28,185 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
The soft X-ray excess AGN RE J2248-511
We model the spectral energy distribution of the ultrasoft broad-line AGN RE
J2248-511 with Comptonised accretion disc models. These are able to reproduce
the steep optical and ultrasoft X-ray slopes, and the derived black hole mass
is consistent with independent mass estimates. This AGN displays properties of
both broad and narrow line Seyfert 1 galaxies, but we conclude that it is
intrinsically a `normal' Seyfert 1 viewed at high inclination angle.Comment: 4 pages, 1 figure. MG10 Proceeding
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&
Optical binding in nanoparticle assembly: Potential energy landscapes
Optical binding is an optomechanical effect exhibited by systems of micro- and nanoparticles, suitably irradiated with off-resonance laser light. Physically distinct from standing-wave and other forms of holographic optical traps, the phenomenon arises as a result of an interparticle coupling with individual radiation modes, leading to optically induced modifications to Casmir-Polder interactions. To better understand how this mechanism leads to the observed assemblies and formation of patterns in nanoparticles, we develop a theory in terms of optically induced energy landscapes exhibiting the three-dimensional form of the potential energy field. It is shown in detail that the positioning and magnitude of local energy maxima and minima depend on the configuration of each particle pair, with regards to the polarization and wave vector of the laser light. The analysis reveals how the positioning of local minima determines the energetically most favorable locations for the addition of a third particle to each equilibrium pair. It is also demonstrated how the result of such an addition subtly modifies the energy landscape that will, in turn, determine the optimum location for further particle additions. As such, this development represents a rigorous and general formulation of the theory, paving the way toward full comprehension of nanoparticle assembly based on optical binding
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.
Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state
GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It
was discovered during an outburst in 1992. During the entire episode a
persistent power-law spectral component extending up to MeV was
observed, which suggests that nonthermal processes should have occurred in the
system. We study relativistic particle interactions and the neutrino production
in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during
its recorded flaring phase. We have developed a magnetized corona model to fit
the spectrum of GRO J0422+32 during the low-hard state. We also estimate
neutrino emission and study the detectability of neutrinos with 1 km
detectors, such as IceCube. The short duration of the flares ( hours) and
an energy cutoff around a few TeV in the neutrino spectrum make neutrino
detection difficult. There are, however, many factors that can enhance neutrino
emission. The northern-sky coverage and full duty cycle of IceCube make it
possible to detect neutrino bursts from objects of this kind through
time-dependent analysis.Comment: 12 pages, 11 figures, accepted for publication in A&
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