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 } .