3,057 research outputs found
On gravitational wave-Cherenkov radiation from photons when passing through diffused dark matters
Analogy to Cherenkov radiation, when a particle moves faster than the
propagation velocity of gravitational wave in matter (), we
expect gravitational wave-Cherenkov radiation (GWCR). In the situation that a
photon travels across diffuse dark matters, the GWCR condition is always
satisfied, photon will thence loss its energy all the path. This effect is long
been ignored in the practice of astrophysics and cosmology, without
justification with serious calculation. We study this effect for the first
time, and shows that this energy loss time of the photon is far longer than the
Hubble time, therefore justify the practice of ignoring this effect in
astrophysics context.Comment: 3 pages; We add a citation to Caves (1980) to the original journal
version, after kindly reminde
A new approach to the GeV flare of PSR B1259-63/LS2883
PSR B1259-63/LS2883 is a binary system composed of a pulsar and a Be star.
The Be star has an equatorial circumstellar disk (CD). The {\it Fermi}
satellite discovered unexpected gamma-ray flares around 30 days after the last
two periastron passages. The origin of the flares remain puzzling. In this
work, we explore the possibility that, the GeV flares are consequences of
inverse Compton-scattering of soft photons by the pulsar wind. The soft photons
are from an accretion disk around the pulsar, which is composed by the matter
from CD captured by the pulsar's gravity at disk-crossing before the
periastron. At the other disk-crossing after the periastron, the density of the
CD is not high enough so that accretion is prevented by the pulsar wind shock.
This model can reproduce the observed SEDs and light curves satisfactorily.Comment: 14 pages, 8 figures, 1 table. Accepted for publication in Ap
Probing the properties of the pulsar wind via studying the dispersive effects in the pulses from the pulsar companion in a double neutron-star binary system
The velocity and density distribution of in the pulsar wind are
crucial distinction among magnetosphere models, and contains key parameters
determining the high energy emission of pulsar binaries. In this work, a direct
method is proposed, which might probe the properties of the wind from one
pulsar in a double-pulsar binary. When the radio signals from the first-formed
pulsar travel through the relativistic flow in the pulsar wind from the
younger companion, the components of different radio frequencies will be
dispersed. It will introduce an additional frequency-dependent time-of-arrival
delay of pulses, which is function of the orbital phase. In this paper, we
formulate the above-mentioned dispersive delay with the properties of the
pulsar wind. As examples, we apply the formula to the double pulsar system PSR
J0737-3039A/B and the pulsar-neutron star binary PSR B1913+16. For PSR
J0737-3039A/B, the time delay in 300\,MHz is s near the
superior-conjunction, under the optimal pulsar wind parameters, which is
half of the current timing accuracy. For PSR B1913+16, with the assumption that
the neutron star companion has a typical spin down luminosity of
\,ergs/s, the time delay is as large as s in 300\,MHz.
The best timing precision of this pulsar is s in 1400\,MHz.
Therefore, it is possible that we can find this signal in archival data.
Otherwise, we can set an upper-limit on the spin down luminosity. Similar
analysis can be apply to other eleven known pulsar-neutron star binariesComment: 6 pages, 6 figures, accepted for publication in MNRA
Modeling the GeV emission of HESS J0632+057
The binary system HESS J0632+057 was recently detected by {Fermi} to possess
orbital modulated GeV emission. In this paper, we study the possibility that
the compact companion of HESS J0632+057 is a pulsar. Under such a presumption,
we focus on the high energy emission mechanism of this system, which is as
follows. The pulsar companion travels through the circumstellar disc of the
main sequence star twice in each orbit, when some of the matter is
gravity-captured. The captured matter develops an accretion disc around the
pulsar, and the soft photons from which are inverse Compton scattered by the
pulsar wind as the GeV emission from the system. With proper choice of
parameters, SED and light curve which are in accordance with observations can
be produced. We predict that the light curve of GeV emission has two peaks, the
larger one is at around 0.4 after the periastron (or 0.1 after the X-ray
maximum), while the smaller one is between phases 0 and 0.1, with integrated
flux one forth of the larger one.Comment: 7pages, 7 figures. Accepted for publication in MNRA
Extraordinary focusing of sound above a soda can array without time reversal
Recently, Lemoult et al. [Phys. Rev. Lett. 107, 064301 (2011)] used time
reversal to focus sound above an array of soda cans into a spot much smaller
than the acoustic wavelength in air. In this study, we show that equally sharp
focusing can be achieved without time reversal, by arranging transducers around
a nearly circular array of soda cans. The size of the focal spot at the center
of the array is made progressively smaller as the frequency approaches the
Helmholtz resonance frequency of a can from below, and, near the resonance,
becomes smaller than the size of a single can. We show that the locally
resonant metamaterial formed by soda cans supports a guided wave at frequencies
below the Helmholtz resonance frequency. The small focal spot results from a
small wavelength of this guided wave near the resonance in combination with a
near field effect making the acoustic field concentrate at the opening of a
can. The focusing is achieved with propagating rather than evanescent waves. No
sub-diffraction-limited focusing is observed if the diffraction limit is
defined with respect to the wavelength of the guided mode in the metamaterial
medium rather than the wavelength of the bulk wave in air
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