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

摘要

Aims. 4U 2206+54 is a wind-fed high-mass X-ray binary with amain-sequence donor star. The nature of its compact object has beenrecently identified as a slow-pulsation magnetized neutron star. Methods. INTEGRAL/IBIS observations have a long-term hard X-raymonitoring of 4U2206+54 and detected a hard X-ray outburst around15 December 2005 combined with the RXTE/ASM data. Results. The hard X-ray outburst had a double-flare feature with a duration of 2days. The first flare showed a fast rise and long-term decaying light curve about 15h with a peak luminosity of ergs-1from 1.5-12keV and a hard spectrum (only significantly seen above5 keV). The second one had the mean hard X-ray luminosity of ergs-1 from 20-150keV with a modulation period at 5550swhich is the pulse period of the neutron star in 4U2206+54. Itshard X-ray spectrum from 20-300keV can be fitted by a brokenpower-law model with the photon indexes ,and ,and the break energy is keV or by a bremsstrahlung model of keV. Conclusions. We suggest that the hard X-ray flare could beinduced by suddenly enhanced accreting dense materials from stellarwinds hitting the polar cap region of the neutron star. This hard X-rayoutburst may be a link to supergiant fast X-ray transients though4U2206+54 has a different type of companion. Key words: stars: individual: 4U 2206+54 - stars: neutron - X-rays: binaries - X-rays: bursts