Evidence for a magnetic neutron star in high-mass X-ray binary 4U 2206+54 with INTEGRAL/IBIS observations

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 star, and variability of its luminosity may be due to changes in its orbital phase. To reveal further the nature of the compact object, we studied 4U 2206+54 with INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL)/IBIS observations over two years, and found that for most of this time 4U 2206+54 was in a quiescent state, sometimes undergoing an active phase. In the quiescent state the spectrum can be fitted by a power-law model of Γ ~ 2.1 with a hard X-ray luminosity of ~5 × 10³⁴ergs^-1 (20-100keV). While the source is in the active state, the 20-100keV hard X-ray luminosity reaches ~ 2 × 10³⁵ergs^-1 and the spectrum is fitted by a thermal bremsstrahlung model of kT ~ 43keV plus two cyclotron absorption lines at ~30 and 60keV. We then derived a magnetic field of 3.3 × 10¹²G for the compact object in 4U 2206+54. During the active state, we found a pulsation period of ~5400s in the light curve of 4U 2206+54. The compact object in 4U 2206+54 should therefore be a magnetic neutron star with slow pulsation. The detection of cyclotron absorption lines in the active state and their non-detection in the quiescent state suggested that the two different accretion states have possible different hard X-ray emission regions: the surface of the neutron star in the active state and the magnetic-accretion pressure equivalent point in the quiescent state. Re-analysis of the RXTE/ASM light curve found modulation periods at ~9.56 and 19.11d, and the orbital period of 4U 2206+54 should be 19.11d.