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

Aims. 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. Methods. 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. Results. The hard X-ray outburst had a double-flare feature with a duration of similar to 2 days. The first flare showed a fast rise and long-term decaying light curve about 15 h with a peak luminosity of similar to 4 x 10(36) erg s(-1) 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 x 10(36) erg s(-1) from 20-150 keV with a modulation period at similar to 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 Gamma(1) similar to 2.3, and Gamma(2) similar to 3.3, and the break energy is E(b) similar to 31 keV or by a bremsstrahlung model of kT similar to 23 keV. Conclusions. 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.