Swift monitoring of the massive X-ray binary SAX J0635.2+0533
Abstract
SAX J0635.2+0533 is a binary pulsar with a very short pulsation period (P = 33.8 ms) and a high long-term spin down ( P > 3.8×10-13 s s-1), which suggests a rotation-powered (instead of an accretion-powered) nature for this source. While it was discovered at a flux level around 10-11 erg cm-2 s-1, between 2003 and 2004 this source was detected with XMM-Newton at an average flux of about 10-13 erg cm-2 s-1; moreover, the flux varied of over one order of magnitude on time scales of a few days, sometimes decreasing below 3×10-14 erg cm-2 s-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 10-12 erg cm-2 s-1.