Monitoring of the peculiar X-ray binary pulsar SAX J0635+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 × 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 it was detected with XMM-Newton with an average flux of ∼ 10^{-13} erg cm^{-2} s^{-1}; moreover, the flux varied of over one order of magnitude on time scales of a few days. This large flux variability is difficult to explain for a rotation-powered pulsar, while the matter accretion onto the NS surface implies a very low magnetic field (B ∼ 10^8 G). Therefore, the nature of SAX J0635.2+0533 remained uncertain.Between 2015 and 2016 we performed a monitoring campaign with Swift, detecting the source between 10^{-13} and 10^{-12} erg cm^{-2} s^{-1}; moreover, with a systematic analysis of all the observations performed with RXTE we revealed that between 1999 and 2001 the source remained active at a flux level above 10^{-12} erg cm^{-2} s^{-1}.Here we report in detail these results and discuss their impact on the assessment of the source nature.