1,112 research outputs found
On the thermal conduction in tangled magnetic fields in clusters of galaxies
Thermal conduction in tangled magnetic fields is reduced because heat
conducting electrons must travel along the field lines longer distances between
hot and cold regions of space than if there were no fields. We consider the
case when the tangled magnetic field has a weak homogeneous component. We
examine two simple models for temperature in clusters of galaxies: a
time-independent model and a time-dependent one. We find that the actual value
of the effective thermal conductivity in tangled magnetic fields depends on how
it is defined for a particular astrophysical problem. Our final conclusion is
that the heat conduction never totally suppressed but is usually important in
the central regions of galaxy clusters, and therefore, it should not be
neglected.Comment: 16 pages, 4 figure
RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster
An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi
X-ray Timing Explorer proportional counter array produced a high quality
spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum
obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging
spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for
non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic
field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for
the cosmic-ray electron energy density. The resulting lower limit to the
central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can
be created by galaxy wakes, a magnetic field of several uG is usually
associated with a cooling flow or, as in the case of the Coma cluster, a
subcluster merger. However, for A2256, the evidence for a merger is weak and
the main cluster shows no evidence of a cooling flow. Thus, there is presently
no satisfactory hypothesis for the origin of an average cluster magnetic field
as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press
Global analysis of muon decay measurements
We have performed a global analysis of muon decay measurements to establish
model-independent limits on the space-time structure of the muon decay matrix
element. We find limits on the scalar, vector and tensor coupling of right- and
left-handed muons to right- and left-handed electrons. The limits on those
terms that involve the decay of right-handed muons to left-handed electrons are
more restrictive than in previous global analyses, while the limits on the
other non-standard model interactions are comparable. The value of the Michel
parameter eta found in the global analysis is -0.0036 \pm 0.0069, slightly more
precise than the value found in a more restrictive analysis of a recent
measurement. This has implications for the Fermi coupling constant G_F.Comment: 5 pages, 3 table
Lambda hyperonic effect on the normal driplines
A generalized mass formula is used to calculate the neutron and proton drip
lines of normal and lambda hypernuclei treating non-strange and strange nuclei
on the same footing. Calculations suggest existence of several bound
hypernuclei whose normal cores are unbound. Addition of Lambda or,
Lambda-Lambda hyperon(s) to a normal nucleus is found to cause shifts of the
neutron and proton driplines from their conventional limits.Comment: 6 pages, 4 tables, 0 figur
Magnetic Field Evolution in Merging Clusters of Galaxies
We present initial results from the first 3-dimensional numerical
magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging
clusters of galaxies. Within the framework of idealized initial conditions
similar to our previous work, we look at the gasdynamics and the magnetic field
evolution during a major merger event in order to examine the suggestion that
shocks and turbulence generated during a cluster/subcluster merger can produce
magnetic field amplification and relativistic particle acceleration and, as
such, may play a role in the formation and evolution of cluster-wide radio
halos. The ICM, as represented by the equations of ideal MHD, is evolved
self-consistently within a changing gravitational potential defined largely by
the collisionless dark matter component represented by an N-body particle
distribution. The MHD equations are solved by the Eulerian, finite-difference
code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We
find significant evolution of the magnetic field structure and strength during
two distinct epochs of the merger evolution.Comment: 21 pages, 7 figures, Figure 2 is color postscript. Accepted for
publication in Ap
Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization
Two possible explanations for the low luminosity of the supermassive black
hole at the center of our galaxy are (1) an accretion rate of order the
canonical Bondi value (roughly 10^{-5} solar masses per year), but a very low
radiative efficiency for the accreting gas or (2) an accretion rate much less
than the Bondi rate. Both models can explain the broad-band spectrum of the
Galactic Center. We show that they can be distinguished using the linear
polarization of synchrotron radiation. Accretion at the Bondi rate predicts no
linear polarization at any frequency due to Faraday depolarization. Low
accretion rate models, on the other hand, have much lower gas densities and
magnetic field strengths close to the black hole; polarization may therefore be
observable at high frequencies. If confirmed, a recent detection of linear
polarization from Sgr A above 150 GHz argues for an accretion rate of order
10^{-8} solar masses per year, much less than the Bondi rate. This test can be
applied to other low-luminosity galactic nuclei.Comment: final version accepted by ApJ; references added, somewhat shortene
Gamma-ray probe of cosmic-ray pressure in galaxy clusters and cosmological implications
Cosmic rays produced in cluster accretion and merger shocks provide pressure
to the intracluster medium (ICM) and affect the mass estimates of galaxy
clusters. Although direct evidence for cosmic-ray ions in the ICM is still
lacking, they produce gamma-ray emission through the decay of neutral pions
produced in their collisions with ICM nucleons. We investigate the capability
of the Gamma-ray Large Area Space Telescope (GLAST) and imaging atmospheric
Cerenkov telescopes (IACTs) for constraining the cosmic-ray pressure
contribution to the ICM. We show that GLAST can be used to place stringent
upper limits, a few per cent for individual nearby rich clusters, on the ratio
of pressures of the cosmic rays and thermal gas. We further show that it is
possible to place tight (<~10%) constraints for distant (z <~ 0.25) clusters in
the case of hard spectrum, by stacking signals from samples of known clusters.
The GLAST limits could be made more precise with the constraint on the
cosmic-ray spectrum potentially provided by IACTs. Future gamma-ray
observations of clusters can constrain the evolution of cosmic-ray energy
density, which would have important implications for cosmological tests with
upcoming X-ray and Sunyaev-Zel'dovich effect cluster surveys.Comment: 12 pages, 5 figures; extended discussions; accepted by MNRA
Nonthermal hard X-ray excess in the cluster Abell 2256 from two epoch observations
After confirmation of the presence of a nonthermal hard X-ray excess with
respect to the thermal emission in the Coma cluster from two independent
observations, obtained using the Phoswich Detection System onboard BeppoSAX, we
present in this Letter also for Abell 2256 the results of two observations
performed with a time interval of about 2.5 yr. In both spectra a nonthermal
excess is present at a confidence level of ~3.3sigma and ~3.7sigma,
respectively. The combined spectrum obtained by adding up the two spectra
allows to measure an excess at the level of ~4.8sigma in the 20-80 keV energy
range. The nonthermal X-ray flux is in agreement with the published value of
the first observation (Fusco-Femiano et al. 2000) and with that measured by a
Rossi X-Ray Timing Explorer observation (Rephaeli & Gruber 2003).Comment: 12 pages, 3 figures, 1 table - ApJL, in pres
On The Origin of Radio Halos in Galaxy Clusters
Previously it has been recognized that radio halos in galaxy clusters are
preferentially associated with merging systems as indicated by substructure in
the X-ray images and temperature maps. Since, however, many clusters without
radio halos also possess substructure, the role of mergers in the formation of
radio halos has remained unclear. By using power ratios to relate gravitational
potential fluctuations to substructure in X-ray images, we provide the first
quantitative comparison of the dynamical states of clusters possessing radio
halos. A correlation between the 1.4 GHz power (P_{1.4}) of the radio halo (or
relic) and the magnitude of the dipole power ratio (P_1/P_0) is discovered such
that approximately P_{1.4} ~ P_1/P_0; i.e., the strongest radio halos appear
only in those clusters currently experiencing the largest departures from a
virialized state. From additional consideration of a small number of highly
disturbed clusters without radio halos detected at 1.4 GHz, and recalling that
radio halos are more common in clusters with high X-ray luminosity (Giovannini,
Tordi, & Feretti), we argue that radio halos form preferentially in massive
(L_x >~ 0.5 x 10^{45} erg/s) clusters experiencing violent mergers (P_1/P_0 >~
0.5 x 10^{-4}) that have seriously disrupted the cluster core. The association
of radio halos with massive, large-P_1/P_0, core-disrupted clusters is able to
account for both the vital role of mergers in accelerating the relativistic
particles responsible for the radio emission as well as the rare occurrence of
radio halos in cluster samples.Comment: 4 pages, 1 figure, Accepted for Publication in The Astrophysical
Journal Letters, updated reference
A View through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN
Results from a survey of the parsec scale Faraday rotation measure properties
for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation
measures for quasars vary from approximately 500 to several thousand radians
per meter squared. Quasar jets have rotation measures which are typically 500
radians per meter squared or less. The cores and jets of the BL Lac objects
have rotation measures similar to those found in quasar jets. The jets of radio
galaxies exhibit a range of rotation measures from a few hundred radians per
meter squared to almost 10,000 radians per meter squared for the jet of M87.
Radio galaxy cores are generally depolarized, and only one of four radio
galaxies (3C-120) has a detectable rotation measure in the core. Several
potential identities for the foreground Faraday screen are considered and we
believe the most promising candidate for all the AGN types considered is a
screen in close proximity to the jet. This constrains the path length to
approximately 10 parsecs, and magnetic field strengths of approximately 1
microGauss can account for the observed rotation measures. For 27 out of 34
quasars and BL Lacs their optically thick cores have good agreement to a lambda
squared law. This requires the different tau = 1 surfaces to have the same
intrinsic polarization angle independent of frequency and distance from the
black hole.Comment: Accepted to the Astrophysical Journal: 71 pages, 40 figure
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