Letter to the Editor

Correlated Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197^*

¹ Universitá di Roma 2, via della Ricerca Scientifica 1, 00133 Roma, Italy e-mail: rea@mporzio.astro.it
² INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
³ Affiliated with International Center for Relativistic Astrophysics
⁴ CNR - IASF, Sezione di Milano “G. Occhialini”, via Bassini 15, 20133 Milano, Italy
⁵ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁶ Università degli Studi di Milano, Dipartimento di Fisica, v. Celoria 16, 20133 Milano, Italy
⁷ CNR - IASF, Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
⁸ Astrophysics Missions Division, Research and Scientific Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
⁹ European Southern Observatory, Karl-Schwarzschild str. 2, 85748 Garching, Germany
¹⁰ European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
¹¹ INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy

Received: 27 May 2004
Accepted: 7 August 2004

Abstract

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. ([CITE], ApJ, 603, L97) was fainter by and 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  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.