Discovery of a new phase-transient cyclotron line in A0535+26: Constraints on the accretion geometry

TL;DR

This paper reports the discovery of a phase-transient cyclotron line in A0535+26 during a bright outburst, revealing complex accretion geometry and magnetic field interactions in a Be/X-ray binary.

Contribution

It presents the first detection of a phase-transient low-energy cyclotron line in A0535+26, providing new insights into the accretion geometry and magnetic field structure.

Findings

Detection of a phase-transient cyclotron line at ~16% pulse phase

Observation of dramatic variations of the line with pulse phase and luminosity

Evidence supporting a sweeping accretion column geometry

Abstract

In November 2020, A0535+26 underwent one of its brightest outbursts, reaching nearly 12 Crab in X-ray flux. Observed by \textit{Insight-HXMT}, \textit{NuSTAR}, \textit{NICER}, and \textit{AstroSat}, this event provided valuable insights into Be/X-ray binaries. The pulse profiles evolved significantly with luminosity, transitioning from pencil-beam to fan-beam geometries. A0535+26, known for its fundamental cyclotron line at $\sim$44 keV, became only the second source to exhibit a negative correlation between cyclotron line energy and flux at high luminosities, with a plateau phase preceding the transition from positive to negative correlation. We report the discovery of a phase-transient low-energy cyclotron line, detected in a narrow phase range ($\sim$16\%) across all seven \textit{NuSTAR} observations during the rising, peak, and declining phases of the outburst. The new line exhibited dramatic variations with pulse phase and luminosity. We explain this behavior using an accretion geometry where the accretion column sweeps across the line of sight.