10,902 research outputs found
Leptonic secondary emission in a hadronic microquasar model
Context: It has been proposed that the origin of the very high-energy photons
emitted from high-mass X-ray binaries with jet-like features, so-called
microquasars (MQs), is related to hadronic interactions between relativistic
protons in the jet and cold protons of the stellar wind. Leptonic secondary
emission should be calculated in a complete hadronic model that include the
effects of pairs from charged pion decays inside the jets and the emission from
pairs generated by gamma-ray absorption in the photosphere of the system. Aims:
We aim at predicting the broadband spectrum from a general hadronic microquasar
model, taking into account the emission from secondaries created by charged
pion decay inside the jet. Methods: The particle energy distribution for
secondary leptons injected along the jets is consistently derived taking the
energy losses into account. We also compute the spectral energy distribution
resulting from these leptons is calculated after assuming different values of
the magnetic field inside the jets. The spectrum of the gamma-rays produced by
neutral pion-decay and processed by electromagnetic cascades under the stellar
photon field. Results: We show that the secondary emission can dominate the
spectral energy distribution at low energies (~1 MeV). At high energies, the
production spectrum can be significantly distorted by the effect of
electromagnetic cascades. These effects are phase-dependent, and some
variability modulated by the orbital period is predicted.Comment: 8 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
In search of an appropriate abstraction level for motif annotations
In: Proceedings of the 2012 Workshop on Computational Models of Narrative, (pp. 22-28).
Constraining the Unitarity Triangle with B -> V gamma
We discuss the exclusive radiative decays , , and in QCD factorization within the Standard
Model. The analysis is based on the heavy-quark limit of QCD. Our results for
these decays are complete to next-to-leading order in QCD and to leading order
in the heavy-quark limit. Special emphasis is placed on constraining the
CKM-unitarity triangle from these observables. We propose a theoretically clean
method to determine CKM parameters from the ratio of the decay
spectrum to the branching fraction of . The method is based on
the cancellation of soft hadronic form factors in the large energy limit, which
occurs in a suitable region of phase space. The ratio of the
and branching fractions determines the side of the
standard unitarity triangle with reduced hadronic uncertainties. The recent
Babar bound on implies , with the
limiting uncertainty coming only from the SU(3) breaking form factor ratio
. This constraint is already getting competitive with the constraint from
- mixing. Phenomenological implications from
isospin-breaking effects are briefly discussed.Comment: 23 pages, 8 figure
Observations of microquasars with the MAGIC telescope
We report on the results from the observations in very high energy band (VHE,
E_gamma > 100GeV) of the black hole X-ray binary (BHXB) Cygnus X-1. The
observations were performed with the MAGIC telescope, for a total of 40 hours
during 26 nights, spanning the period between June and November 2006. We report
on the results of the searches for steady and variable gamma-ray signals,
including the first experimental evidence for an intense flare, of duration
between 1.5 and 24 hours.Comment: Contribution to the 30th ICRC, Merida Mexico, July 2007 on behalf of
the MAGIC Collaboratio
The evolution of the large-scale emission in Fanaroff-Riley type I jets
Recent observations in X-rays and gamma-rays of nearby Fanaroff-Riley type I (FR I) radio galaxies have raised the question of the origin of the emission detected in the termination structures of their jets. The study of these structures can give information on the conditions for particle acceleration and radiation at the front shocks. In addition, an evolutionary scenario can help to disentangle the origin of the detected X-ray emission in young FR I sources, like some gigahertz peaked spectrum active galactic nuclei. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FR I jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FR I jet, coupled with a radiation model, to make predictions for the spectra and light curves of the thermal and non-thermal emission at different stages of the FR I evolution. Our results show that under moderate magnetic fields, the synchrotron radiation would be the dominant non-thermal channel, appearing extended in radio and more compact in X-rays, with relatively small flux variations with time. The shocked jet synchrotron emission would dominate the X-ray band, although the shocked interstellar/intracluster media thermal component alone may be significant in old sources. Inverse Compton (IC) scattering of cosmic microwave background photons could yield significant fluxes in the GeV and TeV bands, with a non-negligible X-ray contribution. The IC radiation would present a bigger angular size in X-rays and GeV than in TeV, with fluxes increasing with time. We conclude that the thermal and non-thermal broad-band emission from the termination regions of FR I jets could be detectable for sources located up to distances of a few 100 Mp
Orbital X-Ray Variability of the Microquasar LS 5039
The properties of the orbit and the donor star in the high mass X-ray binary
microquasar LS 5039 indicate that accretion processes should mainly occur via a
radiatively driven wind. In such a scenario, significant X-ray variability
would be expected due to the eccentricity of the orbit. The source has been
observed at X-rays by several missions, although with a poor coverage that
prevents to reach any conclusion about orbital variability. Therefore, we
conducted RossiXTE observations of the microquasar system LS 5039 covering a
full orbital period of 4 days. Individual observations are well fitted with an
absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line
emission that is probably a diffuse background feature. In addition, we have
taken into account that the continuum is also affected by significant diffuse
background contamination. Our results show moderate power-law flux variations
on timescales of days, as well as the presence of miniflares on shorter
timescales. The new orbital ephemeris of the system recently obtained by
Casares et al. have allowed us to show, for the first time, that an increase of
emission is seen close to the periastron passage, as expected in an accretion
scenario. Moreover, the detected orbital variability is a factor of ~4 smaller
than the one expected by using a simple wind accretion model, and we suggest
that an accretion disk around the compact object could be responsible for this
discrepancy. On the other hand, significant changes in the photon index are
also observed clearly anti-correlated with the flux variations. We interpret
the overall X-ray spectral characteristics of LS 5039 in the context of X-ray
radiation produced by inverse Compton and/or synchrotron processes in the jet
of this microquasar.Comment: published in Astrophysical Journal, submission format (real number of
pages: 7, 4 figures
Synchrotron emission from secondary leptons in microquasar jets
We present a model to estimate the synchrotron radio emission generated in
microquasar (MQ) jets due to secondary pairs created via decay of charged pions
produced in proton-proton collisions between stellar wind ions and jet
relativistic protons. Signatures of electrons/positrons are obtained from
consistent particle energy distributions that take into account energy losses
due to synchrotron and inverse Compton (IC) processes, as well as adiabatic
expansion. The space parameter for the model is explored and the corresponding
spectral energy distributions (SEDs) are presented. We conclude that secondary
leptonic emission represents a significant though hardly dominant contribution
to the total radio emission in MQs, with observational consequences that can be
used to test some still unknown processes occurring in these objects as well as
the nature of the matter outflowing in their jets
B->gamma e nu Transitions from QCD Sum Rules
B->gamma e nu transitions have recently been studied in the framework of QCD
factorization. The attractiveness of this channel for such an analysis lies in
the fact that, at least in the heavy quark limit, the only hadron involved is
the B meson itself, so one expects a very simple description of the form factor
in terms of a convolution of the B meson distribution amplitude with a
perturbative kernel. This description, however, does not include contributions
suppressed by powers of the b quark mass. In this letter, we calculate
corrections to the factorized expression which are induced by the ``soft''
hadronic component of the photon. We demonstrate that the power-suppression of
these terms is numerically not effective for physical values of the quark
mass and that they increase the form factor by about 30% at zero momentum
transfer. We also derive a sum rule for lambda_B, the first negative moment of
the B meson distribution amplitude, and find lambda_B = 0.6 GeV (to leading
order in QCD).Comment: 13 pages, 5 figure
Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?
We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out
with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest
resolution X-ray observation of the source conducted so far. Possible evidence
of an extended structure at a distance between 5 and 12 arcsec towards the
North of LSI+61303 have been found at a significance level of 3.2 sigma. The
asymmetry of the extended emission excludes an interpretation in the context of
a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while
the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12}
ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2},
are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the
hardest ever found. In light of these new results, we briefly discuss the
physics behind the X-ray emission, the location of the emitter, and the
possible origin of the extended emission ~0.1 pc away from LSI+61303.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
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