GRB 201221A: VLT/X-shooter afterglow discovery and spectroscopic redshift z = 5.7
Abstract
We observed the field of GRB 201221A (Page et al., GCN 29096) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Using the acquisition camera, multi-band imaging was secured. Within the XRT error circle (Evans, GCN 29097), no source is detected in an r-band image (taken at mean epoch of 42 min after the GRB) down to limiting magnitude r > 23.5 (AB). However, a source is clearly visible in the z-band image with z = 19.76 +- 0.04 AB (mean time 35 min after the GRB). The corresponding color r-z > 3.7 is exceptionally red. The coordinates of the object are (J2000, 0.3" error): RA = 14:17:55.26 Dec = -45:24:58.0 Spectroscopy of the z-band source was executed covering the wavelength range 3000-25000 AA. Unfortunately due to visibility constraints, only two spectra by 600 s each were secured, plus two extra exposures during twilight which have lower quality. We report on the analysis of the first set only, which has a mean time of 1.65 hr after the GRB. Continuum is well detected in the infrared arm, as well as in the red part of the visible arm. A clear break is detected around 8150 AA, which, if interpreted as the Lyman break, corresponds to z = 5.70. Weak continuum is detected blueward of the break, until it vanishes completely at the corresponding location of the Lyman limit. We thus conclude that z = 5.70 is the redshift of GRB 201221A. Further analysis is undergoing. We acknowledge support from the ESO observing staff at Paranal, in particular Zahed Wahhaj.
- Publication:
-
GRB Coordinates Network
- Pub Date:
- December 2020
- Bibcode:
- 2020GCN.29100....1M