The detection of variability from the candidate IR counterpart to the   Anomalous X-ray Pulsar 1E1048.1-5937
  We report on the detection of variability from the proposed IR counterpart to the Anomalous X-ray Pulsar (AXP) 1E1048.1-5937 based on Chandra and ESO optical/IR deep observations carried out in 2001-2002. Within the narrow Chandra uncertainty region for 1E1048.1-5937 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 about 20.7 (3sigma confidence level). This implies a remarkable IR brightening of this object, Delta Ks>1.3, on a timescale of about 50 days. Although our knowledge of the IR properties of AXPs is rather limited (there is only another source, 1E2259+586, for which IR variability has been detected), the observed IR variability of the proposed counterpart strengthens its association with 1E1048.1-5937. Our results make the IR (and presumably optical) variability a likely common characteristic of AXPs, and provide new constraints on this class of objects. 
G.L. Israel, S. Covino, L. Stella, S. Campana, G. Marconi, S.   Mereghetti, R. Mignani, I. Negueruela, T. Oosterbroek, A.N. ...