Luminosity dependent changes of pulse profiles in the X-ray binary GX 1+4

J.G. Greenhill ,, D. Galloway ,, M.C. Storey, PASA, 15 (2), 254
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Discussion

The explanation for the pulse profile changes is not clear. It seems reasonable to assume that the changes are associated with a change in the accretion flow around the neutron star. Asymmetries in pulse profiles can result from magnetic field asymmetry, caused for example by a non-centred dipole (e.g. Padden and Storey 1986; Kraus et al. 1995), or higher-order multipole field components. In the case of an asymmetric magnetic field distribution the varying strength of the magnetic field sweeping by the accretion disc with phase leads to different rates of accretion onto different regions of the polar cap. Different stable states of accretion could lead to different pulse shapes and different spin-up/spin-down rates.

Asymmetries can also arise because of the relative rotation of the neutron star and the incoming matter. The connection between pulse shape and the relative rotation of the accretion plasma and star has been made previously by Wang and Welter (1981). We have demonstrated a correlation between pulse shape and X-ray luminosity and a linkage between this and the spin-up/spin-down behaviour of GX 1+4. This is consistent with the work of Wang & Welter (1981) who showed that class A or class B pulse profiles can be produced depending on whether the specific angular momentum of the accreting plasma is greater than or less than that of the neutron star. If the specific angular momentum of the accreting plasma is greater than that of the neutron star, the trailing edge of pulse profiles will be brighter (Wang & Welter 1981). If the plasma specific angular momentum is less than that of the star the leading edge of the pulse profile will be brighter.

There has been considerable uncertainty as to whether the deep primary minimum observed in most pulse profiles from GX1+4 is the pulse centre, i.e. the closest passage of the line-of-sight to the magnetic pole of the star where the emission region is located, or whether the secondary minimum sometimes observed is the pulse centre (see, for example Storey, Greenhill & Kotani 1998). The information above can be used to help determine which section of the pulse profile in GX 1+4 represents the pulse centre. If the pulse asymmetry is caused by differential rotation of the accretion plasma and the neutron star, then during the spin-up era of the 1970's (when the accretion plasma specific angular momentum was presumably more than the star specific angular momentum) one would expect trailing-edge bright pulses. An examination of observed pulse profiles from the 1970's shows that the section of pulse immediately before the deep minimum is the brightest. Therefore, from the above analysis, one concludes that the section of pulse immediately before the deep minimum is the trailing edge of the pulse implying that the deep minimum is the edge of the pulse, not the pulse centre. Similarly, during the spin-down era of the 1980's (when the star specific angular momentum was presumably more than that of the accretion plasma) one would expect leading-edge-bright pulses. An examination of observed pulse profiles from the spin-down era shows that the brightest part of the pulse is the part immediately following the deep minimum. Using the analysis of Wang & Welter (1981) this part of the pulse is the leading part of the pulse, which again indicates that the primary minimum is the edge of the pulse.

Evidence has emerged recently that in the spin-down state, at least on some occasions, a retrograde accretion disc is forming in GX1+4. In the standard theory of disc accreting X-ray binaries, assuming a disc rotating in the same direction as the star (Ghosh & Lamb 1979), spin-down is thought to occur when the accretion rate and thus the ram pressure of the flow drops to the point where the inner edge of the accretion disc is pushed outwards by magnetic pressure to a radius where the specific angular momentum of accreting matter is less than that of the neutron star. In this theory one expects that an increase in the spin-down rate would be associated with a decrease in luminosity. However, recently Nelson et al. (1997) have found that GX 1+4 spun down faster during a brief X-ray flare and they point out that this observation could be explained if GX 1+4 had a retrograde accretion disc during the flare, consistent with an earlier suggestion that the accretion disc in GX 1+4 is alternating between retrograde and prograde rotation (Makishima et al. 1988).

Cui (1997) has presented observational evidence for the effect mentioned above where a decrease in accretion rate leads to the inner edge of the accretion disc being pushed outwards from the star until accretion is halted and pulsation stops. Cui derives from the observational results an estimate for the surface magnetic field strength for GX 1+4 of tex2html_wrap_inline208 (tex2html_wrap_inline210 T), accurate to within a factor of two. This estimate is consistent with earlier estimates for the surface field strength based on emission mechanism modelling and pulse phase spectroscopy (Greenhill et al. 1993) and by assuming the formation of a retrograde accretion disc in GX 1+4 (Dotani et al. 1989).

Our demonstration of a correlation between pulse shape and luminosity and the relation to spin-up/spin-down behaviour gives us a new tool with which to explore the accretion disc physics in this, and possibly other, systems. The RXTE results to be presented elsewhere (Giles et al. 1998) show the transition from leading-edge profile to trailing edge profile occurring over a short time period. The fact that the profile shape changed before the rise in luminosity is consistent with the hypothesis that there was an accompanying transition in disc rotation rate. A full analysis of the 1996 RXTE observations of GX 1+4 (Giles et al. 1998) should shed further light on the pulse morphology of GX 1+4.


Next Section: Acknowledgements
Title/Abstract Page: Luminosity dependent changes of
Previous Section: General features of GX
Contents Page: Volume 15, Number 2

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