7,275 research outputs found
Thermal and Magnetic Quantum Discord in Heisenberg models
We investigate how the quantum correlations (quantum discord) of a two-qubit
one dimensional XYZ Heisenberg chain in thermal equilibrium depends on the
temperature (T) of the bath and also on an external magnetic field B. We show
that the behavior of the thermal quantum discord (QD) differs in many
unexpected ways from the thermal entanglement. For example, we show situations
where QD increases with T when entanglement decreases, cases where QD increases
with T even in regions with zero entanglement, and that QD signals a quantum
phase transition even at finite T. We also show that by properly tuning B or
the interaction between the qubits we get non-zero QD for any T and we present
a new effect not seen for entanglement, the regrowth of thermal QD.Comment: 4 pages, 5 figures, RevTex, double column; v2: published versio
Transient gamma-ray emission from Cygnus X-3
The high-mass microquasar Cygnus X-3 has been recently detected in a flaring
state by the gamma-ray satellites Fermi and Agile. In the present contribution,
we study the high-energy emission from Cygnus X-3 through a model based on the
interaction of clumps from the Wolf-Rayet wind with the jet. The clumps inside
the jet act as obstacles in which shocks are formed leading to particle
acceleration and non-thermal emission. We model the high energy emission
produced by the interaction of one clump with the jet and briefly discus the
possibility of many clumps interacting with the jet. From the characteristics
of the considered scenario, the produced emission could be flare-like due to
discontinuous clump penetration, with the GeV long-term activity explained by
changes in the wind properties.Comment: Contribution to the proceedings of the 25th Texas Symposium on
Relativistic Astrophysics - TEXAS 2010, December 06-10, Heidelberg, German
High-energy flares from jet-clump interactions
High-mass microquasars are binary systems composed by a massive star and a
compact object from which relativistic jets are launched. Regarding the
companion star, observational evidence supports the idea that winds of hot
stars are formed by clumps. Then, these inhomogeneities may interact with the
jets producing a flaring activity. In the present contribution we study the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. This interaction produces a shock in the jet, where particles may
be accelerated up to relativistic energies. We calculate the spectral energy
distributions of the dominant non-thermal processes: synchrotron radiation,
inverse Compton scattering, and proton-proton collisions. Significant levels of
X- and gamma-ray emission are predicted, with luminosities in the different
domains up to ~ 10^{34} - 10^{35} erg/s on a timescale of about ~ 1 h. Finally,
jet-clump interactions in high-mass microquasars could be detectable at high
energies. These phenomena may be behind the fast TeV variability found in some
high-mass X-ray binary systems, such as Cygnus X-1, LS 5039 and LS I+61 303. In
addition, our model can help to derive information on the properties of jets
and clumpy winds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200
Gamma rays from cloud penetration at the base of AGN jets
Dense and cold clouds seem to populate the broad line region surrounding the
central black hole in AGNs. These clouds could interact with the AGN jet base
and this could have observational consequences. We want to study the gamma-ray
emission produced by these jet-cloud interactions, and explore under which
conditions this radiation would be detectable. We investigate the
hydrodynamical properties of jet-cloud interactions and the resulting shocks,
and develop a model to compute the spectral energy distribution of the emission
generated by the particles accelerated in these shocks. We discuss our model in
the context of radio-loud AGNs, with applications to two representative cases,
the low-luminous Centaurus A, and the powerful 3C 273. Some fraction of the jet
power can be channelled to gamma-rays, which would be likely dominated by
synchrotron self-Compton radiation, and show typical variability timescales
similar to the cloud lifetime within the jet, which is longer than several
hours. Many clouds can interact with the jet simultaneously leading to fluxes
significantly higher than in one interaction, but then variability will be
smoothed out. Jet-cloud interactions may produce detectable gamma-rays in
non-blazar AGNs, of transient nature in nearby low-luminous sources like Cen A,
and steady in the case of powerful objects of FR II type.Comment: Accepted for publication in A&A (9 pages, 7 figures
Modeling TeV gamma-rays from LS 5039: An active OB star at the extreme
Perhaps the most extreme examples of "Active OB stars" are the subset of
high-mass X-ray binaries -- consisting of an OB star plus compact companion --
that have recently been observed by Fermi and ground-based Cerenkov telescopes
like HESS to be sources of very high energy (VHE; up to 30 TeV) gamma-rays.
This paper focuses on the prominent gamma-ray source, LS5039, which consists of
a massive O6.5V star in a 3.9-day-period, mildly elliptical (e = 0.24) orbit
with its companion, assumed here to be a black-hole or unmagnetized neutron
star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind
onto the companion, we find that the orbital phase variation of the accretion
follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local
radius and wind speed. Moreover, a simple model, wherein intrinsic emission of
gamma-rays is assumed to track this accretion rate, reproduces quite well Fermi
observations of the phase variation of gamma-rays in the energy range 0.1-10
GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov
telescope, it is important to account also for photon-photon interactions
between the gamma-rays and the stellar optical/UV radiation, which effectively
attenuates much of the strong emission near periastron. When this is included,
we find that this simple BHL accretion model also quite naturally fits the HESS
light curve, thus making it a strong alternative to the pulsar-wind-shock
models commonly invoked to explain such VHE gamma-ray emission in massive-star
binaries.Comment: To appear in "Active OB Stars: Structure, Evolution, Mass Loss &
Critical Limits", Proceedings of IAUS 272, held July 2010 in Paris, France. 7
pages; 3 figures. This version 2 corrects an alignment error in figure
The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster
Star formation in galaxies at the center of cooling-flow galaxy clusters is
an important phenomenon in the context of formation and evolution of massive
galaxies in the Universe. Yet, star formation rates (SFRs) in such systems
continue to be elusive. We use our Bayesian-motivated spectral energy
distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR
values in the brightest cluster galaxy of a massive, X-ray luminous galaxy
cluster, Phoenix. Previous studies of Phoenix have resulted in the highest
measurement of SFR for any galaxy, with the estimates reaching up to 1000 solar
masses/yr. However, a very small number of models have been considered in those
studies. BAYESCOOL allows us to probe a large parameter space. We consider two
models for star formation history, instantaneous bursts and continuous star
formation, a wide range of ages for the old and the young stellar population,
along with other discrete parameters, such as the initial mass function,
metallicities, internal extinction and extinction law. We find that in the
absence of any prior except that the maximum cooling rate < 3000 solar
masses/yr, the SFR lies in the range (2230-2890) solar masses/yr. If we impose
an observational prior on the internal extinction, E(B-V) < 0.6, the best-fit
SFR lies in (454-494) solar masses/yr, and we consider this as the most
probable range of SFR values for Phoenix. The SFR dependence on the extinction
is a reflection of the standard age-extinction degeneracy, which can be
overcome by using a prior on one of the two quantities in question.Comment: 12 pages, 4 figures, 1 Table, accepted for publication in MNRA
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