TITLE:   GCN CIRCULAR
NUMBER:  25443
SUBJECT: LIGO/Virgo S190814bv: a potential faint optical counterpart in CFHT imaging
DATE:    19/08/22 04:43:16 GMT
FROM:    Maria R Drout at Carnegie Observatories  <mdrout@carnegiescience.edu>

J. Ruan (McGill U.), N. Vieira (McGill U.), D. Haggard (McGill U.), M. R.
Drout (U. of Toronto), N. Asfari (U. of Toronto), R. Carlberg (U. of
Toronto), R. Doyon (U. de Montreal), R. Fernandez (U. of Alberta), B.
Gaensler (U. of Toronto), V. Kaspi (McGill U.), D. Lafreniere (U. de
Montreal), C. Matzner (U. of Toronto), D-S. Moon (U. of Toronto), C. Ni (U.
of Toronto), M. Nynka (MIT), A. L. Piro (Carnegie Obs.), S. Safi-Harb (U.
of Manitoba), K. Spekkens (RMC/Queen’s U.)

We report on optical imaging obtained on the MegaPrime/MegaCam instrument
(1x1 deg FOV) at the Canada-France-Hawaii Telescope, to search for a
counterpart to the candidate gravitational wave source S190814bv. We
obtained deep images for the entire updated 50% confidence localization
region (GCN 25333) in g-band on UTC 2019-08-16 (1.7 days post-merger), in
i-band on UTC 2019-08-18 (3.6 days post-merger), in i-band on UTC
2019-08-19 (4.6 days post-merger), and in both i- and z-band on 2019-08-21
(6.6 days post-merger). In each of those nights, we also obtained images of
a few additional galaxies in the larger 90% confidence localization region.
We stacked the images from each night and calibrated based on the
Pan-STARRS 3pi Survey. For an initial search, we visually inspected the
galaxies in our stacked images from the GLADE catalog (Dayla et al., 2018),
based on a candidate host-galaxy prioritization scheme.

We find one potential faint counterpart lying near the possible host-galaxy
2MASX J00472894-2526263, which is located at (RA, Dec = 11.870618,
-25.440655) and a distance of 267 Mpc. This galaxy was the 5th
highest-ranked candidate host-galaxy in the LIGO/Virgo localization region
from the GLADE catalog based on our prioritization scheme. The potential
counterpart is at ~17” away from the center of this galaxy, and is located
at (RA, Dec = 11.866781, -25.437264). The potential counterpart was not
detected in our g-band imaging at 1.7 days post-merger, but was
subsequently detected in i-band at 3.6, 4.6, and 6.6 days post-merger, and
in z-band at 6.6 days post-merger. A table of our aperture photometry
measurements is provided below:

UTC, days post-merger, band, mag
2019-08-16, 1.7, g, >22.8
2019-08-18, 3.6, i, 22.86+/-0.19
2019-08-19, 4.6, i, 22.80+/-0.23
2019-08-21, 6.6, i, 22.93+/-0.12
2019-08-21, 6.6, z, 22.24+/-0.30

Cutouts of our images can be found at this link:
http://www.physics.mcgill.ca/~jruan/S190814bv/cutouts_rank4.png

We also visually examined deeper reference images from DECam taken as part
of the DECaLS/DESI Legacy Imaging Surveys, in g-, r- and z-bands. Our
candidate counterpart from CFHT is also not detected in these DECam
reference images. We note that these deep DECam reference images reveal
another source (which we label T1 in our cutouts below) that is ~4.5” away
from our possible counterpart. This T1 source is also clearly detected in
our i-band images at 6.6 days post-merger, and is not the same object as
our potential counterpart. Cutouts showing the clear positional offsets
between T1 and the potential counterpart can be found at this link:
http://www.physics.mcgill.ca/~jruan/S190814bv/cutouts_rank4_DECam.png

Further follow-up of this potential counterpart is encouraged. We are
continuing observations, and analysis will be reported.

We thank the CFHT queued service observing team and the telescope staff for
their help in obtaining these observations.