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

Abstract

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 $\Gamma\sim 2.1$ with a hard X-ray luminosity of $\sim 5\times 10^{34}$ erg s$^{-1}$ (20-- 100 keV). While in the active state, the 20-- 100 keV hard X-ray luminosity reaches $\sim 2\times 10^{35}$ erg s$^{-1}$ and the spectrum is fitted by a thermal bremmstrahlung model of $kT\sim 43$ keV plus two cyclotron absorption lines at $\sim$ 30 and 60 keV. Then we derived a magnetic field of 3.3$\times 10^{12}$ G for the compact object in 4U 2206+54. During the active state, we found a pulsation period of $\sim$ 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 $\sim 9.56$ days and 19.11 days, and the orbit period of 4U 2206+54 should be 19.11 days.