[vsnet-alert 13981] (fwd) XMMSL1 J063045.9-603110 (ATEL)

[Taichi Kato]

ATEL #3813							     ATEL #3813

Title:	GROND observation of optical counterpart of XMMSL1 J063045.9-603110
Author:	D. A. Kann (TLS Tautenburg), J. Greiner, A. Rau (both MPE
		Garching)
Queries:	jcg at mpe.mpg.de
Posted:	18 Dec 2011; 11:39 UT
Subjects:Optical, Transient

We observed the field of the new XMM slew source XMMSL1 J063045.9-603110
(Read et al., ATEL #3811) simultaneously in g'r'i'z'JHK with GROND (Greiner
et al. 2008, PASP, 120, 405) mounted at the 2.2 m ESO/MPG telescope at
La Silla Observatory (Chile). Observations started on December 18, 2011,
at 03:41 UT. Integration times were 284 sec in the optical and 480 sec
in the NIR. The images were taken at an average seeing of 0".8 and at an
airmass of 1.2. 

Within the X-ray error circle of 8.1" (Read et al., ATEL #3811), we detect
one object which is not visible on DSS2 or 2MASS, at position  

RA (2000.0)  =  06:30:45.45      
Decl(2000.0) = -60:31:12.8  

with a conservative error of +-0.3". 

We measure the following magnitudes (all AB magnitude system):  

g' = 18.4+-0.1  
r' = 19.5+-0.1  
i' = 19.6+-0.1  
z' = 20.1+-0.1  
J  = 20.3+-0.2  
H  = 20.9+-0.4  
K  > 20  

Magnitudes were calibrated against GROND zero points in g'r'i'z' and 2MASS
field stars in JHK. The measured g-band magnitude, after color transformation
to DSS2-blue, is about 3 mag brighter than the DSS2-blue limiting magnitude.
We therefore propose this object as the counterpart of XMMSL1 J063045.9-603110.

The brightness of the object is much fainter than expected for a Galactic
nova, as suggested by Read et al. (ATEL #3811), unless we are many months
after maximum light. We note that the Galactic foreground reddening is
very small, E(B-V)= 0.07 (Schlegel et al. 1998). Also the g'-r' color is
very blue, atypical for a nova. 

The blue SED (with slope of F_lambda propto lambda^-2) is, however, reminiscent
of an accretion disk spectrum. The g'-band data point lies about 0.8 mag
above the best-fit SED, suggestive of strong HeII emission. Spectroscopic
observations are encouraged to determine the nature of this source.