XMM-Newton detection of a transient X-ray source in the vicinity of V838
  Monocerotis
  We report the XMM-Newton/EPIC detection in 2008 March of a luminous (L_X ~
10^32-33 erg/s), variable X-ray source in the vicinity (within ~6") of the
enigmatic star V838 Mon, which underwent a spectacular outburst in early 2002.
Spectral modeling of the XMM-Newton X-ray source indicates the presence of two
plasma components with characteristic temperatures of TX ~ 2x10^6K and
~1.5x10^7K, attenuated by an absorbing column (N_H ~ 4 x 10^21cm^-2) that is
consistent with the visual extinction measured toward V838 Mon (A_V ~ 2). No
such luminous source was detected in the immediate vicinity of V838 Mon in
Chandra/ACIS-S observations obtained about one year after outburst or, most
recently, in 2010 January. The two XMM source spectral components appear to be
marginally spatially resolved, with the spatial centroid of the hard component
lying closer to (within ~2" of) the position of V838 Mon than the broad-band
source or the soft source component; however, if there are two sources at or
near V838 Mon, the Chandra nondetections would imply that both of them are
variable. An alternative is that there is a single variable source, and that
the apparent spatial separation may actually be due to photon-counting
statistics or is perhaps instrumental in origin. We consider constraints placed
by the X-ray detection and nondetections on a stellar merger model for the 2002
V838 Mon outburst, in which the spun-up merger product drives a powerful
magnetic dynamo. Alternatively, the transient behavior of the X-ray source
could indicate that the X-rays arose as a consequence of an interaction between
V838 Mon's ejecta and its early-type (B3 V) companion.
