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 ($\dot{P}$ $>$ 3.8$\times 10^{-13}$ 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 $3\times 10^{-14}$ 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${}^{-1}$.

arXiv comment8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy & Astrophysics

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• Kinematic distinction of the two subpopulations of X-ray pulsars