We have investigated the matter distribution and their physical conditions for Vela X-1 and GX 301−2 using X-ray spectroscopy with high energy resolving power. In Fig-ure 8.12, we present a conceptual pictFig-ure of Vela X-1 and GX 301−2 on the basis of the information obtained from our study. The cold and dense cloud is surrounding the neu-tron star of GX 301−2, and prevent the stellar wind from photoionization. This situation makes the heavily absorbed continuum, the fluorescent lines and the Compton shoulder in their X-ray spectrum. On the other hand, the neutron star of Vela X-1 do not have such a dense cloud obscuring over all of the directions, and its X-ray radiation ionize the stellar wind directly. The highly ionized gases in the stellar wind produces the emission lines by recombination and cascades. Additionally, X-ray photoionization by the neutron star affects the flow of the stellar wind.
The two types of HMXBs can be distinguished by the presence or absence of the cold cloud. The one is the “type I HXMB” like Vela X-1, which has highly photoionized plasmas, and the other is the “type II HXMB” represented by GX 301−2, which has heavy absorbers and only cold material.
A mechanism to produce such a cloud is possibly related to the efficiency of the X-ray production. In the case of Vela X-1, the efficiency to capture matter and to convert them into X-ray radiation is high. The luminosity calculated from eq.(2.6) is given as,
LX = (GMns)3M˙∗
Rnsvrel4 D2 = 4.7×1036 erg s−1. (8.1) Here, we assume a reasonable value for this system; Mns = 1.7M¯, Rns = 10 km, vrel = 640 km s−1 (vwind = 570 km s−1, vorbit = 300 km s−1), D = 53.4R¯ and ˙M∗ = 1.5×10−6M¯yr−1. Assuming that the X-ray spectrum extends up to ∼ 20 keV, the observed absorption corrected X-ray luminosity is 3.5×1036erg s−1, which is comparable to the value calculated in eq.(8.1). In other words, almost all of the captured stellar wind material is accreted onto the neutron star, producing strong X-ray radiation. On the other hand, the observed luminosity of GX 301−2 does not reach the value calculated from the stellar wind parameters. We can estimate the luminosity as
LX = (GMns)3M˙∗
Rnsvrel4 D2 = 8.3×1036 erg s−1. (8.2) Here, we assume a value for the IM phase as the most ordinary case; Mns = 1.4M¯, Rns = 10 km,vrel ∼400 km s−1,D∼140R¯and ˙M∗ = 5.0×10−6M¯yr−1. The observed absorption corrected luminosity, ∼7.9 × 1035 erg s−1 in the energy range of 0.5–20 keV, is about an order of magnitude lower the value in eq.(8.2).
Such accretions of matters onto a neutron star should be affected by the physical en-vironment nearby the surface of the neutron star, such as the gravitational field, strength of the magnetic field, and the spin of the neutron star. These physical states would show
their true figures in the hard X-ray spectrum. Therefore, hard X-ray observations with high precision will become important measurements together with high energy resolution X-ray observations.
Cloud
(accretion wake?) absorption +fluorescent line Neutron Star Emission Region of lines from highly ionized ions
Stellar Wind affected by photoionization of the X-ray radiation Companion Star
Stellar Wind
Stellar Wind (low charge state)
Hard X-ray (Heavily absorbed) + fluorescent X-ray Neutron Star
+ Cloud Companion Star
Neutron Star
Ionized gas Cold Cloud (fluorescent X-ray, Compton shoulder)
Vela X-1
GX 301-2
Figure 8.12: Conceptual pictures of Vela X-1 and GX 301−2.
Chapter 9 Conclusion
We have observed two high mass X-ray binaries (Vela X-1 and GX 301−2) with the Chandra HETGS. The observations have been performed at different three orbital phases for each source. From their X-ray spectra with the high energy resolutions, the following results have been newly obtained.
• Vela X-1
– A number of emission lines are detected and clearly resolved. The emission lines from highly ionized S, Si, Mg, and Ne, in addition to fluorescent lines from Fe, Ca, S, and Si ions in lower charge states are detected in the spectra of the eclipse phase and the opposite orbital phase (phase 0.50). The narrow radiative recombination continuum features from H-like ions of Ne are observed in the both phases, and their widths correspond to the electron temperature of 6.6+2.5−1.8eV and 7.4+1.6−1.3 eV for the eclipse and the phase 0.50, respectively. These results indicate that highly ionized plasmas driven through photoionization exist in Vela X-1.
– Multiple Si K fluorescent lines from a wide range of charge states are detected individually for the first time, which is evidence that there exist photoionized plasmas in various ionization degrees in Vela X-1.
– The similar kinds of emission lines are detected in the both spectra of the eclipse phase and the phase 0.50. The intensity ratios of emission lines in phase 0.50 to those in the eclipse are 8–10 for lines from H-like ions and 4–7 for lines from He-like ions.
– From emission lines from highly ionized ions, Doppler shifts are observed.
The lines in the eclipse phase are red-shifted, and those in the phase 0.50 are blue-shifted. The amount of shifts between these two orbital phases ranged in
∼300–600 km s−1.
• GX 301−2
– For all three orbital phases, heavily absorbed (NH ∼ (2–10) × 1023 cm−2) continuum spectra are observed. The emission lines due to fluorescence of Si, S, Ar, Ca ions in low charge states, in addition to an intense iron Kα line are also detected in all phases. In contrast, emission lines from highly ionized ions are entirely absent.
– In the pre-periastron phase observation, the Compton-scattered iron Kα line profile (“Compton shoulder”) is clearly detected and fully resolved for the first time from an astrophysical object.
In order to handle such high energy resolution spectra, we have newly constructed the simulator on the basis of Monte Carlo method. With this simulator, we can deal with situations, which include asymmetrical geometries and not-optically-thin media. In the Monte Carlo part for calculations of photo transport, various physical physical processes are considered, including processes related to highly ionized ions and Doppler effects. By using this simulator, we have explained the observed spectra as follows.
• Vela X-1
– Assuming that the velocity structure of the stellar wind is followed by CAK-model, we calculated the ionization structure of the stellar wind and X-ray spectra of each phase. By using the intensity ratio of Si Lyα between phase 0.50 and eclipse, we found the ionization structure and the matter distribution, which satisfy line intensities and continuum shapes in both phase 0.50 and eclipse.
– However, the amount of observed Doppler shifts in the emission lines were inconsistent with the CAK-model velocity structure, which is assumed in the ionization structure calculations. As a main cause of this inconsistency, we showed that photoionization effects by the X-rays slows the stellar wind veloc-ity and changes the velocveloc-ity structure. There should be a velocveloc-ity structure, which reproduces the observed Doppler shifts leaving the ionization structure and the matter distribution. Further calculations are needed to search the solution.
• GX 301−2
– We have demonstrated that Compton shoulders has become a new probe to investigate the physical conditions of cold material. In fact, we derived from the observed Compton shoulders that a cold and dense cloud is surrounding the neutron star almost spherically.
– The geometry consisting of the CAK-model stellar wind and a cold dense cloud surrounding the neutron star give a good explanation to the observed spectra of GX 301−2 as the Monte Carlo simulation results.
Through the analysis and the consideration, we have established a new method to investigate the physical state of the HMXBs on the basis of the high precision X-ray spectrum. And for the different types of HMXBs, Vela X-1 and GX 301−2, we have specified the physical conditions. GX 301−2 has a cold dense cloud surrounding the neutron star, while Vela X-1 do not have such a cloud obscuring over all direction. This difference is probably induced by the accreting mechanism onto the neutron star. In order to reveal the mechanism, hard X-ray and gamma-ray observations with high precision are needed together with X-ray spectroscopy.
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