NICER / ISS Science Nugget
for July 9, 2020

Type-B QPO detected in NICER observation of MAXI J1348-630

The fast variability observed in the X-ray emission from black-hole (BH) binaries exhibits complex behavior but offers the possibility of directly investigating the accretion flow closest to the BH event horizon. In particular, quasi-periodic "type-B" brightness oscillations (QPOs) in the 2-8 Hz range are thought to be associated with radiation from a relativistic jet emitted perpendicular to the disk of accreting material(see figures).

Artist's impression of a low-mass X-ray binary

Figure 1: Artist's representation of a Low-mass X-ray Binary, where there is mass transfer from the companion star towards the black hole. Not all of the gas falls into the black hole; some is expelled in a perpendicular jet.

The BH binary MAXI J1348-630, discovered by the JAXA ISS payload MAXI on 26 January 2019, was observed by NICER nearly 200 times between the discovery date and 7 February 2020. Timing and spectral analysis of the NICER data in the brightest part of the system's outburst feature a strong type-B QPO at approximately 4.5 Hz and reveal, for the first time, the dependence of the QPO's fractional amplitude and phase on X-ray photon energy, especially below 2 keV: the QPO amplitude decreases from more than 10% at 9 keV to 0.6% at 1.5 keV, and is constant below that energy; with the 2-3 keV band as reference, photons at all other energies show a hard phase lag, increasing with the distance from the reference band.

The energy spectrum can be fitted with a standard model for this state, consisting of a thin disk component and a harder power law, plus an emission line between 6 and 7 keV. Taken together, the results are readily interpreted within a Comptonization model (up-scattering of lower-energy photons in a "corona" of hot plasma surrounding the BH), and form the basis for new numerical simulations to study the interaction between the accretion disk, the corona, and the jet.

Power spectrum and phase lag for the Type-B QPO in MAXI J1348-630

Figure 2: Left: The interaction between disk and jet is thought to produce a quasi-periodic oscillation (QPO) of a kind known as "Type-B" seen at approximately 4.5 Hz in the power spectrum figure. Right: With NICER's unique spectral-timing capabilities, we were able to measure the amplitude of the QPO as a function of energy, from < 1 keV to 12 keV (upper panel). We were also able to measure the phase lag of the oscillation as a function of energy (lower panel). The phase lag can be converted to time lags (delays observed between photons of different energy), and are interpreted as Comptonization of low-energy photons in a hot cloud of electrons (the so-called "corona") that surrounds the black hole.

This peer-reviewed work, led by Tomaso Belloni (INAF, Italy) was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society.

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