TITLE:   GCN CIRCULAR
NUMBER:  28676
SUBJECT: GRB 201015A: MMT afterglow imaging
DATE:    20/10/18 03:28:43 GMT
FROM:    Jillian Rastinejad at Northwestern Univ.  <jillianrastinejad2024@u.northwestern.edu>

J. Rastinejad, K. Paterson, C. D. Kilpatrick and W. Fong (Northwestern) report:

''We observed the location of Swift GRB 201015A (D'Elia et al., GCN 28632; Markwardt et al., GCN 28658) with the Binospec imager and spectrograph mounted on the MMT 6.5-meter telescope on Mount Hopkins, Arizona. We obtain 6x120-sec imaging in the i-band and 9x120-sec imaging in the z-band at mid-times of 2020 October 17.365 (1.41 days post-burst) and 2020 October 17.342 (1.39 days post-burst), respectively. Within the XRT position (Evans et al.  GCN 28647), we detect the previously reported optical afterglow (Lipunov et al. GCN 28633; Malesani et al; GCN 28637, Hu et al.; GCN 28645, Zhu et al.; GCN 28653; Belkin et al. GCN 28656; Jelinek et al., GCN 28664; Belkin et al., GCN 28673). Calibrated to PS1, we measure i = 22.2 +/- 0.1 mag and z = 21.9 +/- 0.1 mag (AB system and not corrected for Galactic extinction in the direction of the burst).

In addition to the afterglow, emission extending directly to the southwest of the afterglow position is weakly detected in both bands, at around RA(J2000) = 23:37:16.35, Dec(J2000)=+53:24:55.7. Given its proximity to the afterglow, this may be the host galaxy of GRB 201015A; however our imaging cannot resolve the afterglow from this extended emission. We also detect the brighter galaxy, previously mentioned in Belkin et al. (GCN 28656) and also detected in archival PS1 imaging at an offset of 2.3'' from the afterglow position (Malesani et al., GCN 28637). Given the large offset from the afterglow (13 kpc at z=0.42; de Ugarte Postigo et al., GCN 28649, Izzo et al., GCN 28661), we consider that this source is less likely to be related to the GRB.

Further observations are planned. We thank Skyler Self and Ben Weiner at the MMT for the rapid scheduling and execution of these observations.''