The infrared counterpart to the anomalous X-ray pulsar 1RXS J170849-400910

We report the discovery of the likely IR counterpart to the anomalous X-ray pulsar (AXP) 1RXS J170849-400910 based on the Chandra High Resolution Camera (imaging detector) X-ray position and the deep optical/IR observations carried out from ESO telescopes and the Canada-France-Hawaii Telescope during 1999-2002. Within the narrow uncertainty region, we found two relatively faint (K' = 20.0 and K' = 17.53) IR objects. Based on their color and position in the J-K' versus J-H diagram, only the brighter object is consistent with the known IR properties of the counterparts to other AXPs. No variability was detected for this source, which is similar to what is observed in the case of 4U 0142+614. Like in other AXPs, we found that the IR flux of 1RXS J170849-400910 is higher than expected for a simple blackbody component extrapolated from the X-ray data. If confirmed, this object would be the fourth IR counterpart to a source of the AXP class and would make the IR excess a likely new characteristic of AXPs.