25,373 research outputs found
Quantum nature of cyclotron harmonics in thermal spectra of neutron stars
Some isolated neutron stars show harmonically spaced absorption features in
their thermal soft X-ray spectra. The interpretation of the features as a
cyclotron line and its harmonics has been suggested, but the usual explanation
of the harmonics as caused by relativistic effects fails because the
relativistic corrections are extremely small in this case. We suggest that the
features correspond to the peaks in the energy dependence of the free-free
opacity in a quantizing magnetic field, known as quantum oscillations. The
peaks arise when the transitions to new Landau levels become allowed with
increasing the photon energy; they are strongly enhanced by the square-root
singularities in the phase-space density of quantum states in the case when the
free (non-quantized) motion is effectively one-dimensional. To explore
observable properties of these quantum oscillations, we calculate models of
hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e.,
electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such
conditions are thought to be typical for the so-called central compact objects
in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that
observable features at the electron cyclotron harmonics form at moderately
large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 -
20. The equivalent widths of the features can reach 100 - 200 eV; they grow
with increasing b_{eff} and are lower for higher harmonics.Comment: 6 pages; shortened, references updated; published in Ap
The Young Pulsar J1357-6429 and Its Pulsar Wind Nebula
We observed the young pulsar J1357--6429 with the {\it Chandra} and {\it
XMM-Newton} observatories. The pulsar spectrum fits well a combination of
absorbed power-law model () and blackbody model
( eV, km at the distance of 2.5 kpc). Strong
pulsations with pulsed fraction of , apparently associated with the
thermal component, were detected in 0.3--1.1 keV. Surprisingly, pulsed fraction
at higher energies, 1.1--10 keV, appears to be smaller, . The small
emitting area of the thermal component either corresponds to a hotter fraction
of the neutron star (NS) surface or indicates inapplicability of the simplistic
blackbody description. The X-ray images also reveal a pulsar-wind nebula (PWN)
with complex, asymmetric morphology comprised of a brighter, compact PWN
surrounded by the fainter, much more extended PWN whose spectral slopes are
and , respectively. The extended PWN with
the observed flux of erg s cm is a factor
of 10 more luminous then the compact PWN. The pulsar and its PWN are located
close to the center of the extended TeV source HESS J1356--645, which strongly
suggests that the VHE emission is powered by electrons injected by the pulsar
long ago. The X-ray to TeV flux ratio, , is similar to those of other
relic PWNe. We found no other viable candidates to power the TeV source. A
region of diffuse radio emission, offset from the pulsar toward the center of
the TeV source, could be synchrotron emission from the same relic PWN rather
than from the supernova remnant.Comment: 18 pages, 12 figures, and 4 tables, Accepted for publication in Ap
CMS barrel Resistive Plate Chambers - tests and results
CMS experiment will use Resistive Plate Chambers (RPCs) as dedicated muon
trigger detectors. The first 24 RPCs for the barrel muon system of the CMS
experiment have been assembled and tested. A brief description of the cosmic
muon telescope used for the tests is given. The preliminary results for the
chamber performance are presented.Comment: Poster session on the XXIII Physics in Collisions Conference (PIC03),
Zeuthen, Germany, June 2003, 3 pages, LaTeX, 3 eps figures. PSN FRAP0
Differential Geometry of Hydrodynamic Vlasov Equations
We consider hydrodynamic chains in dimensions which are Hamiltonian
with respect to the Kupershmidt-Manin Poisson bracket. These systems can be
derived from single equations, here called hydrodynamic Vlasov
equations, under the map For these
equations an analogue of the Dubrovin-Novikov Hamiltonian structure is
constructed. The Vlasov formalism allows us to describe objects like the
Haantjes tensor for such a chain in a much more compact and computable way. We
prove that the necessary conditions found by Ferapontov and Marshall in
(arXiv:nlin.SI/0505013) for the integrability of these hydrodynamic chains are
also sufficient.Comment: 24 page
The jets of the Vela pulsar
Chandra observations of the Vela pulsar-wind nebula (PWN) have revealed a jet
in the direction of the pulsar's proper motion, and a counter-jet in the
opposite direction, embedded in diffuse nebular emission. The jet consists of a
bright, 8''-long inner jet, between the pulsar and the outer arc, and a dim,
curved outer jet that extends up to 100'' in approximately the same direction.
From the analysis of thirteen Chandra observations spread over about 2.5
years we found that this outer jet shows particularly strong variability,
changing its shape and brightness. We observed bright blobs in the outer jet
moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on
time-scales of days to weeks. The spectrum of the outer jet fits a power-law
model with a photon index of 1.3\pm0.1. The X-ray emission of the outer jet can
be interpreted as synchrotron radiation of ultrarelativistic
electrons/positrons. This interpretation allows one to estimate the magnetic
field, ~100 microGauss, maximum energy of X-ray emitting electrons, ~2\times
10^{14} eV, and energy injection rate, ~8\times 10^{33} erg/s, for the outer
jet. In the summed PWN image we see a dim, 2'-long outer counter-jet, which
also shows a power-law spectrum with photon ined of 1.2-1.5. Southwest of the
jet/counter-jet an extended region of diffuse emission is seen. Relativistic
particles responsible for this radiation are apparently supplied by the outer
jet.Comment: 4 pages, including 1 figure, accepted for publication in New
Astronomy Reviews; proceedings of the conference "The Physics of Relativistic
Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologna. The full
resolution versions of the images shown in the fugure are avaliable at
http://www.astro.psu.edu/users/green/vela_jet_proc/vela_jet_proc.htm
Shifts of finite type with nearly full entropy
For any fixed alphabet A, the maximum topological entropy of a Z^d subshift
with alphabet A is obviously log |A|. We study the class of nearest neighbor
Z^d shifts of finite type which have topological entropy very close to this
maximum, and show that they have many useful properties. Specifically, we prove
that for any d, there exists beta_d such that for any nearest neighbor Z^d
shift of finite type X with alphabet A for which log |A| - h(X) < beta_d, X has
a unique measure of maximal entropy. Our values of beta_d decay polynomially
(like O(d^(-17))), and we prove that the sequence must decay at least
polynomially (like d^(-0.25+o(1))). We also show some other desirable
properties for such X, for instance that the topological entropy of X is
computable and that the unique m.m.e. is isomorphic to a Bernoulli measure.
Though there are other sufficient conditions in the literature which guarantee
a unique measure of maximal entropy for Z^d shifts of finite type, this is (to
our knowledge) the first such condition which makes no reference to the
specific adjacency rules of individual letters of the alphabet.Comment: 33 pages, accepted by Proceedings of the London Mathematical Societ
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