Imaging X‐Ray, Optical, and Infrared Observations of the Transient Anomalous X‐Ray Pulsar XTE J1810−197

Abstract

We report X-ray imaging, timing, and spectralstudies of XTE J1810-197, a 5.54 s pulsar discovered by Ibrahim andcoworkers in recent Rossi X-Ray Timing Explorer (RXTE) observations. In aset of short exposures with the High Resolution Camera on board theChandra X-Ray Observatory, we detect a strongly modulated signal(55%+/-4% pulsed fraction) with the expected period located at (J2000)18 h 09 m 51s08, -19 deg 43 ' 51.7",with a uncertainty radius of 0.6" (90% confidence level). Spectraobtained with XMM-Newton are well fitted by a two-component model thattypically describes anomalous X-ray pulsars (AXPs), an absorbed blackbodyplus power law with parameters kT=0.67+/-0.01 keV, Gamma=3.7+/-0.2,N H =(1.05+/-0.05)�10 22 cm -2, andF X (0.5-10keV)=3.98�10 -11 ergs cm -2 s -1.Alternatively, a two-temperature blackbody fit is just as acceptable. Thelocation of CXOU J180951.0-194351 is consistent with a point sourceseen in archival Einstein, ROSAT, and ASCA images, when its flux wasnearly 2 orders of magnitude fainter, and from which no pulsations arefound. The spectrum changed dramatically between the ``quiescent'' and``active'' states; the former can be modeled as a softer blackbody.Using XMM-Newton timing data, we place an upper limit of 0.03 lt-son any orbital motion in the period range 10 minutes-8 hr. Opticaland infrared images obtained on the SMARTS 1.3 m telescope at theCerro Tololo Inter-American Observatory (CTIO) show no object in theChandra error circle to limits V=22.5, I=21.3, J=18.9, and K=17.5.Together, these results argue that CXOU J180951.0-194351 is an isolatedneutron star, one most similar to the transient AXP AX J1844.8-0256.Continuing study of XTE J1810-197 in various states of luminosity isimportant for understanding and possibly unifying a growing class ofisolated, young neutron stars that are not powered by rotation.