摘要：AX J0049.4−7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a ~750 s pulsar which has been observed over the last ~17 years by several X-ray telescopes. Despite numerous observations, little is known about its X-ray behaviour. Therefore, we coherently analysed archivalSwift,Chandra,XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already published ASCA data, to study its X-ray long-term spectral and flux variability. AX J0049.4−7323 shows a high X-ray variability, spanning more than three orders of magnitudes, fromL≈ 1.6 × 1037erg s−1(0.3−8 keV,d= 62 kpc) down toL≈ 8 × 1033erg s−1. RXTE,Chandra,Swift, and ASCA observed, in addition to the expected enhancement of X-ray luminosity at periastron, flux variations by a factor of ~270 with peak luminosities of ≈2.1 × 1036erg s−1far from periastron. These properties are difficult to reconcile with the typical long-term variability of Be/XRBs, traditionally interpreted in terms of type I and type II outbursts. The study of AX J0049.4−7323 is complemented with a spectral analysis ofSwift,Chandra, andXMM-Newtondata which showed a softening trend when the emission becomes fainter, and an analysis of optical/UV data collected by the UVOT telescope on boardSwift. In addition, we measured a secular spin-up rate ofṖ= (−3.00 ± 0.12) × 10−3s day−1, which suggests that the pulsar has not yet achieved its equilibrium period. Assuming spherical accretion, we estimated an upper limit for the magnetic field strength of the pulsar of ≈3 × 1012G.
