GROND observations of MAXI J1836-194

ATel #3619; Arne Rau, Jochen Greiner, Vlad Sudilovsky (all MPE Garching)
on 31 Aug 2011; 19:22 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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We observed the new transient MAXI J1836-194 (Negoro et al. 2011, ATEL 3611) 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) starting Aug 30th, 2011 at 23:19 UT.

The optical counterpart (Kennea et al. 2011, ATEL 3613) is clearly detected in exposures of 16.5min in g' r' i' z' and 21.4min in J H K_s.

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

g' = 16.21 +/- 0.05
r' = 15.92 +/- 0.05
i' = 15.53 +/- 0.05
z' = 15.09 +/- 0.05
J = 14.77 +/- 0.05
H = 14.34 +/- 0.05
K_s = 14.00 +/- 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) and are not corrected for Galactic foreground reddening of E(B-V)~0.50mag (Schlegel et al. 1998).

The g'- K_s (GROND) spectral energy distribution is flat (approx lambda^0.2) and therefore inconsistent with an accretion disk spectrum or with a single star of any temperature. The optical-near-IR emission does not connect smoothly to the XRT spectrum (both in slope and in amplitude) suggesting that it traces a different spectral component. The optical-near-IR slope would be in agreement with optically thin synchrotron emission from a jet.

As noted by Kennea et al. (2011, ATEL 3613), the optical source is not present in archival DSS and 2MASS images, which implies outburst amplitudes of at least 4-5 magnitudes in all bands. This argues against a Be star nature of the optical counterpart as suggested by Cenko et al. (2011, ATEL 3614).

Further observations, in particular in the ultra-violet and at radio wavelengths are encouraged to help resolving the nature of this source.