9,766 research outputs found
Two-level laser light statistics
The statistics of the light emitted by two-level lasers is evaluated on the
basis of generalized rate equations. According to that approach, all
fluctuations are interpreted as being caused by the jumps that occur in active
and detecting atoms. The intra-cavity Fano factor and the photo-current
spectral density are obtained analytically for Poissonian and quiet pumps. The
algebra is simple and the formulas hold for small as well as large pumping
rates. Lasers exhibit excess noise at low pumping levels.Comment: 9 pages, 3 figures, in Optics Communication format (elsevier
Fluorescence from a few electrons
Systems containing few Fermions (e.g., electrons) are of great current
interest. Fluorescence occurs when electrons drop from one level to another
without changing spin. Only electron gases in a state of equilibrium are
considered. When the system may exchange electrons with a large reservoir, the
electron-gas fluorescence is easily obtained from the well-known Fermi-Dirac
distribution. But this is not so when the number of electrons in the system is
prevented from varying, as is the case for isolated systems and for systems
that are in thermal contact with electrical insulators such as diamond. Our
accurate expressions rest on the assumption that single-electron energy levels
are evenly spaced, and that energy coupling and spin coupling between electrons
are small. These assumptions are shown to be realistic for many systems.
Fluorescence from short, nearly isolated, quantum wires is predicted to drop
abruptly in the visible, a result not predicted by the Fermi-Dirac
distribution. Our exact formulas are based on restricted and unrestricted
partitions of integers. The method is considerably simpler than the ones
proposed earlier, which are based on second quantization and contour
integration.Comment: 10 pages, 3 figures, RevTe
The hot gas content of fossil galaxy clusters
We investigate the properties of the hot gas in four fossil galaxy systems
detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey.
XMM-Newton observations reveal overall temperatures of kT ~ 5-6 keV and yield
hydrostatic masses M500,HE > 3.5 x 10e14 Msun, confirming their nature as bona
fide massive clusters. We measure the thermodynamic properties of the hot gas
in X-rays (out to beyond R500 in three cases) and derive their individual
pressure profiles out to R ~ 2.5 R500 with the SZ data. We combine the X-ray
and SZ data to measure hydrostatic mass profiles and to examine the hot gas
content and its radial distribution. The average Navarro-Frenk-White (NFW)
concentration parameter, c500 = 3.2 +/- 0.4, is the same as that of relaxed
`normal' clusters. The gas mass fraction profiles exhibit striking variation in
the inner regions, but converge to approximately the cosmic baryon fraction
(corrected for depletion) at R500. Beyond R500 the gas mass fraction profiles
again diverge, which we interpret as being due to a difference in gas clumping
and/or a breakdown of hydrostatic equilibrium in the external regions. Overall
our observations point to considerable radial variation in the hot gas content
and in the gas clumping and/or hydrostatic equilibrium properties in these
fossil clusters, at odds with the interpretation of their being old, evolved
and undisturbed. At least some fossil objects appear to be dynamically young.Comment: 4 pages, 2 figures. Accepted for publication in A&
Rate-equation approach to atomic-laser light statistics
We consider three- and four-level atomic lasers that are either incoherently
(unidirectionally) or coherently (bidirectionally) pumped, the single-mode
cavity being resonant with the laser transition. The intra-cavity Fano factor
and the photo-current spectral density are evaluated on the basis of rate
equations.
According to that approach, fluctuations are caused by jumps in active and
detecting atoms. The algebra is considerably simpler than the one required by
Quantum-Optics treatments.
Whenever a comparison can be made, the expressions obtained coincide. The
conditions under which the output light exhibits sub-Poissonian statistics are
considered in detail. Analytical results, based on linearization, are verified
by comparison with Monte Carlo simulations. An essentially exhaustive
investigation of sub-Poissonian light generation by three- and four-level atoms
lasers has been performed. Only special forms were reported earlier.Comment: 9 pages, 7 figures, RevTeX
Bent-Double Radio Sources as Probes of Intergalactic Gas
As the most common environment in the universe, groups of galaxies are likely
to contain a significant fraction of the missing baryons in the form of
intergalactic gas. The density of this gas is an important factor in whether
ram pressure stripping and strangulation affect the evolution of galaxies in
these systems. We present a method for measuring the density of intergalactic
gas using bent-double radio sources that is independent of temperature, making
it complementary to current absorption line measurements. We use this method to
probe intergalactic gas in two different environments: inside a small group of
galaxies as well as outside of a larger group at a 2 Mpc radius and measure
total gas densities of and per cubic centimeter (random and systematic
errors) respectively. We use X-ray data to place an upper limit of K on the temperature of the intragroup gas in the small group.Comment: 6 pages, 1 figure, accepted for publication in Ap
Time-Varying Graphs and Dynamic Networks
The past few years have seen intensive research efforts carried out in some
apparently unrelated areas of dynamic systems -- delay-tolerant networks,
opportunistic-mobility networks, social networks -- obtaining closely related
insights. Indeed, the concepts discovered in these investigations can be viewed
as parts of the same conceptual universe; and the formal models proposed so far
to express some specific concepts are components of a larger formal description
of this universe. The main contribution of this paper is to integrate the vast
collection of concepts, formalisms, and results found in the literature into a
unified framework, which we call TVG (for time-varying graphs). Using this
framework, it is possible to express directly in the same formalism not only
the concepts common to all those different areas, but also those specific to
each. Based on this definitional work, employing both existing results and
original observations, we present a hierarchical classification of TVGs; each
class corresponds to a significant property examined in the distributed
computing literature. We then examine how TVGs can be used to study the
evolution of network properties, and propose different techniques, depending on
whether the indicators for these properties are a-temporal (as in the majority
of existing studies) or temporal. Finally, we briefly discuss the introduction
of randomness in TVGs.Comment: A short version appeared in ADHOC-NOW'11. This version is to be
published in Internation Journal of Parallel, Emergent and Distributed
System
Chandra Observations of low velocity dispersion groups
Deviations of galaxy groups from cluster scaling relations can be understood
in terms of an excess of entropy in groups. The main effect of this excess is
to reduce the density and thus luminosity of the intragroup gas. Given this,
groups should also should show a steep relationship between X-ray luminosity
and velocity dispersion. However, previous work suggests that this is not the
case with many measuring slopes flatter than the cluster relation.
Examining the group L_X:\sigma relation shows that much of the flattening is
caused by a small subset of groups which show very high X-ray luminosities for
their velocity dispersions (or vice versa).
Detailed Chandra study of two such groups shows that earlier ROSAT results
were subject to significant (~30-40%) point source contamination, but confirm
that a significant hot IGM is present in these groups, although these are two
of the coolest systems in which intergalactic X-ray emission has been detected.
Their X-ray properties are shown to be broadly consistent with those of other
galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its
X-ray luminosity correspondingly high for its temperature, compared to most
groups.
This leads us to suggest that the velocity dispersion in these systems has
been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap
Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster's Dynamical State
We employ an ensemble of hydrodynamic cluster simulations to create spatially
and spectrally resolved images of quality comparable to Chandra's expected
performance. Emission from simulation mass elements is represented using the
XSPEC mekal program assuming 0.3 solar metallicity, and the resulting spectra
are fit with a single-temperature model. Despite significant departures from
isothermality in the cluster gas, single-temperature models produce acceptable
fits to 20,000 source photon spectra. The spectral fit temperature T_s is
generally lower than the mass weighted average temperature T_m due to the
influence of soft line emission from cooler gas being accreted as part of the
hierarchical clustering process. In a Chandra-like bandpass of 0.5 to 9.5 keV
we find a nearly uniform fractional bias of (T_m-T_s)/T_s = 20% with occasional
large deviations in smaller clusters. In the more traditional 2.0 to 9.5 keV
bandpass, the fractional deviation is scale-dependent and on average follows
the relation (T_m-T_s)/T_s = 0.2 log(T_m). This bias results in a spectral
mass-temperature relationship with slope about 1.6, intermediate between the
virial relation M ~ T_m^{3/2} and the observed relation M_{ICM} ~ T^2. Imaging
each cluster in the ensemble at 16 epochs in its evolutionary history, we
catalogue merger events with mass ratios exceeding 10% in order to investigate
the relationship between spectral temperature and proximity to a major merger
event. Clusters that are very cool relative to the mean mass-temperature
relationship lie preferentially close to a merger, suggesting a viable
observational method to cull a subset of dynamically young clusters from the
general population.Comment: 34 pages, including 2 tables and 14 figures (one in color). Compiled
using LaTeX 2.09 with graphics package and aaspp4 style. The simulated
spectral data files used in this paper are available for public consumption
at http://redshift.stanford.edu/bfm
The stellar mass function of galaxies in Planck-selected clusters at 0.5 < z < 0.7: new constraints on the timescale and location of satellite quenching
We study the abundance of star-forming and quiescent galaxies in a sample of
21 massive clusters at 0.5<z<0.7, detected with the Planck satellite. We
measure the cluster galaxy stellar mass function (SMF), which is a fundamental
observable to study and constrain the formation and evolution of galaxies. Our
measurements are based on homogeneous and deep multi-band photometry spanning
u- to the Ks-band for each cluster and are supported by spectroscopic data from
different programs. The galaxy population is separated between quiescent and
star-forming galaxies based on their rest-frame U-V and V-J colours. The SMF is
compared to that of field galaxies at the same redshifts, using data from the
COSMOS/UltraVISTA survey. We find that the shape of the SMF of star-forming
galaxies does not depend on environment, while the SMF of quiescent galaxies
has a significantly steeper low-mass slope in the clusters compared to the
field. We estimate the environmental quenching efficiency (f_EQ), i.e. the
probability for a galaxy that would normally be star forming in the field, to
be quenched due to its environment. The f_EQ shows no stellar-mass dependence
in any environment, but it increases from 40% in the cluster outskirts to ~90%
in the cluster centres. The radial signature of f_EQ provides constraints on
where the dominant quenching mechanism operates in these clusters and on what
timescale. Exploring these using a simple model based on galaxy orbits obtained
from an N-body simulation, we find a clear degeneracy between both parameters.
For example, the quenching process may either be triggered on a long (~3 Gyr)
time scale at large radii (r~8R_500), or happen well within 1 Gyr at r<R_500.
The radius where quenching is triggered is at least r_quench> 0.67R_500
(95%CL). The ICM density at this location suggests that ram-pressure stripping
of the cold gas is a likely cause of quenching. [Abridged]Comment: 16 pages, 12 figures, accepted for publication in A&
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