A low luminosity state in the massive X-ray binary SAX J0635+0533

TL;DR

This study reports the first observation of SAX J0635+0533 in a low luminosity state, revealing significant flux reduction and implications for understanding its accretion and rotation mechanisms.

Contribution

It provides the first detection of a low luminosity state in SAX J0635+0533, challenging previous assumptions about its emission behavior and aiding in understanding its nature.

Findings

Source was in a low flux state, ~30 times lower than previous observations.

Spectrum consistent with high state, but pulsations not detected.

Reevaluation of the source's accretion and rotation models.

Abstract

The X-ray pulsar SAX J0635+0533 was repeatedly observed with the XMM-Newton satellite in 2003-2004. The precise localization provided by these observations confirms the association of SAX J0635+0533 with a Be star. The source was found, for the first time, in a low intensity state, a factor ~30 lower than that seen in all previous observations. The spectrum, well fitted by an absorbed power law with photon index ~1.7 and N_H = 1.2x10^22 cm^-2, was compatible with that of the high state. The low flux did not allow the detection of the pulsations at 33.8 ms seen BeppoSAX and RXTE data. In view of the small luminosity observed in 2003-2004, we reconsider the peculiarities of this source in both the accretion and rotation powered scenarios.