87,697 research outputs found
Ultra High Energy Cosmic Rays: Strangelets? -- Extra dimensions, TeV-scale black holes and strange matter
The conjecture that ultra high energy cosmic rays (UHECRs) are actually
strangelets is discussed. Besides the reason that strangelets can do as cosmic
rays beyond the GZK-cutoff, another argument to support the conjecture is
addressed in this letter via the study of formation of TeV-scale microscopic
black holes when UHECRs bombarding bare strange stars. It is proposed that the
exotic quark surface of a bare strange star could be an effective
astro-laboratory in the investigations of the extra dimensions and of the
detection of ultra-high energy neutrino fluxes. The flux of neutrinos (and
other point-like particles) with energy >2.3 x 10^{20} eV could be expected to
be smaller than 10^{-26} cm^{-2}$ s^{-1} if there are two extra spatial
dimensions.Comment: accepted by Chin. Phys. Lett., or at
http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.ht
GRB 060206: hints of precession of the central engine?
Aims. The high-redshift (z=4.048) gamma-ray burst GRB 060206 showed unusual behavior, with a significant rebrightening by a factor of ~4 at about 3000 s after the burst. We argue that this rebrightening implies that the central engine became active again after the main burst produced by the first ejecta, then drove another more collimated jet-like ejecta with a larger viewing angle. The two ejecta both interacted with the ambient medium, giving rise to forward shocks that propagated into the ambient medium and reverse shocks that penetrated into the ejecta. The total emission was a combination of the emissions from the reverse- and forward- shocked regions. We discuss how this combined emission accounts for the observed rebrightening.
Methods. We apply numerical models to calculate the light curves from the shocked regions, which include a forward shock originating in the first ejecta and a forward-reverse shock for the second ejecta.
Results. We find evidence that the central engine became active again 2000 s after the main burst. The combined emission produced by interactions of these two ejecta with the ambient medium can describe the properties of the afterglow of this burst. We argue that the rapid rise in brightness at ~3000 s in the afterglow is due to the off-axis emission from the second ejecta. The precession of the torus or accretion disk of the central engine is a natural explanation for the departure of the second ejecta from the line of sight
GRB 060206: Evidence of Precession of Central Engine
The high-redshift (z = 4.048) gamma-ray burst GRB 060206 showed unusual behavior, with a significant re-brightening about 3000 s after the burst. We assume that the central engine became active again 2000 s after the main burst and drove another more collimated off-axis jet. The two jets both interacted with the ambient medium and contributed to the whole emission. We numerically fit this optical afterglow from the two jets using the forward-shock model and the forward-reverse shock model. Combining with the zero time effect, we suggest that the fast rise at ~3000 s in the afterglow was due to the off-axis emission from the second jet. The precession of the torus or accretion disk of the gamma ray burst engine is the natural explanation for the symmetry axes of these two jets not to lie on the same line
GRB 030226 in a Density-Jump Medium
We present an explanation for the unusual temporal feature of the GRB 030226
afterglow. The R-band afterglow of this burst faded as ~ t^{-1.2} in ~ 0.2 days
after the burst, rebrightened during the period of ~ 0.2 - 0.5 days, and then
declined with ~ t^{-2.0}. To fit such a light curve, we consider an
ultrarelativistic jetted blast wave expanding in a density-jump medium. The
interaction of the blast wave with a large density jump produces relativistic
reverse and forward shocks. In this model, the observed rebrightening is due to
emissions from these newly forming shocks, and the late-time afterglow is
caused by sideways expansion of the jet. Our fitting implies that the
progenitor star of GRB 030226 could have produced a stellar wind with a large
density jump prior to the GRB onset.Comment: 9 pages, 1 figure, accepted for publication in ApJ Letter
X-Ray Flares from Postmerger Millisecond Pulsars
Recent observations support the suggestion that short-duration gamma-ray
bursts are produced by compact star mergers. The X-ray flares discovered in two
short gamma-ray bursts last much longer than the previously proposed postmerger
energy release time scales. Here we show that they can be produced by
differentially rotating, millisecond pulsars after the mergers of binary
neutron stars. The differential rotation leads to windup of interior poloidal
magnetic fields and the resulting toroidal fields are strong enough to float up
and break through the stellar surface. Magnetic reconnection--driven explosive
events then occur, leading to multiple X-ray flares minutes after the original
gamma-ray burst.Comment: 10 pages, published in Scienc
Acoustical analysis of gear housing vibration
The modal and acoustical analysis of the NASA gear-noise rig is described. Experimental modal analysis techniques were used to determine the modes of vibration of the transmission housing. The resulting modal data were then used in a boundary element method (BEM) analysis to calculate the sound pressure and sound intensity on the surface of the housing as well as the radiation efficiency of each mode. The radiation efficiencies of the transmission housing modes are compared with theoretical results for finite, baffled plates. A method that uses the measured mode shapes and the BEM to predict the effect of simple structural changes on the sound radiation efficiency of the modes of vibration is also described
Optical Flashes and Very Early Afterglows in Wind Environments
The interaction of a relativistic fireball with its ambient medium is
described through two shocks: a reverse shock that propagates into the
fireball, and a forward shock that propagates into the medium. The observed
optical flash of GRB 990123 has been considered to be the emission from such a
reverse shock. The observational properties of afterglows suggest that the
progenitors of some GRBs may be massive stars and their surrounding media may
be stellar winds. We here study very early afterglows from the reverse and
forward shocks in winds. An optical flash mainly arises from the relativistic
reverse shock while a radio flare is produced by the forward shock. The peak
flux densities of optical flashes are larger than 1 Jy for typical parameters,
if we do not take into account some appropriate dust obscuration along the line
of sight. The radio flare always has a long lasting constant flux, which will
not be covered up by interstellar scintillation. The non-detections of optical
flashes brighter than about 9th magnitude may constrain the GRBs isotropic
energies to be no more than a few ergs and wind intensities to be
relatively weak.Comment: 21 pages, 6 figures, accepted by MNRAS on March 7, 200
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