1,037 research outputs found
Constraints on the Redshift and Luminosity Distributions of Gamma Ray Bursts in an Einstein-de Sitter Universe
Two models of the gamma ray burst population, one with a standard candle
luminosity and one with a power law luminosity distribution, are chi^2-fitted
to the union of two data sets: the differential number versus peak flux
distribution of BATSE's long duration bursts, and the time dilation and energy
shifting versus peak flux information of pulse duration time dilation factors,
interpulse duration time dilation factors, and peak energy shifting factors.
The differential peak flux distribution is corrected for threshold effects at
low peak fluxes and at short burst durations, and the pulse duration time
dilation factors are also corrected for energy stretching and similar effects.
Within an Einstein-de Sitter cosmology, we place strong bounds on the evolution
of the bursts, and these bounds are incompatible with a homogeneous population,
assuming a power law spectrum and no luminosity evolution. Additionally, under
the implied conditions of moderate evolution, the 90% width of the observed
luminosity distribution is shown to be < 10^2, which is less constrained than
others have demonstrated it to be assuming no evolution. Finally, redshift
considerations indicate that if the redshifts of BATSE's faintest bursts are to
be compatible with that which is currently known for galaxies, a standard
candle luminosity is unacceptable, and in the case of the power law luminosity
distribution, a mean luminosity < 10^57 ph s^-1 is favored.Comment: Accepted to the Astrophysical Journal, 18 pages, LaTe
A Planning Pipeline for Large Multi-Agent Missions
In complex multi-agent applications, human operators are often tasked with planning and managing large heterogeneous teams of humans and autonomous vehicles. Although the use of these autonomous vehicles broadens the scope of meaningful applications, many of their systems remain unintuitive and difficult to master for human operators whose expertise lies in the application domain and not at the platform level. Current research focuses on the development of individual capabilities necessary to plan multi-agent missions of this scope, placing little emphasis on the integration of these components in to a full pipeline. The work presented in this paper presents a complete and user-agnostic planning pipeline for large multiagent missions known as the HOLII GRAILLE. The system takes a holistic approach to mission planning by integrating capabilities in human machine interaction, flight path generation, and validation and verification. Components modules of the pipeline are explored on an individual level, as well as their integration into a whole system. Lastly, implications for future mission planning are discussed
Events in the life of a cocoon surrounding a light, collapsar jet
According to the collapsar model, gamma-ray bursts are thought to be produced
in shocks that occur after the relativistic jet has broken free from the
stellar envelope. If the mass density of the collimated outflow is less than
that of the stellar envelope, the jet will then be surrounded by a cocoon of
relativistic plasma. This material would itself be able to escape along the
direction of least resistance, which is likely to be the rotation axis of the
stellar progenitor, and accelerate in approximately the same way as an
impulsive fireball. We discuss how the properties of the stellar envelope have
a decisive effect on the appearance of a cocoon propagating through it. The
relativistic material that accumulated in the cocoon would have enough kinetic
energy to substantially alter the structure of the relativistic outflow, if not
in fact provide much of the observed explosive power. Shock waves within this
plasma can produce gamma-ray and X-ray transients, in addition to the standard
afterglow emission that would arise from the deceleration shock of the cocoon
fireball.Comment: 16 pages, 5 figures, slightly revised version, accepted for
publication in MNRA
Chirality and the origin of atmospheric humic-like substances
Aerosol water extracts and atmospheric humic-like substances (HULIS) obtained from PM2.5-fraction aerosol samples collected in a rural/continental background environment and in an urban environment in spring and summer, and at a tropical site that was heavily impacted by biomass burning were studied. HULIS was obtained as the water-soluble, methanol-elutable material isolated from a solid-phase extraction procedure. The mean organic matter-to-organic carbon mass conversion factor and the standard deviation of 2.04 +/- 0.06 were derived for HULIS from biomass burning. Mean atmospheric concentrations of HULIS for the rural and urban environments and for the biomass burning during daylight periods and nights, were 1.65, 2.2, 43, and 60 mu gm(-3), respectively. This and other abundances indicate that intense emission sources and/or formation mechanisms of HULIS operate in biomass burning. Mean contributions of C in HULIS (HULIS-C) to water-soluble organic carbon (WSOC) were 35, 48, 63, and 76%, respectively, for the sample set listed. HULIS-C is the major component of the WSOC in tropical biomass burning. The data also suggest that HULIS most likely do not share common origin in the three environments studied. Differentiation among the possible formation processes was attempted by investigating the optical activity of HULIS through their (electronic and vibrational) circular dichroism properties. The urban HULIS did not show optical activity, which is in line with the concept of their major airborne formation from anthropogenic aromatics. The rural HULIS revealed weak optical activity, which may be associated with one of their important formation pathways by photo-oxidation and oligomerisation, i.e., with the formation from chiral biogenic precursors with one of the enantiomers slightly enriched. The The biomass burning of HULIS exhibited a strong effect in the vibrational circular dichroism as a clear distinction from the other two types. This was related to the contribution of the thermal degradation products of lignins and cellulose. The biomass burning of HULIS resemble Suwannee River Fulvic Acid standard more closely in some aspects than the urban and rural types of HULIS, which may be related to their common origin from plant material
Predictions for The Very Early Afterglow and The Optical Flash
According to the internal-external shocks model for -ray bursts
(GRBs), the GRB is produced by internal shocks within a relativistic flow while
the afterglow is produced by external shocks with the ISM. We explore the early
afterglow emission. For short GRBs the peak of the afterglow will be delayed,
typically, by few dozens of seconds after the burst. For long GRBs the early
afterglow emission will overlap the GRB signal. We calculate the expected
spectrum and the light curves of the early afterglow in the optical, X-ray and
-ray bands. These characteristics provide a way to discriminate
between late internal shocks emission (part of the GRB) and the early afterglow
signal. If such a delayed emission, with the characteristics of the early
afterglow, will be detected it can be used both to prove the internal shock
scenario as producing the GRB, as well as to measure the initial Lorentz factor
of the relativistic flow. The reverse shock, at its peak, contains energy which
is comparable to that of the GRB itself, but has a much lower temperature than
that of the forward shock so it radiates at considerably lower frequencies. The
reverse shock dominates the early optical emission, and an optical flash
brighter than 15th magnitude, is expected together with the forward shock peak
at x-rays or -rays. If this optical flash is not observed, strong
limitations can be put on the baryonic contents of the relativistic shell
deriving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure
Hydrodynamic Time Scales and Temporal Structure of GRBs
We calculate the hydrodynamic time scales for a spherical ultra-relativistic
shell that is decelerated by the ISM and discuss the possible relations between
these time scales and the observed temporal structure in -ray bursts.
We suggest that the bursts' duration is related to the deceleration time, the
variability is related to the ISM inhomogeneities and precursors are related to
internal shocks within the shell. Good agreement can be achieved for these
quantities with reasonable, not fined tuned, astrophysical parameters. The
difference between Newtonian and relativistic reverse shocks may lead to the
observed bimodal distribution of bursts' durations.Comment: 13 pages, uuencoed including two figures. UU encoded file or
postcript files can also be obtained by anonymous ftp from
ftp://shemesh.fiz.huji.ac.il or from http://shemesh.fiz.huji.ac.il/ . Revised
version, Accepted for Publication in the Astrophysical Journal Letter
Light Curves of GRB Optical Flashes
The standard model of GRB afterglows assumes that relativistically expanding
material is decelerating due to interaction with the surrounding medium. The
afterglows are well described by the synchrotron radiation from a forward
shock, while the strong optical flash associated with GRB 990123 can be
attributed to the emission from a reverse shock. We give a detailed study on
the reverse shock emission. The full light curves are calculated for a long and
a short GRB cases. We discuss the lack of the prompt optical detections by
ROTSE for GRB 981121 and GRB 981223.Comment: 18 pages, 6 figures, ApJ 545 in pres
Variability in GRBs - A Clue
We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.Comment: 15 page latex file with 5 PS figure. Complete uuencoded compressed PS
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/SaP_aclue.u
New direction for gamma-rays
The origin of energetic gamma-ray bursts is still unknown. But the detection
of polarization of gamma-rays provides fresh insight into the mechanism driving
these powerful explosions.Comment: Nature "News & Views", RE Coburn & Boggs (astro-ph/0305377
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