A new investigation of the possible X-ray counterparts of the magnetar candidate AX J1845−0258

Abstract

AX J1845-0258 is a transient X-ray pulsar, with spin period of 6.97s, discovered with the ASCA satellite in 1993. Its soft spectrum and the possible association with a supernova remnant suggest that AX J1845-0258 might be a magnetar, but this has not been confirmed yet. A possible counterpart one order of magnitude fainter, AX J184453-025640, has been found in later X-ray observations, but no pulsations have been detected. In addition, some other X-ray sources are compatible with the pulsar location, which is in a crowded region of the Galactic plane. We have carried out a new investigation of all the X-ray sources in the ASCA error region of AX J1845-0258, using archival data obtained with Chandra in 2007 and 2010, and with XMM-Newton in 2010. We set an upper limit of 6% on the pulsed fraction of AX J184453-025640 and confirmed its rather hard spectrum (power law photon index of 1.2 +\- 0.3). In addition to the other two fainter sources already reported in the literature, we found other X-ray sources positionally consistent with AX J1845-0258. Although many of them are possibly foreground stars likely unrelated to the pulsar, at least another new source, CXOU J184457.5-025823, could be a plausible counterpart of AX J1845-0258. It has a flux of 6x10^{-14} erg cm^{-2} s^{-1} and a spectrum well fit by a power law with photon index ~1.3 and Nh ~ 10^{22} cm^{-2}.