/1551, FeII 1608 and Al II 1671 at that redshift. We therefore propose this as the redshift of the 1000th Swift GRB.
We thank the staff at Paranal, particularly T. Rivinius, for their support in obtaining these observations.