Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748−676

• Published in: Monthly notices of the Royal Astronomical Society Vol. 486; no. 1; pp. L74 - L79
• Main Authors: Mancuso, G C, Altamirano, D, García, F, Lyu, M, Méndez, M, Combi, J A, Díaz-Trigo, M, in’t Zand, J J M
• Format: Journal Article
• Language: English
• Published: Oxford University Press (OUP): Policy P - Oxford Open Option A 01.06.2019
• Subjects: Astrophysics; Physics; accretion; X-rays: binaries; stars: individual: EXO 0748−676; accretion discs; stars: neutron
• ISSN: 1745-3925; 0035-8711; 1745-3933; 1365-2966
• DOI: 10.1093/mnrasl/slz057

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.