1,722 research outputs found
Relativistic Photon Mediated Shocks
A system of equations governing the structure of a steady, relativistic
radiation dominated shock is derived, starting from the general form of the
transfer equation obeyed by the photon distribution function. Closure is
obtained by truncating the system of moment equations at some order. The
anisotropy of the photon distribution function inside the shock is shown to
increase with increasing shock velocity, approaching nearly perfect beaming at
upstream Lorentz factors . Solutions of the shock equations are
presented for some range of upstream conditions. These solutions are shown to
converge as the truncation order is increased.Comment: 5 pages, a shorter version will appear in PR
The Kinematics of High Proper Motion Halo White Dwarfs
We analyse the kinematics of the entire spectroscopic sample of 99 recently
discovered high proper-motion white dwarfs by Oppenheimer et al. using a
maximum-likelihood analysis, and discuss the claim that the high-velocity white
dwarfs are members of a halo population with a local density at least ten times
greater than traditionally assumed. We argue that the observations, as
reported, are consistent with the presence of an almost undetected thin disc
plus a thick disc, with densities as conventionally assumed. In addition, there
is a kinematically distinct, flattened, halo population at the more than 99%
confidence level. Surprisingly, the thick disc and halo populations are
indistinguishable in terms of luminosity, color and apparent age (1-10 Gyr).
Adopting a bimodal, Schwarzschild model for the local velocity ellipsoid, with
the ratios sigma_U:sigma_V:sigma_W=1:2/3:1/2, we infer radial velocity
dispersions of sigma_U=62(+8/-10) km/s and 150(+80/-40) km/s (90% C.L.) for the
local thick disc and halo populations, respectively. The thick disc result
agrees with the empirical relation between asymmetric drift and radial velocity
dispersion, inferred from local stellar populations. The local thick-disc plus
halo density of white dwarfs is n^{td+h}=(1.9+-0.5)x10^-3 pc^-3 (90% C.L.), of
which n^{h}=1.1(+2.1/-0.7)x10^-4 pc^-3 (90% C.L.) belongs to the halo, a
density about five times higher than previously thought. (Abridged)Comment: 19 pages, 11 figures; submitted to MNRA
White Dwarfs: Contributors and Tracers of the Galactic Dark-Matter Halo
We examine the claim by Oppenheimer et al. (2001) that the local halo density
of white dwarfs is an order of magnitude higher than previously thought. As it
stands, the observational data support the presence of a kinematically distinct
population of halo white dwarfs at the >99% confidence level. A
maximum-likelihood analysis gives a radial velocity dispersion of
sigma^h_U=150(+80/-40) km/s and an asymmetric drift of v_a^h=176(+102/-80)
km/s, for a Schwarzschild velocity distribution function with
sigma_U:sigma_V:sigma_W=1:2/3:1/2. Halo white dwarfs have a local number
density of 1.1(+2.1/-0.7)x10^-4 pc-3, which amounts to 0.8(+1.6/-0.5) per cent
of the nominal local dark-matter halo density and is 5.0(+9.5/-3.2) times
higher and thus only marginally in agreement with previous estimates (all
errors indicate the 90% C.L.). We discuss several direct consequences of this
white-dwarf population (e.g. microlensing) and postulate a potential mechanism
to eject young white dwarfs from the disc to the halo, through the orbital
instabilities in triple or multiple stellar systems.Comment: 5 pages, to appear in the proceedings of the Yale Cosmology Workshop
"The Shapes of Galaxies and their Halos" (ed. Priya Natarajan); revised
numerical results, using a corrected likelihood function (thanks to David
Graff and Andy Gould); general conclusions remain simila
Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets
The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap
Thermo-Rotational Instability in Plasma Disks Around Compact Objects
Differentially rotating plasma disks, around compact objects, that are
imbedded in a ``seed'' magnetic field are shown to develop vertically localized
ballooning modes that are driven by the combined radial gradient of the
rotation frequency and vertical gradients of the plasma density and
temperature. When the electron mean free path is shorter than the disk height
and the relevant thermal conductivity can be neglected, the vertical particle
flows produced by of these modes have the effect to drive the density and
temperature profiles toward the ``adiabatic condition'' where
. Here is the plasma temperature and
the particle density. The faster growth rates correspond to steeper
temperature profiles such as those produced by an internal
(e.g., viscous) heating process. In the end, ballooning modes excited for
various values of can lead to the evolution of the disk into a
different current carrying configuration such as a sequence of plasma rings
The X-ray jet in the Crab Nebula: radical implications for pulsar theory?
The recent Chandra image of the Crab nebula shows a striking, axisymmetric
polar jet. It is shown that jets are formed in axisymmetric, magnetized pulsar
winds and that the jet luminosity scales relative to the total as
(\gamma_0\sigma_{eq})^{-4/3}, where \sigma_{eq} is the ratio of Poynting flux
to particle kinetic energy output at the equator at the base of the flow and
\gamma_0 the initial Lorentz factor of the flow. The results are applied to the
image of the Crab nebula, and the limit is set for the Crab pulsar of
\sigma_{eq} \leq 100. It is argued that conventional pulsar theory needs to be
reexamined in light of these limits.Comment: 13 page
Shading and Smothering of Gamma Ray Bursts
The gamma ray burst (GRB) 980425 is distinctive in that it seems to be
associated with supernova (SN) 1998bw, has no X-ray afterglow, and has a single
peak light curve and a soft spectrum. The supernova is itself unusual in that
its expansion velocity exceeds c/6. We suggest that many of these features can
be accounted for with the hypothesis that we observe the GRB along a penumbral
line of sight that contains mainly photons that have scattered off ejected
baryons. The hypothesis suggests a baryon poor jet (BPJ) existing within a
baryon rich outflow. The sharp distinction can be attributed to whether or not
the magnetic field lines thread an event horizon. Such a configuration suggests
that there will be some non-thermal acceleration of pick-up ex-neutrons within
the BPJ. This scenario might produce observable spallation products and
neutrinos.Comment: 7 pages, 2 figures, submitted to ApJ
The Prompt Gamma-Ray and Afterglow Energies of Short-Duration Gamma-Ray Bursts
I present an analysis of the gamma-ray and afterglow energies of the complete
sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80%
of the bursts exhibit a linear correlation between their gamma-ray fluence and
the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of
the blast wave ($F_{X,1}~F_{gamma}^1.01). An even tighter correlation is
evident between E_{gamma,iso} and L_{X,1} for the subset of 13 bursts with
measured or constrained redshifts. The remaining 20% of the bursts have values
of F_{X,1}/F_{gamma} that are suppressed by about three orders of magnitude,
likely because of low circumburst densities (Nakar 2007). These results have
several important implications: (i) The X-ray luminosity is generally a robust
proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a
circumburst density n>0.05 cm^{-3}; (ii) most short GRBs have a narrow range of
gamma-ray efficiency, with ~0.85 and a spread of 0.14 dex; and
(iii) the isotropic-equivalent energies span 10^{48}-10^{52} erg. Furthermore,
I find tentative evidence for jet collimation in the two bursts with the
highest E_{gamma,iso}, perhaps indicative of the same inverse correlation that
leads to a narrow distribution of true energies in long GRBs. I find no clear
evidence for a relation between the overall energy release and host galaxy
type, but a positive correlation with duration may be present, albeit with a
large scatter. Finally, I note that the outlier fraction of 20% is similar to
the proposed fraction of short GRBs from dynamically-formed neutron star
binaries in globular clusters. This scenario may naturally explain the
bimodality of the F_{X,1}/F_{gamma} distribution and the low circumburst
densities without invoking speculative kick velocities of several hundred km/s.Comment: Submitted to ApJ; 9 pages, 2 figures, 1 tabl
Nucleon-Nucleon Scattering in a Strong External Magnetic Field and the Neutrino Emissivity
The nucleon-nucleon scattering in a large magnetic background is considered
to find its potential to change the neutrino emissivity of the neutron stars.
For this purpose we consider the one-pion-exchange approximation to find the NN
cross-section in a background field as large as
. We show that the NN cross-section in
neutron stars with temperatures in the range 0.1-5 \texttt{MeV} can be changed
up to the one order of magnitude with respect to the one in the absence of the
magnetic field. In the limit of the soft neutrino emission the neutrino
emissivity can be written in terms of the NN scattering amplitude therefore the
large magnetic fields can dramatically change the neutrino emissivity of the
neutron stars as well.Comment: 21 pages, 5 figures, to appear in PR
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