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Post Info TOPIC: High mass X-ray binary 4U 2206+54
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4U 2206+54
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Title: On the spin-down mechanism of the X-ray pulsar 4U 2206+54
Authors: N.R. Ikhsanov, N.G. Beskrovnaya

Observations of the X-ray pulsar 4U 2206+54, performed over a 15-year span, show its period, which is now 5555 ± 9 s, to increase drastically. Such behaviour of the pulsar can be hardly explained in the frame of traditional scenarios of spin evolution of compact stars. The observed spin-down rate of the neutron star in 4U 2206+54 proves to be in good agreement with the value expected within magnetic accretion scenario which takes into account that magnetic field of the accretion stream can under certain conditions affect its geometry and type of flow. The neutron star in this case accretes material from a dense gaseous slab with small angular momentum which is kept in equilibrium by the magnetic field of the flow itself. The magnetic accretion scenario can be realised in 4U 2206+54 provided the magnetic field strength on the surface of the optical counterpart to the neutron star is in excess of 70 G. The magnetic field strength on the surface of the neutron star in the frame of this scenario turns out to be 4 x 10^{12} G which is in accordance with estimates of this parameter from the analysis of X-ray spectra of the pulsar.

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Blobrana


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RE: High mass X-ray binary 4U 2206+54
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Title: INTEGRAL/IBIS observations of a hard X-ray outburst in high mass X-ray binary 4U 2206+54
Authors: W. Wang

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 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 ~ 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 \Gamma_1 ~ 2.3,\ \Gamma_2 ~ 3.3, and the break energy is E_b ~ 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.

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Blobrana


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Posts: 131433
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Title: Evidence for a magnetic neutron star in high mass X-ray binary 4U 2206+54 with INTEGRAL/IBIS observations
Authors: W. Wang

The hard X-ray source 4U 2206+54 is a peculiar high mass X-ray binary with a main-sequence donor star. Recent X-ray observations suggested that the compact object in 4U 2206+54 may be a neutron star. The X-ray emission comes from the accretion of stellar winds from the massive donor stars, and variability of luminosity may be due to the changes of its orbit phase. To further reveal the nature of compact object, we studied 4U 2206+54 with INTEGRAL/IBIS observations in two years, and found that in most time, 4U 2206+54 undergone a quiescent state and sometime an active state. In the quiescent state the spectrum can be fitted by a power-law model of \Gamma ~ 2.1 with a hard X-ray luminosity of ~ 5 x 10^{34} erg s^{-1} (20-- 100 keV). While in the active state, the 20-- 100 keV hard X-ray luminosity reaches ~ 2 x 10^{35} erg s^{-1} and the spectrum is fitted by a thermal bremmstrahlung model of kT ~ 43 keV plus two cyclotron absorption lines at ~ 30 and 60 keV. Then we derived a magnetic field of 3.3 x 10^{12} G for the compact object in 4U 2206+54. During the active state, we found a pulsation period of ~ 5400 s in the light curve of 4U 2206+54. So the compact object in 4U 2206+54 should be a magnetic neutron star with a slow pulsation. Cyclotron absorption lines detected in the active state and non-detection in the quiescent state suggested that two different accretion states have possible different hard X-ray emission regions: surface of neutron star in the active state; the magnetic-accretion pressure equivalent point in the quiescent state. The re-analysis of the RXTE/ASM light curve found the modulation periods at ~  9.56 days and 19.11 days, and the orbit period of 4U 2206+54 should be 19.11 days.

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