  We report X-ray imaging, timing, and spectral studies of XTE J1810-197, a
5.54s pulsar discovered by Ibrahim et al. (2003) in recent RXTE observations.
In a set of short exposures with the Chandra HRC camera we detect a strongly
modulated signal (55+/-4% pulsed fraction) with the expected period located at
(J2000) 18:09:51.08, -19:43:51.7, with a uncertainty radius of 0.6 arcsec (90%
C.L.). Spectra obtained with XMM-Newton are well fitted by a two-component
model that typically describes anomalous X-ray pulsars (AXPs), an absorbed
blackbody plus power law with parameters kT = 0.67+/-0.01 keV, Gamma=3.7+/-0.2,
N_H=(1.05+/-0.05)E22 cm^-2, and Fx(0.5-10 keV) = 3.98E-11 ergs/cm2/s.
Alternatively, a 2T blackbody fit is just as acceptable. The location of CXOU
J180951.1-194351 is consistent with a point source seen in archival Einstein,
Rosat, & ASCA images, when its flux was nearly two orders-of-magnitude fainter,
and from which no pulsations are found. The spectrum changed dramatically
between the "quiescent" and "active" states, the former can be modeled as a
softer blackbody. Using XMM timing data, we place an upper limit of 0.03 lt-s
on any orbital motion in the period range 10m-8hr. Optical and infrared images
obtained on the SMARTS 1.3m telescope at CTIO show no object in the Chandra
error circle to limits V=22.5, I=21.3, J=18.9, & K=17.5. Together, these
results argue that CXOU J180951.1-194351 is an isolated neutron star, one most
similar to the transient AXP AX J1844.8-0256. Continuing study of XTE J1810-197
in various states of luminosity is important for understanding and possibly
unifying a growing class of isolated, young neutron stars that are not powered
by rotation.
