Optical/NIR counterpart of Swift J1836.6+0341

ATel #3687; Jochen Greiner (MPE Garching), Marco Nardini (Uni. Milano-Bicocca), Arne Rau, Jonathan Elliott (both MPE Garching)
on 17 Oct 2011; 12:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

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We observed the new transient Swift J1836.6+0341 (Krimm et al. 2011, ATEL 3684) with 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) on Oct. 15, 2011 at 23:51 UT and Oct. 17, 2011 at 00:09 UT, for 35 min each. Observations were performed under poor seeing of 1.5" and airmass of 1.7.

We identify one single object within the 2.2" Swift/XRT error circle (Krimm et al. 2011, ATEL #3684), at coordinates (error of +-0.3")

 
RA  (2000) = 18 36 39.44 
Dec.(2000) = +03 41 00.6 

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

 
g' = 21.5 +/- 0.1 
r' = 20.5 +/- 0.1 
i' = 19.9 +/- 0.1 
z' = 19.4 +/- 0.1 
J  = 19.0 +/- 0.1 
H  = 19.0 +/- 0.2 
K  = 19.0 +/- 0.2 

These magnitudes were derived by calibrating the images against GROND zero points (g'r'i'z') and 2MASS field stars (JHK) and are not corrected for the heavy Galactic foreground reddening of E(B-V)~1.13 mag (Schlegel et al. 1998).

Within the photometric errors, we find no variability between our two observing epochs.

As noted by Krimm et al. (2011, ATEL #3684), this source is not present in DSS2 or 2MASS/DENIS. Comparison with the red DSS2-limit implies a brightening of >2 mag.

The measured hydrogen absorbing column as reported by Krimm et al. (2011, ATEL #3684) suggests that Swift J1836.6+0341 lies near the end or behind the total Galactic column. At a galactic latitude of 5 deg, this corresponds to a distance of at least 1 kpc.

The extinction-corrected g'-K (GROND) spectral energy distribution is very blue (approx lambda^-0.6), consistent with an accretion disk spectrum. The relatively low luminosity of about 10^34 (d/1 kpc)^2 erg/s (if a galactic source) suggests a cataclysmic variable origin for Swift J1836.6+0341. However, an extragalactic origin cannot be excluded at this stage, though the f_x/f_opt ratio is not typical of AGN.

Optical spectroscopy is encouraged.