[vsnet-outburst 13186] (ATEL) MAXI J1836-194: V-16 mag optical counterpart

[Taichi Kato]

   MAXI J1836-194: V-16 mag optical counterpart

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ATEL #3613                                                           ATEL #3613

Title:  MAXI J1836-194: Swift localization and optical counterpart
Author: J. A. Kennea, E. A. Hoversten, M. H. Siegel (PSU), P. A. Evans
                (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano, V. Man
gano
                (INAF-IASFPA), P. Curran (CEA-Saclay), K. Yamaoka (AGU) and H. N
egoro
                (Nihon U.)  
Queries:        kennea at swift.psu.edu
Posted: 31 Aug 2011; 02:01 UT
Subjects:Optical, X-ray, Transient

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<sub>H</sub> = 2.0 +/- 0.4 x 10<sup>21</sup>
cm<sup>-2</sup>, which is consistent with the expected Galactic value of
2.23 x 10<sup>21</sup> cm<sup>-2</sup> in that direction (Kalberla et al.,
2005). The source flux is 1.1 +/- 0.1 x 10<sup>-9</sup> erg/s/cm<sup>2</sup>
(0.3-10 keV, uncorrected for absorption). The corrected flux is ~2 x 10<sup>-9</
sup>
erg/s/cm<sup>2</sup>, which if we assume a distance of 8.5 kpc, equates
to a luminosity of ~10<sup>36</sup> 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). 

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