Time lags of the type-B quasi-periodic oscillation in MAXI J1348−630

• Published in: Monthly notices of the Royal Astronomical Society Vol. 496; no. 4; pp. 4366 - 4371
• Main Authors: Belloni, Tomaso M, Zhang, Liang, Kylafis, Nikolaos D, Reig, Pablo, Altamirano, Diego
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.08.2020
• Subjects: stars: jets; X-rays: individual: GRS 1915+105; stars: black holes; X-rays: binaries; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/staa1843

ABSTRACT The fast variability observed in the X-ray emission from black hole binaries has a very complex phenomenology, but offers the possibility to investigate directly the properties of the inner accretion flow. In particular, type-B oscillations in the 2–8 Hz range, observed in the soft-intermediate state, have been associated with the emission from a relativistic jet. We present the results of the timing and spectral analysis of a set of observations of the bright transient MAXI J1348−630 made with the NICER (Neutron Star Interior Composition Explorer) telescope. The observations are in the brightest part of the outburst and all feature a strong type-B quasi-periodic oscillation (QPO) at ∼4.5 Hz. We compute the energy dependence of the fractional rms and the phase lags at the QPO frequency, obtaining high signal-to-noise data and sampling for the first time at energies below 2 keV. The fractional rms decreases from more than 10 per cent at 9 keV to 0.6 per cent at 1.5 keV, and is constant below that energy. Taking the 2–3 keV band as reference, photons at all energies show a hard lag, increasing with the distance from the reference band. The behaviour below 2 keV has never been observed before, due to the higher energy bandpass of previous timing instruments. The energy spectrum can be fitted with a standard model for this state, consisting of a thin disc component and a harder power law, plus an emission line between 6 and 7 keV. We discuss the results, concentrating on the phase lags, and show that they can be interpreted within a Comptonization model.