Swift monitoring of the massive X-ray binary SAX J0635.2+0533


SAX J0635.2+0533 is a binary pulsar with a very short pulsation period (PP = 33.8 ms) and a high long-term spin down (P˙\dot P >> 3.8×1013\times10^{-13} s s1^{-1}), which suggests a rotation-powered (instead of an accretion-powered) nature for this source. While it was discovered at a flux level around 1011^{-11} erg cm2^{-2} s1^{-1}, between 2003 and 2004 this source was detected with XMM-Newton at an average flux of about 1013^{-13} erg cm2^{-2} s1^{-1}; moreover, the flux varied of over one order of magnitude on time scales of a few days, sometimes decreasing below 3×10143\times10^{-14} erg cm2^{-2} s1^{-1}. Since both the rotation-powered and the accretion-powered scenarios have difficulties to explain these properties, the nature of SAX J0635.2+0533 is still unclear. Here we report on our recent long-term monitoring campaign on SAX J0635.2+0533 carried out with Swift and on a systematic reanalysis of all the RXTE observations performed between 1999 and 2001. We found that during this time interval the source remained almost always active at a flux level above 1012^{-12} erg cm2^{-2} s1^{-1}.Comment: 8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy & Astrophysic

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