243 research outputs found
The spectrum of the Broad Line Region and the high-energy emission of powerful blazars
High-energy emission (from the X-ray through the gamma-ray band) of Flat
Spectrum Radio Quasars is widely associated with the inverse Compton (IC)
scattering of ambient photons, produced either by the accretion disk or by the
Broad Line Region, by high-energy electrons in a relativistic jet. In the
modelling of the IC spectrum one usually adopts a simple black-body
approximation for the external radiation field, though the real shape is
probably more complex. The knowledge of the detailed spectrum of the external
radiation field would allow to better characterize the soft-medium X-ray IC
spectrum, which is crucial to address several issues related to the study of
these sources. Here we present a first step in this direction, calculating the
IC spectra expected by considering a realistic spectrum for the external
radiation energy density produced by the BLR, as calculated with the
photoionization code CLOUDY. We find that, under a wide range of the physical
parameters characterizing the BLR clouds, the IC spectrum calculated with the
black-body approximation reproduces quite well the exact spectrum for energies
above few keV. In the soft energy band, instead, the IC emission calculated
using the BLR emission shows a complex shape, with a moderate excess with
respect to the approximate spectrum, which becomes more important for
decreasing values of the peak frequency of the photoionizing continuum. We also
show that the high-energy spectrum shows a marked steepening, due to the energy
dependence of the scattering cross section, above a characteristic energy of
10-20 GeV, quasi independent on the Lorentz factor of the jet.Comment: 10 pages, 9 figures, accepted for publication in MNRA
Multiwavelength Observations of the BL Lacertae Object PKS 2155-304 with XMM-Newton
The optical-UV and X-ray instruments on-board XMM-Ndewton provide an
excellent opportunity to perform simultaneous observations of violently
variable objects over a broad wavelength range. The UV and X-ray bright BL Lac
object PKS 2155-304 has been repeatedly observed with XMM-Ndewton about twice
per year. In this paper, we present a detailed analysis of the simultaneous
multiwavelength variability of the source from optical to X-rays, based on the
currently available XMM-Ndewton observations. These observations probed the
intra-day multiwavelength variability at optical-UV and X-ray wavelengths of
the source. The UV variability amplitude is substantially smaller than the
X-ray one, and the hardness ratios of the UV to X-rays correlates with the
X-ray fluxes: the brighter the source, the flatter the UV-X-ray spectra. On
2000 May 30-31 the UV and X-ray light curves were weakly correlated, while the
UV variations followed the X-ray ones with no detectable lags on 2000 November
19-21. On 2001 November 30 the source exhibited a major X-ray flare that was
not detected in the optical. The intra-day UV and X-ray variability presented
here is not similar to the inter-day UV and X-ray variability obtained from the
previous coordinated extensive multiwavelength campaigns on the source,
indicating that different ``modes'' of variability might be operating in PKS
2155-304 on different timescales or from epoch to epoch.Comment: Accepted by Ap
A New Equilibrium for Accretion Disks Around Black Holes
Accretion disks around black holes in which the shear stress is proportional
to the total pressure, the accretion rate is more than a small fraction of
Eddington, and the matter is distributed smoothly are both thermally and
viscously unstable in their inner portions. The nonlinear endstate of these
instabilities is uncertain. Here a new inhomogeneous equilibrium is proposed
which is both thermally and viscously stable. In this equilibrium the majority
of the mass is in dense clumps, while a minority reaches temperatures K. The requirements of dynamical and thermal equilibrium completely
determine the parameters of this system, and these are found to be in good
agreement with the parameters derived from observations of accreting black
holes, both in active galactic nuclei and in stellar binary systems.Comment: AAS LaTeX, accepted to Ap. J. Letter
Low energy cut-offs and hard X-ray spectra in high-z radio-loud quasars: the Suzaku view of RBS315
We present the results from the Suzaku observation of the powerful radio-loud
quasar RBS315 (z=2.69), for which a previous XMM-Newton observation showed an
extremely flat X-ray continuum up to 10 keV (photon index Gamma=1.26) and
indications of strong intrinsic absorption (N_H~10^22 cm^{-2} assuming neutral
gas). The instrument for hard X-rays HXD/PIN allows us a detection of the
source up to 50 keV. The broad-band continuum (0.5-50 keV) can be well modeled
with a power-law with slope Gamma=1.5 (definitively softer than the continuum
measured by XMM-Newton) above 1 keV with strong deficit of soft photons. The
low-energy cut-off can be well fitted either with intrinsic absorption (with
column density N_H~10^22 cm^{-2} in the quasar rest frame) or with a break in
the continuum, with an extremely hard (Gamma =0.7) power-law below 1 keV. We
construct the Spectral Energy Distribution of the source, using also optical-UV
measurements obtained through a quasi-simultaneous UVOT/SWIFT observation. The
shape of the SED is similar to that of other Flat Spectrum Radio Quasars
(FSRQs) with similar power, making this source an excellent candidate for the
detection in gamma-rays by GLAST. We model the SED with the synchrotron-Inverse
Compton model usually applied to FSRQs, showing that the deficit of soft
photons can be naturally interpreted as due to an intrinsic curvature of the
spectrum near the low energy end of the IC component rather than to intrinsic
absorption, although the latter possibility cannot be ruled out. We propose
that in at least a fraction of the radio-loud QSOs at high redshift the cut-off
in the soft X-ray band can be explained in a similar way. Further studies are
required to distinguish between the two alternatives.Comment: Accepted for publication in Ap
The ASCA spectrum of the z=4.72 blazar, GB 1428+4217
The X-ray luminous quasar GB 1428+4217 at redshift 4.72 has been observed
with ASCA. The observed 0.5-10 keV flux is 3.2E-12 erg/s/cm2. We report here on
the intrinsic 4-57 keV X-ray spectrum, which is very flat (photon index of
1.29). We find no evidence for flux variability within the ASCA dataset or
between it and ROSAT data. We show that the overall spectral energy
distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars
and present models which fit the available data. The Doppler beaming factor is
likely to be at least 8. We speculate on the number density of such high
redshift blazars, which must contain rapidly-formed massive black holes.Comment: 5 pages, 3 Postscript figures, to appear in MNRA
The redshift-dependence of gamma-ray absorption in the environments of strong-line AGN
The case of gamma-ray absorption due to photon-photon pair production of jet
photons in the external photon environment like accretion disk and broad-line
region radiation field of gamma-ray loud active galactic nuclei (AGN) that
exhibit strong emission lines is considered. I demonstrate that this ''local
opacity'', if detected, will almost unavoidably be redshift-dependent in the
sub-TeV range. This introduces non-negligible biases, and complicates
approaches for studying the evolution of the extragalactic background light
with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space
Telescope (GLAST), etc., where the gamma-ray horizon is probed by means of
statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.)
in AGN spectra at various redshifts. It particularly applies to strong-line
quasars where external photon fields are potentially involved in gamma-ray
production.Comment: 19 pages, 5 figures; accepted for publication in Ap
PMN J0525-3343: soft X-ray spectral flattening in a blazar at z=4.4
We report optical, radio and X-ray observations of a new distant blazar, PMN
J0525-3343, at a redshift of 4.4. The X-ray spectrum measured from ASCA and
BeppoSAX flattens below a few keV, in a manner similar to the spectra of two
other z>4 blazars, GB 1428+4217 (z=4.72) reported by Boller et al and RXJ
1028.6-0844 (z=4.28) by Yuan et al. The spectrum is well fitted by a power-law
continuum which is either absorbed or breaks at a few keV. An intrinsic column
density corresponding to 2 x 10^23 H-atoms cm-2 at solar abundance is required
by the absorption model. This is however a million times greater than the
neutral hydrogen, or dust, column density implied by the optical spectrum,
which covers the rest-frame UV emission of the blazar nucleus. We discuss the
problems raised and suggest that, unless there is intrinsic flattening in the
spectral distribution of the particles/seed photons producing X-rays via
inverse Compton scattering, the most plausible solution is a warm absorber
close to the active nucleus.Comment: 7 pages, 7 figures; MNRAS, in pres
Stochastic wake field particle acceleration in Gamma-Ray Bursts
Gamma-Ray Burst (GRB) prompt emission can, for specific conditions, be so
powerful and short-pulsed to strongly influence any surrounding plasma. In this
paper, we briefly discuss the possibility that a very intense initial burst of
radiation produced by GRBs satisfy the intensity and temporal conditions to
cause stochastic wake-field particle acceleration in a surrounding plasma of
moderate density. Recent laboratory experiments clearly indicate that powerful
laser beam pulses of tens of femtosecond duration hitting on target plasmas
cause efficient particle acceleration and betatron radiation up to tens of MeV.
We consider a simple but realistic GRB model for which particle wake-field
acceleration can first be excited by a very strong low-energy precursor, and
then be effective in producing the observed prompt X-ray and gamma-ray GRB
emission. We also briefly discuss some of the consequences of this novel GRB
emission mechanism.Comment: 5 pages, 1 figure, submitted to MNRA
Effect of manure application timing on roots, canopy and must quality in Vitis vinifera 'Merlot': a case study in Italy, North-East
The maintenance and improvement of soil fertility are among the most important management practices in viticulture. The system efficiency fertilization (SEF) which is a new concept based on a maximum utilization of organic fertilizers (i.e., manure) has become very important, especially within the organic viticulture sector, since other fertilizers are not allowed. The aim of this study was to determine the effect of different manure application timing on the root, shoot, and the grapevine yield, accumulation, and quality of biochemical compounds in the grape must since the timing effect was not previously investigated. The study was carried out on 'Merlot' variety organically cultivated, whose production aims at obtaining high-quality red wines. Three treatments were applied: NT (Non-Treated), T1 (Treated1- manure applied in late October) and T2 (Treated2 - manure applied in late February). After two study-years, the undertaken research has shown positive influences of soil manure application on the canopy features (T1), yield, and yield components (T2), along with a major accumulation of the primary metabolites (T2) (soluble solid, carbohydrates, chlorophyll). Yet, the secondary metabolites (polyphenols and anthocyanins) were promoted in the grape must at harvest time, especially when the manure was applied in late October (T2). Considering the benefits of manure application in the T2, after two study years, this timing is recommended in order to improve 'Merlot' grapes for high-quality red wine production
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