GCN Circular 27826

W. Fong (Northwestern), E. Berger (Harvard), J. Rastinejad (Northwestern) and C. D. Kilpatrick (UCSC) report on behalf of a larger collaboration:

"We initiated Hubble Space Telescope (HST) observations of the field of GRB 200522A (Evans et al., GCN 27778
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; Beardmore et al., GCN 27780
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) under Program 15964 (PI: Berger). We obtained WFC3/IR imaging in the F160W band, at a mid-time on 2020 May 26.077 UT (3.6 days post-burst). A total of 5223-sec of exposure time was obtained.

Coincident with the XRT position (90% confidence), we detect two clearly extended sources, Sources A and B. We perform absolute astrometry with SDSS DR12 and use WFC3 tabulated zeropoints to determine the following:

Source A:
RA(J2000) = 00:22:43.72
Dec(J2000) = -00:16:57.46
m(F160W) = 20.7 AB mag

Source B:
RA(J2000) = 00:22:43.82
Dec(J2000) = -00:16:59.54
m(F160W) = 23.7 AB mag.

Source A is the catalogued SDSS galaxy (Fong et al., GCN 27786
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) and exhibits a disturbed morphology, while Source B is considerably fainter. The angular offsets from the XRT positional center are ~2.0" and ~2.1", and the light of both galaxies are ~50% encompassed by the XRT position (90% confidence). Using NIR number counts, we estimate Pchance ~ 0.01 and 0.11 for Sources A and B, respectively.

We do not detect any additional sources, point-like or extended, within the XRT position or at the position of the additional LCO optical source (Strausbaugh et al., GCN 27794
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) to a 3-sigma limiting magnitude of m(F160W)>27.2 mag.

Assuming an optical decline rate of F~t^-1 from the time of the LCO faint source detection (R~22.57; Strausbaugh et al., GCNs 27792
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, 27794
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) to 3.6 days, and typical afterglow colors, the expected afterglow magnitude is m(F160W)~23.6 mag, which is >3 mag brighter than the HST limit. If we assume that a jet break occurred at t~1 day, this still results in expected afterglow magnitudes of ~25.5 mag, >1 mag brighter than the HST limit. We thus conclude that either the LCO source is not real, or that the afterglow exhibited a very steep decline of t^-3.5 or steeper beyond ~1 day.

At the redshift of the putative host galaxy of z~0.4 (Fong et al., GCN 27786
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), F160W roughly corresponds to rest-frame J-band. Scaling the J-band kilonova light curve of GW170817 (Villar et al. 2017: ApJL, 851, L21) to z~0.4, we find that the HST limit is slightly deeper than the kilonova of GW170817 at the same time.

The results of WFC3/F125W observations, which have been taken but are not yet available, will be discussed in an upcoming circular. We thank the HST staff for quickly implementing these observations."