46 research outputs found
Energy Extraction from Spinning Stringy Black Holes
We perform the first numerical simulations modeling the inflow and outflow of
magnetized plasma in the Kerr-Sen spacetime, which describes classical spinning
black holes (BHs) in string theory. We find that the Blandford-Znajek (BZ)
mechanism, which is believed to power astrophysical relativistic outflows or
``jets'', is valid even for BHs in an alternate theory of gravity, including
near the extremal limit. The BZ mechanism releases outward
Poynting-flux-dominated plasma as frame-dragging forces magnetic field lines to
twist. However, for nonspinning BHs, where the frame-dragging is absent, we
find an alternate powering mechanism through the release of gravitational
potential energy during accretion. Outflows from non-spinning stringy BHs can
be approximately more powerful as compared to Schwarzschild BHs, due to
their relatively smaller event horizon sizes and, thus, higher curvatures.
Finally, by constructing the first synthetic images of near-extremal non-Kerr
BHs from time-dependent simulations, we find that these can be ruled out by
horizon-scale interferometric images of accreting supermassive BHs.Comment: To be submitted to journal. Comments are welcom
On the Universality of Energy Extraction from Black Hole Spacetimes
The launching of astrophysical jets provides the most compelling
observational evidence for direct extraction of black hole (BH) spin energy via
the Blandford-Znajek (BZ) mechanism. Whilst it is know that spinning Kerr BHs
within general relativity (GR) follow the BZ jet power relation, the nature of
BH energy extraction in general theories of gravity has not been adequately
addressed. This study performs the first comprehensive investigation of the BZ
jet power relation by utilising a generalised BH spacetime geometry which
describes parametric deviations from the Kerr metric of GR, yet recovers the
Kerr metric in the limit that all deviation parameters vanish. Through
performing and analysing an extensive suite of three-dimensional covariant
magnetohydrodynamics (MHD) simulations of magnetised gas accretion onto these
generalised BH spacetimes we find that the BZ jet power relation still holds,
in some instances yielding jet powers far in excess of what can be produced by
even extremal Kerr BHs. It is shown that the variation of the quadrupole moment
of the BH can enhance or suppress the effects of BH spin, and by extension of
frame-dragging. This variation greatly enhances or suppresses the observed jet
power and underlying photon ring image asymmetry, introducing a previously
unexplored yet important degeneracy in BH parameter inference.Comment: To be submitted to journal. Comments are welcom
On the stability of a superspinar
The superspinar proposed by Gimon and Horava is a rapidly rotating compact
entity whose exterior is described by the over-spinning Kerr geometry. The
compact entity itself is expected to be governed by superstringy effects, and
in astrophysical scenarios it can give rise to interesting observable
phenomena. Earlier it was suggested that the superspinar may not be stable but
we point out here that this does not necessarily follow from earlier studies.
We show, by analytically treating the Teukolsky equations by Detwiler's method,
that in fact there are infinitely many boundary conditions that make the
superspinar stable, and that the modes will decay in time. It follows that we
need to know more on the physical nature of the superspinar in order to decide
on its stability in physical reality.Comment: 5 page
Constraints on black-hole charges with the 2017 EHT observations of M87*
Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes