The likely IR counterpart to the Anomalous X-ray Pulsar XTE J1810-197

ATel #203; GianLuca Israel, Nanda Rea, Vincenzo Testa, Vanessa Mangano (INAF Roma), Nancy Ageorges, Wolfgang Hummel, Gaspare Lo Curto, Olivier Marco, Roberto Mignani (ESO) on behalf of a larger Collaboration
on 4 Nov 2003; 15:12 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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We report on IR VLT adaptive optics (2003 October 7th; H and Ks band) and Chandra (2003 November 1st; HRC-I) ToO observations of the field of the new candidate Anomalous X--ray Pulsar (AXP) XTE J1810-197 (also known as CXOU J180951.1-194351; IAUC 8168 , IAUC 8190 , ATEL #193, ATEL #195, Ibrahim et al. 2003, astro-ph/0310665, and Gotthelf et al. 2003, astro-ph/0309745).

Images in the H and Ks band were obtained (effective exposure time of 42 minutes) with NAOS-CONICA (1024x1024 pixels, 0.027" pixel scale; 28"x28" Field of View) under very good sky and seeing condition (about 0.5"), resulting in an on-axis FWHM of about 0.15" for a point-like source. Limiting magnitudes (S/N~3) of about 23 and 22 (H and Ks, respectively) were inferred and only a few faint sources found within the 2.5" (1sigma c.l) radius uncertainty region obtained based on a previous Chandra HRC-I observation (Gotthelf et al. 2003).

Within the new Chandra image (about 2.9ks of effective exposure time) we found only one source (on-axis), at a count rate level of about 1cts/s , fully consistent with that reported by Gotthelf et al. (2003). A 49+\-2% pulsed fraction pulsation at a period of 5.5391(4)s was found, confirming that the source is indeed XTE J1810-197. The Chandra position is (J2000) 18h09m51.08s, -19d43'51.74", 90% uncertainty radius of about 0.8" (astrometric uncertainty included).

Only one IR Ks=20.8 magnitude object (H-Ks=1.2) was found to be consistent with the Chandra position, implying a (absorbed) F_X/F_IR larger than 10^3. Additional i-Gunn images were obtained from the 3.6m ESO telescope (EFOSC2; 2048x2048 pixels; 0.157" pixel scale; 5'x5' FOV; 84 minutes of effective exposure time; seeing of about 1") on 2003 October 12th. No object was found down to a limiting magnitude of about 24 (i-Ks>3). These optical/IR properties, similar to those of other IR counterparts to AXPs, suggests that we have indeed discovered the IR counterpart to XTE J1810-197.

All these findings, definitively establish the AXP nature of XTE J1810-197, the first confirmed example of transient (or at least highly variable) object of the class. We note that the time-scale of the (X-ray) flux variability detected from this new AXP (of the order of months) suggests a considerably larger number of AXPs in a "low" state likely exist in our Galaxy. Pulsations in the "low" state (if any; see Gotthelf et al. 2003) would easily remain undetected, thus preventing identification of these sources as AXPs.