Swift J1357.2-0933: Optical/NIR counterpart from GROND

ATel #3140; A. Rau, J. Greiner, R. Filgas (MPE Garching)
on 1 Feb 2011; 12:47 UT
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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GROND (Greiner et al. 2008, PASP 120, 405), the 7-channel imager mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile) started follow-up observations of the transient source Swift J1357.2-0933 (Krimm et al. 2011, ATEL #3138) on 1 Feb 2011, 08:28 UT simultaneously in the g'r'i'z'JHK bands. Within the 3.0 arcmin Swift/BAT error circle (Krimm et al. 2011, ATEL #3138) we detect a new, bright, source compared to SDSS DR7. The position is RA(2000.0) = 13:57:16.8, Decl.(2000.0) = -08:32:39 with an error of 0.3 arcsec.

We measure the following magnitudes (all in AB system), not corrected for Galactic foreground reddening of E(B-V)=0.04mag (Schlegel et al. 1998):

g' = 15.99±0.05
r' = 16.30±0.05
i' = 16.49±0.05
z' = 16.57±0.05
J = 16.99±0.07
H = 17.18±0.07
K = 17.40±0.10

At the GROND position, we find a faint source in the SDSS DR7 images taken on 26 May 2006 with the following (AB) magnitudes:

u = 22.83±0.63
g = 22.75±0.16
r = 21.96±0.14
i = 21.60±0.14
z = 20.71±0.22

The source is also detected near the limit of the DSS image taken on 24 Feb 1988 (limiting magnitude ~20.3), suggesting that it has been brighter than during the SDSS observations in 2006.

The large outburst amplitude in the optical (~6 mag) makes an extra-galactic origin of Swift J1357.2-0933 very unlikely.

Instead, the GROND and pre-outburst photometry suggest it to be a dwarf nova outburst. The SDSS photometry indicates that the quiescent counterpart was very red, and the i-z color and apparent magnitude fit well with an M4 star at a distance of 1.5 kpc. The very blue SED measured by GROND is compatible with an accretion disk. The lack of curvature in the blue implies a lower limit on the temperature of 10,000K.

Alternatively, Swift J1357.2-0933 could also be a Low-Mass X-ray Binary, although the X-ray luminosity at 1.5 kpc (1.5x10^35 erg/s; assuming BAT covered the outburst maximum) would be atypically low.

We urge for spectroscopic observations.