We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX J2103.5+4545 at a time when the Be star ’ s circumstellar disk had disappeared and thus the main reservoir of material available for accretion had extinguished .
In this very low optical state , pulsed X-ray emission was detected at a level of L _ { X } \sim 10 ^ { 33 } erg s ^ { -1 } .
This is the lowest luminosity at which pulsations have ever been detected in an accreting pulsar .
The derived spin period is 351.13 s , consistent with previous observations .
The source continues its overall long-term spin-up , which reduced the spin period by 7.5 s since its discovery in 1997 .
The X-ray emission is consistent with a purely thermal spectrum , represented by a blackbody with kT = 1 keV .
We discuss possible scenarios to explain the observed quiescent luminosity and conclude that the most likely mechanism is direct emission resulting from the cooling of the polar caps , heated either during the most recent outburst or via intermittent accretion in quiescence .