Partial accretion in the propeller stage of low mass X-ray binary Aql X--1

TL;DR

This study analyzes the spectral and light curve data of Aql X--1's outbursts to understand the transition to a partial propeller stage, introducing a new method to estimate accretion fractions based on the fastness parameter.

Contribution

It presents a novel approach to infer the fraction of inflowing matter accreted onto the neutron star during the propeller transition using light curve analysis.

Findings

Different outbursts follow a similar path in accreted mass flux versus fastness parameter space.

The fastness parameter range indicates the disk's scale-height relative to the inner radius.

The method links spectral states to the disk-magnetosphere interaction modes.

Abstract

Aql X--1 is one of the most prolific low mass X-ray binary transients (LMXBTs) showing outbursts almost annually. We present the results of our spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts. We investigate the spectral changes related to the changing disk-magnetosphere interaction modes of Aql X--1. The X-ray light curves of the outbursts of LMXBTs typically show phases of fast rise and exponential decay. The decay phase shows a "knee" where the flux goes from the slow decay to the rapid decay stage. We assume that the rapid decay corresponds to a weak propeller stage at which a fraction of the inflowing matter in the disk accretes onto the star. We introduce a novel method for inferring, from the light curve, the fraction of the inflowing matter in the disk that accretes onto the NS depending on the fastness parameter. We determine the fastness parameter range within which the transition from the accretion to the partial propeller stage is realized. This fastness parameter range is a measure of the scale-height of the disk in units of the inner disk radius. We applied the method to a sample of outbursts of Aql X--1 with different maximum flux and duration times. We show that different outbursts with different maximum luminosity and duration follow a similar path in the parameter space of accreted/inflowing mass flux fraction versus fastness parameter.