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

Mancuso, G. C., Altamirano, D., García, F., Lyu, M., Méndez, M., Combi, J. A., Díaz-Trigo, M. & in't Zand, J. J. M., 1-Jun-2019, In : Monthly Notices of the Royal Astronomical Society: Letters. 486, 1

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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.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
Publication statusPublished - 1-Jun-2019


  • accretion, accretion discs, stars: individual: EXO 0748-676, stars: neutron, X-rays: binaries

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