10,013 research outputs found
X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?
We present two XMM-Newton observations of the luminous (L_x > 10^46 erg/s),
radio-quiet quasar, PG 1407+265, separated by eleven months. The data indicate
two distinct states: a highly variable, bright state (first epoch); and a
quiescent, low-flux one (second epoch). During the low-flux state the spectrum
is consistent with a single, unabsorbed power law. However, during the brighter
state a highly variable, steep component is statistically required.
Contemporaneous UV data from the Optical Monitor allow an estimate of the
optical-to-X-ray spectral index (alpha_ox), which appears typical of
radio-quiet quasars during the low-flux state, but extremely flat during the
high-flux state. The XMM-Newton data can be described as originating from a
combination of jet and accretion disc processes, in which the (relativistic)
X-ray jet only works intermittently. The scenario could help describe some of
the complexities seen in the broadband spectral energy distribution of PG
1407+265, such as weak high-ionisation emission lines, strong Fe II, unbeamed
continuum, and the weak radio emission relative to the optical.Comment: 9 pages, 8 figures. Accepted for publication in MNRAS. Correction
made to reported aox value. No changes in conclusion
Dynamics in the Ising field theory after a quantum quench
We study the real-time dynamics of the order parameter . Our main result is the development of
a method for treating divergences associated with working directly in the field
theory limit. We recover the scaling limit of the corresponding result by
Calabrese et al. [Phys. Rev. Lett. \textbf{106}, 227203 (2011)], which was
obtained for the lattice model. Our formalism generalizes to integrable quantum
quenches in other integrable models
The origin of blue-shifted absorption features in the X-ray spectrum of PG 1211+143: Outflow or disc?
In some radio-quiet active galaxies (AGN), high-energy absorption features in
the x-ray spectra have been interpreted as Ultrafast Outflows (UFOs) -- highly
ionised material (e.g. Fe XXV and Fe XXVI) ejected at mildly relativistic
velocities. In some cases, these outflows can carry energy in excess of the
binding energy of the host galaxy. Needless to say, these features demand our
attention as they are strong signatures of AGN feedback and will influence
galaxy evolution. For the same reason, alternative models need to be discussed
and refuted or confirmed. Gallo & Fabian proposed that some of these features
could arise from resonance absorption of the reflected spectrum in a layer of
ionised material located above and corotating with the accretion disc.
Therefore, the absorbing medium would be subjected to similar blurring effects
as seen in the disc. A priori, the existence of such plasma above the disc is
as plausible as a fast wind. In this work, we highlight the ambiguity by
demonstrating that the absorption model can describe the ~7.6 keV absorption
feature (and possibly other features) in the quasar PG 1211+143, an AGN that is
often described as a classic example of an UFO. In this model, the 2-10 keV
spectrum would be largely reflection dominated (as opposed to power law
dominated in the wind models) and the resonance absorption would be originating
in a layer between about 6 and 60 gravitational radii. The studies of such
features constitutes a cornerstone for future X-ray observatories like Astro-H
and Athena+. Should our model prove correct, or at least important in some
cases, then absorption will provide another diagnostic tool with which to probe
the inner accretion flow with future missions.Comment: 4 pages. Accepted for publication in MNRAS Letter
Spectrum of Low-Lying Excitations in a Supersymmetric Extended Hubbard Model
We continue the study of the -supersymmetric extension of the Hubbard
model in one dimension. We determine the excitation spectrum at zero
temperature even in the sectors where the ground states are
-descendants of Bethe states. The excitations include spinons, holons,
electrons, localons (local electrons pairs, moving coherently through the
lattice) and their bound states. The spectra are found to be very different for
repulsive and attractive on-site interaction. We also study the thermodynamics
of the model.Comment: 37 pages, uuencoded compressed postscript fil
Unlocking the Potential of Flexible Energy Resources to Help Balance the Power Grid
Flexible energy resources can help balance the power grid by providing
different types of ancillary services. However, the balancing potential of most
types of resources is restricted by physical constraints such as the size of
their energy buffer, limits on power-ramp rates, or control delays. Using the
example of Secondary Frequency Regulation, this paper shows how the flexibility
of various resources can be exploited more efficiently by considering multiple
resources with complementary physical properties and controlling them in a
coordinated way. To this end, optimal adjustable control policies are computed
based on robust optimization. Our problem formulation takes into account power
ramp-rate constraints explicitly, and accurately models the different
timescales and lead times of the energy and reserve markets. Simulations
demonstrate that aggregations of select resources can offer significantly more
regulation capacity than the resources could provide individually.Comment: arXiv admin note: text overlap with arXiv:1804.0389
The profile of an emission line from relativistic outflows around a black hole
Recent observations show strong evidence for the presence of Doppler-shifted
emission lines in the spectrum of both black hole candidates and active
galactic nuclei. These lines are likely to originate from relativistic outflows
(or jets) in the vicinity of the central black hole. Consequently, the profile
of such a line should be distorted by strong gravitational effects near the
black hole, as well as special relativistic effects. In this paper, we present
results from a detailed study on how each process affects the observed line
profile. We found that the profile is sensitive to the intrinsic properties of
the jets (Lorentz factor, velocity profile, and emissivity law), as well as to
the spin of the black hole and the viewing angle (with respect to the axis of
the jets). More specifically, in the case of approaching jets, an intrisically
narrow line (blue-shifted) is seen as simply broadened at small viewing angles,
but it shows a doubly peaked profile at large viewing angles for extreme Kerr
black holes (due to the combination of gravitational focusing and Doppler
effects); the profile is always singly peaked for Schwarzschild black holes.
For receding jets, however, the line profile becomes quite complicated owing to
complicated photon trajectories. To facilitate comparison with observations, we
searched a large parameter space to derive representative line profiles. We
show the results and discuss how to use emission lines as a potential tool for
probing the inner region of a black hole jet system.Comment: 16 pages in emulateapj style, 11 figure
Applications of Massive Integrable Quantum Field Theories to Problems in Condensed Matter Physics
We review applications of the sine-Gordon model, the O(3) non-linear sigma
model, the U(1) Thirring model, and the O(N) Gross--Neveu model to quasi
one-dimensional quantum magnets, Mott insulators, and carbon nanotubes. We
focus upon the determination of dynamical response functions for these
problems. These quantities are computed by means of form factor expansions of
quantum correlation functions in integrable quantum field theories. This
approach is reviewed here in some detail.Comment: 150 pages, 35 figures, published in the I. Kogan Memorial Volume by
World Scientifi
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