Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..aprs11009p&link_type=abstract
American Physical Society, April Meeting, Jointly Sponsored with the High Energy Astrophysics Division (HEAD) of the American As
Astronomy and Astrophysics
Astrophysics
Scientific paper
The main emission mechanism responsible for the X-ray production in accreting objects such as black holes and neutron stars is believed to be Comptonization of soft photons by the hot electron plasmas. In the case of black holes which do not have hard surface, the exact geometry of the emitting region is largerly unknown. In accreting X-ray pulsars, the situation is dramatically different. From the basic physical arguments it is clear that the hard emission is produced in the accretion column on the neutron star magnetic poles. Due to the misalignment of the magnetic dipole and the rotation axis, the emission is strongly variable. Recent RXTE observations of the millisecond (period 2.5 ms) pulsar SAX J1808.4-3658 with high temporal resolution revealed that the shape of the pulse profiles in the soft X-rays is dramatically different from that in the hard X-rays (above 15 keV). From the shape of the spectrum, one sees that the emission below 5 keV is dominated by the black body like radiation from the surface of the neutron star, while the high energy emission is produced by Comptonization in the accretion column. We construct a detailed model of the X-ray production. We show'that the emission pattern of the black body and the Comptonized radiation are dramatically different: "knife" and "fan" like, respectively. This results in the dramatic difference of the observed pulse profile. Constraints on the inclination angle, the angle between magnetic and rotational axes, the temperature of the emitting plasma, and its optical depth are obtained by comparing the model with the data. The Doppler effect distortes the shape of the pulse profile and can be used to determine the rotation velocity and, therefore, the radius of the neutron star. This is used then to put constaints on the neutron star equation of state.
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