Probing Circumstellar Matter with X-ray Binary Pulsars

Takao Endo

We have observed X-ray emission from the binary system GX301-2/Wray 977 at the orbital phases intermediate (Θorb ∼ 0.31), near apastron (Θorb∼ 0.47), and near periastron (Θorb ∼ 0.97) with the ASCA satellite. Owing to the high spectral resolution of ASCA, we have discovered that a iron Kα line possesses the finite energy width of 40-80 eV in Gaussian σ in all the three orbital phases. This width is far larger than that expected from the terminal velocity of the stellar wind. Quantitative evaluation of the accreting matter velocity leads us to conclude that the iron Kα emission line from GX 301-2 originates from a region typically around several Alfvén radii from the neutron star. The discovery of the finite width requires a significant change in the old picture in which the iron Kα emission line from the HMXB pulsars originates from the stellar wind that is extended on the scale of the binary orbit.

In addition to the finite width of the line core, we have also discovered a broad tail component in the iron Kα emission line in the spectrum taken at the periastron phase. The average energy width of σ = 150 eV implies that the broad tail component originates from matter within the Alfvén radius via fluorescence. As candidates for the emission site, we have considered the pre-shock accretion column and the inner edge of the accretion disk. Both candidates can explain the width of the broad tail, the amount of the observed equivalent width, and the presence of the broad tail only in the periastron phase, but fail to explain the observed low ξ parameter. To resolve this problem, it is necessary to invoke a mechanism of density enhancement by a factor of 103 at least from the uniform accreting plasma, such as a blobby accretion.

The continuum spectrum of GX 301-2 is expressed by a power-law undergoing photoelectric absorption with dual hydrogen column densities. We have detected a significant pulsation not only from the more absorbed component but also from the less absorbed component. From this fact, we conclude that both the less and more absorbed continuum components are the direct emission from the pulsar, and have denied the old identification that the less absorbed component is the scattered one from the matter surrounding the central pulsar.

In the SIS spectra, we have resolved iron Kβ emission line for the first time. By taking this line into account appropriately in the spectral model, it is revealed that the ionization states of iron in the fluorescing matter and in the line of sight are consistent with each other and close to neutral. The old result obtained from the Tenma observation that the matter on the line of sight is more ionized than that responsible for the fluorescent iron line is found to be false, as a result of the detection of the iron Kβ line.