1,456 research outputs found
Expected Number and Flux Distribution of Gamma-Ray-Burst Afterglows with High Redshifts
If Gamma-Ray-Bursts (GRBs) occur at high redshifts, then their bright
afterglow emission can be used to probe the ionization and metal enrichment
histories of the intervening intergalactic medium during the epoch of
reionization. In contrast to other sources, such as galaxies or quasars, which
fade rapidly with increasing redshift, the observed infrared flux from a GRB
afterglow at a fixed observed age is only a weak function of its redshift. This
results from a combination of the spectral slope of GRB afterglows and the
time-stretching of their evolution in the observer's frame. Assuming that the
GRB rate is proportional to the star formation rate and that the characteristic
energy output of GRBs is ~10^{52} ergs, we predict that there are always ~15
GRBs from redshifts z>5 across the sky which are brighter than ~100 nJy at an
observed wavelength of ~2 \mu m. The infrared spectrum of these sources could
be taken with the future Next Generation Space Telescope, as a follow-up on
their early X-ray localization with the Swift satellite.Comment: 29 pages, 14 figures; submitted to Ap
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
The Link between Warm Molecular Disks in Maser Nuclei and Star Formation near the Black Hole at the Galactic Center
The discovery of hundreds of young, bright stars within a parsec from the
massive black hole at the center of the Galaxy presents a challenge to star
formation theories. The requisite Roche densities for the gravitational
collapse of gas clouds are most naturally achieved in accretion disks. The
water maser sources in Keplerian rotation in the nuclei of NGC4258, NGC1068,
and the Circinus galaxy indicate the presence of warm, extended, molecular
accretion disks around black holes similar in mass to the one at the Galactic
center. Here we argue that the current conditions in the maser nuclei, and
those near the Galactic center, represent two consecutive, recurrent phases in
the life cycle of the nucleus of a typical gas-rich spiral bulge. The warm
molecular disks that give rise to the observed maser emission fragment into
stellar-size objects. The stellar masses, their orbital geometry, and the total
number of stars thus formed are consistent with the values identified at the
Galactic center. The stars tend to form in compact groups resembling the IRS 13
complex that dominates the stellar light in the neighborhood of the black hole.Comment: ApJ Letters, in pres
Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks
We examine the electromagnetic (EM) and gravitational wave (GW) signatures of
stellar-mass compact objects (COs) spiraling into a supermassive black hole
(extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure
dominated, accretion disk. At large separations, the tidal effect of the
secondary CO clears a gap. We show that the gap refills during the late
GW-driven phase of the inspiral, leading to a sudden EM brightening of the
source. The accretion disk leaves an imprint on the GW through its angular
momentum exchange with the binary, the mass increase of the binary members due
to accretion, and its gravity. We compute the disk-modified GWs both in an
analytical Newtonian approximation and in a numerical effective-one-body
approach. We find that disk-induced migration provides the dominant
perturbation to the inspiral, with weaker effects from the mass accretion onto
the CO and hydrodynamic drag. Depending on whether a gap is present, the
perturbation of the GW phase is between 10 and 1000 radians per year,
detectable with the future Laser Interferometer Space Antenna (LISA) at high
significance. The Fourier transform of the disk-modified GW in the stationary
phase approximation is sensitive to disk parameters with a frequency trend
different from post-Newtonian vacuum corrections. Our results suggest that
observations of EMRIs may place new sensitive constraints on the physics of
accretion disks.Comment: 42 pages, 8 figures, 3 tables, submitted to Phys. Rev.
Toward an Ising Model of Cancer and Beyond
Theoretical and computational tools that can be used in the clinic to predict
neoplastic progression and propose individualized optimal treatment strategies
to control cancer growth is desired. To develop such a predictive model, one
must account for the complex mechanisms involved in tumor growth. Here we
review resarch work that we have done toward the development of an "Ising
model" of cancer. The review begins with a description of a minimalist
four-dimensional (three in space and one in time) cellular automaton (CA) model
of cancer in which healthy cells transition between states (proliferative,
hypoxic, and necrotic) according to simple local rules and their present
states, which can viewed as a stripped-down Ising model of cancer. This model
is applied to model the growth of glioblastoma multiforme, the most malignant
of brain cancers. This is followed by a discussion of the extension of the
model to study the effect on the tumor dynamics and geometry of a mutated
subpopulation. A discussion of how tumor growth is affected by chemotherapeutic
treatment is then described. How angiogenesis as well as the heterogeneous and
confined environment in which a tumor grows is incorporated in the CA model is
discussed. The characterization of the level of organization of the invasive
network around a solid tumor using spanning trees is subsequently described.
Then, we describe open problems and future promising avenues for future
research, including the need to develop better molecular-based models that
incorporate the true heterogeneous environment over wide range of length and
time scales (via imaging data), cell motility, oncogenes, tumor suppressor
genes and cell-cell communication. The need to bring to bear the powerful
machinery of the theory of heterogeneous media to better understand the
behavior of cancer in its microenvironment is presented.Comment: 55 pages, 21 figures and 3 tables. To appear in Physical Biology.
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