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Session 104 - Accretion-Powered X-Ray Sources.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
We systematically analyzed the spectra of X-ray binary pulsars observed with GINGA. A new model NPEX (Negative and Positive power-laws EXponential) was introduced to represent the concave continuum. \hspace*1.5cm NPEX(E) = (A_1 E^-\alpha_1 + A_2 E^+\alpha_2 ) \ \exp \-E/(kT)\ kT is the typical temperature of the X-ray emitting plasma, and \alpha_1 and \alpha_2 are the negative and positive power-law indices, respectively. From the study of Her X-1, \alpha_2 was found to be 2.0\pm 0.3, which expresses the Wien spectrum. Therefore \alpha_2 was fixed to 2.0 for the other sources.
Combining the NPEX continuum with the CYAB factor (cyclotron resonance scattering model), we successfully fit the whole-band spectra of all the pulsars. For sources without cyclotron structure, no CYAB model was used, and for those with the second harmonic: 4U 0115+63 in 1990 and possibly Vela X-1, another CYAB was multiplied.
A possible physical meaning of the NPEX model is the Comptonized spectra. The obtained temperatures might be determined by the magnetic fields with the relation of kT \approx 0.25 E_res.
Using the smooth and slightly concave NPEX model, we found new cyclotron structures from LMC X-4 and GS 1843+00 and finally from 11 sources among 23 X-ray pulsars and pulsar-candidates. Adding A0535+26 discovered with HEXE, the cyclotron structures were detected from 12 pulsars, about a half of the 23 sources. The magnetic fields were scattered in the range of 4\times10^11 -- 5\times10^12 G. The distribution shown for the first time is remarkably similar to that of radio pulsars.
The harmonic cyclotron structures of 4U 0115+63 in 1990 changed to a single structure in 1991. The resonance energy also increased by 40 %. If we attribute this change to the height of the scattering region in a dipole magnetic field, the height change is as much as \sim 1.1 km. Such changes of the resonance energies with luminosities are observed from 5 pulsars and are well explained by the accretion column height model.
