GROND observations of Swift J063933.6+054918: Likely a flare star

ATel #4634; Arne Rau, Patricia Schady, Jochen Greiner (all MPE Garching), and Thomas Kruehler (DARK/NBI):
on 10 Dec 2012; 14:06 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We observed the new transient Swift J063933.6+054918 (Page et al. 2012, ATel #4613) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) at the 2.2m MPG/ESO telescope at La Silla Observatory (Chile). A single 10min exposure was obtained on December 10th 06:38 UT.

Within the 2"5 Swift/XRT error circle (Page et al. 2012, ATel #4613) we find a single bright source at the following position

RA(J2000) = 06:39:33.68 (99.89032 deg)

Dec(J2000) = 05:49:16.8 ( 5.82132 deg)

with an uncertainty of 0"08 in RA and 0"11 in Dec.

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

g' = 19.0 +/- 0.1
r' = 17.5 +/- 0.1
i' = 16.2 +/- 0.1
z' = 15.7 +/- 0.1
J = 14.93 +/- 0.05
H = 14.75 +/- 0.05
K_s = 14.94 +/- 0.07

These magnitudes were derived by calibrating the images against GROND zero points (g' r' i' z') and 2MASS field stars (J H K_s).

The GROND spectral energy distribution is best fit with a M2V stellar template with a Galactic foreground reddening of E(B-V)=0.2mag. This is in agreement with absorbing column distribution along the line of sight (total E(B-V)~0.77mag; Schlafly et al. 1998) and implies a distance of ~330pc. The source is also listed in the 2MASS and USNO B1.0 catalogs at similar brightness as measured by GROND, suggesting that we observed it at its quiescent level. The source is not in the GCVS database (Version 2012Jan; Samus N.N. et al.)

While we can not exclude that the source is a chance coincidence and not related to the X-ray/UV transient, the considerable empty field (next source is 10" away) makes this rather unlikely. Instead, the GROND observations suggest that transient detected by Swift was an energetic flare from an M dwarf. The X-ray spectrum (thin-thermal, Page et al. #4613) and luminosity (3.3x10^31 erg/s at 330pc), are consistent with this interpretation. Similarly, the flare star scenario can explain the very bright, but short-lived UV emission (Page et al. #4613).