705 research outputs found
Development and evaluation of controlled viscosity coatings for superalloys
Controlled viscosity glass based protective coatings for superalloys for turbine blade application
High temperature glass coatings for superalloys and refractory metals
New glasses are used as protective coatings on metals and alloys susceptible to oxidation at high temperatures in oxidizing atmospheres. Glasses are stable and solid at temperatures up to 1000 deg C, adhere well to metal surfaces, and are usable for metals with broad range of expansion coefficients
The Gamma Ray Pulsar Population
We apply a likelihood analysis to pulsar detections, pulsar upper limits, and
diffuse background measurements from the OSSE and EGRET instruments on the
Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray
pulsars and some properties of the gamma-ray pulsar population. We find that
the dependence of luminosity on spin period and dipole magnetic field is much
steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively),
suggesting that different emission mechanisms are responsible for low- and
high-energy gamma-ray emission. Incorporating a spin-down model and assuming a
pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray
background due to unidentified pulsars and find that pulsars may be an
important component of the OSSE diffuse flux, but are most likely not important
at EGRET energies. Using measurements of the diffuse background flux from these
instruments, we are able to place constraints on the braking index, initial
spin period, and magnetic field of the Galactic pulsar population. We are also
able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr).
Our results are based on a large gamma-ray beam, but they do not scale in a
simple way with beam size. With our assumed beam size, the implied gamma-ray
efficiency for the EGRET detections is no more than 20%. We estimate that about
20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use
our model to predict the pulsar population that will be seen by future
gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray
pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
The Identification of EGRET Sources with Flat-Spectrum Radio Sources
We present a method to assess the reliability of the identification of EGRET
sources with extragalactic radio sources. We verify that EGRET is detecting the
blazar class of AGN. However, many published identifications are found to be
questionable. We provide a table of 42 blazars which we expect to be robust
identifications of EGRET sources. This includes one previously unidentified
EGRET source, the lensed AGN PKS 1830-210 near the direction of the Galactic
center. We provide the best available positions for 16 more radio sources which
are also potential identifications for previously unidentified EGRET sources.
All high Galactic latitude EGRET sources (b>3 degrees) which demonstrate
significant variability can be identified with flat spectrum radio sources.
This suggests that EGRET is not detecting any type of AGN other than blazars.
This identification method has been used to establish with 99.998% confidence
that the peak gamma-ray flux of a blazar is correlated with its average 5 GHz
radio flux. An even better correlation is seen between gamma-ray flux and the
2.29 GHz flux density measured with VLBI at the base of the radio jet. Also,
using high confidence identifications, we find that the radio sources
identified with EGRET sources have larger correlated VLBI flux densities than
the parent population of flat radio spectrum sources.Comment: To be published in Ap J, May 20, 1997 issue, Vol. 481. The complete
paper is available at http://bu-ast.bu.edu/~mattox/src_id.htm
High-Energy Gamma-Ray Observations of Two Young, Energetic Radio Pulsars
We present results of Compton Gamma-Ray Observatory EGRET observations of the
unidentified high-energy gamma-ray sources 2EG J1049-5847 (GEV J1047-5840, 3EG
J1048-5840) and 2EG J1103-6106 (3EG J1102-6103). These sources are spatially
coincident with the young, energetic radio pulsars PSRs B1046-58 and
J1105-6107, respectively. We find evidence for an association between PSR
B1046-58 and 2EG J1049-5847. The gamma-ray pulse profile, obtained by folding
time-tagged photons having energies above 400 MeV using contemporaneous radio
ephemerides, has probability of arising by chance of 1.2E-4 according to the
binning-independent H-test. A spatial analysis of the on-pulse photons reveals
a point source of equivalent significance 10.2 sigma. Off-pulse, the
significance drops to 5.8 sigma. Archival ASCA data show that the only hard
X-ray point source in the 95% confidence error box of the gamma-ray source is
spatially coincident with the pulsar within the 1' uncertainty (Pivovaroff,
Kaspi & Gotthelf 1999). The double peaked gamma-ray pulse morphology and
leading radio pulse are similar to those seen for other gamma-ray pulsars and
are well-explained in models in which the gamma-ray emission is produced in
charge-depleted gaps in the outer magnetosphere. The inferred pulsed gamma-ray
flux above 400 MeV, (2.5 +/- 0.6) x 10E-10 erg/cm^2/s, represents 0.011 +/-
0.003 of the pulsar's spin-down luminosity, for a distance of 3 kpc and 1 sr
beaming. For PSR J1105-6107, light curves obtained by folding EGRET photons
using contemporaneous radio ephemerides show no significant features. We
conclude that this pulsar converts less than 0.014 of its spin-down luminosity
into E > 100 MeV gamma-rays beaming in our direction (99% confidence), assuming
a distance of 7 kpc, 1 sr beaming and a duty cycle of 0.5.Comment: Accepted for publication in the Astrophysical Journa
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