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

## TL;DR

This study presents a comprehensive monitoring of the binary pulsar SAX J0635.2+0533, revealing its persistent activity at certain flux levels and highlighting the challenges in understanding its emission mechanisms.

## Contribution

It provides the first long-term monitoring data with Swift and a systematic reanalysis of RXTE observations, offering new insights into the source's flux variability and activity state.

## Key findings

- Source remained active above 10^{-12} erg cm^{-2} s^{-1} during 1999-2001
- Flux varied over an order of magnitude on days
- Flux levels were sometimes below 3×10^{-14} erg cm^{-2} s^{-1}

## 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$\times10^{-13}$ 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 $3\times10^{-14}$ 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$^{-1}$.