5,309 research outputs found
Fermi acceleration along the orbit of {\eta} Carinae
The {\eta} Carinae binary system hosts the most massive stars with the
highest known mass-loss rate. Its dense wind encounters the faster wind
expelled by the companion, dissipating mechanical energy in the shock,
accelerating particles up to relativistic energies and producing high-energy
(HE) {\gamma}-rays. We used the first 7-year data of the Fermi LAT which span
two passages of {\eta} Carinae at periastron. We extracted low and HE light
curves and spectra in different orbital phase bins using the new PASS8
pipeline. We used particle acceleration in hydrodynamic simulations of the
system in a multi-cell geometry and compared the prediction with the
observations. The {\gamma}-ray emission location is compatible with {\eta}
Carinae. Two emission components are distinguished. The low-energy (LE) one
cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by
a factor < 2. Short-term variability occurs at periastron. The HE component
flux varies by a factor 3-4 but differently during the two periastrons. The
variabilities observed at LE and HE during the first half of the observations,
match the prediction of the simulation, assuming a surface magnetic field of
500 G. The HE component and the thermal X-ray emission were weaker than
expected around the second periastron suggesting a modification of the wind
density in the inner wind collision region (WCR). Diffuse shock acceleration in
the WCR provides a convincing match to the observations and new diagnostic
tools to probe the geometry and energetics of the system. Further observations
are required to explain the periastron-to-periastron HE variability and to
associate it firmly with hadronic origin. {\eta} Carinae is a pevatron at
periastron. Its flux can be detected by IceCube after many years of
observations. Orbital modulations of the HE component can be distinguished from
those of photo absorption by CTA.Comment: 11 pages, 8 figure
Unbeamed tidal disruption events at hard X-rays
Owing to their thermal emission, tidal disruption events (TDEs) were
regularly detected in the soft X-rays and sometimes in the optical. Only a few
TDEs have been detected at hard X-rays: two are high redshift beamed events,
one of which occurred at the core of a nearby galaxy, and the most recent one
is of a different nature, involving a compact object in the Milky Way. The aims
of this work are to obtain a first sample of hard X-ray-selected unbeamed TDEs,
to determine their frequency and to probe whether TDEs usually or exceptionally
emit at hard X-ray energies. We performed extensive searches for hard X-ray
flares at positions in over 53000 galaxies, up to a distance of 100 Mpc in the
Swift BAT archive. Light curves were extracted and parametrized. The quiescent
hard X-ray emission was used to exclude persistently active galactic nuclei.
Significant flares from non-active galaxies were derived and checked for
possible contamination. We found a sample of nine TDE candidates, which
translates into a rate of
above the BAT detection limit. This rate is consistent with those observed by
XMM-Newton at soft X-rays and in the optical from SDSS observations, and is as
expected from simulations. We conclude that hard X-ray emission should be
ubiquitous in un-beamed TDEs and that electrons should be accelerated in their
accretion flow.Comment: accepted for publication in A&A; 14 pages, 6 figures, 2 table
The environment of the wind-wind collision region of Carinae
Carinae is a colliding wind binary hosting two of the most massive
stars and featuring the strongest wind collision mechanical luminosity. The
wind collision region of this system is detected in X-rays and -rays
and offers a unique laboratory for the study of particle acceleration and wind
magneto-hydrodynamics. Our main goal is to use X-ray observations of
Carinae around periastron to constrain the wind collision zone geometry and
understand the reasons for its variability. We analysed 10 Nuclear
Spectroscopic Telescope Array (NuSTAR) observations, which were obtained around
the 2014 periastron. The NuSTAR array monitored the source from 3 to 30 keV,
which allowed us to grasp the continuum and absorption parameters with very
good accuracy. We were able to identify several physical components and probe
their variability. The X-ray flux varied in a similar way as observed during
previous periastrons and largely as expected if generated in the wind collision
region. The flux detected within ~10 days of periastron is lower than expected,
suggesting a partial disruption of the central region of the wind collision
zone. The Fe K line is likely broadened by the electrons heated along
the complex shock fronts. The variability of its equivalent width indicates
that the fluorescence region has a complex geometry and that the source
obscuration varies quickly with the line of sight.Comment: to be published in A&A, 7 pages, 9 figure
Tidal disruption of a super-Jupiter by a massive black hole
Aims: A strong, hard X-ray flare was discovered (IGR J12580+0134) by INTEGRAL
in 2011, and is associated to NGC 4845, a Seyfert 2 galaxy never detected at
high-energy previously. To understand what happened we observed this event in
the X-ray band on several occasions. Methods: Follow-up observations with
XMM-Newton, Swift, and MAXI are presented together with the INTEGRAL data. Long
and short term variability are analysed and the event wide band spectral shape
modelled. Results: The spectrum of the source can be described with an absorbed
(N_H ~ 7x10^22 cm^{-2}) power law (\Gamma \simeq 2.2), characteristic of an
accreting source, plus a soft X-ray excess, likely to be of diffuse nature. The
hard X-ray flux increased to maximum in a few weeks and decreased over a year,
with the evolution expected for a tidal disruption event. The fast variations
observed near the flare maximum allowed us to estimate the mass of the central
black hole in NGC 4845 as ~ 3x10^5 Msun. The observed flare corresponds to the
disruption of about 10% of an object with a mass of 14-30 Jupiter. The hard
X-ray emission should come from a corona forming around the accretion flow
close to the black hole. This is the first tidal event where such a corona has
been observed.Comment: 8 pages, 10 figures, 4 table
Translocation and insertion of precursor proteins into isolated outer membranes of mitochondria
Nuclear-encoded proteins destined for mitochondria must cross the outer or both outer and inner membranes to reach their final sub- mitochondrial locations. While the inner membrane can translocate preproteins by itself, it is not known whether the outer membrane also contains an endogenous protein translocation activity which can function independently of the inner membrane. To selectively study the protein transport into and across the outer membrane of Neurospora crassa mitochondria, outer membrane vesicles were isolated which were sealed, in a right-side-out orientation, and virtually free of inner membranes. The vesicles were functional in the insertion and assembly of various outer membrane proteins such as porin, MOM19, and MOM22. Like with intact mitochondria, import into isolated outer membranes was dependent on protease-sensitive surface receptors and led to correct folding and membrane integration. The vesicles were also capable of importing a peripheral component of the inner membrane, cytochrome c heme lyase (CCHL), in a receptor-dependent fashion. Thus, the protein translocation machinery of the outer mitochondrial membrane can function as an independent entity which recognizes, inserts, and translocates mitochondrial preproteins of the outer membrane and the intermembrane space. In contrast, proteins which have to be translocated into or across the inner membrane were only specifically bound to the vesicles, but not imported. This suggests that transport of such proteins involves the participation of components of the intermembrane space and/or the inner membrane, and that in these cases the outer membrane translocation machinery has to act in concert with that of the inner membrane
The NLMS algorithm with time-variant optimum stepsize derived from a Bayesian network perspective
In this article, we derive a new stepsize adaptation for the normalized least
mean square algorithm (NLMS) by describing the task of linear acoustic echo
cancellation from a Bayesian network perspective. Similar to the well-known
Kalman filter equations, we model the acoustic wave propagation from the
loudspeaker to the microphone by a latent state vector and define a linear
observation equation (to model the relation between the state vector and the
observation) as well as a linear process equation (to model the temporal
progress of the state vector). Based on additional assumptions on the
statistics of the random variables in observation and process equation, we
apply the expectation-maximization (EM) algorithm to derive an NLMS-like filter
adaptation. By exploiting the conditional independence rules for Bayesian
networks, we reveal that the resulting EM-NLMS algorithm has a stepsize update
equivalent to the optimal-stepsize calculation proposed by Yamamoto and
Kitayama in 1982, which has been adopted in many textbooks. As main difference,
the instantaneous stepsize value is estimated in the M step of the EM algorithm
(instead of being approximated by artificially extending the acoustic echo
path). The EM-NLMS algorithm is experimentally verified for synthesized
scenarios with both, white noise and male speech as input signal.Comment: 4 pages, 1 page of reference
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