GROND observation of optical counterpart of XMMSL1 J063045.9-603110

ATel #3813; D. A. Kann (TLS Tautenburg), J. Greiner, A. Rau (both MPE Garching)
on 18 Dec 2011; 11:39 UT
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Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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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.