36,783 research outputs found
Discovery of X-ray Emission from the Galactic Supernova Remnant G32.8-0.1 with Suzaku
We present the first dedicated X-ray study of the supernova remnant (SNR)
G32.8-0.1 (Kes 78) with Suzaku. X-ray emission from the whole SNR shell has
been detected for the first time. The X-ray morphology is well correlated with
the emission from the radio shell, while anti-correlated with the molecular
cloud found in the SNR field. The X-ray spectrum shows not only conventional
low-temperature (kT ~ 0.6 keV) thermal emission in a non-equilibrium ionization
state, but also a very high temperature (kT ~ 3.4 keV) component with a very
low ionization timescale (~ 2.7e9 cm^{-3}s), or a hard non-thermal component
with a photon index Gamma~2.3. The average density of the low-temperature
plasma is rather low, of the order of 10^{-3}--10^{-2} cm^{-3}, implying that
this SNR is expanding into a low-density cavity. We discuss the X-ray emission
of the SNR, also detected in TeV with H.E.S.S., together with multi-wavelength
studies of the remnant and other gamma-ray emitting SNRs, such as W28 and RCW
86. Analysis of a time-variable source, 2XMM J185114.3-000004, found in the
northern part of the SNR, is also reported for the first time. Rapid time
variability and a heavily absorbed hard X-ray spectrum suggest that this source
could be a new supergiant fast X-ray transient.Comment: 20 pages, 14 figures, ApJ, in pres
Latent Gaussian Count Time Series Modeling
This paper develops theory and methods for the copula modeling of stationary
count time series. The techniques use a latent Gaussian process and a
distributional transformation to construct stationary series with very flexible
correlation features that can have any pre-specified marginal distribution,
including the classical Poisson, generalized Poisson, negative binomial, and
binomial count structures. A Gaussian pseudo-likelihood estimation paradigm,
based only on the mean and autocovariance function of the count series, is
developed via some new Hermite expansions. Particle filtering methods are
studied to approximate the true likelihood of the count series. Here,
connections to hidden Markov models and other copula likelihood approximations
are made. The efficacy of the approach is demonstrated and the methods are used
to analyze a count series containing the annual number of no-hitter baseball
games pitched in major league baseball since 1893
Spin-phonon coupling in Gd(Co1/2Mn1/2)O3 perovskite
We have investigated the temperature-dependent Raman-active phonons and the
magnetic properties of Gd(Co1/2Mn1/2)O3 perovskite ceramics in the temperature
range from 40 K to 300 K. The samples crystallized in an orthorhombic distorted
simple perovskite, whose symmetry belongs to the Pnma space group. The data
reveals spin-phonon coupling near the ferromagnetic transition occurring at
around 120 K. The correlation of the Raman and magnetization data suggests that
the structural order influences the magnitude of the spin-phonon coupling.Comment: 3 Figures, suplementary materia
Raman study of the Verwey transition in Magnetite at high-pressure and low-temperature; effect of Al doping
We report high-pressure low-temperature Raman studies of the Verwey
transition in pure and Al-doped magnetite (Fe_3O_4). The low temperature phase
of magnetite displays a number of additional Raman modes that serve as
transition markers. These transition markers allow one to investigate the
effect of hydrostatic pressure on the Verwey transition temperature. Al-doped
magnetite Fe_2.8Al_0.2O_4 (TV=116.5K) displays a nearly linear decrease of the
transition temperature with an increase of pressure yielding dP/dT_V = -0.096
GPa/K. In contrast pure magnetite displays a significantly steeper slope of the
PT equilibrium line with dP/dT_V = -0.18 GPa/K. The slope of the PT equilibrium
lines is related to the changes of the molar entropy and molar volume at the
transition. We compare our spectroscopic data with that obtained from the
ambient pressure specific heat measurements and find a good agreement in the
optimally doped magnetite. Our data indicates that Al doping leads to a smaller
entropy change and larger volume expansion at the transition. Our data displays
the trends that are consistent with the mean field model of the transition that
assumes charge ordering in magnetite.Comment: 17 pages, 3 figure
Examining the time dependence of DAMA's modulation amplitude
If dark matter is composed of weakly interacting particles, Earth's orbital
motion may induce a small annual variation in the rate at which these particles
interact in a terrestrial detector. The DAMA collaboration has identified at a
9.3 confidence level such an annual modulation in their event rate over
two detector iterations, DAMA/NaI and DAMA/LIBRA, each with years of
observations. We statistically examine the time dependence of the modulation
amplitudes, which "by eye" appear to be decreasing with time in certain energy
ranges. We perform a chi-squared goodness of fit test of the average modulation
amplitudes measured\ by the two detector iterations which rejects the
hypothesis of a consistent modulation amplitude at greater than 80\%, 96\%, and
99.6\% for the 2--4~keVee, 2--5~keVee and 2--6~keVee energy ranges,
respectively. We also find that among the 14 annual cycles there are three
departures from the average in the 5-6~keVee energy range. In
addition, we examined several phenomenological models for the time dependence
of the modulation amplitude. Using a maximum likelihood test, we find that
descriptions of the modulation amplitude as decreasing with time are preferred
over a constant modulation amplitude at anywhere between 1 and
3, depending on the phenomenological model for the time dependence and
the signal energy range considered. A time dependent modulation amplitude is
not expected for a dark matter signal, at least for dark matter halo
morphologies consistent with the DAMA signal. New data from DAMA/LIBRA--phase2
will certainly aid in determining whether any apparent time dependence is a
real effect or a statistical fluctuation.Comment: 13 pages, 1 figur
Discovery of Broad Molecular lines and of Shocked Molecular Hydrogen from the Supernova Remnant G357.7+0.3: HHSMT, APEX, Spitzer and SOFIA Observations
We report a discovery of shocked gas from the supernova remnant (SNR)
G357.7+0.3. Our millimeter and submillimeter observations reveal broad
molecular lines of CO(2-1), CO(3-2), CO(4-3), 13CO (2-1) and 13CO (3-2), HCO^+
and HCN using HHSMT, Arizona 12-Meter Telescope, APEX and MOPRA Telescope. The
widths of the broad lines are 15-30 kms, and the detection of such broad lines
is unambiguous, dynamic evidence showing that the SNR G357.7+0.3 is interacting
with molecular clouds. The broad lines appear in extended regions (>4.5'x5').
We also present detection of shocked H2 emission in mid-infrared but lacking
ionic lines using the Spitzer IRS observations to map a few arcmin area. The H2
excitation diagram shows a best-fit with a two-temperature LTE model with the
temperatures of ~200 and 660 K. We observed [C II] at 158um and high-J
CO(11-10) with the GREAT on SOFIA. The GREAT spectrum of [C II], a 3 sigma
detection, shows a broad line profile with a width of 15.7 km/s that is similar
to those of broad CO molecular lines. The line width of [C~II] implies that
ionic lines can come from a low-velocity C-shock. Comparison of H2 emission
with shock models shows that a combination of two C-shock models is favored
over a combination of C- and J-shocks or a single shock. We estimate the CO
density, column density, and temperature using a RADEX model. The best-fit
model with n(H2) = 1.7x10^{4} cm^{-3}, N(CO) = 5.6x10^{16} cm^{-2}, and T = 75
K can reproduce the observed millimeter CO brightnesses.Comment: 19 pages, 22 figure
Primordial Black Holes: Observational Characteristics of The Final Evaporation
Many early universe theories predict the creation of Primordial Black Holes
(PBHs). PBHs could have masses ranging from the Planck mass to 10^5 solar
masses or higher depending on the size of the universe at formation. A Black
Hole (BH) has a Hawking temperature which is inversely proportional to its
mass. Hence a sufficiently small BH will quasi-thermally radiate particles at
an ever-increasing rate as emission lowers its mass and raises its temperature.
The final moments of this evaporation phase should be explosive and its
description is dependent on the particle physics model. In this work we
investigate the final few seconds of BH evaporation, using the Standard Model
and incorporating the most recent Large Hadron Collider (LHC) results, and
provide a new parameterization for the instantaneous emission spectrum. We
calculate for the first time energy-dependent PBH burst light curves in the
GeV/TeV energy range. Moreover, we explore PBH burst search methods and
potential observational PBH burst signatures. We have found a unique signature
in the PBH burst light curves that may be detectable by GeV/TeV gamma-ray
observatories such as the High Altitude Water Cerenkov (HAWC) observatory. The
implications of beyond the Standard Model theories on the PBH burst
observational characteristics are also discussed, including potential
sensitivity of the instantaneous photon detection rate to a squark threshold in
the 5 -10 TeV range.Comment: Accepted to Astroparticle Physics Journal (71 Pages, 22 Figures
The Three-Dimensional Expansion of the Ejecta from Tycho's Supernova Remnant
We present the first three-dimensional measurements of the velocity of
various ejecta knots in Tycho's supernova remnant, known to result from a Type
Ia explosion. Chandra X-ray observations over a 12-year baseline from 2003 to
2015 allow us to measure the proper motion of nearly 60 "tufts" of Si-rich
ejecta, giving us the velocity in the plane of the sky. For the line of sight
velocity, we use two different methods: a non-equilibrium ionization model fit
to the strong Si and S lines in the 1.2-2.8 keV regime, and a fit consisting of
a series of Gaussian lines. These methods give consistent results, allowing us
to determine the red or blue shift of each of the knots. Assuming a distance of
3.5 kpc, we find total velocities that range from 2400 to 6600 km s,
with a mean of 4430 km s. We find several regions where the ejecta knots
have overtaken the forward shock. These regions have proper motions in excess
of 6000 km s. Some Type Ia supernova explosion models predict a velocity
asymmetry in the ejecta. We find no such velocity asymmetries in Tycho, and
discuss our findings in light of various explosion models, favoring those
delayed detonation models with relatively vigorous and symmetrical
deflagrations. Finally, we compare measurements with models of the remnant's
evolution that include both smooth and clumpy ejecta profiles, finding that
both ejecta profiles can be accommodated by the observations.Comment: Accepted for publication in ApJ. Some figures slightly degraded to
reduce file siz
A Study of Dark Matter and QCD-Charged Mediators in the Quasi-Degenerate Regime
We study a scenario in which the only light new particles are a Majorana
fermion dark matter candidate and one or more QCD-charged scalars, which couple
to light quarks. This scenario has several interesting phenomenological
features if the new particles are nearly degenerate in mass. In particular, LHC
searches for the light scalars have reduced sensitivity, since the visible and
invisible products tend to be softer. Moreover, dark matter-scalar
co-annihilation can allow even relatively heavy dark matter candidates to be
consistent thermal relics. Finally, the dark matter nucleon scattering cross
section is enhanced in the quasi-degenerate limit, allowing direct detection
experiments to use both spin-independent and spin-dependent scattering to probe
regions of parameter space beyond those probed by the LHC. Although this
scenario has broad application, we phrase this study in terms of the MSSM, in
the limit where the only light sparticles are a bino-like dark matter candidate
and light-flavored squarks.Comment: 24 pages, 5 figures; as published in PRD with significant revision
Realization of random-field dipolar Ising ferromagnetism in a molecular magnet
The longitudinal magnetic susceptibility of single crystals of the molecular
magnet Mn-acetate obeys a Curie-Weiss law, indicating a transition to a
ferromagnetic phase due to dipolar interactions. With increasing magnetic field
applied transverse to the easy axis, the transition temperature decreases
considerably more rapidly than predicted by mean field theory to a T=0 quantum
critical point. Our results are consistent with an effective Hamiltonian for a
random-field Ising ferromagnet in a transverse field, where the randomness is
induced by an external field applied to Mn-acetate crystals that are
known to have an intrinsic distribution of locally tilted magnetic easy axes.Comment: 4 pages, 4 figure
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