Sidebands to mHz QPOs in 4U 1626$-$67 in the second spin-down state

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 ${\nu }_{\mathrm{NS}}\sim$130 mHz X-ray pulsations along with a prominent quasi-periodic oscillation (QPO) feature at ${\nu }_{\mathrm{QPO}}\sim$46 mHz with $\sim$20\% rms amplitude, which was positively correlated with energy. We also report the detection of sidebands to QPO occurring at a beat frequency (${\nu }_{\mathrm{NS}}-{\nu }_{\mathrm{QPO}}$) of $\sim$83 mHz with $\sim$8\% rms amplitude, having $>3\sigma$ detection significance. Additionally, we utilized $NuclearSpectroscopicTelescopeARray$ ($NuSTAR$) 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.

In press MNRAS

References in corpus (14)

• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Large Area X-ray Proportional Counter (LAXPC) Instrument on AstroSat and Some Preliminary Results from its performance in the orbit
• Rapid spectral and timing variability of Be/X-ray binaries during type II outbursts
• A study of the long term evolution of quasi periodic oscillations in the accretion powered X-ray pulsar 4U 1626-67
• Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR
• A broadband look of the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton
• LAXPC / AstroSat Study of ~ 1 and ~ 2 mHz Quasi-periodic Oscillations in the Be/X-ray Binary 4U 0115+63 During its 2015 Outburst
• Probing the Cyclotron line characteristics of 4U 1538-522 using AstroSat-LAXPC
• Broadband mHz QPOs and spectral study of LMC X$-$4 with AstroSat
• Detection of a quasi-periodic oscillation at $\sim$40 mHz in Cen X-3 with Insight-HXMT
• 4U 1626-67 Returns to Spin-Down: Timing features toe the line
• AstroSat detection of a mHz QPO and cyclotron line in IGR J19294+1816 during the 2019 outburst
• Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal
• The ongoing spin-down episode of 4U 1626-67

Cited by in corpus (2)

• The NuSTAR view of Ultra-Compact X-ray Binaries
• Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5-6932 during its 2024 Outburst