Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748−676. (10th May 2019)

• Record Type: Journal Article
• Authors: Mancuso, G C; Altamirano, D; García, F; Lyu, M; Méndez, M; Combi, J A; Díaz-Trigo, M; in't Zand, J J M
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 486:Number 1(2019)
• Page Start: L74
• Page End: L79
• Publication Date: 2019-05-10
• Subjects: accretion, accretion discs -- stars: individual: EXO 0748−676 -- stars: neutron -- X-rays: binaries; Astronomy -- Research -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnrasl/slz057
• Languages: English
• ISSNs: 1745-3933

Abstract

Abstract: We report the discovery of millihertz quasi-periodic oscillations (mHz QPOs) from the bursting, high-inclination atoll neutron star low-mass X-ray binary (NS LMXB) EXO 0748−676 with the Rossi X-ray Time Explorer (RXTE). This class of QPO, originally discovered in three NS LMXBs, has been interpreted as a consequence of a special mode of nuclear burning on the NS surface. Using all the RXTE archival observations of the source, we detected significant (>3σ) mHz QPOs in 11 observations. The frequency of the oscillations was between ∼5 and ∼13 mHz. We also found a decrease of the QPO frequency with time in two occasions; in one of these the oscillations disappeared with the onset of an X-ray burst, similar to what was reported in other sources. Our analysis of the X-ray colours revealed that EXO 0748−676 was in a soft spectral state when it exhibited the QPOs. This makes EXO 0748−676 the sixth source with mHz oscillations associated with marginally stable burning, and the second one that shows a systematic frequency drift. Our results suggest that the mechanism that produces the drift might always be present if the mHz QPOs are observed in the so-called intermediate state.