393 research outputs found
High-energy emission from Gamma-Ray Bursts
The number of Gamma-Ray Bursts (GRBs) detected at high energies
( GeV) has seen a rapid increase over the last decade, thanks to
observations from the Fermi-Large Area Telescope. The improved statistics and
quality of data resulted in a better characterisation of the high-energy
emission properties and in stronger constraints on theoretical models. In spite
of the many achievements and progresses, several observational properties still
represent a challenge for theoretical models, revealing how our understanding
is far from being complete. This paper reviews the main spectral and temporal
properties of GeV emission from GRBs and summarises the most
promising theoretical models proposed to interpret the observations. Since a
boost for the understanding of GeV radiation might come from observations at
even higher energies, the present status and future prospects for observations
at very-high energies (above 100 GeV) are also discussed. The improved
sensitivity of upcoming facilities, coupled to theoretical predictions,
supports the concrete possibility for future ground GRB detections in the
high/very-high energy domain.Comment: Invited review article, IJMPD (in press
Anisotropic CR diffusion and gamma-ray production close to supernova remnants, with an application to W28
Cosmic rays that escape their acceleration site interact with the ambient
medium and produce gamma rays as the result of inelastic proton-proton
collisions. The detection of such diffuse emission may reveal the presence of
an accelerator of cosmic rays, and also constrain the cosmic ray diffusion
coefficient in its vicinity. Preliminary results in this direction have been
obtained in the last years from studies of the gamma-ray emission from
molecular clouds located in the vicinity of supernova remnants, which are the
prime candidate for cosmic ray production. Hints have been found for a
significant suppression of the diffusion coefficient with respect to the
average one in the Galaxy. However, most of these studies rely on the
assumption of isotropic diffusion, which may not be very well justified. Here,
we extend this study to the case in which cosmic rays that escape an
accelerator diffuse preferentially along the magnetic field lines. As a first
approximation, we further assume that particles are strongly magnetized and
that their transport perpendicular to the magnetic field is mainly due to the
wandering of the field lines. The resulting spatial distribution of runaway
cosmic rays around the accelerator is, in this case, strongly anisotropic. An
application of the model to the case of the supernova remnant W28 demonstrates
how the estimates of the diffusion coefficient from gamma-ray observations
strongly depend on the assumptions made on the isotropy (or anisotropy) of
diffusion. For higher levels of anisotropy of the diffusion, larger values of
the diffusion coefficient are found to provide a good fit to data. Thus,
detailed models for the propagation of cosmic rays are needed in order to
interpret in a correct way the gamma-ray observations.Comment: 10 pages, 5 figures, submitted to MNRA
Linear and Circular polarization in ultra-relativistic synchrotron sources - implications to GRB afterglows
Polarization measurements from relativistic outflows are a valuable tool to
probe the geometry of the emission region and the microphysics of the particle
distribution. Indeed, the polarization level depends on: (i) the local magnetic
field orientation, (ii) the geometry of the emitting region with respect to the
line of sight, and (iii) the electron pitch-angle distribution. Here we
consider optically thin synchrotron emission and we extend the theory of
circular polarization from a point source to an extended radially expanding
relativistic jet. We present numerical estimates for both linear and circular
polarization in such systems. We consider different configurations of the
magnetic field, spherical and jetted outflows, isotropic and anisotropic
pitch-angle distributions, and outline the difficulty in obtaining the reported
high level of circular polarization observed in the afterglow of GRB 121024A.
We conclude that the origin of the observed polarization cannot be intrinsic to
an optically thin synchrotron process, even when the electron pitch-angle
distribution is extremely anisotropic.Comment: This version matches the version published in MNRAS (moderate
revisions
A Revised Analysis of Gamma Ray Bursts' prompt efficiencies
The prompt Gamma-Ray Bursts' (GRBs) efficiency is an important clue on the
emission mechanism producing the -rays. Previous estimates of the
kinetic energy of the blast waves, based on the X-ray afterglow luminosity
, suggested that this efficiency is large, with values above 90\% in some
cases. This poses a problem to emission mechanisms and in particular to the
internal shocks model. These estimates are based, however, on the assumption
that the X-ray emitting electrons are fast cooling and that their Inverse
Compton (IC) losses are negligible. The observed correlations between
(and hence the blast wave energy) and , the isotropic
equivalent energy in the prompt emission, has been considered as observational
evidence supporting this analysis. It is reasonable that the prompt gamma-ray
energy and the blast wave kinetic energy are correlated and the observed
correlation corroborates, therefore, the notion is indeed a valid proxy
for the latter. Recent findings suggest that the magnetic field in the
afterglow shocks is significantly weaker than was earlier thought and its
equipartition fraction, , could be as low as or even
lower. Motivated by these findings we reconsider the problem, taking now IC
cooling into account. We find that the observed
correlation is recovered also when IC losses are significant. For small
values the blast wave must be more energetic and we find that the
corresponding prompt efficiency is significantly smaller than previously
thought. For example, for we infer a typical prompt
efficiency of .Comment: 10 pages, 4 figures. Accepted for publication in MNRA
Shedding light on the prompt high efficiency paradox - self consistent modeling of GRB afterglows
We examine GRBs with both Fermi-LAT and X-ray afterglow data. Assuming that
the 100MeV (LAT) emission is radiation from cooled electrons accelerated by
external shocks, we show that the kinetic energy of the blast wave estimated
from the 100MeV flux is 50 times larger than the one estimated from the X-ray
flux. This can be explained if either: i) electrons radiating at X-rays are
significantly cooled by SSC (suppressing the synchrotron flux above the cooling
frequency) or ii) if the X-ray emitting electrons, unlike those emitting at
100MeV energies, are in the slow cooling regime. In both cases the X-ray flux
is no longer an immediate proxy of the blast wave kinetic energy. We model the
LAT, X-ray and optical data and show that in general these possibilities are
consistent with the data, and explain the apparent disagreement between X-ray
and LAT observations. All possible solutions require weak magnetic fields:
(where is the fraction of shocked
plasma energy in magnetic fields). Using the LAT emission as a proxy for the
blast wave kinetic energy we find that the derived prompt efficiencies are of
order 15%. This is considerably lower compared with previous estimates (87% and
higher for the same bursts). This provides at least a partial solution to the
"prompt high efficiency paradox".Comment: 8 pages, 2 figures, proceedings of "Swift: 10 Years of Discovery
Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows
We consider a sample of ten GRBs with long lasting ()
emission detected by Fermi/LAT and for which X-ray data around day are
also available. We assume that both the X-rays and the GeV emission are
produced by electrons accelerated at the external forward shock, and show that
the X-ray and the GeV fluxes lead to very different estimates of the initial
kinetic energy of the blast wave. The energy estimated from GeV is on average
times larger than the one estimated from X-rays. We model the data
(accounting also for optical detections around day, if available) to
unveil the reason for this discrepancy and find that good modelling within the
forward shock model is always possible and leads to two possibilities: either
the X-ray emitting electrons (unlike the GeV emitting electrons) are in the
slow cooling regime or ii) the X-ray synchrotron flux is strongly suppressed by
Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is
much smaller at GeV energies. In both cases the X-ray flux is no longer a
robust proxy for the blast wave kinetic energy. On average, both cases require
weak magnetic fields () and
relatively large isotropic kinetic blast wave energies
corresponding to large lower limits
on the collimated energies, in the range
for an ISM environment
with n\sim 1\mbox{cm}^{-3} and
for a wind environment with
. These energies are larger than those estimated from the X-ray
flux alone, and imply smaller inferred values of the prompt efficiency
mechanism, reducing the efficiency requirements on the still uncertain
mechanism responsible for prompt emission.Comment: 14 pages, 7 figures. Accepted for publication in MNRA
La lengua imita a la música: comentario al cap. 18 del "Perì diaÃtes" hipocrático
El escrito médico hipocrático Peri\ diai/thj cuyo tema es, grosso modo, la infl
uencia de la dieta en la salud del hombre, la interacción entre régimen alimenticio
y hábitos de vida con la salud, es uno de los más representativos del grupo de textos
hipocráticos infl uenciados por doctrinas fi losófi cas. En términos generales se le sitúa
cronológicamente a fi nales del s. V o comienzos del IV. En sus primeros capÃtulos el
escrito contiene una serie de refl exiones muy próximas a planteamientos fi losófi cos del
momento, o anteriores, que le llevan a aplicar una doctrina del micro-macrocosmos, en
la que es dominante el infl ujo pitagórico y de los presocráticos.Peer reviewe
Non-linear Cosmic Ray propagation close to the acceleration site
Recent advances on gamma-ray observations from SuperNova Remnants and
Molecular Clouds offer the possibility to study in detail the properties of the
propagation of escaping Cosmic Rays (CR). However, a complete theory for CR
transport outside the acceleration site has not been developed yet. Two
physical processes are thought to be relevant to regulate the transport: the
growth of waves caused by streaming instability, and possible wave damping
mechanisms that reduce the growth of the turbulence. Only a few attempts have
been made so far to incorporate these mechanisms in the theory of CR diffusion.
In this work we present recent advances in this subject. In particular, we show
results obtained by solving the coupled equations for the diffusion of CRs and
the evolution of Alfven waves. We discuss the importance of streaming
instabilities and wave damping in different ISM phases.Comment: Contribution to the Proceedings of the 34th International Cosmic Ray
Conference (ICRC 2015), The Hague, The Netherland
Non-linear diffusion of cosmic rays escaping from supernova remnants - I. The effect of neutrals
Supernova remnants are believed to be the main sources of galactic Cosmic
Rays (CR). Within this framework, particles are accelerated at supernova
remnant shocks and then released in the interstellar medium. The mechanism
through which CRs are released and the way in which they propagate still remain
open issues. The main difficulty is the high non-linearity of the problem: CRs
themselves excite the magnetic turbulence that confines them close to their
sources. We solve numerically the coupled differential equations describing the
evolution in space and time of the escaping particles and of the waves
generated through the CR streaming instability. The warm ionized and warm
neutral phases of the interstellar medium are considered. These phases occupy
the largest fraction of the disc volume, where most supernovae explode, and are
characterised by the significant presence of neutral particles. The friction
between those neutrals and ions results in a very effective wave damping
mechanism. It is found that streaming instability affects the propagation of
CRs even in the presence of ion-neutral friction. The diffusion coefficient can
be suppressed by more than a factor of over a region of few tens of pc
around the remnant. The suppression increases for smaller distances. The
propagation of GeV particles is affected for several tens of
kiloyears after escape, while TeV particles are affected for few
kiloyears. This might have a great impact on the interpretation of gamma-ray
observations of molecular clouds located in the vicinity of supernova remnants.Comment: Revised to match the version published in MNRA
Undocumented Chicanx/Latinx Graduate Students: Illuminating Home-Based Sources Of Support
This qualitative research study explores the experiences of Chicanx/Latinx undocumented graduate students in higher education and specifically examines the home-based teachings and learnings employed by their parents to access and navigate higher education institutions. The study is guided by the research question: How do undocumented Latinx graduate students make sense of the forms of support and participation they receive from their families around education? The authors used Bernal’s (2001) Pedagogies of the Home as a guiding framework. The findings revealed important learnings from their family’s migration, laboral experiences and struggles, and strategies to resist marginalization. Through this study, student experiences highlight important considerations for policy and practice that validate the teachings and learnings that occur in their families and homes
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