656 research outputs found
Neutrino afterglow from Gamma-Ray Bursts: ~10^{18} eV
We show that a significant fraction of the energy of a gamma-ray burst(GRB)
is probably converted to a burst of 10^{17}-10^{19} eV neutrinos and multiple
GeV gammas that follow the GRB by > 10 s . If, as previously suggested, GRB's
accelerate protons to ~10^{20} eV, then both the neutrinos and the gammas may
be detectable.Comment: Accepted ApJ; added sentence re: sterile neutrinos; related material
at http://www.sns.ias.edu/~jn
Limits on Supersymmetric Dark Matter From EGRET Observations of the Galactic Center Region
In most supersymmetic models, neutralino dark matter particles are predicted
to accumulate in the Galactic center and annihilate generating, among other
products, gamma rays. The EGRET experiment has made observations in this
region, and is sensitive to gamma rays from 30 MeV to 30 GeV. We have
used an improved point source analysis including an energy dependent point
spread function and an unbinned maximum likelihood technique, which has allowed
us to significantly lower the limits on gamma ray flux from the Galactic
center. We find that the present EGRET data can limit many supersymmetric
models if the density of the Galactic dark matter halo is cuspy or spiked
toward the Galactic center. We also discuss the ability of GLAST to test these
models.Comment: 4 pages, 3 figure
The Effects of Fatigue on Driver Performance for Single and Team Long-Haul Truck Drivers
Driver fatigue is an important safety issue for long-haul truck drivers. To provide an efficient means of obtaining sleep, long-haul truck drivers often use tractors equipped with sleeper berth units. Depending on the type of cargo and distances traveled, long-haul truck drivers either drive in teams or alone as single drivers. Team drivers, therefore, typically sleep in a moving truck whereas single drivers sleep in a stationary truck. It has been hypothesized that sleeping in a moving truck could adversely affect the sleep quality and, therefore, the alertness level of team drivers. A naturalistic data collection system was developed and installed in two Class 8 heavy trucks. This trigger-based system consisted of vehicle sensors and cameras that allowed the experimenters to obtain the driving performance and driver alertness data for analysis of fatigue. Fatigue was measured using both objective and subjective measures that were recorded before and after sleep and while driving. Fatigue and driving performance were compared for single versus team drivers to determine which driver type acquired the greatest sleep deficit during a trip. Results suggest that single drivers were more frequently involved in critical incidents while exhibiting extreme drowsiness than were team drivers by a factor of 4 to 1. These results will be discussed in relation to the general safety of single versus team trucking operations
The Effects of Fatigue on Driver Performance for Single and Team Long-Haul Truck Drivers
Driver fatigue is an important safety issue for long-haul truck drivers. To provide an efficient means of obtaining sleep, long-haul truck drivers often use tractors equipped with sleeper berth units. Depending on the type of cargo and distances traveled, long-haul truck drivers either drive in teams or alone as single drivers. Team drivers, therefore, typically sleep in a moving truck whereas single drivers sleep in a stationary truck. It has been hypothesized that sleeping in a moving truck could adversely affect the sleep quality and, therefore, the alertness level of team drivers. A naturalistic data collection system was developed and installed in two Class 8 heavy trucks. This trigger-based system consisted of vehicle sensors and cameras that allowed the experimenters to obtain the driving performance and driver alertness data for analysis of fatigue. Fatigue was measured using both objective and subjective measures that were recorded before and after sleep and while driving. Fatigue and driving performance were compared for single versus team drivers to determine which driver type acquired the greatest sleep deficit during a trip. Results suggest that single drivers were more frequently involved in critical incidents while exhibiting extreme drowsiness than were team drivers by a factor of 4 to 1. These results will be discussed in relation to the general safety of single versus team trucking operations
Multi-wavelength observations of 2HWC J1928+177: dark accelerator or new TeV gamma-ray binary?
2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the High
Altitude Water Cherenkov (HAWC) Observatory up to ~ 56 TeV. The HAWC source,
later confirmed by H.E.S.S., still remains unidentified as a dark accelerator
since there is no apparent supernova remnant or pulsar wind nebula detected in
the lower energy bands. The radio pulsar PSR J1928+1746, coinciding with the
HAWC source position, has no X-ray counterpart. Our SED modeling shows that
inverse Compton scattering in the putative pulsar wind nebula can account for
the TeV emission only if the unseen nebula is extended beyond r ~ 4 [arcmin].
Alternatively, TeV gamma rays may be produced by hadronic interactions between
relativistic protons from an undetected supernova remnant associated with the
radio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandra
observations detected a variable X-ray point source within the HAWC error
circle, potentially associated with a bright IR source. The X-ray spectra can
be fitted with an absorbed power-law model with cm and and exhibit
long-term X-ray flux variability over the last decade. If the X-ray source,
possibly associated with the IR source (likely an O star), is the counterpart
of the HAWC source, it may be a new TeV gamma-ray binary powered by collisions
between the pulsar wind and stellar wind. Follow-up X-ray observations are
warranted to search for diffuse X-ray emission and determine the nature of the
HAWC source.Comment: accepted to ApJ, 8 pages, 7 figure
Recommended from our members
Gruneisen-stress induced ablation of biological tissue
The objective of biomedical applications of lasers is frequently to remove tissue in a controlled manner. However, for ablation induced by thermal- or photo-decomposition, damage to surrounding tissue may be excessive in some instances. Tissue can also be ablated by a hydrodynamic process referred to as front surface spallation, in which a thin layer next to a free surface is heated to levels, below vaporization but, so rapidly that it cannot undergo thermal expansion during laser heating. This generates a stress pulse, which propagates away from the heated region, with an initial amplitude that can be calculated using the Grueneisen coefficient. As the pulse reflects from the free surface, a tensile tail can develop of sufficient amplitude, exceeding the material strength, that a layer will be spalled off, taking much of the laser-deposited energy with it. Because tissue is generally a low strength material, this process has the potential of producing controlled ablation with reduced damage to the remaining tissue. However, to achieve these conditions, the laser pulse length, absorption depth and fluence must be properly tailored. This paper presents hydrodynamic calculations and analytical modeling relating to both stress- and thermal-induced ablation as a function of laser and tissue properties to illustrate the potential benefits of stress induced ablation. Also, guidance is given for tailoring the exposure parameters to enhance front surface spallation. 8 refs., 6 figs
Observations of GRB 990123 by the Compton Gamma-Ray Observatory
GRB 990123 was the first burst from which simultaneous optical, X-ray and
gamma-ray emission was detected; its afterglow has been followed by an
extensive set of radio, optical and X-ray observations. We have studied the
gamma-ray burst itself as observed by the CGRO detectors. We find that
gamma-ray fluxes are not correlated with the simultaneous optical observations,
and the gamma-ray spectra cannot be extrapolated simply to the optical fluxes.
The burst is well fit by the standard four-parameter GRB function, with the
exception that excess emission compared to this function is observed below ~15
keV during some time intervals. The burst is characterized by the typical
hard-to-soft and hardness-intensity correlation spectral evolution patterns.
The energy of the peak of the nu f_nu spectrum, E_p, reaches an unusually high
value during the first intensity spike, 1470 +/- 110 keV, and then falls to
\~300 keV during the tail of the burst. The high-energy spectrum above ~MeV is
consistent with a power law with a photon index of about -3. By fluence, GRB
990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly
placing it on the -3/2 power-law portion of the intensity distribution.
However, the redshift measured for the afterglow is inconsistent with the
Euclidean interpretation of the -3/2 power-law. Using the redshift value of >=
1.61 and assuming isotropic emission, the gamma-ray fluence exceeds 10E54 ergs.Comment: Submitted to The Astrophysical Journal. 16 pages including 4 figure
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