GCN Circular 20168
Subject
GRB 161104A: Magellan imaging and spectroscopy
Date
2016-11-11T02:15:11Z (8 years ago)
From
Wen-fai Fong at U of Arizona <wfong@email.arizona.edu>
W. Fong (University of Arizona) and R. Chornock (Ohio University) report:
We imaged the location of the short-duration GRB 161104A (Mingo et al., GCN
20213) with the Inamori-Magellan Areal Camera and Spectrograph (IMACS)
mounted on the Magellan/Baade 6.5-m telescope at a mid-time of 2016 Nov
7.170 UT (2.77 days post-burst). We obtained 6x360-sec of r-band exposures
in 1.0" seeing at an airmass of 1.3. We clearly detect the four optical
sources reported from GROND (Nicuesa Guelbenzu et al.; GCN 20132) and
Gemini (Troja et al.; GCN 20137) imaging. Given the relatively poor seeing,
accurate photometry of the individual sources is challenging. However, the
sources appear to have similar relative flux levels as previously reported,
suggesting a lack of significant variability in any of these sources. We
note that Source A (Nicuesa Guelbenzu et al.; GCN 20132) is fully
coincident with the latest XRT position*, while the three remaining sources
are on the outskirts of the position.
In addition, we obtained 3x1800-sec of spectroscopy with Magellan/IMACS on
2016 Nov 8 UT in 0.6" seeing at an airmass of 1.1. We used the 200 line
grism and the f/2 camera to cover 4000-10000 Angstroms. The slit was
aligned through Sources A and B (Nicuesa Guelbenzu et al.; GCN 20132) and
also serendipitously passed through a nearby extended source 6.1" from the
center of the XRT position (Source X; RA=5:11:34.77, Dec=-51:27:41.8). The
spectrum of Source A exhibits a red continuum indicative of an early-type
galaxy with prominent Ca II H+K stellar absorption lines at z=0.793. Source
B has a faint trace with a single possible emission line of unknown
identification near 9780 Angs. The spectrum of Source X is also that of an
early-type galaxy at z=0.788. We note that both Sources A and X lack
prominent emission lines of either [O II] 3727 or H-beta. The similarity
in the redshifts of Sources A and X, their early-type galaxy spectra, and
the abundance of extended sources in the vicinity of the XRT position may
signify an origin from a cluster or group.
We thank Magellan telescope operators Hernan, Jorge and Mauricio for their
assistance with these observations.
*http://www.swift.ac.uk/xrt_positions/