1,094 research outputs found
Neutrinos from the Galactic Center in the Light of its Gamma-Ray Detection at TeV Energy
We re-evaluate the event rate expected in km^3-scale detectors for neutrinos
from the direction of the Galactic Center (GC) in light of recent spectral
measurements obtained by the HESS instrument for ~TeV gamma-radiation from this
direction. In the most plausible scenario the re-evaluated event rate is
smaller than that previously calculated--and here re-calculated--on the basis
of EGRET data. However, the GC TeV gamma-ray detections by the Whipple,
CANGAROO, and HESS instruments, together with the strong indications for an
overabundance of cosmic rays coming from the GC at EeV energies, strengthen the
expectation for a detectable, TeV-PeV GC neutrino signal from proton-proton
interactions in that region. If the TeV gamma-ray--EeV cosmic ray anisotropy
connection is correct, this signal will be detectable within a year and half
for km^3-scale neutrino detectors in the Northern Hemisphere at super-TeV
energies and, significantly, should also be detectable in 1.6 years by the
South Polar IceCube detector at energies > 10^14 eV. The GC neutrino signal
should also produce a detectable signal from neutrino showering and resonant
W^- production by anti-electron-neutrinos in the volume of a km^3-scale
detector.Comment: 12 pages, 1 figure. Version accepted to ApJ Letters. Minor amendment
The Gravitational Horizon for a Universe with Phantom Energy
The Universe has a gravitational horizon, coincident with the Hubble sphere,
that plays an important role in how we interpret the cosmological data.
Recently, however, its significance as a true horizon has been called into
question, even for cosmologies with an equation-of-state w = p/rho > -1, where
p and rho are the total pressure and energy density, respectively. The claim
behind this argument is that its radius R_h does not constitute a limit to our
observability when the Universe contains phantom energy, i.e., when w < -1, as
if somehow that mitigates the relevance of R_h to the observations when w > -1.
In this paper, we reaffirm the role of R_h as the limit to how far we can see
sources in the cosmos, regardless of the Universe's equation of state, and
point out that claims to the contrary are simply based on an improper
interpretation of the null geodesics.Comment: 9 pages, 1 figure. Slight revisions in refereed version. Accepted for
publication in JCAP. arXiv admin note: text overlap with arXiv:1112.477
Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*
Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have
yielded several strong constraints on the transient energization mechanism,
justifying a re-examination of the stochastic acceleration model proposed
previously for these events. We here demonstrate that the new results are fully
consistent with the acceleration of electrons via the transit-time damping
process. But more importantly, these new NIR and X-ray flares now can constrain
the source size, the gas density, the magnetic field, and the wave energy
density in the turbulent plasma. Future simultaneous multi-wavelength
observations with good spectral information will, in addition, allow us to
study their temporal evolution, which will eventually lead to an accurate
determination of the behavior of the plasma just minutes prior to its
absorption by the black hole.Comment: 13 pages, 4 figures, Submitted to ApJ
A fit to the simultaneous broadband spectrum of Cygnus X-1 using the transition disk model
We have used the transition disk model to fit the simultaneous broad band
( keV) spectrum of Cygnus X-1 from OSSE and Ginga observations. In this
model, the spectrum is produced by saturated Comptonization within the inner
region of the accretion disk, where the temperature varies rapidly with radius.
In an earlier attempt, we demonstrated the viability of this model by fitting
the data from EXOSAT, XMPC balloon and OSSE observations, though these were not
made simultaneously. Since the source is known to be variable, however, the
results of this fit were not conclusive. In addition, since only once set of
observations was used, the good agreement with the data could have been a
chance occurrence. Here, we improve considerably upon our earlier analysis by
considering four sets of simultaneous observations of Cygnus X-1, using an
empirical model to obtain the disk temperature profile. The vertical structure
is then obtained using this profile and we show that the analysis is self-
consistent. We demonstrate conclusively that the transition disk spectrum is a
better fit to the observations than that predicted by the soft photon
Comptonization model. Since the temperature profile is obtained by fitting the
data, the unknown viscosity mechanism need not be specified. The disk structure
can then be used to infer the viscosity parameter , which appears to
vary with radius and luminosity. This behavior can be understood if
depends intrinsically on the local parameters such as density, height and
temperature. However, due to uncertainties in the radiative transfer,
quantitative statements regarding the variation of cannot yet be made.Comment: 8 figures. uses aasms4.sty, accepted by ApJ (Mar 98
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Polarimetric Imaging of the Massive Black Hole at the Galactic Center
The radio source Sgr A* in the Galactic center emits a polarized spectrum at
millimeter and sub-millimeter wavelengths that is strongly suggestive of
relativistic disk accretion onto a massive black hole. We use the
well-constrained mass of Sgr A* and a magnetohydrodynamic model of the
accretion flow to match both the total flux and polarization from this object.
Our results demonstrate explicitly that the shift in the position angle of the
polarization vector, seen at wavelengths near the peak of the mm to sub-mm
emission from this source, is a signal of relativistic accretion flow in a
strong gravitational field. We provide maps of the polarized emission to
illustrate how the images of polarized intensity from the vicinity of the black
hole would appear in upcoming observations with very long baseline radio
interferometers (VLBI). Our results suggest that near-term VLBI observations
will be able to directly image the polarized Keplerian portion of the flow near
the horizon of the black hole.Comment: 12 pages, 2 figures, Accepted for publciation in ApJ Letter
Big Blue Bump and Transient Active Regions in Seyfert Galaxies
An important feature of the EUV spectrum (known as the Big Blue Bump,
hereafter BBB) in Seyfert Galaxies is the narrow range in its cutoff energy E0
from source to source, even though the luminosity changes by 4 orders of
magnitude. Here we show that if the BBB is due to accretion disk emission, then
in order to account for this ``universality'' in the value of E0, the emission
mechanism is probably optically thin bremsstrahlung. In addition, we
demonstrate that the two-phase model with active regions localized on the
surface of the cold disk is consistent with this constraint if the active
regions are very compact and are highly transient, i.e., they evolve faster
than one dynamical time scale.Comment: Accepted for publication in ApJL, uses aas2pp4.st
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