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GCN Circular 28492

Subject
GRB 200922A: chance superposition between optical afterglow and archival star
Date
2020-09-24T21:15:28Z (3 years ago)
From
Daniele B Malesani at DTU Space <malesani@space.dtu.dk>
D. B. Malesani (DTU Space), L. Izzo (DARK/NBI), D. A. Kann (HETH, 
IAA/CSIC), A. de Ugarte Postigo (HETH, IAA/CSIC), A. J. Levan (Radboud 
Univ.), D. Xu (NAOC), P. D'Avanzo (INAF/OABr), and A. Rossi (INAF/OAS) 
report on behalf of the Stargate collaboration:

For the recent GRB 200922A, several authors have noted the presence of a 
bright optical afterglow, initially detected by UVOT onboard the Neil 
Gehrels Swift observatory (Moss et al., GCN 28471; Izzo et al., GCN 
28478; Siegel & Moss, GCN 28482), superimposed on a point-like object, 
seen e.g. in the DSS, DECam, and SkyMapper surveys.

We retrieved the white-band UVOT images (Siegel & Moss, GCN 28482). We 
focused on the initial image (344 s exposure starting at 12:08:32 UT on 
Sep 22) and on the image taken roughly one day after (274 s exposure 
starting at 10:05:38 UT on Sep 23). We also retrieved for comparison the 
archival DECam i-band images of the same region (from 2017 August).

We remark that the position of the UVOT object is entirely consistent 
with the latest XRT position of the afterglow (Evans et al., GCN 28483; 
see also https://www.swift.ac.uk/xrt_positions/), strengthening the 
association between the optical transient and GRB 200922A.

We then cross-matched the astrometry between the UVOT and the archival 
DECam images. The scatter in the astrometric solution, computed on stars 
of brightness similar to the UVOT afterglow, is 0.06 and 0.10 arcsec for 
the initial and late exposure, respectively.

In the initial UVOT image, we measure an offset of 0.62" compared to the 
object seen in the archival data. While small, this offset is 
significant. The same comparison carried out in the late time image, 
however, returns no measurable offset to within the errors. 
Independently, we also checked that the proper motion of the star 
(catalogued in the Gaia DR2; Brown et al. 2018, A&A, 616, A1) is too 
small to play any effect in producing the offset.

We thus conclude that this is a rare case of near-perfect chance 
superposition between a background afterglow and a bright foreground 
star. In the initial UVOT image, the light was dominated by the 
afterglow (thus yielding the offset in the position), which subsequently 
faded to a level fainter than the foreground star. The light curve of 
the X-ray afterglow (D'Ai et al., GCN 28476) is also consistent with 
this interpretation, as it shows the typical behaviour of a cosmological 
GRB.


A spectrum of the optical transient was taken using the ESO VLT1 UT1 
(Antu) equipped with the FORS2 instrument. Observations were carried out 
using the grisms 300V and 300I (900 s exposure each), covering the 
wavelength range 3500-9200 AA, and started on 2020 Sep 22.997 UT (11.8 
hr after the trigger). In the acquisition image, using (old) archival 
zeropoints, we measure V = 18.0 (Vega). This value is roughly consistent 
with the archival values of the star (for example the Gaia catalog gives 
G = 17.85), and indicates only little contribution from the afterglow at 
the epoch of our spectrum (at least in the V band).

The spectrum we observe is consistent with the one of a G-type star. 
Absorption features are observed at z = 0 from Ca H and K, Mg I, Na I D, 
Hdelta, Hgamma, Hbeta, and Halpha. The ordinary type of the star 
supports the lack of a connection with the GRB. A search was conducted 
for potential emission lines from the background GRB host galaxy, but 
the intense and spatially-variable glare from the star does not allow us 
to place quantitative limits. We are thus unfortunately unable to 
provide constraints on the GRB redshift.


We welcome the reopening of the ESO observatory at Paranal, even if at 
reduced capacity. We thank the ESO staff for carrying out our 
observations, in particular Steffen Mieske, Claudia Cid, and Romain 
Thomas, as well as and the entire support team on- and off-site.
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