An unexpected outburst from A0535+262

A0535+262 is a transient Be/X-ray binary system which was in a quiescent phase from 1994 to 2005. In this paper we report on the timing and spectral properties of the INTEGRAL detection of the source in 2003 October. The source is detected for ∼6000 s in the 18–100 keV energy band at a luminosity of  ∼3.8 × 10³⁵ erg s⁻¹  ; this is compatible with the high end of the range of luminosities expected for quiescent emission. The system is observed to be outside of the centrifugal inhibition regime and pulsations are detected with periodicity,   P = 103.7 ± 0.1 s  . An examination of the pulse history of the source shows that it had been in a constant state of spin-down since it entered the quiescent phase in 1994. The rate of spin-down implies the consistent presence of an accretion disc supplying torques to the pulsar. The observations show that the system is still active and highly variable even in the absence of recent Type I or Type II X-ray outbursts.