Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608--52 and Aql X--1

Abstract

We detect millihertz quasi-periodic oscillations (mHz QPOs) using the Rossi X-ray Time Explorer (RXTE) from the atoll neutron-star (NS) low-mass X-ray binaries 4U 1608--52 and Aql X--1. From the analysis of all RXTE observations of 4U 1608--52 and Aql X--1, we find mHz QPOs with a significance level $>3\sigma$ in 49 and 47 observations, respectively. The QPO frequency is constrained between $\sim$ 4.2 and 13.4 mHz. These types of mHz QPOs have been interpreted as being the result of marginally stable nuclear burning of He on the NS surface. We also report the discovery of a downward frequency drift in three observations of 4U 1608--52, making it the third source that shows this behaviour. We only find strong evidence of frequency drift in one occasion in Aql X--1, probably because the observations were too short to measure a significant drift. Finally, the mHz QPOs are mainly detected when both sources are in the soft or intermediate states; the cases that show frequency drift only occur when the sources are in intermediate states. Our results are consistent with the phenomenology observed for the NS systems 4U 1636--53 and EXO 0748--676, suggesting that all four sources can reach the conditions for marginally stable burning of He on the NS surface. These conditions depend on the source state in the same manner in all four systems.

8 pages, 5 figures, accepted for publication by MNRAS

References in corpus (12)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Discovery of coherent millisecond X-ray pulsations in Aql X-1
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• First superburst from a classical low-mass X-ray binary transient
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• Millihertz quasi-periodic oscillations in 4U 1636-53 associated with bursts with positive convexity only
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• Maxi observations of long X-ray bursts
• Discovery of a soft X-ray 8 mHz QPO from the accreting millisecond pulsar IGR J00291+5934
• XMM-Newton and NICER measurement of the rms spectrum of the millihertz quasi-periodic oscillations in the neutron-star low-mass X-ray binary 4U 1636-53

Cited by in corpus (5)

• Discovery of transition from marginally stable burning to unstable burning after a superburst in Aql X-1
• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER
• A Temporal Scale to Track the Spectral Transitions in Low-Mass X-ray Binaries
• Thermonuclear Type-I X-ray Bursts and Burst Oscillations from the Eclipsing AMXP Swift J1749.4-2807
• Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT