Precursors and Outbursts of A 0535+26 in 2009-2011 observed by the MAXI/GSC and the Swift/BAT

TL;DR

This study monitors the outburst behavior of the Be/X-ray binary A 0535+26 over three years, identifying nine outbursts, precursors, and analyzing their periodicity and possible disc structures.

Contribution

It provides new insights into the recurrence periods and precursor phenomena of outbursts, suggesting disc structures influence outburst timing.

Findings

Nine outbursts observed over three years.

Precursor activity detected before some giant outbursts.

Recurrence period of ~115 days, longer than orbital period.

Abstract

Over the 3-year active period from 2008 September to 2011 November, the outburst behavior of the Be/X-ray binary A 0535+26 was continuously monitored with the MAXI/GSC and the Swift/BAT. The source exhibited nine outbursts, every binary revolution of 111.1 days, of which two are categorized into the giant (type-II) outbursts. The recurrence period of these outbursts is found to be $\sim115$ days, significantly longer than the orbital period of 111.1 days. With the MAXI/GSC, a low-level active period, or a "precursor", was detected prior to at least four giant outbursts. The precursor recurrence period agrees with that of the giant outbursts. The period difference of the giant outbursts from the orbital period is possibly related with some structures in the circumstellar disc formed around the Be companion. Two scenarios, one based on a one-armed disc structure and the other a Be-disc precession, are discussed.