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Astron. Astrophys. 319, 507-510 (1997)

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4. Discussion

A continuous observation of this pulsar in its present bright state may improve our understanding of the disk-magnetosphere interactions in neutron stars and will also lead to verification of the same. As the luminosity in the present state is comparable to its luminosity in the earlier spin-up phase, a reversal of the spin change will help to determine the critical value of fastness parameter at which the resultant torque on the neutron star changes its sign. GX 1+4 being a hard X-ray object, most of its luminosity lies in the 20-100 keV region. To study the [FORMULA] Vs [FORMULA] relation regular hard X-ray observations are needed. In our two observations, the period values indicate a very small spin-down rate. Compared to our previous observation of the same source with the same telescope, a period change rate of [FORMULA] is obtained which is the lowest rate of change of period for this source since its discovery. A reversal in the spin change, from spin-down to spin-up in between our two observations is consistent with our period determinations. BATSE observations (Chakrabarty et al., 1994) in this period has also indicated the same.

In the recent 1995 observation the pulse profile is double peaked, which may be an indication of increased mass accretion on the other magnetic pole. A gradual change in the beam pattern from a pencil beam to a fan beam, also may explain this pulse pattern. We note that the fan beam appeared in the relatively lower luminosity state contrary to the present understanding. Fan beam is more likely to occur in a luminosity state of [FORMULA] erg s-1 but in the 1993 December observation it showed a simple pencil beam pattern in spite of a very high luminosity of [FORMULA] erg s-1. The pencil beam in the December 1993 observation can be explained if the time scale of change in the beam pattern is long (100 days). The anti correlation between the luminosity and the pulse fraction, that was noticed in an earlier observation is still persistent and it will be interesting to observe whether this feature is present in other pulsars.

Compared to the earlier observations (Laurent et al. 1993) the spectrum is much harder which is evident from a power law photon index of 1.67. The thermal Bremsstrahlung model also fits well with a temperature of 99 keV which is higher than that measured from the earlier high energy observations.

Significant difference between the pulsed and unpulsed spectra was not observed. To detect any small difference in the hardness or temperature, longer duration observations or much reduced background level and bigger effective area telescope will be required.

A simple modeling of the double peaked pulse profile gave similar values for the inclination angles for the magnetic axis and the line of sight with the spin axis of the neutron star. The geometrical description that fits the present observation also supports the explanation given by Dotani et al. (1989) for the dip structure seen in two GINGA observations. Our earlier observation of the source profile with single peak also can be explained if a pencil beam emission is considered. One may also note that sometimes in fan beam configuration a larger value of the angular decay scale may hide the valley in between the two peaks corresponding to the two magnetic equator crossings. More frequent long duration observation of the source to determine accurate value of the luminosity, period, period derivative and epoch will help in establishing this possibility of change in the beam pattern.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998