GCN Circular 21584
Subject
LIGO/Virgo G298048: GROND photometry of candidate optical counterpart reveals brightening in the NIR
Event
Date
2017-08-19T01:58:14Z (8 years ago)
From
Philip Wiseman at MPE/Garching <wiseman@mpe.mpg.de>
P. Wiseman, T.-W. Chen, J. Greiner, and P. Schady (all MPE Garching)
report on behalf of the GROND team:
We observed the field of the of the transient source SSS17a in NCG 4993
(RA =
13:09:48.09 DEC = -23:22:53.35, Coulter et al. GCN #21529, Allam et al.
GCN
#21530, Yang et al. GCN #21531, Melandri et al. GCN #21532), possibly
associated
with LIGO/Virgo trigger G298048, simultaneously in g'r'i'z'JHK with
GROND (Greiner
et al. 2008,PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La
Silla
Observatory(Chile).
Observations started at 23:15 UT on August 18th 2017. They were
performed at an
average seeing of 1.8" and at an average airmass of 1.4.
We clearly detect a single point source at the location of the
transient, although
the strong host contribution hinders photometric accuracy.
Based on the first 13.8 min of total exposures in g'r'i'z' and 12.4 min
in JHK, we
estimate preliminary magnitudes (all in AB system) of
g' = 17.9 +/- 0.1 mag,
r' = 17.2 +/- 0.1 mag,
i' = 17.2 +/- 0.1 mag,
z' = 16.8 +/- 0.1 mag,
J = 16.6 +/- 0.2 mag,
H = 16.9 +/- 0.2 mag, and
K = 16.8 +/- 0.2 mag.
There appears to be little change in the optical brightness of the
source: we
compare our magnitudes to the g = 17.76 mag and r = 17.20 mag from 14
hours
previous,reported by SkyMapper (Wolf et al. GCN #21560); and to the i,z
= 17.25
mag of Pan-STARRS (Chambers et al. GCN #21553) and z = 17.3 from HSC-z
(Yoshida
et al. GCN #21561) from 19 hours previous.
On the other hand, the object has brightened significantly in the NIR
over the
last 24 hours. We compare the J band to the value of 17.5 mag (AB) from
VISTA/VIRCAM reported by Tanvir et al. (GCN #21544), while the K band
(now 14.9
in Vega) is much brighter than the 16.9 mag (Vega) reported by Singer
et al.
(GCN #21552).
Given magnitudes are calibrated against Pan-STARRS zeropoints/2MASS
field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)= 0.1 mag in the direction of
the
candidate (Schlafly & Finkbeiner 2011).
After the Milky Way extinction correction, we fitted our colour SED
assuming a
black body (no K-correction applied), which indicates a temperature of
5333 +/-
243 K.