202 research outputs found
Convergence and stability theorems for the Picard-Mann hybrid iterative scheme for a general class of contractive-like operators
In this paper we use the general class of contractive-like operators introduced by
Bosede and Rhoades (J. Adv. Math. Stud. 3(2):1-3, 2010) to prove strong convergence
and stability results for Picard-Mann hybrid iterative schemes considered in a real
normed linear space. We establish the strong convergence and stability of the Picard
iterative scheme as a corollary. Our results generalize and improve a multitude of
results in the literature, including the recent results of Chidume (Fixed Point Theory
Appl. 2014:233, 2014)
X-ray spectropolarimetric measurements of the Kerr metric
It is thought that the spacetime geometry around black hole candidates is
described by the Kerr solution, but an observational confirmation is still
missing. Today, the continuum-fitting method and the analysis of the iron
K line cannot unambiguously test the Kerr paradigm because of the
degeneracy among the parameters of the system, in the sense that it is
impossible with current X-ray data to distinguish a Kerr black hole from a
non-Kerr object with different values of the model parameters. In this paper,
we study the possibility of testing the Kerr nature of black hole candidates
with X-ray spectropolarimetric measurements. Within our simplified model that
does not include the effect of returning radiation, we find that it is
impossible to test the Kerr metric and the problem is still the strong
correlation between the spin and possible deviations from the Kerr geometry.
Moreover, the correlation is very similar to that of other two techniques,
which makes the combination of different measurements not very helpful.
Nevertheless, our results cannot be taken as conclusive and, in order to arrive
at a final answer, the effect of returning radiation should be properly taken
into account.Comment: 12 pages, 8 figures. v2: refereed versio
Accreting Black Holes
This chapter provides a general overview of the theory and observations of
black holes in the Universe and on their interpretation. We briefly review the
black hole classes, accretion disk models, spectral state classification, the
AGN classification, and the leading techniques for measuring black hole spins.
We also introduce quasi-periodic oscillations, the shadow of black holes, and
the observations and the theoretical models of jets.Comment: 41 pages, 18 figures. To appear in "Tutorial Guide to X-ray and
Gamma-ray Astronomy: Data Reduction and Analysis" (Ed. C. Bambi, Springer
Singapore, 2020). v3: fixed some typos and updated some parts. arXiv admin
note: substantial text overlap with arXiv:1711.1025
New Insights into X-ray Binaries
X-ray binaries are excellent laboratories to study collapsed objects. On the
one hand, transient X-ray binaries contain the best examples of stellar-mass
black holes while persistent X-ray binaries mostly harbour accreting neutron
stars. The determination of stellar masses in persistent X-ray binaries is
usually hampered by the overwhelming luminosity of the X-ray heated accretion
disc. However, the discovery of high-excitation emission lines from the
irradiated companion star has opened new routes in the study of compact
objects. This paper presents novel techniques which exploits these irradiated
lines and summarises the dynamical masses obtained for the two populations of
collapsed stars: neutron stars and black holes.Comment: 12 pages, 5 figures, 2 tables, Invited review to plenary session in
"Highlights of Spanish Astrophysics V", Proceedings of the VIII Scientific
Meeting of the Spanish Astronomical Society (SEA) held in Santander, 7-11
July, 2008. Edited by J. Gorgas, L. J. Goicoechea, J. I. Gonzalez-Serrano, J.
M. Dieg
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