Title:	Discovery of a New Phase-transient Cyclotron Line in A0535+26: Constraints on the Accretion Geometry
Authors:	Devaraj, Ashwin Paul, Biswajit Bhalerao, Varun Mukherjee, Dipanjan
Keywords:	x-ray binary stars high mass x-ray binary stars x-ray astronomy binary pulsars neutron stars
Issue Date:	10-Mar-2025
Publisher:	IOP Publishing Ltd
Citation:	The Astrophysical Journal, 2025, Vol. 981, p205
Abstract:	In 2020 November, A0535+26 underwent one of its brightest outbursts, reaching nearly 12 Crab in X-ray flux. Observed by Insight-HXMT, NuSTAR, NICER, and 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 ∼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 (∼16%) across all seven 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.
Description:	Open Access
URI:	http://hdl.handle.net/2289/8386
Alternative Location:	https://arxiv.org/abs/2501.15537 https://inspirehep.net/literature/2872694 https://ui.adsabs.harvard.edu/abs/2025ApJ...981..205D/abstract https://doi.org/10.3847/1538-4357/adb0c2
Copyright:	2025 The Author(s)
Appears in Collections:	Research Papers (A&A)

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