TITLE:   GCN CIRCULAR
NUMBER:  24337
SUBJECT: LIGO/Virgo S190425z: DECam Observations of the UVOT Candidate Region
DATE:    19/04/29 05:06:28 GMT
FROM:    Josh Bloom at UC Berkeley  <jbloom@astron.berkeley.edu>

Joshua S. Bloom (UC Berkeley), Catherine Zucker (Harvard), Eddie Schlafly
(LBNL), Doug Finkbeiner (Harvard), Jorge Martínez-Palomera (UC Berkeley),
Daniel A. Goldstein (Caltech), and Igor Andreoni (Caltech) report:

"Starting at 2019-04-28 5:10 UTC, we observed the region of the UVOT
transient (Breeveld et al. GCN #24296) candidate counterpart of the
gravitational wave event S190425z (LIGO/VIRGO Collaboration, GCN #24168)
with the Victor M. Blanco 4m Telescope at Cerro Tololo Inter-American
Observatory, equipped with the Dark Energy Camera (DECam). We observed in
four bands (g for 6 minutes, r for 6 minutes, i for 8 minutes, z for 8
minutes).

Consistent with previous reports (Kong et al. GCN #24301, Arcavi et al. GCN
#24307, Im et al. GCN #24318, Shappee et al. GCN #24313, Hu et al. GCN
#24324, Chang et al. GCN #24325, Morihana et al. GCN #24328, Tanvir et al.
GCN #24334, Troja et al. GCN #24335), in 1.2 arcsec seeing, we find no
source at the reported position of the UVOT source, placing the following
upper limits, calibrated to PS1 photometry (Chambers et al. 2016,
arXiv:1612.05560C):

  g > 24.0
  r > 24.0
  i > 23.7
  z > 23.1

The nearby source noted in Palmese et al. (GCN #24312) at position
ra=17:02:19.19, dec=-12:29:07.3 is detected in r, i, z and not detected in
g. We report the following magnitudes for this source:

  z = 21.50
  i = 22.04
  r = 23.03
  g > 24.0

All detection magnitudes inhere a ~0.1 mag uncertainty currently. The
r-band detection is consistent with Palmese et al. (GCN #24312) and thus
does not appear to be variable between the two observations. We find no
evidence for any spatial extent beyond the stellar PSF. The photometric
measurements (albeit not corrected for the Galactic extinction along the
line of sight) suggest the source to be a M2-dwarf star (see West et al.,
2011).

The currently reported astrometric location of the bright UV transient is
not precisely consistent with the location of the source discussed above.
However, if they are indeed physically associated (through a refined
astrometric study, especially since the UVOT source appears trailed) a
consistent picture emerges: the UVOT detection was due to a flare from an
M2-dwarf star in our galaxy. We thus agree with the hypothesis advanced by
Lipunov et al. (GCN #24326) that the UVOT event is unrelated to GW
S190425z."

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