Note on the effect of a massive accretion disk in the measurements of black hole spins

J. Cosmol. Astropart. Phys.

2016

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Abstract. The spacetime geometry around astrophysical black holes is supposed to be well approximated by the Kerr metric, but deviations from the Kerr solution are predicted in a number of scenarios involving new physics. Broad iron Kα lines are commonly observed in the X-ray spectrum of black holes and originate by X-ray fluorescence of the inner accretion disk. The profile of the iron line is sensitively affected by the spacetime geometry in the strong gravity region and can be used to test the Kerr black hole hypothesis. In this paper, we extend previous work in the literature. In particular: i) as test-metric, we employ the parametrization recently proposed by Konoplya, Rezzolla, and Zhidenko, which has a number of subtle advantages with respect to the existing approaches; ii) we perform simulations with specific X-ray missions, and we consider NuSTAR as a prototype of current observational facilities and eXTP as an example of the next generation of X-ray observatories. We find a significant difference between the constraining power of NuSTAR and eXTP. With NuSTAR, it is difficult or impossible to constrain deviations from the Kerr metric. With eXTP, in most cases we can obtain quite stringent constraints (modulo we have the correct astrophysical model).

Section: Introductionmentioning

confidence: 99%

Testing the Kerr metric with the iron line and the KRZ parametrization

Jiang

J. Cosmol. Astropart. Phys.

2016

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“…The strong field regime is another unexplored area and today there is an increasing interest in the possibility of testing the actual nature of astrophysical black hole (BH) candidates [3]. In classical general relativity, astrophysical BHs should be well described by the Kerr solution; for instance, the presence of accretion disks is usually completely negligible [4]. However, for the time being astrophysical BH candidates are just objects too heavy and compact to be neutron stars or clusters of neutron stars [5].…”

mentioning

confidence: 99%

Note on the Cardoso-Pani-Rico parametrization to test the Kerr black hole hypothesis

2014

Phys. Rev. D

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The construction of a generic parametrization to describe the spacetime geometry around astrophysical black hole candidates is an important step to test the Kerr black hole hypothesis. In the last few years, the Johannsen-Psaltis metric has been the most common framework to study possible deviations from the Kerr solution with present and near future observations. Recently, Cardoso, Pani and Rico have proposed a more general parametrization. The aim of the present paper is to study this new metric in a specific context, namely the thermal spectrum of geometrically thin and optically thick accretion disks. The most relevant finding is that the spacetime geometry around objects that look like very fast-rotating Kerr black holes may still have large deviations from the Kerr solution. This is not the case with the Johannsen-Psaltis metric, which means the latter is missing an important class of non-Kerr spacetimes.

“…If we can measure both M and a * of a Kerr BH, we know all the properties of the spacetime geometry. The effect of the accretion disk on the background metric is indeed negligible [4]. However, it is not easy to estimate the BH spin: the spin has no effects in Newtonian gravity and therefore it is necessary to probe the spacetime close to the object.…”

Section: Introductionmentioning

confidence: 99%

Attempt to explain black hole spin in X-ray binaries by new physics

2015

Eur. Phys. J. C

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It is widely believed that the spin of black holes in X-ray binaries is mainly natal. A significant spin-up from accretion is not possible. If the secondary has a low mass, the black hole spin cannot change too much even if the black hole swallows the whole stellar companion. If the secondary has a high mass, its lifetime is too short to transfer the necessary amount of matter and spin the black hole up. However, while black holes formed from the collapse of a massive star with solar metallicity are expected to have low birth spin, current spin measurements show that some black holes in X-ray binaries are rotating very rapidly. Here we show that, if these objects are not the Kerr black holes of general relativity, the accretion of a small amount of matter (∼2 M ) can make them look like very fast-rotating Kerr black holes. Such a possibility is not in contradiction with any observation and it can explain current spin measurements in a very simple way.