2,867 research outputs found
Gamma-ray flares from black hole coronae
We present results of a study of non-thermal, time-dependent particle
injection in a corona around an accreting black hole. We model the spectral
energy distribution of high-energy flares in this scenario. We consider
particle interactions with magnetic, photon and matter fields in the black hole
magnetosphere. Transport equations are solved for all species of particles and
the electromagnetic output is predicted. Photon annihilation is taken into
account for the case of systems with early-type donor stars.Comment: 7 pages, 7 figures. Accepted for publication in the Proceedings of
the 25th Texas Symposium on Relativistic Astrophysics, held in Heidelberg,
December 06-10, 201
Gravitational Lensing of Neutrino from Collapsars
We study neutrino emission from long gamma-ray bursts. The collapse of very
massive stars to black holes, and the consequent jet formation, are expected to
produce high-energy neutrinos through photomeson production. Such neutrinos can
escape from the source and travel up to the Earth. We focus on the case of
Population III progenitors for gamma-ray bursts. Neutrinos can be the only
source of information of the first stars formed in the universe. The expected
signal is rather weak, but we propose that gravitational lensing by nearby
supermassive black holes might enhance the neutrino emission in some cases. We
implement a Monte Carlo analysis to ponder the statistical significance of this
scenario. We suggest that an observational strategy based on gravitational
lensing could lead to the detection of neutrinos from the re-ionization era of
the universe with current instrumentation.Comment: 5 pages, 3 figures. Accepted for publication in the Proceedings of
the First Argentine-Brazilian Meeting on Gravitation, Astrophysics and
Cosmology, held in Foz do Igua\c{c}u, October, 201
A model for the polarization of the high-energy radiation from accreting black holes: the case of XTE J1118+480
The high-energy emission ( keV MeV) of Cygnus X-1 --the most
well-studied Galactic black hole-- was recently found to be strongly polarized.
The origin of this radiation is still unknown. In this work, we suggest that it
is the result of non-thermal processes in the hot corona around the accreting
compact object, and study the polarization of high-energy radiation expected
for black hole binaries. Two contributions to the total magnetic field are
taken into account in our study, a small scale random component related to the
corona, and an ordered magnetic field associated with the accretion disk. The
degree of polarization of gamma-ray emission for this particular geometry is
estimated, as well as the angle of the polarization vector. We obtain that the
corona+disk configuration studied in this work can account for the high degree
of polarization of gamma-rays detected in galactic black holes without the need
of a relativistic jet; specific predictions are made for sources in a low-hard
state. In particular, the model is applied to the transient source \xtee; we
show that if a new outburst of \xte is observed, then its gamma-ray
polarization should be measurable by future instruments, such as ASTRO-H or the
proposed ASTROGAM.Comment: 8 pages, 6 figures, accepted for publication in A&
Constraining f(R) theories with cosmography
A method to set constraints on the parameters of extended theories of
gravitation is presented. It is based on the comparison of two series
expansions of any observable that depends on H(z). The first expansion is of
the cosmographical type, while the second uses the dependence of H with z
furnished by a given type of extended theory. When applied to f(R) theories
together with the redshift drift, the method yields limits on the parameters of
two examples (the theory of Hu and Sawicki (2007), and the exponential gravity
introduced by Linder (2009)) that are compatible with or more stringent than
the existing ones, as well as a limit for a previously unconstrained parameter.Comment: 9 pages, 2 figures; v2: modifications in title and presentation,
results unchange
Primordial black hole evolution in two-fluid cosmology
Several processes in the early universe might lead to the formation of
primordial black holes with different masses. These black holes would interact
with the cosmic plasma through accretion and emission processes. Such
interactions might have affected the dynamics of the universe and generated a
considerable amount of entropy. In this paper we investigate the effects of the
presence of primordial black holes on the evolution of the early universe. We
adopt a two-fluid cosmological model with radiation and a primordial black hole
gas. The latter is modelled with different initial mass functions taking into
account the available constraints over the initial primordial black hole
abundances. We find that certain populations with narrow initial mass functions
are capable to produce significant changes in the scale factor and the entropy.Comment: 8 pages, 7 figures. Modified to match the published versio
A mechanism for fast radio bursts
Fast radio bursts are mysterious transient sources likely located at
cosmological distances. The derived brightness temperatures exceed by many
orders of magnitude the self-absorption limit of incoherent synchrotron
radiation, implying the operation of a coherent emission process. We propose a
radiation mechanism for fast radio bursts where the emission arises from
collisionless Bremsstrahlung in strong plasma turbulence excited by
relativistic electron beams. We discuss possible astrophysical scenarios in
which this process might operate. The emitting region is a turbulent plasma hit
by a relativistic jet, where Langmuir plasma waves produce a concentration of
intense electrostatic soliton-like regions (cavitons). The resulting radiation
is coherent and, under some physical conditions, can be polarised and have a
power-law distribution in energy. We obtain radio luminosities in agreement
with the inferred values for fast radio bursts. The timescale of the radio
flare in some cases can be extremely fast, of the order of s. The
mechanism we present here can explain the main features of fast radio bursts
and is plausible in different astrophysical sources, such as gamma-ray bursts
and some Active Galactic Nuclei.Comment: 6 pages, 1 figure. Accepted for publication in Phys. Rev.
Non-thermal radiation from Cygnus X-1 corona
Cygnus X-1 was the first X-ray source widely accepted to be a black hole
candidate and remains among the most studied astronomical objects in its class.
The detection of non-thermal radio, hard X-rays and gamma rays reveals the fact
that this kind of objects are capable of accelerating particles up to very high
energies.
In order to explain the electromagnetic emission from Cygnus X-1 in the
low-hard state we present a model of a black hole corona with both relativistic
lepton and hadron content. We characterize the corona as a two-temperature hot
plasma plus a mixed non-thermal population in which energetic particles
interact with magnetic, photon and matter fields. Our calculations include the
radiation emitted by secondary particles (pions, muons and electron/positron
pairs). Finally, we take into account the effects of photon absorption. We
compare the results obtained from our model with data of Cygnus X-1 obtained by
the COMPTEL instrument.Comment: 6 pages, 10 figures, presented as a poster in HEPRO II, Buenos Aires,
Argentina, October 26-30 2009 / accepted for publication in Int. Jour. Mod.
Phys.
Decolonizing Native Histories
Decolonizing Native Histories is an interdisciplinary collection that grapples with the racial and ethnic politics of knowledge production and indigenous activism in the Americas. It analyzes the relationship of language to power and empowerment, and advocates for collaborations between community members, scholars, and activists that prioritize the rights of Native peoples to decide how their knowledge is used. The contributors—academics and activists, indigenous and nonindigenous, from disciplines including history, anthropology, linguistics, and political science—explore the challenges of decolonization.
These wide-ranging case studies consider how language, the law, and the archive have historically served as instruments of colonialism and how they can be creatively transformed in constructing autonomy. The collection highlights points of commonality and solidarity across geographical, cultural, and linguistic boundaries and also reflects deep distinctions between North and South. Decolonizing Native Histories looks at Native histories and narratives in an internationally comparative context, with the hope that international collaboration and understanding of local histories will foster new possibilities for indigenous mobilization and an increasingly decolonized future
Episodic gamma-ray and neutrino emission from the low mass X-ray binary GRO J0422+32
GRO J0422+32 is a member of the class of low-mass X-ray binary (LMXB)
sources, discovered during an outburst in 1992. Along the entire episode ( days) a persistent power-law spectral component extending up to
MeV was observed. These results suggest that non-thermal processes must be at
work in the system. We apply a corona model to describe the spectrum of GRO
J0422+32 during the flaring phase. We study relativistic particle interactions
solving the transport equations for all type of particles self-consistently. We
fit the electromagnetic emission during the plateau phase of the event and
estimate the emission during an energetic episode, as well as the neutrino
production. Our work leads to predictions that can be tested by the new
generation of very high energy gamma-ray instruments.Comment: HIGH ENERGY GAMMA-RAY ASTRONOMY: 5th International Meeting on High
Energy Gamma-Ray Astronomy. AIP Conference Proceedings, Volume 1505, pp.
410-413 (2012
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