Accurate X-ray position of the Anomalous X-ray Pulsar XTE J1810-197 and identification of its likely IR counterpart

摘要

We report the accurate sub-arcsec X-ray position of the new Anomalous X-rayPulsar (AXP) XTE J1810-197, derived with a Chndra-HRC Target of Opportunityobservation carried out in November 2003. We also report the discovery of alikely IR counterpart based on a VLT (IR band) Target of Opportunityobservation carried out in October 2003. Our proposed counterpart is the onlyIR source (Ks=20.8) in the X-ray error circle. Its IR colors as well as theX-ray/IR flux ratio, are consistent with those of the counterparts of all otherAXPs (at variance with field star colors). Deep Gunn-i band images obtained atthe 3.6m ESO telescope detected no sources down to a limiting magnitude of24.3. Moreover, we find that the pulsed fraction and count rates of XTEJ1810-197 remained nearly unchanged since the previous Chandra and XMM-Newtonobservations (2003 August 27th and September 8th, respectively). We brieflydiscuss the implications of these results. In particular, we note that thetransient (or at least highly variable) nature of this AXP might imply arelatively large number of hidden members of this class.