1,039 research outputs found
Polarized Scattering in the Vicinty of Galaxies
Some bright cD galaxies in cluster cooling flows have Thomson optical depths
exceeding 0.01. A few percent of their luminosity is scattered and appears as
diffuse polarized emission. We calculate the scattering process for different
geometric combinations of luminosity sources and scattering media. We apply our
results to galaxies, with and without active nuclei, immersed in cooling flows.
We model observations of NGC 1275 and M87 (without active nuclei) in the
presence of sky and galactic background fluxes which hinder the measurement of
the scattered light at optical wavelengths. Current instruments are unable to
detect the scattered light from such objects. However, when a galaxy has an
active nucleus of roughly the same luminosity as the remainder of the galaxy in
V, both the total and polarized scattered intensity should observable on large
scales (5--30kpc), meaning intensity levels greater than 1% of the background
level. For typical AGN and galaxy spectral distributions, the scattering is
most easily detected at short (U) wavelengths. We point out that a number of
such cases will occur. We show that the radiation pattern from the central
nuclear region can be mapped using the scattering. We also show that the
scattered light can be used to measure inhomogeneities in the cooling flow.Comment: 29 pages of TEX, 14 figs, CRSR-1046, in ApJ Nov 20, 199
Cosmic String Loop Microlensing
Cosmic superstring loops within the galaxy microlens background point sources
lying close to the observer-string line of sight. For suitable alignments,
multiple paths coexist and the (achromatic) flux enhancement is a factor of
two. We explore this unique type of lensing by numerically solving for
geodesics that extend from source to observer as they pass near an oscillating
string. We characterize the duration of the flux doubling and the scale of the
image splitting. We probe and confirm the existence of a variety of fundamental
effects predicted from previous analyses of the static infinite straight
string: the deficit angle, the Kaiser-Stebbins effect, and the scale of the
impact parameter required to produce microlensing. Our quantitative results for
dynamical loops vary by O(1) factors with respect to estimates based on
infinite straight strings for a given impact parameter. A number of new
features are identified in the computed microlensing solutions. Our results
suggest that optical microlensing can offer a new and potentially powerful
methodology for searches for superstring loop relics of the inflationary era.Comment: 20 pages, 19 figure
Black Hole-Neutron Star Mergers in Globular Clusters
We model the formation of black hole-neutron star (BH-NS) binaries via
dynamical interactions in globular clusters. We find that in dense, massive
clusters, 16-61% of the BH-NS binaries formed by interactions with existing BH
binaries will undergo mergers driven by the emission of gravitational
radiation. If the BHs are retained by the cluster after merging with a NS, the
BHs acquire subsequent NS companions and undergo several mergers. Thus, the
merger rate depends critically upon whether or not the BH is retained by the
cluster after the merger. Results from numerical relativity suggest that kick
imparted to a ~7 M_sun BH after it merges with a NS will greatly exceed the
cluster's escape velocity. In this case, the models suggest that the majority
of BH-NS mergers in globular clusters occur within 4 Gyrs of the cluster's
formation and would be unobservable by Advanced LIGO. For more massive BHs, on
the other hand, the post merger kick is suppressed and the BH is retained.
Models with 35 M_sun BHs predict Advanced LIGO detection rates in the range
0.04 - 0.7 per year. On the pessimistic end of this range, BH-NS mergers
resulting from binary-single star interactions in globular clusters could
account for an interesting fraction of all BH-NS mergers. On the optimistic
end, this channel may dominate the rate of detectable BH-NS mergers.Comment: 13 pages, 7 figures, 3 tabels, accepted for publication in MNRA
Dynamically formed black hole+millisecond pulsar binaries in globular clusters
The discovery of a binary comprising a black hole (BH) and a millisecond
pulsar (MSP) would yield insights into stellar evolution and facilitate
exquisitely sensitive tests of general relativity. Globular clusters (GCs) are
known to harbor large MSP populations and recent studies suggest that GCs may
also retain a substantial population of stellar mass BHs. We modeled the
formation of BH+MSP binaries in GCs through exchange interactions between
binary and single stars. We found that in dense, massive clusters most of the
dynamically formed BH+MSP binaries will have orbital periods of 2 to 10 days,
regardless of the mass of the BH, the number of BHs retained by the cluster,
and the nature of the GC's binary population. The size of the BH+MSP population
is sensitive to several uncertain parameters, including the BH mass function,
the BH retention fraction, and the binary fraction in GCs. Based on our models,
we estimate that there are dynamically formed BH+MSP binaries in
the Milky Way GC system, and place an upper limit on the size of this
population of . Interestingly, we find that BH+MSP binaries will be
rare even if GCs retain large BH populations.Comment: 15 pages, 5 figures, 2 tables, accepted for publication in MNRAS,
updated to match published versio
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