Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations

Abstract

The accretion-ejection activities of black holes play a vital role in shaping the Universe. Bright and recurrent black hole X-ray binaries are ideal objects for studying accretion physics across a wide range of accretion rates, providing insights into the understanding of their supermassive counterparts. This short review summarizes X-ray techniques capable of measuring accretion geometry, our current understanding, and open questions. In particular, X-ray spectroscopic studies indicate that the accretion disk can extend close to the innermost stable circular orbit in the bright hard state. Some hints of disk-corona-jet connections are also discussed.

Figures (7)

• Figure 1: (Left) Artist's impression of a black hole X-ray binary system (courtesy of JPL/NASA). (Middle) A zoom-in sketch of the inner accretion flow near a BH. Arrows with different colors represent emission components: (red) the disk thermal emission; (blue) the coronal emission; (green) the reflected emission of the disk. (Right) The Insight--HXMT observation of the low mass BH XRB GX 339--4 in the intermediate state on 2021 March 27. The red, blue and green lines show spectral components corresponding to the middle panel. The sum is shown as the black solid line. Figure adapted from Liu2024thesis.
• Figure 2: Lightcurves of a transient (GX 339-4) and a persistent (Cygnus X-1) BH XRBs by the Monitor of All-sky X-ray Image (MAXI) in the 2--20 keV band.
• Figure 3: (Left) Hardness-intensity diagrams of three recent outbursts from transient BH XRBs plotted using MAXI data. The dashed line denotes the launch of ballistic jets. Figure adapted from Liu2024thesis. (Right) X-ray spectra of different states from NuSTAR and NICER observations of MAXI J1820+070.
• Figure 4: (Left) The broad iron line profiles as a function of the inner disk radius ($R_{\rm in}$). The profiles are calculated using the relline model assuming a black spin $a_*=0.998$, an inclination angle $i=30^{\circ}$, an power-law emissivity profile with the index $q=3$ and an outer disk radius $R_{\rm out}=400~R_{\rm g}$. (Right) Rest-frame reflection (black) and relativistic reflection (red) spectra for an accretion disk extending to the innermost circular orbit.
• Figure 5: (Upper Left) Lag-frequency spectrum between the 0.5--1 keV and 1--10 keV bands for the black hole X-ray binary MAXI J1820+070 from a NICER observation. (Upper Right) Lag-energy spectrum from the same data in the 0.1-1.0 Hz frequency range, using the 0.5--10 keV band as a reference. (Lower) Lag-energy spectrum in the 2-20 Hz frequency range.