15,641 research outputs found
Soft gamma repeaters outside the Local group
We propose that the best sites to search for SGRs outside the Local group are
galaxies with active massive star formation. Different possibilities to observe
SGR activity from these sites are discussed. In particular we searched for
giant flares from nearby galaxies ( -- 4 Mpc) M82,
M83, NGC 253, and NGC 4945 in the BATSE data. No candidates alike giant SGR
flares were found. The absence of such detections implies that the rate of
giant flares with energy release in the initial spike above
erg is less then 1/25 yr in our Galaxy. However, hyperflares similar to
the one of 27 December 2004 can be observed from larger distances.
Nevertheless, we do not see any significant excess of short GRBs from the Virgo
galaxy cluster and from galaxies Arp 299 and NGC 3256 with extremely high star
formation rate. This implies that the galactic rate of hyperflares with energy
release erg is less than yr. With this
constraint the fraction of possible extragalactic SGR hyperflares among BATSE
short GRBs should not exceed few percents. We present a list of short GRBs
coincident with galaxies mentioned above, and discuss the possibility that some
of them are SGR giant flares. We propose that the best target for observations
of extragalactic SGR flares by {\it Swift} is the Virgo cluster.Comment: 14 pages with 3 figures; accepted to MNRAS (final version
An Off-line Scan of the BATSE Daily Records and a Large Uniform Sample of Gamma-Ray Bursts
During a scan of the archival BATSE daily records covering the entire 9.1
years (TJD 8369-11690) of the BATSE operation, 3906 gamma-ray bursts (GRBs)
have been detected. 2068 of these GRBs are previously known BATSE triggers
while 1838 of them are new non-triggered bursts. It is important that all
events were detected in the same type of data and were processed with the same
procedure. Therefore these 3906 GRBs constitute a uniform sample. We have
created a publically available electronic data base containing this sample. We
describe the procedures of the data reduction, the selection of the GRB
candidates, and the statistical tests for possible non-GRB contaminations. We
also describe a novel test burst method used to measure the scan efficiency and
the information obtained using the test bursts. Our scan decreases the BATSE
detection threshold to ~0.1 photons/sec/cm2. As a first result, we show that
the differential log N - log P distribution corrected for the detection
efficiency extends to low brightnesses without any indication of a turn-over.
Any reasonable extrapolation of the new log N - log P to lower brightnesses
imply a rate of several thousands of GRBs in the Universe per year.Comment: 14 pages; 23 figures; revised version accepted to ApJ; electronic
version of Table 2 is available at
ftp://ftp.astro.su.se/pub/head/grb/catalogs/etable2.txt The GRB archive is
available at http://www.astro.su.se/groups/head/grb_archive.htm
The Vacuum Structure of Light-Front -Theory
We discuss the vacuum structure of -theory in 1+1 dimensions
quantised on the light-front . To this end, one has to solve a
non-linear, operator-valued constraint equation. It expresses that mode of the
field operator having longitudinal light-front momentum equal to zero, as a
function of all the other modes in the theory. We analyse whether this zero
mode can lead to a non-vanishing vacuum expectation value of the field
and thus to spontaneous symmetry breaking. In perturbation theory, we get no
symmetry breaking. If we solve the constraint, however, non-perturbatively,
within a mean-field type Fock ansatz, the situation changes: while the vacuum
state itself remains trivial, we find a non-vanishing vacuum expectation value
above a critical coupling. Exactly the same result is obtained within a
light-front Tamm-Dancoff approximation, if the renormalisation is done in the
correct way.Comment: 28 pages LaTeX, 1 Postscript figur
Blind search for the real sample: Application to the origin of ultra-high energy cosmic rays
We suggest a method for statistical tests which does not suffer from a
posteriori manipulations with tested samples (e.g. cuts optimization) and does
not require a somewhat obscure procedure of the penalty estimate. The idea of
the method is to hide the real sample (before it has been studied) among a
large number of artificial samples, drawn from a random distribution expressing
the null hypothesis, and then to search for it as the one demonstrating the
strongest hypothesized effect. The statistical significance of the effect in
this approach is the inverse of the maximal number of random samples at which
the search was successful. We have applied the method to revisit the problem of
correlation between the arrival directions of ultra-high energy cosmic rays and
BL Lac objects. No significant correlation was found.Comment: 4 pages, 1 figure, accepted to ApJ Letter
ON THE GEOMETRY OF THE X-RAY EMITTING REGION IN SEYFERT GALAXIES
For the first time, detailed radiative transfer calculations of Comptonized
X-ray and gamma-ray radiation in a hot pair plasma above a cold accretion disk
are performed using two independent codes and methods. The simulations include
both energy and pair balance as well as reprocessing of the X- and gamma-rays
by the cold disk. We study both plane-parallel coronae as well as active
dissipation regions having shapes of hemispheres and pill boxes located on the
disk surface. It is shown, contrary to earlier claims, that plane-parallel
coronae in pair balance have difficulties in selfconsistently reproducing the
ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses
inferred from observations of type 1 Seyfert galaxies. Instead, the
observations are consistent with the X-rays coming from a number of individual
active regions located on the surface of the disk.
A number of effects such as anisotropic Compton scattering, the reflection
hump, feedback to the soft photon source by reprocessing, and an active region
in pair equilibrium all conspire to produce the observed ranges of X-ray
slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray
slopes can be due to a spread in the properties of the active regions such as
their compactnesses and their elevations above the disk surface. Simplified
models invoking isotropic Comptonization in spherical clouds are no longer
sufficient when interpreting the data.Comment: 9 pages, 3 postscript figures, figures can be obtained from the
authors via e-mail: [email protected]
Photon breeding mechanism in relativistic jets: astrophysical implications
Photon breeding in relativistic jets involves multiplication of high-energy
photons propagating from the jet to the external environment and back with the
conversion into electron-positron pairs. The exponential growth of the energy
density of these photons is a super-critical process powered by the bulk energy
of the jet. The efficient deceleration of the jet outer layers creates a
structured jet morphology with the fast spine and slow sheath. In initially
fast and high-power jets even the spine can be decelerated efficiently leading
to very high radiative efficiencies of conversion of the jet bulk energy into
radiation. The decelerating, structured jets have angular distribution of
radiation significantly broader than that predicted by a simple blob model with
a constant Lorentz factor. This reconciles the discrepancy between the high
Doppler factors determined by the fits to the spectra of TeV blazars and the
low apparent velocities observed at VLBI scales as well as the low jet Lorentz
factors required by the observed statistics and luminosity ratio of
Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a
population of high-energy leptons in agreement with the constraints on the
electron injection function required by spectral fits of the TeV blazars.
Relativistic pairs created outside the jet and emitting gamma-rays by inverse
Compton process might explain the relatively high level of the TeV emission
from the misaligned jet in the radio galaxies. The mechanism reproduces basic
spectral features observed in blazars including the blazar sequence (shift of
the spectral peaks towards lower energies with increasing luminosity). The
mechanism is very robust and can operate in various environments characterised
by the high photon density.Comment: 6 pages, 3 figures, to appear in the proceedings of the HEPRO
conference, September 24-28, 2007, Dublin, Irelan
Superlong GRBs
We searched for anomalously long GRBs (GRBs) in the archival records of the
Burst and Transient Sources Experiment (BATSE). Ten obvious superlong (>500 s)
GRBs with almost continuous emission episodes were found. Nine of these events
are known from the BATSE catalog, but five have no duration estimates; we found
one burst for the first time. We also detected events with emission episodes
separated by a long period of silence (up to 1000 s) with a total duration of
1000--2000 s. In the latter case, we cannot reach an unequivocal conclusion
about a common origin of the episodes due to the BATSE poor angular resolution.
However, for most of these pairs, the probability of a coincidence of
independent GRBs is much lower than unity, and the probability that all of
these are coincidences is 10E-8. All of the events have a hardness ratio (the
ratio of the count rates in different energy channels) typical of GRBs, and
their unique duration is unlikely to be related to their high redshifts.
Superlong bursts do not differ in their properties from typical long (>2 s)
GRBs. We estimated the fraction of superlong GRBs (>500 s) among the long GRBs
in the BATSE sample with fluxes up to 0.1 ph cm^{-2} s^{-1} to be between 0.3
and 0.5%, which is higher than the estimate based on the BATSE catalog.Comment: 11 pages, 4 figures, 3 tables, translation is made by Astronomy
Letter
Electronic dummy for acoustical testing
Electronic Dummy /ED/ used for acoustical testing represents the average male torso from the Xiphoid process upward and includes an acoustic replica of the human head. This head simulates natural flesh, and has an artificial voice and artificial ears that measure sound pressures at the eardrum or the entrance to the ear canal
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