X-Ray and Optical Observations of A 0535+26

TL;DR

This paper presents multi-wavelength observations of the Be/X-ray binary A 0535+26, revealing the neutron star's interaction with the Be disk, peculiar pulse profile evolution, and correlations between optical and X-ray activity over decades.

Contribution

It provides new contemporaneous X-ray and optical data, analyzes the neutron star-Be disk interaction, and identifies patterns in disk variability and outburst timing.

Findings

Large Be disk before the 2009 outburst may explain X-ray activity.

Detected a 30-70 mHz X-ray QPO that weakens at lower energies.

Observed a correlation between optical brightness and X-ray activity, with specific anti-correlations post-2002.

Abstract

We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A\,0535+26 with the \textit{Fermi}/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since $\sim$1978) and discussed in terms of the neutron star Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray QPO was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25\,keV. In the long-term a strong optical/X-ray correlation was found for this system, however in the medium-term the H$_\alpha$ EW and the V-band brightness showed an anti-correlation after $\sim$2002 Agust. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H$_\alpha$ showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H$_\alpha$ line. These had a period of $\sim$\,25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning $\sim$\,6 months following a giant outburst.