15 research outputs found
Effective sound speed in relativistic accretion discs around rotating black holes
For axially symmetric accretion maintained in the hydrostatic equilibrium
along the vertical direction in the Kerr metric, the radial Mach number does
not become unity at the critical point. The sonic points are, thus, formed at a
radial distance different from that where the critical points are formed. We
propose that a modified dynamical sound speed can be defined through the linear
perturbation of the full space-time dependent equations describing the
aforementioned accretion flow structure. The linear stability analysis of such
fluid equations leads to the formation of an wave equation which describes the
propagation of linear acoustic perturbation. The speed of propagation of such
perturbation can be used as the effective sound speed which makes the value of
the Mach number to be unity when evaluated at the critical points. This allows
the critical points to coalesce with the sonic points. We study how the spin
angular momentum of the black hole (the Kerr parameter) influences the value of
the effective sound speed
Dependence of acoustic surface gravity on disc thickness for accreting astrophysical black holes
For axially symmetric accretion maintained in hydrostatic equilibrium along
the vertical direction, we investigate how the characteristic features of the
embedded acoustic geometry depends on the background Kerr metric, and how such
dependence is governed by three different expressions of the thickness of the
matter flow. We first obtain the location of the sonic points and stationary
shock between the sonic points. We then linearly perturb the flow to obtain the
corresponding metric elements of the acoustic space-time. We thus construct the
causal structure to establish that the sonic points and the shocks are actually
the analogue black hole type and white hole type horizons, respectively. We
finally compute the value of the acoustic surface gravity as a function of the
spin angular momentum of the rotating black hole for three different flow
thicknesses considered in the present work. We find that for some flow models,
the intrinsic acoustic geometry, although in principle may be extended up to
the outer gravitational horizon of the astrophysical black hole, cannot be
constructed beyond a certain truncation radius as imposed by the expressions of
the thickness function of the corresponding flow.Comment: 22 pages, 9 figure
Carter-Penrose diagrams for emergent spacetime in axisymmetrically accreting black hole systems
For general relativistic, inviscid, axisymmetric flow around Kerr black hole
one may choose different flow thickness. The stationary flow equations can be
solved using methods of dynamical system to get transonic accretion flows ,
i.e, flow infalling in the blackhole that turns supersonic from subsonic with
decreasing radial distance, or vice versa. This transonic flows are obtained by
choosing the particular flow passing through critical points of phase portrait.
For certain flow thickness like the one maintaining conical shape, the sonic
point coincide with the critical point. But there are certain flows maintaining
hydrostatic equilibrium, such as the one described by Novikov-Thorne, where the
sonic point is not same as the critical point. We perturb the flow for both
kind of flow and study the behaviour of linear perturbation which behaves like
massless scalar field in some curved spacetime, known as, analogue space time.
We draw the compactified causal structure, i.e, Penrose Carter diagram for both
kind of analogue metric and prove that for both cases critical points are the
acoustic horizons, whereas in the case where sonic points do not coincide with
critical points, the sonic points are not the acoustic horizon, as one may
expect from the definition of sound speed.Comment: arXiv admin note: text overlap with arXiv:1811.0497
Challenges in QCD matter physics - The Compressed Baryonic Matter experiment at FAIR
Substantial experimental and theoretical efforts worldwide are devoted to
explore the phase diagram of strongly interacting matter. At LHC and top RHIC
energies, QCD matter is studied at very high temperatures and nearly vanishing
net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was
created at experiments at RHIC and LHC. The transition from the QGP back to the
hadron gas is found to be a smooth cross over. For larger net-baryon densities
and lower temperatures, it is expected that the QCD phase diagram exhibits a
rich structure, such as a first-order phase transition between hadronic and
partonic matter which terminates in a critical point, or exotic phases like
quarkyonic matter. The discovery of these landmarks would be a breakthrough in
our understanding of the strong interaction and is therefore in the focus of
various high-energy heavy-ion research programs. The Compressed Baryonic Matter
(CBM) experiment at FAIR will play a unique role in the exploration of the QCD
phase diagram in the region of high net-baryon densities, because it is
designed to run at unprecedented interaction rates. High-rate operation is the
key prerequisite for high-precision measurements of multi-differential
observables and of rare diagnostic probes which are sensitive to the dense
phase of the nuclear fireball. The goal of the CBM experiment at SIS100
(sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD
matter: the phase structure at large baryon-chemical potentials (mu_B > 500
MeV), effects of chiral symmetry, and the equation-of-state at high density as
it is expected to occur in the core of neutron stars. In this article, we
review the motivation for and the physics programme of CBM, including
activities before the start of data taking in 2022, in the context of the
worldwide efforts to explore high-density QCD matter.Comment: 15 pages, 11 figures. Published in European Physical Journal