Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730–22 with NICER

Abstract

We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs) from the neutron star (NS) low-mass X-ray binary 4U 1730–22 using the Neutron Star Interior Composition Explorer (NICER). After being inactive for almost 50 years, 4U 1730–22 went into outburst twice between June and August 2021, and between February and July 2022. We analyse all the NICER observations of this source, and detect mHz QPOs with a significance > 4 σ in 35 observations. The QPO frequency of the full data set ranged between ∼ 4.5 and ∼ 8.1 mHz with an average fractional rms amplitude of the order of ∼2 per cent. The X-ray colour analysis strongly suggests that 4U 1730–22 was in a soft spectral state during the QPO detections. Our findings are consistent with those reported for other sources where the mHz QPOs have been interpreted as the result of a special mode of He burning on the NS surface called marginally stable nuclear burning (MSNB). We conclude that the mHz QPOs reported in this work are also associated with the MSNB, making 4U 1730–22 the eighth source that shows this phenomenology. We discuss our findings in the context of the heat flux from the NS crust.

X-Ray and Radio Monitoring of the Neutron Star Low-mass X-Ray Binary 1A 1744-361: Quasiperiodic Oscillations, Transient Ejections, and a Disk Atmosphere

“…Atoll sources show a variety of QPO phenomena, including mHz QPOs (e.g., Revnivtsev et al 2001;Heger et al 2007;Mancuso et al 2023), ∼1 Hz QPOs seen in some atolls exhibiting absorption dips (e.g., Homan et al 1999;Jonker et al 1999), and hecto-Hz QPOs (e.g., Ford & van der Klis 1998;Altamirano et al 2008;Bult & van der Klis 2015). A class of atoll sources also show NBO-like QPOs near the highest source luminosities; they include XTE J1806−246, the ultracompact X-ray binary 4U 1820-30, and the transient source Aql X−1 Reig et al 2004).…”