Infrared and X–ray Variability of the Transient Anomalous X-ray Pulsar Xte J1810-197 ⋆

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X–ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM–Newton observation carried out the day before the VLT observations. The 0.5-10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM–Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X–ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.