GROND and Swift observations of XMMU J115113.3-623730

ATel #2746; J. Greiner, T. Kruehler, P. Schady, A. Rau, F. Olivares (all MPE)
on 19 Jul 2010; 18:24 UT
Distributed as an Instant Email Notice Novae
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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We observed XMMU J115113.3-623730, a bright, soft X-ray source discovered during a slew maneuvre of XMM, with the 7-channel imager GROND (Greiner et al. 2008, PASP 120, 405). Observations started on 2010 Jul 19, 00:04 UT at airmass of 1.45, and under 1.5 arcsec seeing.

We find a single optical/NIR source in the Swift X-ray error circle, at position RA(2000.0) = 11:51:13.00, Decl.(2000.0) = -62:37:28.8 (+-0.3" in both coordinates). We measure the following magnitudes (all in the AB system):
g' = 13.9 +- 0.1
r' = 13.8 +- 0.1
i' = 13.9 +- 0.1
z' = 13.9 +- 0.1
J = 13.9 +- 0.1
H = 14.1 +- 0.1
K = 14.3 +- 0.1
calibrated against GROND zeropoints.

This object coincides with 2MASS object 11511302-6237292, with catalog magnitudes of
J = 14.97 +- 0.04
H = 14.82 +- 0.06
K = 14.43 +- 0.09
and USNO B-1 object 0273-0386669 with B2=15.51 mag and R2=16.05 mag.

Comparison to our GROND magnitudes shows this object to be about 2 mag brighter than these archival measurements. John Greaves called attention to the ASAS-3 light curve, when this source was monitored to steadily decline from V=10.5 mag (2008 Nov 23) by 2 mag until Aug 2009 (S. Otero 2010, VSNET Alert 12084). An extrapolation of that same decay slope would predict V~15.2 for now, 1.5 mag fainter than observed.

Swift observed this object on July 16/17 for a total of 2.15 ksec. The source is bright (varying between 0.3 and 4 cts/s) with a very soft spectrum which can be adequately fit with a 40 eV blackbody spectrum. The best-fit absorbing column is NH=(1.9+-0.1)*10^21 cm^-2, only about 10% of the total Galactic column of 1.3*10^22 cm^-2 (Kalberla et al. 2005). For the solar neighbourhood, this corresponds to AV~1 mag, and a rough distance of 1 kpc. The unabsorbed bolometric luminosity of the blackbody model amounts to 2*10^36 (D/1kpc)^2 erg/s.

An archival ROSAT image (http://www.xray.mpe.mpg.de/rosat/images/rgb/p/1802/180252p-rgb.jpg) suggests bright and extended (~15 arcmin across) soft X-ray emission - the relation to XMMU J115113.3-623730 remains to be clarified.

UVOT detects this object at uvw2 = 13 mag, with clear variability of +-0.3 mag over the duration of the observation, and roughly in proportion to the X-ray count rate. The UV flux is about a factor 500 above the extrapolation of the best-fit blackbody model, suggesting a different component.

This multi-wavelength behaviour, together with the ASAS-3 light curve, suggests that XMMU J115113.3-623730 is a nova which presently is in its supersoft X-ray phase, similar to V598 Pup = XMMSL1 J070542.7-381442 as already suggested by T. Kato (2010, VSNET Alert 12085). The excess UV/optical emission then could be due to reprocessing in the system. Only the relatively bright 2MASS and USNO magnitudes are somewhat at odds with a quiescent pre-nova at 1 kpc distance.

We encourage optical spectroscopic observations.