TITLE:   GCN CIRCULAR
NUMBER:  26968
SUBJECT: ZTF20aajnksq: Gemini-North r-band photometry
DATE:    20/02/03 02:54:07 GMT
FROM:    Leo Singer at GSFC  <leo.p.singer@nasa.gov>

ZTF20aajnksq: Gemini-North optical photometry

Leo P. Singer (NASA/GSFC), Tomas Ahumada (UMD), Anna Y. Q. Ho (Caltech)
report on behalf of the Zwicky Transient Facility (ZTF) and Global Relay
of Observatories Watching Transients Happen (GROWTH) collaborations:

Starting at 2020-02-01T12:25:57 UTC, we imaged ZTF20aajnksq/AT2020blt
(Ho et al., ATel #13429, GCN 26966) using the Gemini Multi-Object
Spectrograph (GMOS) mounted on the Gemini North 8-meter telescope on
Mauna Kea.

We combined 8 r-band 200s exposures using DRAGONS, a Python-based data
reduction platform provided by the Gemini Observatory. We detect a source
at the location of the transient. The aperture photometry calibrated
against Pan-STARRS DR1 magnitudes (Chambers et al., 2016) of the object is
r = 25.2 +- 0.05 AB mag.

We note that for the redshift (z = 2.9) and apparent magnitude
(r = 19.6 AB mag) reported in GCN 26966, the absolute magnitude of the
source (M_r = -27.4) would be significantly brighter than afterglows of
short GRBs at one day: short GRBs are typically M_B = -17.34 +- 0.50 AB
mag whereas long GRBs are typically M_B = -23.17 +- 0.21 AB mag (Kann et
al., 2011). We therefore suggest three alternative interpretations:
(a) GRB 200128A is not a typical short GRB, (b) ZTF20aajnksq is the
afterglow of a different (long) GRB, or (c) ZTF20aajnksq is an orphan
afterglow.

We thank S. Stewart and the Gemini North staff for executing these
observations.