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Please use this identifier to cite or link to this item: http://hdl.handle.net/2289/8384
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dc.contributor.authorSharma, Rahul-
dc.contributor.authorJain, Chetana-
dc.contributor.authorPaul, Biswajit-
dc.contributor.authorBeri, Aru-
dc.date.accessioned2025-03-19T05:37:04Z-
dc.date.available2025-03-19T05:37:04Z-
dc.date.issued2025-03-04-
dc.identifier.citationMonthly Notices of the Royal Astronomical Society, 2025, Vol. 538, p1046en_US
dc.identifier.urihttp://hdl.handle.net/2289/8384-
dc.descriptionOpen Accessen_US
dc.description.abstractWe 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.en_US
dc.language.isoenen_US
dc.publisherOxford University Press on behalf of Royal Astronomical Society.en_US
dc.relation.urihttps://arxiv.org/abs/2503.05444en_US
dc.relation.urihttps://ui.adsabs.harvard.edu/abs/2025MNRAS.538.1046S/abstracten_US
dc.relation.urihttps://inspirehep.net/literature/2898013en_US
dc.relation.urihttps://doi.org/10.1093/mnras/staf379en_US
dc.rights2025 The Author(s)en_US
dc.subjectaccretionen_US
dc.subjectaccretion discsen_US
dc.subjectstars: neutronen_US
dc.subjectX-rays: binariesen_US
dc.subjectX-rays: individual: 4U 1626−67en_US
dc.titleSidebands to mHz QPOs in 4U 1626-67 in the second spin-down stateen_US
dc.typeArticleen_US
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