In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AXJ0049.4-7323

AXJ0049.4-7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a similar to 750 s pulsar which has been observed over the last similar to 17 years by several X-ray telescopes. Despite numerous observations, little is known about its X-ray behaviour. Therefore, we coherently analysed archival Swift, 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. AXJ0049.4-7323 shows a high X-ray variability, spanning more than three orders of magnitudes, from L approximate to 1.6 x 10(37) erg s(-1) (0.3-8 keV, d = 62 kpc) down to L approximate to 8 x 10(33) erg 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 similar to 270 with peak luminosities of approximate to 2.1 x 10(36) erg s(-1) far 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 AXJ0049.4-7323 is complemented with a spectral analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend when the emission becomes fainter, and an analysis of optical/UV data collected by the UVOT telescope on board Swift. In addition, we measured a secular spin-up rate of. P = (-3-00 +/- 0.12) x 10(-3) s 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 approximate to 3 x 10(12) G.