177 research outputs found
Towards Locating the Brightest Microlensing Events on the Sky
It is estimated that a star brighter than visual magnitude 17 is undergoing a
detectable gravitational microlensing event, somewhere on the sky, at any given
time. It is assumed that both lenses and sources are normal stars drawn from a
standard Bahcall-Soneira model of our Galaxy. Furthermore, over the time scale
of a year, a star 15th magnitude or brighter should undergo a detectable
gravitational lens amplification. Detecting and studying the microlensing event
rate among the brightest 10 stars could yield a better understanding of
Galactic stellar and dark matter distributions. Diligent tracking of bright
microlensing events with even small telescopes might detect planets orbiting
these stellar lenses.Comment: 19 pages, 4 figures, accepted by Ap
X-ray study of low-temperature annealed arsenic-implanted silicon
Low-temperature anneals (500â650 °C) of 2, 4, and 8Ă10^15 cm^â2 As+ implanted in silicon at 50 keV were studied by x-ray double crystal diffraction. The rocking curves were analyzed by a kinematical model. Two regions of strain were found in the solid-phase epitaxially regrown layer. One layer was uniform and positively strained. The other was nonuniform and negatively strained. By comparing rocking curves of repeatedly etched layers it was found that the surface layer is negatively strained, corresponding largely to the substitutional As in the regrown layer. The positively strained region lies at the interface between the implanted layer and the undamaged silicon substrate
Probing For Machos of Mass - with Gamma-Ray Burst Parallax Spacecraft
Two spacecraft separated by \sim 1\,\au and equipped with gamma-ray burst
(GRB) detectors could detect or rule out a cosmological density of Massive
Compact Halo Objects (MACHOs) in the mass range 10^{-15} M_{\odot}\lsim M
\lsim 10^{-7} M_{\odot} provided that GRBs prove to be cosmological.
Previously devised methods for detecting MACHOs have spanned the mass range
10^{-16} M_{\odot}\lsim M \lsim 10^{7} M_{\odot}, but with a gap of several
orders of magnitude near . For MACHOs and sources both at a
cosmological distance, the Einstein radius is \sim 1\,\au\,(M/10^{-7}
M_\odot)^{1/2}. Hence, if a GRB lies within the Einstein ring of a MACHO of
mass M\lsim 10^{-7}M_\odot as seen by one detector, it will not lie in the
Einstein ring as seen by a second detector \sim 1\,\au away. This implies
that if GRBs are measured to have significantly different fluxes by the two
detectors, this would signal the presence of a MACHO \lsim 10^{-7}M_\odot. By
the same token, if the two detectors measured similar fluxes for several
hundred events a cosmological abundance of such low-mass MACHOs would be ruled
out. The lower limit of sensitivity, M\lsim 10^{-15}M_\odot is set by the
finite size of the source. If low-mass MACHOs are detected, there are tests
which can discriminate among events generated by MACHOs in the three mass
ranges M\lsim 10^{-12}\,M_\odot, 10^{-12}\,M_\odot\lsim M\lsim
10^{-7}\,M_\odot, and M\gsim 10^{-7}\ M_\odot. Further experiments would
then be required to make more accurate mass measurements.Comment: 8 pages, uuencoded postscript, no figure
Bright Lenses and Optical Depth
In gravitational lensing, the concept of optical depth assumes the lens is
dark. Several microlensing detections have now been made where the lens may be
bright. Relations are developed between apparent and absolute optical depth in
the regime of the apparent and absolute brightness of the lens. An apparent
optical depth through bright lenses is always less than the true, absolute
optical depth. The greater the intrinsic brightness of the lens, the more
likely it will be found nearer the source.Comment: 18 pages including 4 figures, AASTeX, ApJ in pres
Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data
A new search for the gravitational lens effects of a significant cosmological
density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded
a null result. We inspected the timing data of 774 BATSE-triggered GRBs for
evidence of millilensing: repeated peaks similar in light-curve shape and
spectra. Our null detection leads us to conclude that, in all candidate
universes simulated, is favored for , while in some universes and mass ranges the density
limits are as much as 10 times lower. Therefore, a cosmologically significant
population of SCOs near globular cluster mass neither came out of the
primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2
Adventures in Friedmann Cosmology: An Educationally Detailed Expansion of the Cosmological Friedmann Equations
The general relativistic cosmological Friedmann equations which describe how
the scale factor of the universe evolves are expanded explicitly to include
energy forms not usually seen. The evolution of the universe as predicted by
the Friedmann equations when dominated by a single, isotropic, stable, static,
perfect-fluid energy form is discussed for different values of its
gravitational pressure to density ratio . These energy forms include phantom
energy (), cosmological constant (), domain walls (),
cosmic strings (), normal matter (), radiation and
relativistic matter (), and a previously little-discussed form of
energy called "ultralight" (). A brief history and possible futures of
Friedmann universes dominated by a single energy form are discussed.Comment: 32 pages, 4 tables; modified version accepted for publication in the
American Journal of Physic
Severe New Limits on the Host Galaxies of Gamma Ray Bursts
The nature of Gamma Ray Bursts (GRBs) remains a complete mystery, despite the
recent breakthrough discovery of low energy counterparts, although it is now
generally believed that at least most GRBs are at cosmological distances.
Virtually all proposed cosmological models require bursters to reside in
ordinary galaxies. This can be tested by looking inside the smallest GRB error
boxes to see if ordinary galaxies appear at the expected brightness levels.
This letter reports on an analysis of the contents of 26 of the smallest
regions, many from the brightest bursts. These events will have and
small uncertainties about luminosity functions, K corrections and galaxy
evolutions; whereas the recent events with optical transients are much fainter
and hence have high redshifts and grave difficulties in interpretation. This
analysis strongly rejects the many models with peak luminosities of as deduced from the curve with no evolution.
Indeed, the lower limit on acceptable luminosities is . The only possible solution is to either place GRBs at
unexpectedly large distances (with for the faint BATSE bursts) or to
require bursters to be far outside any normal host galaxy.Comment: 17 pages, to be published by ApJ
The Minimum Variability Time Scale and its Relation to Pulse Profiles of Fermi GRBs
We present a direct link between the minimum variability time scales
extracted through a wavelet decomposition and the rise times of the shortest
pulses extracted via fits of 34 Fermi GBM GRB light curves comprised of 379
pulses. Pulses used in this study were fitted with log-normal functions whereas
the wavelet technique used employs a multiresolution analysis that does not
rely on identifying distinct pulses. By applying a corrective filter to
published data fitted with pulses we demonstrate agreement between these two
independent techniques and offer a method for distinguishing signal from noise.Comment: Accepted for publication in MNRAS Letters. 4 pages, 4 figure
The Hurst Exponent of Fermi GRBs
Using a wavelet decomposition technique, we have extracted the Hurst exponent
for a sample of 46 long and 22 short Gamma-ray bursts (GRBs) detected by the
Gamma-ray Burst Monitor (GBM) aboard the Fermi satellite. This exponent is a
scaling parameter that provides a measure of long-range behavior in a time
series. The mean Hurst exponent for the short GRBs is significantly smaller
than that for the long GRBs. The separation may serve as an unbiased criterion
for distinguishing short and long GRBs.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Extreme gravitational lensing in vicinity of Schwarzschild-de Sitter black holes
We have developed a realistic, fully general relativistic computer code to
simulate optical projection in a strong, spherically symmetric gravitational
field. The standard theoretical analysis of optical projection for an observer
in the vicinity of a Schwarzschild black hole is extended to black hole
spacetimes with a repulsive cosmological constant, i.e, Schwarzschild-de Sitter
spacetimes. Influence of the cosmological constant is investigated for static
observers and observers radially free-falling from the static radius.
Simulations include effects of the gravitational lensing, multiple images,
Doppler and gravitational frequency shift, as well as the intensity
amplification. The code generates images of the sky for the static observer and
a movie simulations of the changing sky for the radially free-falling observer.
Techniques of parallel programming are applied to get a high performance and a
fast run of the BHC simulation code
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