370 research outputs found
On the formation of TeV radiation in LS 5039
The recent detections of TeV gamma-rays from compact binary systems show that
relativistic outflows (jets or winds) are sites of effective acceleration of
particles up to multi-TeV energies. In this paper, we discuss the conditions of
acceleration and radiation of ultra-relativistic electrons in LS 5039, the
gamma-ray emitting binary system for which the highest quality TeV data are
available. Assuming that the gamma-ray emitter is a jet-like structure, we
performed detailed numerical calculations of the energy spectrum and
lightcurves accounting for the acceleration efficiency, the location of the
accelerator, the speed of the emitting flow, the inclination angle of the
system, as well as specific features related to anisotropic inverse Compton
scattering and pair production. We conclude that the accelerator should not be
deep inside the binary system unless we assume a very efficient acceleration
rate. We show that within the IC scenario both the gamma-ray spectrum and flux
are strongly orbital phase dependent. Formally, our model can reproduce, for
specific sets of parameter values, the energy spectrum of gamma-rays reported
by HESS for wide orbital phase intervals. However, the physical properties of
the source can be constrained only by observations capable of providing
detailed energy spectra for narrow orbital phase intervals ().Comment: 14 pages, 26 figures, accepted for publication in MNRAS, submitted on
July 11, 200
Unraveling the high-energy emission components of gamma-ray binaries
The high and very-high energy spectrum of gamma-ray binaries has become a
challenge for all theoretical explanations since the detection of powerful,
persistent GeV emission from LS 5039 and LS I +61 303 by Fermi/LAT. The
spectral cutoff at a few GeV indicates that the GeV component and the fainter,
hard TeV emission above 100 GeV are not directly related. We explore the
possible origins of these two emission components in the framework of a young,
non-accreting pulsar orbiting the massive star, and initiating the non-thermal
emission through the interaction of the stellar and pulsar winds. The
pulsar/stellar wind interaction in a compact orbit binary gives rise to two
potential locations for particle acceleration: the shocks at the head-on
collision of the winds and the termination shock caused by Coriolis forces on
scales larger than the binary separation. We explore the suitability of these
two locations to host the GeV and TeV emitters, respectively, through the study
of their non-thermal emission along the orbit. We focus on the application of
this model to LS 5039 given its well determined stellar wind with respect to
other gamma-ray binaries. The application of the proposed model to LS 5039
indicates that these two potential emitter locations provide the necessary
conditions for reproduction of the two-component high-energy gamma-ray spectrum
of LS 5039. In addition, the ambient postshock conditions required at each of
the locations are consistent with recent hydrodynamical simulations. The
scenario based on the interaction of the stellar and pulsar winds is compatible
with the GeV and TeV emission observed from gamma-ray binaries with unknown
compact objects, such as LS 5039 and LS I +61 303.Comment: Version as published in A&
Gamma-ray flares from red giant/jet interactions in AGN
Non-blazar AGN have been recently established as a class of gamma-ray
sources. M87, a nearby representative of this class, show fast TeV variability
on timescales of a few days. We suggest a scenario of flare gamma-ray emission
in non-blazar AGN based on a red giant interacting with the jet at the base. We
solve the hydrodynamical equations that describe the evolution of the envelope
of a red giant blown by the impact of the jet. If the red giant is at least
slightly tidally disrupted by the supermassive black hole, enough stellar
material will be blown by the jet, expanding quickly until a significant part
of the jet is shocked. This process can render suitable conditions for energy
dissipation and proton acceleration, which could explain the detected day-scale
TeV flares from M87 via proton-proton collisions. Since the produced radiation
would be unbeamed, such an events should be mostly detected from non-blazar
AGN. They may be frequent phenomena, detectable in the GeV-TeV range even up to
distances of Gpc for the most powerful jets. The counterparts at lower
energies are expected to be not too bright.} {M87, and nearby non-blazar AGN in
general, can be fast variable sources of gamma-rays through red giant/jet
interactions.Comment: 8 pages, 4 figure
Clues to unveil the emitter in LS 5039: powerful jets vs colliding winds
LS 5039 is among the most interesting VHE sources in the Galaxy. Two
scenarios have been put forward to explain the observed TeV radiation: jets vs
pulsar winds. The source has been detected during the superior conjunction of
the compact object, when very large gamma-ray opacities are expected. In
addition, electromagnetic cascades, which may make the system more transparent
to gamma-rays, are hardly efficient for realistic magnetic fields in massive
star surroundings. All this makes unlikely the standard pulsar scenario for LS
5039, in which the emitter is the region located between the star and the
compact object, where the opacities are the largest. Otherwise, a jet-like flow
can transport energy to regions where the photon-photon absorption is much
lower and the TeV radiation is not so severely absorbed.Comment: 3 pages, 3 Figures, contribution to the "Fourth Heidelberg
International Symposium on High-Energy Gamma-Ray Astronomy 2008
Towards a Formalism-Based Toolkit for Automotive Applications
The success of a number of projects has been shown to be significantly
improved by the use of a formalism. However, there remains an open issue: to
what extent can a development process based on a singular formal notation and
method succeed. The majority of approaches demonstrate a low level of
flexibility by attempting to use a single notation to express all of the
different aspects encountered in software development. Often, these approaches
leave a number of scalability issues open. We prefer a more eclectic approach.
In our experience, the use of a formalism-based toolkit with adequate notations
for each development phase is a viable solution. Following this principle, any
specific notation is used only where and when it is really suitable and not
necessarily over the entire software lifecycle. The approach explored in this
article is perhaps slowly emerging in practice - we hope to accelerate its
adoption. However, the major challenge is still finding the best way to
instantiate it for each specific application scenario. In this work, we
describe a development process and method for automotive applications which
consists of five phases. The process recognizes the need for having adequate
(and tailored) notations (Problem Frames, Requirements State Machine Language,
and Event-B) for each development phase as well as direct traceability between
the documents produced during each phase. This allows for a stepwise
verification/validation of the system under development. The ideas for the
formal development method have evolved over two significant case studies
carried out in the DEPLOY project
Detection of gamma rays of likely jet origin in Cygnus X-1
Aims: Probe the high-energy (60 MeV) emission from the black hole X-ray
binary system, Cygnus X-1, and investigate its origin. Methods: We analysed 7.5
yr of data by Fermi/LAT with the latest PASS8 software version. Results: We
report the detection of a signal at 8 statistical significance
spatially coincident with Cygnus X-1 and a luminosity above 60 MeV of
5.510 erg s. The signal is correlated with the hard X-ray
flux: the source is observed at high energies only during the hard X-ray
spectral state, when the source is known to display persistent, relativistic
radio emitting jets. The energy spectrum, extending up to 20 GeV without
any sign of spectral break, is well fitted by a power-law function with a
photon index of 2.30.2. There is a hint of orbital flux variability, with
high-energy emission mostly coming around the superior conjunction.
Conclusions: We detected GeV emission from Cygnus X-1 and probed that the
emission is most likely associated with the relativistic jets. The evidence of
flux orbital variability points to the anisotropic inverse Compton on stellar
photons as the mechanism at work, thus constraining the emission region to a
distance cm from the black hole.Comment: accepted A\&A (9 pages and 7 figures
Study of the Spectral and Temporal Characteristics of X-Ray Emission of the Gamma-Ray Binary LS 5039 with Suzaku
We report on the results from Suzaku broadband X-ray observations of the
galactic binary source LS5039. The Suzaku data, which have continuous coverage
of more than one orbital period, show strong modulation of the X-ray emission
at the orbital period of this TeV gamma-ray emitting system.The X-ray emission
shows a minimum at orbital phase ~ 0.1, close to the so-called superior
conjunction of the compact object, and a maximum at phase ~0.7, very close to
the inferior conjunction of the compact object. The X-ray spectral data up to
70 keV are described by a hard power-law with a phase-dependent photon index
which varies within Gamma ~1.45 - 1.61. The amplitude of the flux variation is
a factor of 2.5, but is significantly less than that of the factor ~8 variation
in the TeV flux. Otherwise the two light curves are similar, but not identical.
Although periodic X-ray emission has been found from many galactic binary
systems, the Suzaku result implies a phenomenon different from the "standard"
origin of X-rays related to the emission of the hot accretion plasma formed
around the compact companion object. The X-ray radiation of LS5039is likely to
be linked to very-high-energy electrons which are also responsible for the TeV
gamma-ray emission. While the gamma-rays are the result of inverse Compton
scattering by electrons on optical stellar photons, X-rays are produced via
synchrotron radiation. Yet, while the modulation of the TeV gamma-ray signal
can be naturally explained by the photon-photon pair production and anisotropic
inverse Compton scattering, the observed modulation of synchrotron X-rays
requires an additional process, the most natural one being adiabatic expansion
in the radiation production region.Comment: 9 pages, 7 figures, Accepted for publication in ApJ, references
fixed, a few typos correcte
Conflict of Interest in Science Communication: More than a Financial Issue Report from Esteve Foundation Discussion Group, April 2009
A systematic review and meta-analysis suggests that around 2% of scientists admit to have falsified research at least once (1). Up to 33% admit other questionable practices such as plagiarism, duplicate publication, undisclosed changes in pre-research protocols or dubious ethical behavior (1). There can be no doubt that discovered cases of
research and publication misconduct represent a tip of an iceberg and many cases go unreported (2). Experienced biomedical journal editors are aware of a
âroguesâ galleryâ of major fraudsters, such as Schoen, Hwang, Sudbo, Poehlman, Singh, and Chandra (3-8). Much more common are the less dramatic, because more subtle but probably more dangerous, examples; these are more dangerous because they remain undiscovered so may
feed into meta-analyses and guidelines. A seminar organized by the Esteve Foundation, held in Sitges in April 2009, concentrated on conflicts of interest (COI, sometimes also referred to as Competing Interests,
CI), which underlie so much research and publication misconduct.
All attendants of the meeting agreed that there were many
sources of COI in the general process of scientific communication
(Figure 1). The meeting was mainly focused on non-financial COI. Three introductory presentations highlighted some of the topics related to COI in the contemporary scientific publishing enterprise
Interpretation of the flares of M87 at TeV energies in the cloud-jet interaction scenario
Active galactic nuclei with misaligned jets have been recently established as
a class of high-energy gamma-ray sources. M87, a nearby representative of this
class, shows fast TeV variability on timescales less than one day. We present
calculations performed in the framework of the scenario in which gamma-ray
flares in non-blazar active galactic nuclei are produced by a red giant or a
gas cloud interacting with the jet. We show that both the light curve and
energy spectrum of the spectacular April 2010 flare can be reproduced by this
model, assuming that a relatively massive cloud of approx 1.e29 g penetrates
into the jet at few tens of Schwarzschild radii from the super-massive black
hole.Comment: 8 pages, 8 figures, accepted by Ap
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