1,488 research outputs found
The higher grading structure of the WKI hierarchy and the two-component short pulse equation
A higher grading affine algebraic construction of integrable hierarchies,
containing the Wadati-Konno-Ichikawa (WKI) hierarchy as a particular case, is
proposed. We show that a two-component generalization of the Sch\" afer-Wayne
short pulse equation arises quite naturally from the first negative flow of the
WKI hierarchy. Some novel integrable nonautonomous models are also proposed.
The conserved charges, both local and nonlocal, are obtained from the Riccati
form of the spectral problem. The loop-soliton solutions of the WKI hierarchy
are systematically constructed through gauge followed by reciprocal B\" acklund
transformation, establishing the precise connection between the whole WKI and
AKNS hierarchies. The connection between the short pulse equation with the
sine-Gordon model is extended to a correspondence between the two-component
short pulse equation and the Lund-Regge model
The algebraic structure behind the derivative nonlinear Schroedinger equation
The Kaup-Newell (KN) hierarchy contains the derivative nonlinear Schr\"
odinger equation (DNLSE) amongst others interesting and important nonlinear
integrable equations. In this paper, a general higher grading affine algebraic
construction of integrable hierarchies is proposed and the KN hierarchy is
established in terms of a Kac-Moody algebra and principal
gradation. In this form, our spectral problem is linear in the spectral
parameter. The positive and negative flows are derived, showing that some
interesting physical models arise from the same algebraic structure. For
instance, the DNLSE is obtained as the second positive, while the Mikhailov
model as the first negative flows, respectively. The equivalence between the
latter and the massive Thirring model is explicitly demonstrated also. The
algebraic dressing method is employed to construct soliton solutions in a
systematic manner for all members of the hierarchy. Finally, the equivalence of
the spectral problem introduced in this paper with the usual one, which is
quadratic in the spectral parameter, is achieved by setting a particular
automorphism of the affine algebra, which maps the homogeneous into principal
gradation.Comment: references adde
The contribution of faint AGN to the hard X-ray background
Hard X-ray selection is the most efficient way to discriminate between
accretion-powered sources, such as AGN, from sources dominated by starlight.
Hard X-rays are also less affected than other bands by obscuration. We have
then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the
largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with
flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage
this corresponds to resolving about 30 % of the hard Cosmic X-ray Background
(XRB). Here we report on a first optical spectroscopic identification campaign,
finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for
obscuration in X-ray and optical bands, a fraction higher than in previous
ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level
respectively), thus supporting the scenario in which a significant fraction of
the XRB is made by obscured AGN.Comment: MNRAS, revised version after minor referee comment
Low Input Techniques for Firebreak Covering: Agronomic Aspects
In Sardinia, wild fire prevention is traditionally passive , based on mechanical removal of vegetation and upper soil layers. This has a dramatic negative effect on plant diversity and soil erosion. This experiment concerns active prevention of wildfires, based on green covering and grazing of firebreaks. The objective was the establishment of a persistent green cover, to be grazed by animals to reduce fuel accumulation
AGN counts at 15um. XMM observations of the ELAIS-S1-5 sample
Context: The counts of galaxies and AGN in the mid infra-red (MIR) bands are
important instruments for studying their cosmological evolution. However, the
classic spectral line ratios techniques can become misleading when trying to
properly separate AGN from starbursts or even from apparently normal galaxies.
Aims: We use X-ray band observations to discriminate AGN activity in
previously classified MIR-selected starburst galaxies and to derive updated
AGN1 and (Compton thin) AGN2 counts at 15 um.
Methods: XMM observations of the ELAIS-S1 15um sample down to flux limits
~2x10^-15 erg cm^-2 s^-1 (2-10 keV band) were used. We classified as AGN all
those MIR sources with a unabsorbed 2-10 keV X-ray luminosity higher that
~10^42 erg/s.
Results: We find that at least about 13(+/-6) per cent of the previously
classified starburst galaxies harbor an AGN. According to these figures, we
provide an updated estimate of the counts of AGN1 and (Compton thin) AGN2 at 15
um. It turns out that at least 24% of the extragalactic sources brighter than
0.6 my at 15 um are AGN (~13% contribution to the extragalactic background
produced at fluxes brighter than 0.6 mJy).Comment: Accepted for publication on A&
The BeppoSAX HELLAS survey: on the nature of faint hard X-ray selected sources
The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80
square degrees of sky down to a flux of F(5-10keV)~5E-14 cgs. Optical
spectroscopic identification of about half of the sources in the sample (62)
shows that many (~50%) are highly obscured AGN, in line with the predictions of
AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et
al. 1995). The X-ray data, complemented by optical, near-IR and radio
follow-up, indicate that the majority of these AGN are ``intermediate''
objects, i.e. type 1.8-1.9 AGN,`red' quasars, and even a few broad line, blue
continuum quasars, obscured in X-rays by columns of the order of
logNH=22.5-23.5 cm-2, but showing a wide dispersion in optical extinction. The
optical and near-IR photometry of the obscured objects are dominated by galaxy
starlight, indicating that a sizeable fraction of the accretion power in the
Universe may actually have been missed in optical color surveys. This also
implies that multicolor photometry techniques may be efficiently used to assess
the redshift of the hard X-ray selected sources.Comment: 9 pages, Invited talk to appear in the Proceedings of the Conference
X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray
Background. (September 6-10 - 1999
Avaliação dos conceitos de quantidade e intensidade de mineralização de nitrogenio para trinta solos do Rio Grande do Sul.
Visando testar aplicabilidade do modelo de mineralizacao de N, sugerido por Stanford & Smith, a alguns solos brasileiros, foram utilizados os dados de incubacao por trinta semanas obtidos por Pottker & Tedescoem trinta solos do Rio Grande do Sul. O modelo matematico utilizado parte da premissa de que apenas uma fracao do N total do solo e potencialmente mineralizavel e que esse fenomeno segue uma cinetica de primeira ordem, ou seja: dN/dt=-kN, onde N = N0 e k para os grandes grupos de solos estudados foram avaliados, sendo tecidas consideracoes sobre ambos os fatores. O modelo de mineralizacao ajustou-se perfeitamente aos solos estudados (r2 >0,99). Mesmo dentro dos grandes grupos de solos, a variacao dos valores de N0 e k e grande. A meia-vida da mineralizacao de N a 24 0c foi de 11,6 semanas. Para estimar N0 por meio de regressoes, e vantajoso trabalhar com os grandes grupos de solos isoladamente
The HELLAS2XMM survey. IX. Spectroscopic identification of super-EROs hosting AGNs
We present VLT near-IR spectroscopic observations of three X-ray sources
characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red
colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3).
These objects are very faint in the optical, making their spectroscopic
identification extremely challenging. Instead, our near-IR spectroscopic
observations have been successful in identifying the redshift of two of them
(z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When
combined with the X-ray properties, our results clearly indicate that all these
objects host obscured QSOs (4e44 < L(2-10keV) < 1.5e45 erg/s, 2e22 < N_H < 4e23
cm-2) at high redshift. The only object with unresolved morphology in the K
band shows broad Halpha emission, but not broad Hbeta, implying a type 1.9 AGN
classification. The other two objects are resolved and dominated by the host
galaxy light in the K band, and appear relatively quiescent: one of them has a
LINER-like emission line spectrum and the other presents only a single, weak
emission line which we tentatively identify with Halpha. The galaxy
luminosities for the latter two objects are an order of magnitude brighter than
typical local L* galaxies and the derived stellar masses are well in excess of
10^11 Msun. For these objects we estimate black hole masses higher than 10^9
Msun and we infer that they are radiating at Eddington ratios L/L_Edd < 0.1. We
discuss the implications of these findings for the coevolution of galaxies and
black hole growth. Our results provide further support that X-ray sources with
high X/O ratios and very red colors tend to host obscured QSO in very massive
galaxies at high redshift.Comment: 7 pages, 5 figures, accepted for publication in A&
- …