Makoto Tashiro

Ginga observations of the X-ray binary pulsar GX301 - 2 are analysed for their temporal and spectral properties. We report the unambiguous detection of aperiodic intensity variations of 7-10 per cent rms relative amplitude over time-scales covering approximately 16 to 0.1 s. These variations are self-similar, being well fitted by a power law in power spectral density versus frequency space. The pulse period measured is 689.80 s, the shortest yet measured for this source. The fraction of X-rays which is pulsed is energy dependent, being greater at higher energies. The fractional aperiodic variability shows no such energy dependence, nor do they appear to be significantly dependent on source intensity, pulse phase or absorbing column measured. These facts strongly suggest that the aperiodic variations are not caused by absorption variations, but are intrinsic to the source. The iron-line intensity shows little pulsations, but surprisingly shows significant aperiodic variability down to a time-scale as short as several seconds, thus giving a constraint to the size of the line emission region.

Ginga observations of the X-ray binary pulsar GX301 - 2 are analysed for their temporal and spectral properties. We report the unambiguous detection of aperiodic intensity variations of 7-10 per cent rms relative amplitude over time-scales covering approximately 16 to 0.1 s. These variations are self-similar, being well fitted by a power law in power spectral density versus frequency space. The pulse period measured is 689.80 s, the shortest yet measured for this source. The fraction of X-rays which is pulsed is energy dependent, being greater at higher energies. The fractional aperiodic variability shows no such energy dependence, nor do they appear to be significantly dependent on source intensity, pulse phase or absorbing column measured. These facts strongly suggest that the aperiodic variations are not caused by absorption variations, but are intrinsic to the source. The iron-line intensity shows little pulsations, but surprisingly shows significant aperiodic variability down to a time-scale as short as several seconds, thus giving a constraint to the size of the line emission region.

Observations of a cyclotron absorption feature in the spectrum of X-ray pulsars are reported. The cyclotron absorption features have been discovered with Ginga in two X-ray pulsars, 4U 1538-52 (at 21keV) and X0331+53 (at 28keV). Ginga data indicate that the known cyclotron feature in the spectrum from Her X-1 should be regarded as an absorption feature at ∿34keV rather than an emission feature at ∿50keV. These results indicate that he cyclotron absorption feature should be common in binary X-ray pulsars. We infer that the high-energy spectral break, characteristic of X-ray pulsars, is a direct consequence of the cyclotron resonance, and closely related to the absorption feature. This implies that the spectral break, instead of the absorption feature, can be used as a convenient measure of the field strength for many X-ray pulsars. It is suggested that the X-ray pulsars exhibit a rather narrow scatter in the field strength, namely in the range of (1-4)×10^<12> G.