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 (Ṗ> 3.8 × 10<SUP>-13</SUP> s s<SUP>-1</SUP>), which suggests a rotation-powered (instead of an accretion-powered) nature for this source. While it was discovered at a flux level around 10<SUP>-11</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>, between 2003 and 2004, this source was detected with XMM-Newton at an average flux of about 10<SUP>-13</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>; moreover, the flux varied by over one order of magnitude on timescales of a few days, sometimes decreasing to below 3 × 10<SUP>-14</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>. 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<SUP>-12</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>.