Modelling the outburst profile of X-ray powered millisecond pulsars

TL;DR

This paper models the decay profiles of X-ray outbursts in millisecond pulsars, linking light curve features to accretion disc radii, enabling parameter estimation even for distant sources.

Contribution

It introduces a method to estimate accretion disc radii from X-ray light curves, improving distance and system parameter determinations for millisecond pulsars.

Findings

Strong agreement between two independent disc radius estimates for specific pulsars

New distance and disc radius measurements derived from light curve analysis

Potential to determine disc radii in extragalactic sources

Abstract

The outbursts of low mass X-ray binaries are prolonged relative to those of dwarf nova cataclysmic variables as a consequence of X-ray irradiation of the disc. We show that the time-scale of the decay light curve and its luminosity at a characteristic time are linked to the radius of the accretion disc. Hence a good X-ray light curve permits two independent estimates of the disc radius. In the case of the millisecond pulsars SAX J1808.4-3658 and XTE J0929-314 the agreement between these estimates is very strong. Our analysis allows new determinations of distances and accretion disc radii. Our analysis will allow determination of accretion disc radii for sources in external galaxies, and hence constrain system parameters where other observational techniques are not possible. We also use the X-ray light curves to estimate the mass transfer rate. The broken exponential decay observed in the 2002 outburst of SAX J1808.4-3658 may be caused by the changing self-shadowing of the disc.