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GCN Circular 28100

GRB 200522A: Candidate Optical/nIR Counterpart from Gemini and HST Imaging
2020-07-14T19:19:42Z (4 years ago)
Brendan O'Connor at UMD <>
B. O'Connor (GWU, UMD), S. Dichiara (UMD, NASA-GSFC),
E. Troja (UMD, NASA-GSFC), S.B. Cenko (UMD, NASA-GSFC)
report on behalf of a larger collaboration:

We report on further observations of the short GRB 200522A (Evans et al.,
GCN Circ. 27778) taken at 3.1 and 9.1 d post-burst in the r-band filter
with the Gemini Multi-Object Spectrograph (GMOS) mounted on the 8.1m Gemini
North telescope. Image subtraction with HOTPANTS (Becker 2015) identifies
a low significance residual close to the candidate host galaxy's center
(Fong et al., GCN Circ. 27779; Strausbaugh et al., GCN Circ. 27792),
and at the edge of the XRT enhanced position (Beardmore et al., GCN Circ.
27780). We independently reanalyzed the HST images (Fong et al., GCN Circ.
27827; Kilpatrick et al., GCN Circ. 27904), and performed image subtraction
between the two epochs at 3.5 and 16.3 d. Although the analysis is
complicated by the presence of a bright galaxy's nucleus, a residual signal
of brightness F125W ~ 25.6 AB mag is present at the location of the Gemini
optical candidate. We suggest this to be the candidate optical/nIR
counterpart of GRB 200522A. The observed magnitude is much brighter than
expected for a kilonova like AT2017gfo, and suggests that the afterglow
emission is the dominant component. The projected offset from the galaxy's
nucleus is ~0.23 arcseconds, corresponding to 1.5 kpc at redshift z = 0.554
(Dichiara et al., GCN Circ. 28038). This physical offset is low yet not
unprecedented, falling in the bottom 15% of the short GRB offset
(Fong & Berger, 2013).

We thank the Gemini North staff for efficiently executing
these observations.
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