Abstract for Proposal 088157
Searching for tightly-bound, transient X-ray binaries in the Galactic Center
We propose XMM ToO observations of new X-ray transients to be detected
by the daily Swift/XRT monitoring in the central ~50 pc region of
our Galaxy. As demonstrated for one of the black hole transients,
XMM can probe a population of tightly-bound X-ray binaries (with short
orbital periods < 12 hours) in the vicinity of Sgr A*, as predicted by
recent N-body simulations and X-ray outburst history data. In addition,
XMM can resolve Fe atomic features with the EPIC CCD spectrometers,
including narrow Fe absorption lines formed in accretion disk winds from
highly-inclined X-ray transients. The proposed XMM observations will
provide important and complementary X-ray data to the approved Chandra
and NuSTAR ToO program and investigate X-ray binary formation in the
Galactic Center.
Details on Observing Strategy and Trigger Criteria
Our collaboration monitors the Galactic Center with Swift/XRT daily
and we have approved Chandra + NuSTAR ToOs to follow up with new
Swift transients. Upon Swift/XRT detection of an X-ray transient,
its source position will be measured to 0.3" precision by Chandra
ToO within less than one week. We will then know whether a new X-ray
outburst is recurrent or from a new transient and make a decision on
triggering XMM observation. We will trigger XMM observation for a new
Swift transient above Lx ~ 10^35 erg/s as long as it falls into the XMM
s visibility windows. We do not observe any of the six known NS-LMXBs
within 25 arcmin from Sgr A* (e.g., AXJ J1745-2901). Historically,
every 3 years on average, Swift has detected an X-ray transient that
satisfies our criteria, i.e. a bright and long outburst which overlaps
with the XMM's visibility window. The proposed XMM observation does not
need to coordinate with Chandra, NuSTAR or Swift.