In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4-7323

Ducci, LorenzoLorenzoDucciROMANO, PatriziaPatriziaROMANOMalacaria, C.C.MalacariaJi, L.L.JiBozzo, EnricoEnricoBozzoSantangelo, AndreaAndreaSantangelo2020-11-252020-11-2520180004-6361http://hdl.handle.net/20.500.12386/28545AX 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 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. AX J0049.4-7323 shows a high X-ray variability, spanning more than three orders of magnitudes, from L ≈ 1.6 × 1037 erg s-1 (0.3-8 keV, d = 62 kpc) down to L ≈ 8 × 1033 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 270 with peak luminosities of ≈2.1 × 1036 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 AX J0049.4-7323 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 Ṗ = (-3.00 ± 0.12) × 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 ≈3 × 1012 G.STAMPAenIn-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4-7323Article10.1051/0004-6361/2017319222-s2.0-85048868600000434857600003https://www.aanda.org/articles/aa/abs/2018/06/aa31922-17/aa31922-17.html2018A&A...614A..34DFIS/05 - ASTRONOMIA E ASTROFISICA