3,608 research outputs found
Synchrotron and Synchrotron Self-Compton Spectral Signatures and Blazar Emission Models
We find that energy losses due to synchrotron self-Compton (SSC) emission in
blazar jets can produce distinctive signatures in the time-averaged synchrotron
and SSC spectra of these objects. For a fairly broad range of particle
injection distributions, SSC-loss dominated synchrotron emission exhibits a
spectral dependence . The presence or absence of this
dependence in the optical and ultraviolet spectra of flat spectrum radio
quasars such as 3C~279 and in the soft X-ray spectra of high frequency BL Lac
objects such as Mrk 501 gives a robust measure of the importance of SSC losses.
Furthermore, for partially cooled particle distributions, spectral breaks of
varying sizes can appear in the synchrotron and SSC spectra and will be related
to the spectral indices of the emission below the break. These spectral
signatures place constraints on the size scale and the non-thermal particle
content of the emitting plasma as well as the observer orientation relative to
the jet axis.Comment: 4 pages, 1 figure, LaTeX2e, emulateapj5.sty, accepted for publication
in Ap
Line emission from gamma-ray burst environments
The time and angle dependent line and continuum emission from a dense torus
around a cosmological gamma-ray burst source is simulated, taking into account
photoionization, collisional ionization, recombination, and electron heating
and cooling due to various processes. The importance of the hydrodynamical
interaction between the torus and the expanding blast wave is stressed. Due to
the rapid deceleration of the blast wave as it interacts with the dense torus,
the material in the torus will be illuminated by a drastically different photon
spectrum than observable through a low-column-density line of sight, and will
be heated by the hydrodynamical interaction between the blast wave and the
torus. A model calculation to reproduce the Fe K-alpha line emission observed
in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~
10^{-4} solar masses of iron must be concentrated in a region of less than
10^{-3} pc. The illumination of the torus material due to the hydrodynamic
interaction of the blast wave with the torus is the dominant heating and
ionization mechanism leading to the formation of the iron line. These results
suggest that misaligned GRBs may be detectable as X-ray flashes with pronounced
iron emission line features.Comment: Accepted for publication in ApJ. Updated recombination rate data;
discussion on element abundances added; references update
The Spectral Energy Distribution of the High-Z Blazar Q0906+693
We describe further observations of QSO J0906+6930, a z=5.48 blazar likely to
be detected in gamma-rays. New radio and X-ray data place significant
constraints on any kpc-scale extension of the VLBA-detected jet. Improved
optical spectroscopy detects absorption from an intervening galaxy at z=1.849
and raise the possibility that this distant, bright source is lensed. We
combine the new data into an improved SED for the blazar core and comment on
the Compton keV-GeV flux component.Comment: 10pp, 3 figures, accpeted for publication in the Astronomical Journa
On the singular values and eigenvalues of the Fox–Li and related operators
The Fox–Li operator is a convolution operator over a finite
interval with a special highly oscillatory kernel. It plays an important
role in laser engineering. However, the mathematical analysis of its spectrum
is still rather incomplete. In this expository paper we survey part
of the state of the art, and our emphasis is on showing how standard
Wiener–Hopf theory can be used to obtain insight into the behaviour of
the singular values of the Fox–Li operator. In addition, several approximations
to the spectrum of the Fox–Li operator are discussed and results
on the singular values and eigenvalues of certain related operators are
derived
Dust sublimation by GRBs and its implications
The prompt optical flash recently detected accompanying GRB990123 suggests
that, for at least some GRBs, gamma-ray emission is accompanied by prompt
optical-UV emission with luminosity L(1-7.5eV)=10^{49}(\Delta\Omega/4\pi)erg/s,
where \Delta\Omega is the solid angle into which gamma-ray and optical-UV
emission is beamed. Such an optical-UV flash can destroy dust in the beam by
sublimation out to an appreciable distance, approximately 10 pc, and may clear
the dust out of as much as 10^7(\Delta\Omega/4\pi)M_sun of molecular cloud
material on an apparent time scale of 10 seconds. Detection of time dependent
extinction on this time scale would therefore provide strong constraints on the
GRB source environment. Dust destruction implies that existing, or future,
observations of not-heavily-reddened fireballs are not inconsistent with GRBs
being associated with star forming regions. In this case, however, if gamma-ray
emission is highly beamed, the expanding fireball would become reddened on a 1
week time scale.
If the optical depth due to dust beyond approximately 8 pc from the GRB is
0.2<\tau_V<2, most of the UV flash energy is converted to infra-red, \lambda
\sim 1 micron, radiation with luminosity \sim 10^{41} erg/s extending over an
apparent duration of \sim 20(1+z)(\Delta\Omega/0.01) day. Dust infra-red
emission may already have been observed in GRB970228 and GRB980326, and may
possibly explain their unusual late time behavior.Comment: 16 pages, including 1 figure, submitted to Ap
A Case Study of Co-teacher Relationships for English Language Learners in a Suburban Elementary School
English Language Learners (ELLs) in the United States are faced with many academic challenges including those of language acquisition, lack of background knowledge, fear of participation and unknown academic language. These challenges mixed with the ever-growing demands of the educational system, resulted in State and local educational agencies to make a distinct shift away from segregated instructional services for ELLs, by either requiring or recommending integrated co-teaching as an optimal way to educate them. Although a large number of teachers and administrators are unfamiliar with collaborative practices for ELLs, school districts have nonetheless implemented integrated co-teaching models to comply with guidelines. However, the basic establishment of professionals in an assigned classroom does not create a collaborative teaching partnership. The development and success of these partnerships relied on many different factors.
This study explored the development of co-teaching relationships in an elementary public school setting organized to serve ELL students through an integrated model. This study examined the co-teaching relationships between general education (GE) and Teachers to Speakers of Other Languages (TESOL) in an elementary school and described how these teachers constructed collective efficacy beliefs that affected the development, implementation, and sustainment of a successful co-teaching relationship.
This research utilized a qualitative case study methodology. Data was collected through focus groups with co-teachers, an interview with administration, observations of collaborative sessions and a review of documents (i.e. lesson plans, school improvement plan). This study provided guidance on how teachers of ELLs construct collective efficacy beliefs that affect the development, implementation and sustainment of a successful integrated co-teaching model. Furthermore, this study provided information for school administrators so that they may recognize key elements and strategies that will guide them in fostering successful co-teaching partnerships amongst their staff. The outcome of this study, identified key elements and strategies to guide administrators and teachers in fostering successful co-teaching relationships that benefit all students
Crystal structure of Schmallenberg orthobunyavirus nucleoprotein-RNA complex reveals a novel RNA sequestration mechanism
Schmallenberg virus (SBV) is a newly emerged orthobunyavirus (family Bunyaviridae) that has caused severe disease in the offspring of farm animals across Europe. Like all orthobunyaviruses, SBV contains a tripartite negative-sense RNA genome that is encapsidated by the viral nucleocapsid (N) protein in the form of a ribonucleoprotein complex (RNP). We recently reported the three-dimensional structure of SBV N that revealed a novel fold. Here we report the crystal structure of the SBV N protein in complex with a 42-nt-long RNA to 2.16 Å resolution. The complex comprises a tetramer of N that encapsidates the RNA as a cross-shape inside the protein ring structure, with each protomer bound to 11 ribonucleotides. Eight bases are bound in the positively charged cleft between the N- and C-terminal domains of N, and three bases are shielded by the extended N-terminal arm. SBV N appears to sequester RNA using a different mechanism compared with the nucleoproteins of other negative-sense RNA viruses. Furthermore, the structure suggests that RNA binding results in conformational changes of some residues in the RNA-binding cleft and the N- and C-terminal arms. Our results provide new insights into the novel mechanism of RNA encapsidation by orthobunyaviruses
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Assessment For Learning: The Role of Feedback
Formative assessment (FA) and learning-oriented assessment (LOA) focus on the learning process. These ongoing assessments for learning can be an integral part of instruction and may add a new dimension to the learning mandate inherent in the classroom. Assessment for learning is founded on the notion that learners can raise their consciousness of their learning aims through the process of assessment and work towards improvement (Luoma & Tarnanen, 2003). That is, FA or LOA enables the students to gather “positive evidence” about what they know, and “negative evidence” about what is still not being incorporated into their interlanguage correctly (Purpura, 2004). Such assessment practices not only provide students with critical information about what they know but also enable teachers to assess their students‟ ongoing progress at various stages of the learning process. Leung (2004) defines classroom-based FA as “the noticing and gathering of information about student language use in ordinary (noncontrived) classroom activities, and the use of that information to make decisions about teaching (to promote learning) without necessarily quantifying it or using it for reporting purposes” (p. 20). Indeed, FA or LOA serves both the teacher in terms of planning future curricula as well as the students in giving them feedback so that they become aware of their strengths, weaknesses, and what needs to be implemented to make improvement. This discussion will focus on the important role of feedback in FA or LOA
Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst
The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is
analyzed in terms of different variations of the homogeneous leptonic jet model
for the production of high-energy radiation from blazars. We find that a
two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and
an external Compton component, is required in order to yield an acceptable fit
to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other
BL Lac objects, the broad emission line region plays an important role for the
high-energy emission. Several alternative blazar jet models are briefly
discussed. In the appendix, we describe the formalism in which the process of
Comptonization of reprocessed accretion disk photons is treated in the
previously developed blazar jet simulation code which we use.Comment: Now accepted for publication in The Astronomical Journal.
Significantly extended discussion w.r.t. original version. 3 Figures included
using epsf.sty, rotate.st
Spherical magnetic nanoparticles: magnetic structure and interparticle interaction
The interaction between spherical magnetic nanoparticles is investigated from
micromagnetic simulations and ananlysed in terms of the leading dipolar
interaction energy between magnetic dipoles. We focus mainly on the case where
the particles present a vortex structure. In a first step the local magnetic
structure in the isolated particle is revisited. For particles bearing a
uniaxial magnetocrystaline anisotropy, it is shown that the vortex core
orientation relative to the easy axis depends on both the particle size and the
anisotropy constant. When the particles magnetization present a vortex
structure, it is shown that the polarization of the particles by the dipolar
field of the other one must be taken into account in the interaction. An
analytic form is deduced for the interaction which involves the vortex core
magnetization and the magnetic susceptibility which are obtained from the
magnetic properties of the isolated particle.Comment: 20 pages, 10 figures Published in Journal of Applied Physics. To be
found at: http://link.aip.org/link/?jap/105/07391
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