7,170 research outputs found
Variations in the Cyclotron Resonant Scattering Features during 2011 outburst of 4U 0115+63
We study the variations in the Cyclotron Resonant Scattering Feature (CRSF)
during 2011 outburst of the high mass X-ray binary 4U 0115+63 using
observations performed with Suzaku, RXTE, Swift and INTEGRAL satellites. The
wide-band spectral data with low energy coverage allowed us to characterize the
broadband continuum and detect the CRSFs. We find that the broadband continuum
is adequately described by a combination of a low temperature (kT ~ 0.8 keV)
blackbody and a power-law with high energy cutoff (Ecut ~ 5.4 keV) without the
need for a broad Gaussian at ~ 10 keV as used in some earlier studies. Though
winds from the companion can affect the emission from the neutron star at low
energies (< 3 keV), the blackbody component shows a significant presence in our
continuum model. We report evidence for the possible presence of two
independent sets of CRSFs with fundamentals at ~ 11 keV and ~ 15 keV. These two
sets of CRSFs could arise from spatially distinct emitting regions. We also
find evidence for variations in the line equivalent widths, with the 11 keV
CRSF weakening and the 15 keV line strengthening with decreasing luminosity.
Finally, we propose that the reason for the earlier observed anti-correlation
of line energy with luminosity could be due to modelling of these two
independent line sets (~ 11 keV and ~ 15 keV) as a single CRSF.Comment: 12 pages, 8 figures (4 in colour), 6 tables. Accepted for publication
in MNRAS. Typos corrected, Figure 8 changed and some changes to draf
First law of black hole mechanics in Einstein-Maxwell and Einstein-Yang-Mills theories
The first law of black hole mechanics is derived from the Einstein-Maxwell
(EM) Lagrangian by comparing two infinitesimally nearby stationary black holes.
With similar arguments, the first law of black hole mechanics in
Einstein-Yang-Mills (EYM) theory is also derived.Comment: Modified version, major changes made in the introduction. 14 pages,
no figur
Tracing very high energy neutrinos from cosmological distances in ice
Astrophysical sources of ultrahigh energy neutrinos yield tau neutrino fluxes
due to neutrino oscillations. We study in detail the contribution of tau
neutrinos with energies above PeV relative to the contribution of the other
flavors. We consider several different initial neutrino fluxes and include tau
neutrino regeneration in transit through the Earth and energy loss of charged
leptons. We discuss signals of tau neutrinos in detectors such as IceCube, RICE
and ANITA.Comment: 27 pages, 19 figure
Magnetic anomalies in Gd6Co1.67Si3 and Tb6Co1.67Si3
The compounds, Gd6Co1.67Si3 and Tb6Co1.67Si3, recently reported to form in a
Ce6Ni2Si3-derived hexagonal structure (space group: P6_3 / m) and to order
magnetically below 295 and 190 K respectively, have been investigated by
detailed magnetization (M) studies in the temperature interval 1.8-330 K as a
function of magnetic field (H). The points of emphasis are: We observe multiple
steps in the M(H) curve for the Tb compound at 1.8 K while increasing H, but
these steps do not appear in the reverse cycle of H. At higher temperatures,
such steps are absent. However, this 'staircase' behavior of M(H) is not
observed for the Gd compound at any temperature and the isothermal
magnetization is not hysteretic unlike in Tb compound. From the M(H) data
measured at close intervals of temperature, we have derived isothermal entropy
change (Delta S) and it is found that Delta S follows a theoretically predicted
H^2/3-dependence
Homogeneous Relaxation at Strong Coupling from Gravity
Homogeneous relaxation is a ubiquitous phenomenon in semiclassical kinetic
theories where the quasiparticles are distributed uniformly in space, and the
equilibration involves only their velocity distribution. For such solutions,
the hydrodynamic variables remain constant. We construct asymptotically AdS
solutions of Einstein's gravity dual to such processes at strong coupling,
perturbatively in the amplitude expansion, where the expansion parameter is the
ratio of the amplitude of the non-hydrodynamic shear-stress tensor to the
pressure. At each order, we sum over all time derivatives through exact
recursion relations. We argue that the metric has a regular future horizon,
order by order in the amplitude expansion, provided the shear-stress tensor
follows an equation of motion. At the linear order, this equation of motion
implies that the metric perturbations are composed of zero wavelength
quasinormal modes. Our method allows us to calculate the non-linear corrections
to this equation perturbatively in the amplitude expansion. We thus derive a
special case of our previous conjecture on the regularity condition on the
boundary stress tensor that endows the bulk metric with a regular future
horizon, and also refine it further. We also propose a new outlook for
heavy-ion phenomenology at RHIC and ALICE.Comment: 60 pages, a section titled "Outlook for RHIC and ALICE" has been
added, accepted for publication in Physical Review
Entropy of Constant Curvature Black Holes in General Relativity
We consider the thermodynamic properties of the constant curvature black hole
solution recently found by Banados. We show that it is possible to compute the
entropy and the quasilocal thermodynamics of the spacetime using the
Einstein-Hilbert action of General Relativity. The constant curvature black
hole has some unusual properties which have not been seen in other black hole
spacetimes. The entropy of the black hole is not associated with the event
horizon; rather it is associated with the region between the event horizon and
the observer. Further, surfaces of constant internal energy are not isotherms
so the first law of thermodynamics exists only in an integral form. These
properties arise from the unusual topology of the Euclidean black hole
instanton.Comment: 4 pages LaTeX2e (RevTeX), 2 PostScript figures. Small corrections in
the text and the reference
A Mott Glass to Superfluid Transition for Random Bosons in Two Dimensions
We study the zero temperature superfluid-insulator transition for a
two-dimensional model of interacting, lattice bosons in the presence of
quenched disorder and particle-hole symmetry. We follow the approach of a
recent series of papers by Altman, Kafri, Polkovnikov, and Refael, in which the
strong disorder renormalization group is used to study disordered bosons in one
dimension. Adapting this method to two dimensions, we study several different
species of disorder and uncover universal features of the superfluid-insulator
transition. In particular, we locate an unstable finite disorder fixed point
that governs the transition between the superfluid and a gapless, glassy
insulator. We present numerical evidence that this glassy phase is the
incompressible Mott glass and that the transition from this phase to the
superfluid is driven by percolation-type process. Finally, we provide estimates
of the critical exponents governing this transition.Comment: (24 pages + 7 page appendix, 28 figures) This version has been
accepted to PRB. We have acquired new data that resolves the contradiction
between two estimates of the critical exponents in the earlier version of the
pape
On the Noether charge form of the first law of black hole mechanics
The first law of black hole mechanics was derived by Wald in a general
covariant theory of gravity for stationary variations around a stationary black
hole. It is formulated in terms of Noether charges, and has many advantages. In
this paper several issues are discussed to strengthen the validity of the
Noether charge form of the first law. In particular, a gauge condition used in
the derivation is justified. After that, we justify the generalization to
non-stationary variations done by Iyer-Wald.Comment: Latex, 16 pages, arguments on gauge conditions and near-stationary
entropy are added, accepted for publication in Physical Review
Quasi-normal modes of charged, dilaton black holes
In this paper we study the perturbations of the charged, dilaton black hole,
described by the solution of the low energy limit of the superstring action
found by Garfinkle, Horowitz and Strominger. We compute the complex frequencies
of the quasi-normal modes of this black hole, and compare the results with
those obtained for a Reissner-Nordstr\"{o}m and a Schwarzschild black hole. The
most remarkable feature which emerges from this study is that the presence of
the dilaton breaks the \emph{isospectrality} of axial and polar perturbations,
which characterizes both Schwarzschild and Reissner-Nordstr\"{o}m black holes.Comment: 15 pages, 5 figure
Lagrangian perfect fluids and black hole mechanics
The first law of black hole mechanics (in the form derived by Wald), is
expressed in terms of integrals over surfaces, at the horizon and spatial
infinity, of a stationary, axisymmetric black hole, in a diffeomorphism
invariant Lagrangian theory of gravity. The original statement of the first law
given by Bardeen, Carter and Hawking for an Einstein-perfect fluid system
contained, in addition, volume integrals of the fluid fields, over a spacelike
slice stretching between these two surfaces. When applied to the
Einstein-perfect fluid system, however, Wald's methods yield restricted
results. The reason is that the fluid fields in the Lagrangian of a gravitating
perfect fluid are typically nonstationary. We therefore first derive a first
law-like relation for an arbitrary Lagrangian metric theory of gravity coupled
to arbitrary Lagrangian matter fields, requiring only that the metric field be
stationary. This relation includes a volume integral of matter fields over a
spacelike slice between the black hole horizon and spatial infinity, and
reduces to the first law originally derived by Bardeen, Carter and Hawking when
the theory is general relativity coupled to a perfect fluid. We also consider a
specific Lagrangian formulation for an isentropic perfect fluid given by
Carter, and directly apply Wald's analysis. The resulting first law contains
only surface integrals at the black hole horizon and spatial infinity, but this
relation is much more restrictive in its allowed fluid configurations and
perturbations than that given by Bardeen, Carter and Hawking. In the Appendix,
we use the symplectic structure of the Einstein-perfect fluid system to derive
a conserved current for perturbations of this system: this current reduces to
one derived ab initio for this system by Chandrasekhar and Ferrari.Comment: 26 pages LaTeX-2
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