63 research outputs found
The energy spectrum observed by the AGASA experiment and the spatial distribution of the sources of ultra-high energy cosmic rays
Seven and a half years of continuous monitoring of giant air showers
triggered by ultra high-energy cosmic rays have been recently summarized by the
AGASA collaboration. The resulting energy spectrum indicates clearly that the
cosmic ray spectrum extends well beyond the Greisen-Zatsepin-Kuzmin (GZK)
cut-off at eV. Furthermore, despite the small number
statistics involved, some structure in the spectrum may be emerging. Using
numerical simulations, it is demonstrated in the present work that these
features are consistent with a spatial distribution of sources that follows the
distribution of luminous matter in the local Universe. Therefore, from this
point of view, there is no need for a second high-energy component of cosmic
rays dominating the spectrum beyond the GZK cut-off.Comment: 14 pages, 4 figures, Astrophys. J. Letters (submitted
COSMOLOGICAL GAMMA RAY BURSTS AND THE HIGHEST ENERGY COSMIC RAYS
We discuss a scenario in which the highest energy cosmic rays (CR's) and
cosmological -ray bursts (GRB's) have a common origin. This scenario is
consistent with the observed CR flux above , provided that
each burst produces similar energies in -rays and in CR's above
. Protons may be accelerated by Fermi's mechanism to energies
in a dissipative, ultra-relativistic wind, with
luminosity and Lorentz factor high enough to produce a GRB. For a homogeneous
GRB distribution, this scenario predicts an isotropic, time-independent CR
flux.Comment: Phys. Rev. Lett. in press (Received: March 22, 1995; Accepted: May
17, 1995
Relativity at Action or Gamma-Ray Bursts
Gamma ray Bursts (GRBs) - short bursts of few hundred keV -rays -
have fascinated astronomers since their accidental discovery in the sixties.
GRBs were ignored by most relativists who did not expect that they are
associated with any relativistic phenomenon. The recent observations of the
BATSE detector on the Compton GRO satellite have revolutionized our ideas on
these bursts and the picture that emerges shows that GRBs are the most
relativistic objects discovered so far.Comment: 7 pages, 4th prize in this years gravity essay competition to appear
in General Relativity and Gravitation. Complete PS file is available at
ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/essay96.u
Hydrogen Phases on the Surface of a Strongly Magnetized Neutron Star
The outermost layers of some neutron stars are likely to be dominated by
hydrogen, as a result of fast gravitational settling of heavier elements. These
layers directly mediate thermal radiation from the stars, and determine the
characteristics of X-ray/EUV spectra. For a neutron star with surface
temperature T\lo 10^6 K and magnetic field B\go 10^{12} G, various forms of
hydrogen can be present in the envelope, including atom, poly-molecules, and
condensed metal. We study the physical properties of different hydrogen phases
on the surface of a strongly magnetized neutron star for a wide range of field
strength and surface temperature . Depending on the values of and
, the outer envelope can be either in a nondegenerate gaseous phase or in a
degenerate metallic phase. For T\go 10^5 K and moderately strong magnetic
field, B\lo 10^{13} G, the envelope is nondegenerate and the surface material
gradually transforms into a degenerate Coulomb plasma as density increases. For
higher field strength, G, there exists a first-order phase
transition from the nondegenerate gaseous phase to the condensed metallic
phase. The column density of saturated vapor above the metallic hydrogen
decreases rapidly as the magnetic field increases or/and temperature decreases.
Thus the thermal radiation can directly emerge from the degenerate metallic
hydrogen surface. The characteristics of surface X-ray/EUV emission for
different phases are discussed. A separate study concerning the possibility of
magnetic field induced nuclear fusion of hydrogen on the neutron star surface
is also presented.Comment: TeX, 35 pages including 6 postscript figures. To be published in Ap
Can a Large Neutron Excess Help Solve the Baryon Loading Problem in Gamma-Ray Burst Fireballs?
We point out that the baryon-loading problem in Gamma-Ray Burst (GRB) models
can be amelioriated if a significant fraction of the baryons which inertially
confine the fireball are converted to neutrons. A high neutron fraction in some
circumstances can result in a reduced transfer of energy from relativistic
light particles in the fireball to baryons. The energy needed to produce the
required relativistic flow in the GRB is consequently reduced, in some cases by
orders of magnitude. This could be relevant to GRB models because a high
neutron-to-proton ratio has been calculated in neutron star-merger fireball
environments. Significant neutron excess also could occur near compact objects
with high neutrino fluxes.Comment: 5 pages, 2 figures, to appear in Phys. Rev. Let
Chandra and ASCA Observations of the X-ray-brightest T-Tauri Stars in the Rho Ophiuchi Cloud
We present the Chandra ACIS and ASCA GIS results for a series of four
long-term observations on DoAr 21, ROXs 21 and ROXs 31; the X-ray brightest
T-Tauri stars (TTSs) in the Rho Ophiuchi cloud. In the four observations with a
net exposure of ~600 ksec, we found six, three and two flares from DoAr 21,
ROXs 21 and ROXs 31, respectively; hence the flare rate is fairly high. The
spectra of DoAr 21 are well fitted with a single-temperature plasma model,
while those of ROXs 21 and ROXs 31 need an additional soft plasma component.
Since DoAr 21 is younger (~10^5 yr) than ROXs 21 and ROXs 31 (~10^6 yr), these
results may indicate that the soft component gradually increases as T-Tauri
stars age. The abundances are generally sub-solar and vary from element to
element. Both high-FIP (first ionization potential) and low-FIP elements show
enhancement over the mean abundances. An unusual giant flare is detected from
ROXs 31. The peak luminosity and temperature are ~10^33 ergs s^-1 and ~10 keV,
respectively. The temperature reaches its peak value before the flux maximum,
and is nearly constant (4--5 keV) during the decay phase, indicating successive
energy release during the flare. The abundances and absorption show dramatic
variability from the quiescent to flare phase.Comment: 13 pages, 18 figures, accepted for publication in Ap
Fractional Action Cosmology with Power Law Weight Function
Motivated by an earlier work on fractional-action cosmology with a periodic
weight function [1], we extend it by choosing a power-law weight function in
the action. In this approach, we obtain a varying gravitational coupling
constant. We then model dark energy in this paradigm and obtain relevant
cosmological parameters.Comment: 12 pages, 9 figures, Contributed talk published in the proceedings of
"3rd Italian-Pakistani Workshop on Relativistic Astrophysics
Electron-positron outflow from black holes
Gamma-ray bursts (GRBs) appear as the brightest transient phenomena in the
Universe. The nature of the central engine in GRBs is a missing link in the
theory of fireballs to their stellar mass progenitors. Here it is shown that
rotating black holes produce electron-positron outflow when brought into
contact with a strong magnetic field. The outflow is produced by a coupling of
the spin of the black hole to the orbit of the particles. For a nearly extreme
Kerr black hole, particle outflow from an initial state of electrostatic
equilibrium has a normalized isotropic emission of erg/s, where B is the
external magnetic field strength, B_c=4.4 x 10^{13}G, and M is the mass of the
black hole. This initial outflow has a half-opening angle
. A connection with fireballs in -ray bursts is
given.Comment: 10 pages LaTe
The initial Lorentz factors of fireballs inferred from the early X-ray data of SWIFT GRBs
We intend to determine the type of circumburst medium and measure directly
the initial Lorentz factor of GRB outflows. If the early X-ray
afterglow lightcurve has a peak and the whole profile across the peak is
consistent with the standard external shock model, the early rise profile of
light curves can be used to differentiate whether the burst was born in
interstellar medium (ISM) or in stellar wind. In the thin shell case, related
to a sub-relativistic reverse shock, the peak time occurring after the end of
the prompt emission, can be used to derive an accurate , especially
for the ISM case. The afterglow lightcurves for a flat electron spectrum
have been derived analytically. In our GRB sample, we obtain for the bursts born in ISM. We did not find any good case for bursts
born in stellar wind and behaving as a thin shell that can be used to constrain
reliably.Comment: 6 pages,1 figure,3 tables,accepted for publication in Astronomy &
Astrophysic
The Connection Between Thermal and Non-Thermal Emission in Gamma-ray Bursts: General Considerations and GRB090902B as a Case Study
Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB)
"fireball" model. We show here, that inclusion of this component in the
analysis of the GRB prompt emission phase naturally explains some of the prompt
GRB spectra seen by the Fermi satellite over its entire energy band. The
sub-MeV peak is explained as multi-color black body emission, and the high
energy tail, extending up to the GeV band, results from roughly similar
contributions of synchrotron emission, synchrotron self Compton(SSC) and
Comptonization of the thermal photons by energetic electrons originating after
dissipation of the kinetic energy above the photosphere. We show how this
analysis method results in a complete, self consistent picture of the physical
conditions at both emission sites of the thermal and non-thermal radiation. We
study the connection between the thermal and non-thermal parts of the spectrum,
and show how the values of the free model parameters are deduced from the data.
We demonstrate our analysis method on GRB090902B: We deduce a Lorentz factor in
the range 920 <= \eta <= 1070, photospheric radius r_{ph} ~ 7.2 - 8.4 * 10^{11}
cm and dissipation radius r_\gamma >= 3.5 - 4.1 * 10^{15} cm. By comparison to
afterglow data, we deduce that a large fraction, epsilon_d ~85% - 95% of the
kinetic energy is dissipated, and that large fraction, ~equipartition of this
energy is carried by the electrons and the magnetic field. This high value of
epsilon_d questions the "internal shock" scenario as the main energy
dissipation mechanism for this GRB.Comment: 15 pages, 5 figures; minor revisions, typos corrected. Accepted for
publication in MNRA
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