2,524 research outputs found
Phenomenology of Gamma-Ray Jets
We discuss some phenomenological aspects of -ray emitting jets. In
particular, we present calculations of the -sphere and -sphere for
various target photon fields, and employ them to demonstrate how -ray
observations at very high energies can be used to constraint the Doppler factor
of the emitting plasma and the production of VHE neutrinos. We also consider
the implications of the rapid TeV variability observed in M87 and the TeV
blazars, and propose a model for the very rapid TeV flares observed with HESS
and MAGIC in some blazars,that accommodates the relatively small Doppler
factors inferred from radio observations. Finally, we briefly discuss the
prospects for detecting VHE neutrinos from relativistic jets.Comment: Proceedings, Huangshan meeting on "Astrophysics of Compact Objects
Possible evidence that pulsars are quark stars
It is a pity that the real state of matter in pulsar-like stars is still not
determined confidently because of the uncertainty about cold matter at
supranuclear density, even 40 years after the discovery of pulsar. Nuclear
matter (related to neutron stars) is one of the speculations for the inner
constitution of pulsars even from the Landau's time more than 70 years ago, but
quark matter (related to quark stars) is an alternative due to the fact of
asymptotic freedom of interaction between quarks as the standard model of
particle physics develops since 1960s. Therefore, one has to focus on
astrophysical observations in order to answer what the nature of pulsars is. In
this presentation, I would like to summarize possible observational
evidence/hints that pulsar-like stars could be quark stars, and to address
achievable clear evidence for quark stars in the future experiments.Comment: 6 pages, 2 figures; a talk at the international conference
"Astrophysics of Compact Objects" (July 1-7, 2007; Huangshan, China);
http://vega.bac.pku.edu.cn/rxxu/publications/index_C.htm. A mistake in Fig.1
is corrected; Correction of typo
Dynamics of Magnetized Spherical Accretion Flows
Transonic accretion flow with self-consistent treatment of random magnetic
field is presented.Comment: in proceedings to "Astrophysics of Compact Objects", Huangshan,
China, 200
"Black Star" or Astrophysical Black Hole?
Recently wide publicity has been given to a claim by T. Vachaspati that
"black holes do not exist", that the objects known as black holes in
astrophysics should rather be called "black stars" and they not only do not
have event horizons but actually can be the source of spectacular gamma ray
bursts. In this short essay (no flimsier than the original preprint where these
extravagant claims appeared) I demonstrate that these ill-considered claims are
clearly wrong. Yet they present a good occasion to reflect on some well known
but little discussed conceptual difficulties which arise when applying
relativistic terminology in an astrophysical context.Comment: Poster presented at "Compact Objects" meeting in Hunagshan, China,
2-7 July 2007. To be published in the AIP Conference Proceeding serie
Phase-locked scroll waves defy turbulence induced by negative filament tension
Scroll waves in a three-dimensional media may develop into turbulence due to negative tension of the filament. Such negative tension-induced instability of scrollwaves has been observed in the Belousov-Zhabotinsky reaction systems. Here we propose a method to restabilize scroll wave turbulence caused by negative tension in three-dimensional chemical excitable media using a circularly polarized (rotating) external field. The stabilization mechanism is analyzed in terms of phase-locking caused by the external field, which makes the effective filament tension positive. The phase-locked scrollwaves that have positive tension and higher frequency defy the turbulence and finally restore order. A linear theory for the change of filament tension caused by a generic rotating external field is presented and its predictions closely agree with numerical simulations
A simple proof of exponential decay in the two dimensional percolation model
Kesten showed the exponential decay of percolation probability in the
subcritical phase for the two-dimensional percolation model. This result
implies his celebrated computation that for bond percolation in the
square lattice, and site percolation in the triangular lattice, respectively.
In this paper, we present a simpler proof for Kesten's theorem.Comment: 9 pages and one figur
Neutron star magnetospheres: the binary pulsar, Crab and magnetars
A number of disparate observational and theoretical pieces of evidence
indicate that, contrary to the conventional wisdom, neutron stars' closed field
lines are populated by dense, hot plasma and may be responsible for producing
some radio and high energy emission. This conclusion is based on eclipse
modeling of the binary pulsar system PSR J0737-3039A/B (Lyutikov & Thompson
2005), a quantitative theory of Crab giant pulses (Lyutikov 2007) and a number
of theoretical works related to production of non-thermal spectra in magnetars
through resonant scattering. In magnetars, dense pair plasma is produced by
twisting magnetic field lines and associated electric fields required to lift
the particles from the surface. In long period pulsars, hot particles on closed
field lines can be efficiently trapped by magnetic mirroring, so that
relatively low supply rate, e.g. due to a drift from open field lines, may
result in high density. In short period pulsars, magnetic mirroring does not
work; large densities may still be expected at the magnetic equator near the
Y-point.Comment: Proceedings, Huangshan meeting "Astrophysics of Compact Objects
Density alteration in non-physiological cells
In the present study an important phenomenon of cells was discovered: the change of intracellular density in cell's response to drug and environmental factors. For convenience, this phenomenon is named as "density alteration in non-physiological cells" ( DANCE). DANCE was determined by discontinuous sucrose gradient centrifugation (DSGC), in which cells were separated into several bands. The number and position of the bands in DSGC varied with the change of cell culture conditions, drugs, and physical process, indicating that cell's response to these factors was associated with alteration of intracellular density. Our results showed that the bands of cells were molecularly different from each other, such as the expression of some mRNAs. For most cells tested, intracellular density usually decreased when the cells were in bad conditions, in presence of drugs, or undergoing pathological changes. However, unlike other tissue cells, brain cells showed increased intracellular density in 24 hrs after the animal death. In addition, DANCE was found to be related to drug resistance, with higher drug-resistance in cells of lower intracellular density. Further study found that DANCE also occurred in microorganisms including bacteria and fungus, suggesting that DANCE might be a sensitive and general response of cells to drugs and environmental change. The mechanisms for DANCE are not clear. Based on our study the following causes were hypothesized: change of metabolism mode, change of cell membrane function, and pathological change. DANCE could be important in medical and biological sciences. Study of DANCE might be helpful to the understanding of drug resistance, development of new drugs, separation of new subtypes from a cell population, forensic analysis, and importantly, discovery of new physiological or pathological properties of cells
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