TITLE:   GCN CIRCULAR
NUMBER:  25911
SUBJECT: GRB 191001A: DDOTI Optical Counterpart Candidate
DATE:    19/10/01 22:28:50 GMT
FROM:    Emma Margarita Pereyra Talamantes at IA-UNAM Ensenada  <mpereyra@astro.unam.mx>

Margarita Pereyra (UNAM), Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L.
Becerra (UNAM), Diego Gonzalez (UNAM), Alexander Kutyrev (GSFC/UMD),
illiam H. Lee (UNAM), Eleonora Troja (GSFC/UMD), Gabriele Minervini
(INAF/IAPS-Rome)  and Tanner Wolfram (ASU), Simone Dichiara (GSFC/UMD)
report:

We observed the field of GRB 191001A (trigger 591604915, Fermi GBM Team,
GCN Circ. 25893) from 2019 Oct 1 06:48:21 UTC (391 seconds after the
trigger) to 06:59:52 UTC with the DDOTI wide-field imager at the
Observatorio Astronomico Nacional on Sierra San Pedro Martir (
http://ddoti.astroscu.unam.mx/). We calibrated our images against the APASS
catalog and obtained a 10-sigma limiting magnitude of w = 18.6.

Comparing our 10-sigma detections against the USNO-B1 and Pan-STARRS DR1
catalogues, we detect an uncatalogued source at 20:20:47.65 +15:05:03.4
that fades at the 2.4 sigma level from w = 16.7 to 17.0. Specifically, it
fades as a power-law in time since trigger with an index of -0.53 +/- 0.23.

Independently and roughly simultaneously, the source was detected at a
similar magnitude by ATLAS as ATLAS19wxr and is cataloged in the Transient
Name Server as AT2019rog (TNS Astronomical Transient Report 48331). The
discovery report states that it was not detected on 2019 Sep 29.

No known minor planets, brighter than V = 24.0, were found in the 5 arcmin
region around the source position at the time our observations were
performed.

The source is about 1 sigma from the Fermi Final Real-Time Localization and
BALROG positions (Fermi GBM Team, GCN Circ. 25893; Burgess et al., GCN
Circ. 25894). We suggest that it might be the optical counterpart of the
GRB and encourage further observations.

We thank the staff of the Observatorio Astronomico Nacional in San Pedro
Martir.
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