Testing the Kerr Nature of Black Hole Candidates Using Iron Line Spectra in the CPR Framework

Abstract

The iron Kα line commonly observed in the X-ray spectrum of both stellar-mass and supermassive black hole (BH) candidates originates from X-ray fluorescence of the inner accretion disk. Accordingly, it can be used to map the spacetime geometry around these objects. In this paper, we extend previous work using the iron Kα line to test the Kerr BH hypothesis. We adopt the Cardoso-Pani-Rico parametrization and we test the possibility of constraining possible deviations from the Kerr solution that can be obtained from observations across the range of BH spins and inclination angles. We confirm previous claims that the iron Kα line is potentially a quite powerful probe for testing the Kerr metric given sufficiently high quality data and with systematics under control, especially in the case of fast-rotating BHs and high inclination angles since both conditions serve to maximize relativistic effects. We find that some geometric perturbations from Kerr geometry manifest more strongly in the iron line profile than others. While the perturbation parameter t 3  can be well constrained by the iron line profile, an orthogonal data set is necessary to constrain departures from Kerr geometry in .

Search for astrophysical rotating Ellis wormholes with x-ray reflection spectroscopy

“…This technique has been applied to different scenarios and, compared with other approaches, can also provide strong constraints, see for instance, Refs. [26][27][28][29].…”

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

“…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]. Among these techniques, x-ray reflection spectroscopy is the only one that can be already used to test astrophysical black holes and promise to be able to provide stringent constraints with the next generation of x-ray facilities [26][27][28].…”

Testing the Kerr nature of black hole candidates using iron line reverberation mapping in the Cardoso-Pani-Rico framework

“…In the case of a rotating BH, it is still true that g tt is only altered by t 3 and that g rr is only altered by r 3 , but now both the deformation parameters enter g tφ and g φφ in a non-trivial way. We found that the CPR deformation parameter t 3 is relatively easy to constrain with a high-count spectrum, while r 3 is much more elusive and even the iron line of a fast-rotating Kerr BH may be mimicked by a slowrotating non-Kerr object with a large positive r 3 [17]. Since r 3 mainly affects the photon propagation, there is reason to expect that the time information provided by reverberation might significantly improve the constraint.…”

“…[17]. In the non-rotating limit, t 3 only alters the metric coefficient g tt , while r 3 only the metric coefficient g rr .…”

“…The thermal spectrum of thin accretion disks [11,12] has a simple shape, and, in general, it is difficult to measure the spin parameter while simultaneously constraining possible deviations from the Kerr solution [13]. The iron Kα technique [14,15] is potentially more powerful, but the limitation preventing current observations from being used to test the Kerr paradigm is a combination of the limited signal strength and the degree of sophistication of present models [16,17]. Current data can exclude that BH candidates are exotic dark stars [18] or traversable wormholes [19], because the iron line does not match in the available data.…”