5,618 research outputs found
Derived categories of hyper-K\"ahler manifolds via the LLV Algebra
We mostly review work of Taelman (Derived equivalences of hyperk ̈ahlervarieties, 2019, arXiv:1906.08081) on derived categories of hyper-K ̈ahlermanifolds. We study the LLV algebra using polyvector fields to prove that itis a derived invariant. Applications to the action of derived equivalences on coho-mology and to the study of their Hodge structures are given
BeppoSAX observations of the quasar Markarian 205
We present the first BeppoSAX observation (0.1 to 220 keV) of the quasar Mrk
205. We have searched for the unusual Fe line profile claimed in the XMM-Newton
spectrum which has been widely discussed in recent literature. We find no
evidence for a broad, ionized Fe line component in our data. We detect for the
first time a Compton hump in this object. Besides, when this component is
included in the fit, the line strength diminishes, in agreement with a recent
re-analysis of the XMM-Newton data, but with better constraints on the
reflection component thanks to the PDS instrument (15-220 keV). We interpret
this fact as another indication for illumination of a distant and cold material
rather than reprocessing in the highly ionized inner parts of an accretion
disk. We cannot constrain the presence of a high energy cutoff but we confirm
the existence of a variable soft excess (one year timescale).Comment: 13 pages, 12 figures, accepted for publication in A&
Equivariant categories of symplectic surfaces and fixed loci of Bridgeland moduli spaces
Given an action of a finite group on the derived category of a smooth projective variety we relate the fixed loci of the induced -action on moduli spaces of stable objects in with moduli spaces of stable objects in the equivariant category . As an application we obtain a criterion for the equivariant category of a symplectic action on the derived category of a symplectic surface to be equivalent to the derived category of a surface. This generalizes the derived McKay correspondence, and yields a general framework for describing fixed loci of symplectic group actions on moduli spaces of stable objects on symplectic surfaces
Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators
Whispering gallery resonators (WGR's), based on total internal reflection,
possess high quality factors in a broad spectral range. Thus, nonlinear optical
processes in such cavities are ideally suited for the generation of broadband
or tunable electromagnetic radiation. Experimentally and theoretically, we
investigate the tunability of optical parametric oscillation in a radially
structured WGR made of lithium niobate. With a 1.04 /mum pump wave, the signal
and idler waves are tuned from 1.78 to 2.5 \mum - including the point of
degeneracy - by varying the temperature between 20 and 62 {\deg}C. A weak
off-centering of the radial domain structure extends considerably the tuning
capabilities. The oscillation threshold lies in the mW-power range.Comment: 4 pages, 5 figure
The 1 keV to 200 keV X-ray Spectrum of NGC 2992 and NGC 3081
The Seyfert 2 galaxies NGC 2992 and NGC 3081 have been observed by INTEGRAL
and Swift. We report about the results and the comparison of the spectrum above
10 keV based on INTEGRAL IBIS/ISGRI, Swift/BAT, and BeppoSAX/PDS. A spectrum
can be extracted in the X-ray energy band ranging from 1 keV up to 200 keV.
Although NGC 2992 shows a complex spectrum below 10 keV, the hard tail observed
by various missions exhibits a slope with photon index = 2, independent on the
flux level during the observation. No cut-off is detectable up to the detection
limit around 200 keV. In addition, NGC 3081 is detected in the INTEGRAL and
Swift observation and also shows an unbroken Gamma = 1.8 spectrum up to 150
keV. These two Seyfert galaxies give further evidence that a high-energy
cut-off in the hard X-ray spectra is often located at energies E_C >> 100 keV.
In NGC 2992 a constant spectral shape is observed over a hard X-ray luminosity
variation by a factor of 11. This might indicate that the physical conditions
of the emitting hot plasma are constant, while the amount of plasma varies, due
to long-term flaring activity.Comment: 8 pages, 4 figures, accepted for publication in Ap
Spin Transport in Disordered Two-Dimensional Hopping Systems with Rashba Spin-Orbit Interaction
The influence of Rashba spin-orbit interaction on the spin dynamics of a
topologically disordered hopping system is studied in this paper. This is a
significant generalization of a previous investigation, where an ordered
(polaronic) hopping system has been considered instead. It is found, that in
the limit, where the Rashba length is large compared to the typical hopping
length, the spin dynamics of a disordered system can still be described by the
expressions derived for an ordered system, under the provision that one takes
into account the frequency dependence of the diffusion constant and the
mobility (which are determined by charge transport and are independent of
spin). With these results we are able to make explicit the influence of
disorder on spin related quantities as, e.g., the spin life-time in hopping
systems.Comment: 12 pages, 6 figures, some clarifications adde
Hard X-ray Variability of AGN
Aims: Active Galactic Nuclei are known to be variable throughout the
electromagnetic spectrum. An energy domain poorly studied in this respect is
the hard X-ray range above 20 keV.
Methods: The first 9 months of the Swift/BAT all-sky survey are used to study
the 14 - 195 keV variability of the 44 brightest AGN. The sources have been
selected due to their detection significance of >10 sigma. We tested the
variability using a maximum likelihood estimator and by analysing the structure
function.
Results: Probing different time scales, it appears that the absorbed AGN are
more variable than the unabsorbed ones. The same applies for the comparison of
Seyfert 2 and Seyfert 1 objects. As expected the blazars show stronger
variability. 15% of the non-blazar AGN show variability of >20% compared to the
average flux on time scales of 20 days, and 30% show at least 10% flux
variation. All the non-blazar AGN which show strong variability are
low-luminosity objects with L(14-195 keV) < 1E44 erg/sec.
Conclusions: Concerning the variability pattern, there is a tendency of
unabsorbed or type 1 galaxies being less variable than the absorbed or type 2
objects at hardest X-rays. A more solid anti-correlation is found between
variability and luminosity, which has been previously observed in soft X-rays,
in the UV, and in the optical domain.Comment: 9 pages, 7 figures, accepted for publication in A&
- …