Testing black hole candidates with electromagnetic radiation

Abstract

Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but there is not yet direct observational evidence that the spacetime geometry around these objects is described by the Kerr solution. The study of the properties of the electromagnetic radiation emitted by gas or stars orbiting these objects can potentially test the Kerr black hole hypothesis. This paper reviews the state of the art of this research field, describing the possible approaches to test the Kerr metric with current and future observational facilities and discussing current constraints.

Testing Einstein-dilaton-Gauss-Bonnet gravity with the reflection spectrum of accreting black holes

“…The intensity profile for a corona with arbitrary geometry is often approximated by a power law (∝ 1=r q , where q is the emissivity index) or by a broken power law (∝ 1=r q 1 for r < r br and ∝ 1=r q 2 for r > r br , where q 1 and q 2 are, respectively, the inner and the outer emissivity indices and r br is the breaking radius). Figure 2 shows the iron line shapes calculated in black hole solutions 1-12, assuming that the intensity profile scales as 1=r 3 (Newtonian limit at large radii for a lamppost corona), that the inclination angle of the disk with respect to the line of sight of the distant observer is i ¼ 45°, and that the rest frame energy of the line is 6.4 keV. The calculations are done with the code described in Refs.…”

“…There are two main lines of research to test the nature of astrophysical black holes: the study of the properties of the electromagnetic radiation emitted by gas or stars orbiting these objects [3,4] or the analysis of the gravitational wave signal emitted by a system with a black hole [5,6]. Tests with electromagnetic radiation include, but are not limited to, the study of the thermal spectrum of thin accretion disks [7][8][9][10], the analysis of the reflection spectrum of thin disks [11][12][13][14], the measurements of the frequencies of quasiperiodic oscillations [15][16][17][18], and the possible future detection of black hole shadows [19][20][21][22][23][24][25].…”

Testing General Relativity with the Reflection Spectrum of the Supermassive Black Hole in 1H0707-495

“…The study of the properties of the electromagnetic radiation emitted by the gas in the accretion disk can potentially probe the spacetime metric around astrophysical black holes and test the Kerr nature of these objects [4]. Previous work has shown that x-ray reflection spectroscopy (the so-called iron line method) [5] is a promising technique to do this job [6].…”

On the Calculation of the Fe K-alpha Line Emissivity of Black Hole Accretion Disks

“…We close with a brief summary and outlook in Section 3. Although we limit the following discussion to General Relativity's Kerr metric in Boyer-Lindquist (BL Boyer & Lindquist 1967) coordinates, the results can easily be adapted to work for other stationary, axisymmetric, and asymptotically flat black hole spacetimes (see Johannsen 2016;Bambi 2017, for recent reviews describing observational constraints on alternative black hole spacetimes).…”