The detection of variability from the candidate infrared counterpart to the anomalous X-ray pulsar 1E 1048.1-59371

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-K-s = 2.4) source, while the recently proposed IR counterpart was not detected down to a limiting K-s magnitude of similar to20.7 (3 sigma confidence level). This implies a remarkable IR brightening of this object, DeltaK(s) > 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.