3,735 research outputs found
Proton Cyclotron Features in Thermal Spectra of Ultra-magnetized Neutron Stars
A great deal of interest has been recently raised in connection with the
possibility that soft -ray repeaters (SGRs) and anomalous X-ray pulsars
(AXPs) contain {\em magnetars}, young neutron stars endowed with magnetic
fields G. In this paper we calculate thermal spectra from
ultra-magnetized neutron stars for values of the luminosity and magnetic field
believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very
close to a blackbody at the star effective temperature and exhibit a
distinctive absorption feature at the proton cyclotron energy keV. The proton cyclotron features (PCFs) are
conspicuous (equivalent width of up to many hundreds eV) and relatively broad
(). The detection of the PCFs is well within the
capabilities of present X-ray spectrometers, like the HETGS and METGS on board
Chandra. Their observation might provide decisive evidence in favor of the
existence of magnetars.Comment: 7 pages, 4 figures, minor changes included, typos corrected. Accepted
for publication in Ap
High-energy electron-induced damage production at room temperature in aluminum-doped silicon
DLTS and EPR measurements are reported on aluminum-doped silicon that was irradiated at room temperature with high-energy electrons. Comparisons are made to comparable experiments on boron-doped silicon. Many of the same defects observed in boron-doped silicon are also observed in aluminum-doped silicon, but several others were not observed, including the aluminum interstitial and aluminum-associated defects. Damage production modeling, including the dependence on aluminum concentration, is presented
The Accretion Rates and Spectral Energy Distributions of BL Lacertae Objects
We investigate the relationship between accretion rates and the spectral
energy distributions (SEDs) of BL Lac objects, using a sample of objects for
which published information on the host galaxies, emission-line luminosities,
and peak frequencies and luminosities of their SEDs are available. The sample
is composed of 43 BL Lac objects which have a relatively continuous
distribution of peak frequencies. Under the assumption that the observed
emission lines are photoionized by the central accretion disk, we use the line
luminosities to estimate the accretion luminosities and hence accretion rates.
We find that low frequency-peaked BL Lac objects (LBLs) span a wide range of
accretion rates, whereas high frequency-peaked BL Lac objects (HBLs) cover a
more restricted range of lower values. There appears to be a continuous
distribution of accretion rates between the two subclasses of BL Lac objects.
We find that the peak frequency of the SED, \pknu, correlates with the
accretion rate, approximately with the form \pknu\propto \Lambda^{-3} in HBLs
and \pknu \propto \Lambda^{-0.25} in LBLs, where . The peak luminosity of the SED is also correlated with .
These results suggest that the accretion rate influences the shape of the SED
in BL Lac objects. They also support models which couple the jet and the
accretion disk. We present a physical scenario to account for the empirical
trends.Comment: 6 pages in emulateapj.sty, 3 figures 1 table. The Astrophysical
Journal (in press
ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac)
We present ASCA observations of the radio-selected BL Lacertae objects
1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept and
Nov, respectively. The ASCA spectra of both sources can be described as a first
approximation by a power law with photon index Gamma ~ 2. This is flatter than
for most X-ray-selected BL Lacs observed with ASCA, in agreement with the
predictions of current blazar unification models. While 1749+096 exhibits
tentative evidence for spectral flattening at low energies, a concave continuum
is detected for 2200+420: the steep low-energy component is consistent the
high-energy tail of the synchrotron emission responsible for the longer
wavelengths, while the harder tail at higher energies is the onset of the
Compton component. The spectral energy distributions from radio to gamma-rays
are consistent with synchrotron-self Compton emission from a single homogeneous
region shortward of the IR/optical wavelengths, with a second component in the
radio domain related to a more extended emission region. For 2200+420,
comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we find
that models requiring inverse Compton scattering of external photons provide a
viable mechanism for the production of the highest (GeV) energies during the
flare. An increase of the external radiation density and of the power injected
in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A
directly testable prediction of this model is that the line luminosity in
2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for
publication in the Astrophysical Journa
Markarian 421's Unusual Satellite Galaxy
We present Hubble Space Telescope (HST) imagery and photometry of the active
galaxy Markarian 421 and its companion galaxy 14 arcsec to the ENE. The HST
images indicate that the companion is a morphological spiral rather than
elliptical as previous ground--based imaging has concluded. The companion has a
bright, compact nucleus, appearing unresolved in the HST images. This is
suggestive of Seyfert activity, or possibly a highly luminous compact star
cluster. We also report the results of high dynamic range long-slit
spectroscopy with the slit placed to extend across both galaxies and nuclei. We
detect no emission lines in the companion nucleus, though there is evidence for
recent star formation. Velocities derived from a number of absorption lines
visible in both galaxies indicate that the two systems are probably tidally
bound and thus in close physical proximity. Using the measured relative
velocities, we derive a lower limit on the MKN 421 mass within the companion
orbit (R \sim 10 kpc) of 5.9 \times 10^{11} solar masses, and a mass-to-light
ratio of >= 17. Our spectroscopy also shows for the first time the presence of
H\alpha and [NII] emission lines from the nucleus of MKN 421, providing another
example of the appearance of new emission features in the previously
featureless spectrum of a classical BL Lac object. We see both broad and narrow
line emission, with a velocity dispersion of several thousand km s^{-1} evident
in the broad lines.Comment: LaTeX (aaspp4 style), 28 pages, 8 figures, to appear in AJ. Revised
text from ref. comments; new & modified figures; new photometry included;
minor corrections of typos. Color version of Fig. 1 to appear in Feb. 2000
Sky & Telescop
Predicting the long-term impact of antiretroviral therapy scale-up on population incidence of tuberculosis.
OBJECTIVE: To investigate the impact of antiretroviral therapy (ART) on long-term population-level tuberculosis disease (TB) incidence in sub-Saharan Africa. METHODS: We used a mathematical model to consider the effect of different assumptions about life expectancy and TB risk during long-term ART under alternative scenarios for trends in population HIV incidence and ART coverage. RESULTS: All the scenarios we explored predicted that the widespread introduction of ART would initially reduce population-level TB incidence. However, many modelled scenarios projected a rebound in population-level TB incidence after around 20 years. This rebound was predicted to exceed the TB incidence present before ART scale-up if decreases in HIV incidence during the same period were not sufficiently rapid or if the protective effect of ART on TB was not sustained. Nevertheless, most scenarios predicted a reduction in the cumulative TB incidence when accompanied by a relative decline in HIV incidence of more than 10% each year. CONCLUSIONS: Despite short-term benefits of ART scale-up on population TB incidence in sub-Saharan Africa, longer-term projections raise the possibility of a rebound in TB incidence. This highlights the importance of sustaining good adherence and immunologic response to ART and, crucially, the need for effective HIV preventive interventions, including early widespread implementation of ART
Moving from a Product-Based Economy to a Service-Based Economy for a More Sustainable Future
Traditionally, economic growth and prosperity have been linked with the availability, production and distribution of tangible goods as well as the ability of consumers to acquire such goods. Early evidence regarding this connection dates back to Adam Smith's Wealth of Nations (1776), in which any activity not resulting in the production of a tangible good is characterized as unproductive of any value." Since then, this coupling of economic value and material production has been prevalent in both developed and developing economies throughout the world. One unintended consequence of this coupling has been the exponential increase in the amount of solid waste being generated. The reason is that any production and consumption of material goods eventually generates the equivalent amount of (or even more) waste. Exacerbating this problem is the fact that, with today's manufacturing and supply chain management technologies, it has become cheaper to dispose and replace most products rather than to repair and reuse them. This has given rise to what some call a disposable society." To put things in perspective: In 2012 households in the U.K. generated approximately 22 thousand tons of waste, which amounted to 411 kg of waste generated per person (Department for Environment, Food & Rural Affairs, 2015). During the same time period, households in the U.S. generated 251 million tons of waste, which is equivalent to a person generating approximately 2 kg of waste every day (U.S. Environmental Protection Agency, 2012). Out of these 251 million tons of total waste generated, approximately 20% of the discarded items were categorized as durable goods. The disposal of durable goods is particularly worrisome because they are typically produced using material from non- renewable resources such as iron, minerals, and petroleum-based raw materials
Historic Light Curve and Long-term Optical Variation of BL Lacertae 2200+420
In this paper, historical optical(UBVRI) data and newly observed data from
the Yunnan Observatory of China(about100 years) are presented for BL Lacertae.
Maximum variations in UBVRI: 5.12, 5.31, 4.73, 2.59, and 2.54 and color indices
of U-B = -0.11 +/- 0.20, B-V= 1.0 +/- 0.11, V-R= 0.73 +/- 0.19, V-I= 1.42 +/-
0.25, R-I= 0.82 +/- 0.11, and B-I= 2.44 +/- 0.29 have been obtained from the
literature; The Jurkevich method is used to investigate the existence of
periods in the B band light curve, and a long-term period of 14 years is found.
The 0.6 and 0.88 year periods reported by Webb et al.(1988) are confirmed. In
addition, a close relation between B-I and B is found, suggesting that the
spectra flattens when the source brightens.Comment: 21 pages, 6 figures, 2 table, aasms4.sty, to be published in ApJ,
Vol. 507, 199
A Viewing Angle - Kinetic Luminosity Unification Scheme For BL Lacertae Objects
We propose a unified classification for BL Lac objects (BLs), focusing on the
synchrotron peak frequency of the spectral energy distribution. The unification
scheme is based on the angle Theta that describes the orientation of the
relativistic jet and on the electron kinetic luminosity Lambda of the jet. We
assume that Lambda scales with the size of the jet r in a self-similar fashion
(Lambda propto r^2), as supported by observational data. The jets are
self-similar in geometry and have the same pressure and median magnetic field
at the inlet, independent of size. The self-similarity is broken for the
highest energy electrons, which radiate mainly at high frequencies, since for
large sources they suffer more severe radiative energy losses over a given
fraction of the jet length. We calculate the optically thin synchrotron
spectrum using an accelerating inner jet model based on simple relativistic gas
dynamics and show that it can fit the observed infrared to X-ray spectrum of
PKS 2155--304. We couple the accelerating jet model to the unification scheme
and compare the results to complete samples of
BLs. The negative apparent evolution of X-ray selected BLs is explained as a
result of positive evolution of the jet electron kinetic luminosity
. We review observational arguments in favor of the existence of
scaled-down accretion disks and broad emission-line regions in BLs. The
proposed unification scheme can explain the lack of observed broad emission
lines in X-ray selected BLs, as well as the existence of those lines
preferentially in luminous radio-selected BLs. Finally, we review observational
arguments that suggest the extension of this unification scheme to all blazars.Comment: 32 pages, 8 figures, to be published in the ApJ (Oct 20, 1998
Dynamic Key-Value Memory Networks for Knowledge Tracing
Knowledge Tracing (KT) is a task of tracing evolving knowledge state of
students with respect to one or more concepts as they engage in a sequence of
learning activities. One important purpose of KT is to personalize the practice
sequence to help students learn knowledge concepts efficiently. However,
existing methods such as Bayesian Knowledge Tracing and Deep Knowledge Tracing
either model knowledge state for each predefined concept separately or fail to
pinpoint exactly which concepts a student is good at or unfamiliar with. To
solve these problems, this work introduces a new model called Dynamic Key-Value
Memory Networks (DKVMN) that can exploit the relationships between underlying
concepts and directly output a student's mastery level of each concept. Unlike
standard memory-augmented neural networks that facilitate a single memory
matrix or two static memory matrices, our model has one static matrix called
key, which stores the knowledge concepts and the other dynamic matrix called
value, which stores and updates the mastery levels of corresponding concepts.
Experiments show that our model consistently outperforms the state-of-the-art
model in a range of KT datasets. Moreover, the DKVMN model can automatically
discover underlying concepts of exercises typically performed by human
annotations and depict the changing knowledge state of a student.Comment: To appear in 26th International Conference on World Wide Web (WWW),
201
- âŠ