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

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 $\sim$ 2 days. The first flare showed a fast rise and long time decaying light curve about 15 hours with a peak luminosity of $\sim 4\times 10^{36}$ 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 $1.3\times 10^{36}$ erg s$^{-1}$ from 20 -- 150 keV with a modulation period at $\sim 5550$ 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 $Γ_1 \sim 2.3,\ Γ_2 \sim 3.3$, and the break energy is $E_b \sim 31$ keV or a bremsstrahlung model of $kT\sim 23$ 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.

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