Accretion geometry and spectral evolution in 1A 1118-61: a comparison of the 2009 and 2026 outbursts

TL;DR

This study analyzes the 2026 outburst of the Be X-ray binary pulsar 1A 1118-61 using Swift and NuSTAR, revealing spectral and timing evolution, including pulsations, QPOs, and a stable cyclotron line, compared to the 2009 outburst.

Contribution

It provides the first detailed comparison of the spectral and timing properties of 1A 1118-61 across two major outbursts, highlighting changes in accretion geometry.

Findings

Detection of coherent pulsations at ~400 s with energy-dependent profiles.

Observation of a transient QPO at ~0.11 Hz during the rising phase.

Spectral hardening at higher luminosities and a nearly constant cyclotron line energy.

Abstract

We present a detailed spectro-temporal study of the Be X-ray binary pulsar $1A$ $1118-61$ during its brightest recorded outburst in 2026, using \textit{Swift} and \textit{NuSTAR} observations, and compare its properties with the 2009 outburst. Coherent pulsations at $\sim400$ s are detected throughout the outburst, with pulse profiles evolving strongly with energy and luminosity, indicating changes in emission geometry. A transient quasi-periodic oscillation (QPO) at $\sim$0.11 Hz is observed during the rising phase. The luminosity dependence of the QPO frequency during the current and previous outbursts suggests an origin associated with instabilities near the magnetospheric radius. The broadband spectra are well described by thermal Comptonization and show clear spectral hardening at higher luminosities. A cyclotron line is detected in the two \textit{NuSTAR} observations, with its energy remaining nearly constant despite a factor of $\sim25$ change in luminosity. Long-term monitoring reveals that the 2026 outburst is systematically harder and brighter, suggesting significant difference in the accretion structure and emission regions between the two outbursts.