Spectral line

Spectral density

Turbulence

Statistics

Computational physics

Radiation

Astrophysics

Photon

Optical depth

Physics

Mathematics

Optics

Astronomy

Flow (mathematics)

Aerosol

Stratified flow

Meteorology

Black-body radiation

Mechanics

Accretion (finance)

Binary number

Accretion disc

Stratified flows

Arithmetic

Monthly Notices of the Royal Astronomical Society

ABSTRACT We study the structure of the accretion flow in the hard state of the black hole X-ray binary MAXI J1820+070 with NICER data. The power spectra show broad-band variability which can be fit with four Lorentzian components peaking at different time-scales. Extracting power spectra as a function of energy enables the energy spectra of these different power spectral components to be reconstructed. We found significant spectral differences among Lorentzians, with the one corresponding to the shortest variability time-scales displaying the hardest spectrum. Both the variability spectra and the time-averaged spectrum are well-modelled by a disc blackbody and thermal Comptonization, but the presence of (at least) two Comptonization zones with different temperatures and optical depths is required. The disc blackbody component is highly variable, but only in the variability components peaking at the longest time-scales ($\lower.5ex\hbox{$\,\, \buildrel\gt \over \sim \,\,$}1$ s). The seed photons for the spectrally harder zone come predominantly from the softer Comptonization zone. Our results require the accretion flow in this source to be structured, and cannot be described by a single Comptonization region upscattering disc blackbody photons, and reflection from the disc.

A spectrally stratified hot accretion flow in the hard state of MAXI J1820+070