The millisecond X-ray pulsar/burster SAX J1808.4-3658: the outburst light curve and the power law spectrum

TL;DR

This paper analyzes the spectral and light curve properties of the millisecond X-ray pulsar SAX J1808.4-3658 during its 1998 outburst, revealing a stable power-law spectrum and discussing possible mechanisms and magnetic field constraints.

Contribution

It provides detailed spectral analysis and insights into the outburst behavior, including the stability of the spectrum and the potential mechanisms behind spectral formation and accretion inhibition.

Findings

Spectral stability with a photon index of ~2 during outburst

Power law spectrum with a lower limit on e-folding energy ~270 keV

Evidence for centrifugal inhibition of accretion at low luminosity

Abstract

The X-ray light curve and broad band spectral properties of the millisecond X-ray pulsar/burster SAX J1808.4-3658 are reported. In the course of RXTE observations in April--May 1998 the 3--150 keV luminosity of the source decreased by a factor of ~100 from the peak value of ~9E36 erg/s (for a 4 kpc distance). However, the spectrum was remarkably stable and maintained a roughly power law shape with a photon index of ~2 without a strong high energy cut-off below 100 keV, similar to that sometimes observed in the low spectral state of X-ray bursters. An approximation of the averaged spectrum with an exponentially cut-off power law with a superimposed reflected component yields the 90% lower limit on the e-folding energy ~270 keV. We speculate that Comptonization on the bulk motion in the radiation dominated shock might be a possible mechanism of spectral formation. The decaying part of the X-ray light curve was cut off abruptly at luminosity ~few 1E35 erg/s. Such behavior might be due to centrifugal inhibition of accretion (transition to the ``propeller'' regime) as well as to disk instability. In either case an upper limit on the neutron star magnetic field strength is B < few 1E7 Gauss.