GCN Circular 18486
Subject
LIGO/Virgo G194575: GROND multi-color observations
Event
Date
2015-10-27T13:38:55Z (10 years ago)
From
Sylvio Klose at Thuringer Landessternwarte Tautenburg <klose@tls-tautenburg.de>
Jochen Greiner (MPE Garching), Sylvio Klose (TLS Tautenburg), and Phil
Wiseman (MPE Garching) report:
We observed the optical transient LSQ15bjb (Rabinowitz et al., GCN 18473)
simultaneously in g'r'i'z'JHK using the multi-channel imager GROND
(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at
ESO La Silla Observatory (Chile). Observations started on October 27 at
01:09 UT. In total 18 short exposures were taken until 06:40 UT.
At a mean time of 01:34 UT we measure the following AB magnitudes:
g' = 16.8 +/- 0.1,
r' = 17.1 +/- 0.1,
i' = 17.4 +/- 0.1,
z' = 17.5 +/- 0.1,
J = 18.2 +/- 0.1,
H = 19.0 +/- 0.2,
K = 19.8 +/- 0.3,
calibrated against SDSS in the optical and 2MASS in the NIR.
The source appears very blue. After correcting for the (tiny) Galactic
reddening along the line of sight, E(B-V) = 0.03 mag (Schlegel et al.
1998), the spectral energy distribution (SED) can be fit with a power law
(F_nu \sim nu**(-beta) ), with a slope beta = -0.33 +/- 0.02, consistent
with an accretion disk spectrum (and ruling out a GRB afterglow SED, which
would have beta > 0).
Despite the still fast rise with respect to earlier observations
(Rabinowitz et al., GCN 18473, Smith et al., GCN 18484) the source appears
to be constant within +/- 0.05 mag over the course of our 5.5 hrs
observation (consistent with D'Avanzo et al., GCN 18476).
Assuming SDSS upper limits of g'>23.1, r'>24.2, i'>23.6 and z'>22.3 (based
on some residual flux of the nearby galaxy), we note that LSQ15bjb
exhibits a >7 mag amplitude. Typically, such amplitudes and several days
time scale are found only in novae, black hole transients ("X-ray novae"),
or tidal disruption events. The latter is an unlikely explanation due to
the offset to the galaxy. Even if this source is unrelated to G194575, its
light curve evolution appears to be peculiar.