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GCN Circular 20319

Subject
GRB 161214B: optical spectroscopy of the foreground SDSS object
Date
2016-12-21T10:31:39Z (7 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
D. Malesani (DARK/NBI and DTU Space), J. P. U. Fynbo (DARK/NBI), B. 
Milvang-Jensen (DARK/NBI), N. R. Tanvir (Univ. Leicester), M. I. 
Andersen (DARK/NBI), H. Korhonen (DARK/NBI), D. A. Perley (DARK/NBI), T. 
Kruehler (MPE), D. Xu (NAOC/CAS), G. Fedorets (NOT and Univ. Helsinki), 
report on behalf of a larger collaboration:

We report on further analysis of our data (Malesani et al., GCN 20260) 
taken at the Nordic Optical Telescope for GRB 161214B (D'Avanzo et al., 
GCN 20257). The best-seeing images (~0.9") clearly reveal two blended 
objects within the XRT error circle, separated by <1 arcsec, with 
comparable brightness (at ~2.2 hr after the GRB). As first suggested in 
GCN 20260, the likely interpretation is the near-superposition of the 
afterglow of GRB 161214B and an unrelated, foreground object. 
Astrometric registration of our images with the SDSS frames shows that 
the afterglow is slightly south of the SDSS source.

A spectrum of the complex was secured using the AlFOSC spectrograph, 
covering both objects. Observations started on 2016 Dec 14.82 UT (2.42 
hr after the GRB) and consisted of 2x20 min exposures using grism #4, 
covering the wavelength range 3500-9400 AA. A few absorption lines are 
apparent, and there is a hint that their spatial extent on the trace is 
offset to the North (consistent with the SDSS source location). The two 
most prominent features can be identified as the Mg I b triplet and the 
Na I D doublet at redshift ~0, confirming that the SDSS object is a 
Galactic star (probably of K or early M type). One more feature is 
observed at ~4238 AA, whose identification is unclear. One possibility 
is the Ca I 4226 A resonant line at z = 0, sometimes observed in K 
stars, which would however require an offset of the wavelength 
calibration in the blue part of our spectrum.

No features which can be clearly attributed to the afterglow are 
detected in our spectrum. Marshall & D'Avanzo (GCN 20268) report 
detection of the afterglow in the UVOT UVW2 filter, at a flux level much 
brighter than the SDSS object (u ~ 22.5). This allows to set an upper 
limit to the redshift z <~ 1.2 from the absence of the Lyman limit dropout.
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