2011ATel.3613....1K

At 16:46 UT on August 30th, 2011 Swift began a 1ks target of opportunity observation of the newly discovered X-ray transient MAXI J1836-194 (Negoro et al., ATEL #3611). Analysis of Swift/XRT Photon Counting mode data reveal a bright (∼36 XRT count/s), uncatalogued X-ray source at the following UVOT enhanced location: RA/Dec (J2000) = 278.93097, -19.32004, which is equivalent to:

RA(J2000) = 18h 35m 43.43s

Dec(J2000) = -19d 19m 12.1s

with an estimated uncertainty of 1.8 arc-seconds radius (90% confidence). This position lies 12 arcminutes from the center of the 0.2 degree MAXI error circle, consistent with that position, and 4.2 arc-minutes from the BAT position, 1.2 arc-minutes outside the BAT 90% confidence error circle. However, given the brightness of this source and the fact that it matches no catalogued X-ray source, we conclude that the XRT detected source is MAXI J1836-194.

The spectrum can be described by an absorbed power-law model with photon index 2.34 ± 0.15, and N_H = 2.0 ± 0.4 x 10²¹ cm^-2, which is consistent with the expected Galactic value of 2.23 x 10²¹ cm^-2 in that direction (Kalberla et al., 2005). The source flux is 1.1 ± 0.1 x 10^-9 erg/s/cm² (0.3-10 keV, uncorrected for absorption). The corrected flux is ∼2 x 10^-9 erg/s/cm², which if we assume a distance of 8.5 kpc, equates to a luminosity of ∼10³⁶ erg/s (0.3 - 10 keV). The brightness is consistent with a constant level during the 1ks observation.

Analysis of UVOT data finds a V=16.20 ± 0.04, U= 16.36 ± 0.04 counterpart which does not appear in the to be present in the DSS or 2MASS archival images of the field. The refined UVOT position is RA/Dec (J2000) = 278.93104, -19.31967, equivalent to:

RA(J2000) = 18h 35m 43.45s

Dec(J2000) = -19d 19m 10.8s

with an estimated uncertainty of 0.5 arc-seconds (radius, 90% confidence, statistical + systematic). This position lies 1.35 arc-seconds from the center of the UVOT enhanced XRT position, inside the 90% confidence error circle. This source is not consistent with the position of any catalogued optical source, and therefore we suggest that this is the optical counterpart of MAXI J1836-194.

Multi-wavelength follow-up observations to determine the nature of this source are encouraged.

This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (Evans et al. 2009, MNRAS, 397, 1177).