The Detection of Variability from the Candidate Infrared Counterpart to the Anomalous X-Ray Pulsar 1E 1048.1−5937

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Abstract
We report on the detection of variability from the proposed IR counterpart to the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 based on Chandra and ESO optical/IR deep observations carried out in 2001-2002. Within the narrow Chandra uncertainty region for 1E 1048.1-5937, we found only one relatively faint (J = 22.1 ± 0.3, J-Ks = 2.4) source, while the recently proposed IR counterpart was not detected down to a limiting Ks magnitude of ~20.7 (3 σ confidence level). This implies a remarkable IR brightening of this object, ΔKs > 1.3, on a timescale of about 50 days. Although our knowledge of the IR properties of AXPs is rather limited (there is only one other source, 1E 2259+586, for which IR variability has been detected), the observed IR variability of the proposed counterpart strengthens its association with 1E 1048.1-5937. Our results make the IR (and presumably optical) variability a likely common characteristic of AXPs and provide new constraints on this class of objects.
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