INTEGRAL/IBIS observations of a hard X-ray outburst in high-mass X-ray binary 4U 2206+54

Abstract

<i>Aims. <i/>4U 2206+54 is a wind-fed high-mass X-ray binary with a main-sequence donor star. The nature of its compact object has been recently identified as a slow-pulsation magnetized neutron star. <i>Methods. <i/>INTEGRAL/IBIS observations have a long-term hard X-ray monitoring of 4U 2206+54 and detected a hard X-ray outburst around 15 December 2005 combined with the RXTE/ASM data. <i>Results. <i/>The hard X-ray outburst had a double-flare feature with a duration of ~2 days. The first flare showed a fast rise and long-term decaying light curve about 15 h with a peak luminosity of ~4 × 10<sup>36<sup/> erg s<sup>-1<sup/> from 1.5–12 keV and a hard spectrum (only significantly seen above 5 keV). The second one had the mean hard X-ray luminosity of 1.3 × 10<sup>36<sup/> erg s<sup>-1<sup/> from 20–150 keV with a modulation period at ~5550 s which is the pulse period of the neutron star in 4U 2206+54. Its hard X-ray spectrum from 20–300 keV can be fitted by a broken power-law model with the photon indexes <i>Γ<i/><sub>1<sub/> ~ 2.3, and <i>Γ<i/><sub>2<sub/> ~ 3.3, and the break energy is <i>E<i/><sub><i>b<i/><sub/> ~ 31 keV or by a bremsstrahlung model of <i>kT<i/> ~ 23 keV.<i>Conclusions. <i/>We suggest that the hard X-ray flare could be induced by suddenly enhanced accreting dense materials from stellar winds hitting the polar cap region of the neutron star. This hard X-ray outburst may be a link to supergiant fast X-ray transients though 4U 2206+54 has a different type of companion.