INTEGRAL/IBIS observations of a hard X-ray outburst in high mass X-ray binary 4U 2206+54
Abstract
U 2206+54 is a wind-fed high mass X-ray binary with a main-sequence donor star. The nature of its compact object was 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 time decaying light curve about 15 hours with a peak luminosity of 4× 1036 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× 1036 erg s-1 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 with a broken power-law model with the photon indexes 1 2.3,\ 2 3.3, and the break energy is Eb 31 keV or a bremsstrahlung model of kT 23 keV. We suggest that the hard X-ray flare could be induced by suddenly enhanced accretion 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.