2010A&A...520A..22W

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. 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. 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 ∼4x10³⁶erg/s from 1.5-12keV and a hard spectrum (only significantly seen above 5keV). The second one had the mean hard X-ray luminosity of 1.3x10³⁶erg/s from 20-150keV with a modulation period at ∼5550s which is the pulse period of the neutron star in 4U 2206+54. Its hard X-ray spectrum from 20-300keV can be fitted by a broken power-law model with the photon indexes Γ₁∼2.3, and Γ₂∼3.3, and the break energy is E_b∼31keV or by a bremsstrahlung model of kT∼23keV. 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.