Title:	Sidebands to mHz QPOs in 4U 1626-67 in the second spin-down state
Authors:	Sharma, Rahul Jain, Chetana Paul, Biswajit Beri, Aru
Keywords:	accretion accretion discs stars: neutron X-rays: binaries X-rays: individual: 4U 1626−67
Issue Date:	4-Mar-2025
Publisher:	Oxford University Press on behalf of Royal Astronomical Society.
Citation:	Monthly Notices of the Royal Astronomical Society, 2025, Vol. 538, p1046
Abstract:	We report results from an AstroSat target-of-opportunity (ToO) observation of 4U 1626−67, performed on 2023 May 18, soon after the discovery of torque reversal to spin-down in the source. The X-ray emission exhibited significant dependence on both energy and torque state. This work highlights the comparison of timing features of 4U 1626−67 with a previous AstroSat observation from 2018, when the neutron star was in the spin-up state. The power density spectrum (PDS) of the 2023 observation comprised a sharp peak corresponding to 130 mHz X-ray pulsations along with a prominent quasi-periodic oscillation (QPO) feature at 46 mHz with 20 per cent rms amplitude, which was positively correlated with energy. We also report the detection of sidebands to QPO occurring at a beat frequency (⁠⁠) of 83 mHz with 8 per cent rms amplitude, having detection significance. Additionally, we utilized Nuclear Spectroscopic Telescope ARray (NuSTAR) observations from the same torque state (2023 May–July) to analogize the presence and energy dependence of sidebands. The source retains timing properties in this spin-down torque state, similar to those seen in the previous spin-down phase. In sharp contrast, PDS from the 2018 observation was dominated by red noise, an absence of QPOs and a broadening in the wings of the pulse frequency peak, indicating a coupling between periodic and low-frequency aperiodic variability. Furthermore, we detected the known cyclotron resonance scattering feature (CRSF) at 37 keV in the Large Area X-ray Proportional Counter (LAXPC) spectrum. We explore various mechanisms that could possibly explain the presence of QPOs exclusively during the spin-down state.
Description:	Open Access
URI:	http://hdl.handle.net/2289/8384
Alternative Location:	https://arxiv.org/abs/2503.05444 https://ui.adsabs.harvard.edu/abs/2025MNRAS.538.1046S/abstract https://inspirehep.net/literature/2898013 https://doi.org/10.1093/mnras/staf379
Copyright:	2025 The Author(s)
Appears in Collections:	Research Papers (A&A)

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