2,865 research outputs found
2003--2005 INTEGRAL and XMM-Newton observations of 3C 273
The aim of this paper is to study the evolution of the broadband spectrum of
one of the brightest and nearest quasars 3C 273.
We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM
monitoring of the blazar 3C 273 in 2003--2005 in the UV, X-ray and soft
gamma-ray bands and study the results in the context of the long-term evolution
of the source.
The 0.2-100 keV spectrum of the source is well fitted by a combination of a
soft cut-off power law and a hard power law. No improvement of the fit is
achieved if one replaces the soft cut-off power law by either a blackbody, or a
disk reflection model. During the observation period the source has reached the
historically softest state in the hard X-ray domain with a photon index
. Comparing our data with available archived X-ray data
from previous years, we find a secular evolution of the source toward softer
X-ray emission (the photon index has increased by
over the last thirty years). We argue that existing theoretical models have to
be significantly modified to account for the observed spectral evolution of the
source.Comment: 11 pages, accepted to A&
Synchrotron radio emission in radio-quiet AGNs
The basic mechanism responsible for radio emission in radio-loud active
galactic nuclei (AGNs) is assumed to be synchrotron radiation. We suggest here
that radio emission in radio-quiet objects is also due to synchrotron radiation
of particles accelerated in shocks. We consider generic shocks and study the
resulting synchrotron properties. We estimate the synchrotron radio luminosity
and compare it with the X-ray component produced by inverse Compton emission.
We obtain that the radio to X-ray luminosity ratio is much smaller than unity,
with values typical of radio-quiet sources. The predicted trends on source
parameters, black hole mass and accretion rate, may account for the
anticorrelation between radio-loudness and Eddington ratio observed in
different AGN samples.Comment: 5 pages, accepted for publication in Astronomy and Astrophysic
Nuclear and gravitational energies in stars
The force that governs the evolution of stars is gravity. Indeed this force
drives star formation, imposes thermal and density gradients into stars at
hydrostatic equilibrium and finally plays the key role in the last phases of
their evolution. Nuclear power in stars governs their lifetimes and of course
the stellar nucleosynthesis. The nuclear reactions are at the heart of the
changes of composition of the baryonic matter in the Universe. This change of
composition, in its turn, has profound consequences on the evolution of stars
and galaxies.
The energy extracted from the gravitational, respectively nuclear reservoirs
during the lifetimes of stars of different masses are estimated. It is shown
that low and intermediate mass stars (M < 8 Msol) extract roughly 90 times more
energy from their nuclear reservoir than from their gravitational one, while
massive stars (M > 8 Msol), which explode in a supernova explosion, extract
more than 5 times more energy from the gravitational reservoir than from the
nuclear one. We conclude by discussing a few important nuclear reactions and
their link to topical astrophysical questions.Comment: 8 pages, 2 figures, Seventh European Summer School on Experimental
Nuclear Astrophysics, Santa Tecla, AIP proceeding
The physical origin of the X-ray power spectral density break timescale in accreting black holes
X-ray variability of active galactic nuclei (AGN) and black hole binaries can
be analysed by means of the power spectral density (PSD). The break observed in
the power spectrum defines a characteristic variability timescale of the
accreting system. The empirical variability scaling that relates characteristic
timescale, black hole mass, and accretion rate () extends from supermassive black holes in AGN down
to stellar-mass black holes in binary systems. We suggest that the PSD break
timescale is associated with the cooling timescale of electrons in the
Comptonisation process at the origin of the observed hard X-ray emission. We
obtain that the Compton cooling timescale directly leads to the observational
scaling and naturally reproduces the functional dependence on black hole mass
and accretion rate (). This result simply
arises from general properties of the emission mechanism and is independent of
the details of any specific accretion model.Comment: 4 pages, accepted for publication in Astronomy and Astrophysics,
Letters to the Edito
Relationship between X-ray and ultraviolet emission in 3C 273
In 3C 273, ultraviolet flux and X-ray flux measured by BATSE are not well
correlated, contrarily to predictions of several models, unless the X-ray flux
lags the UV emission by 1.75 yr. The absence of observed correlation at small
lag cannot be due to spectral variability. A Comptonizing corona model is
however compatible with all UV and X-ray observations covering the BATSE
period.Comment: LaTeX, 4 pages, 6 figures. espcrc2.sty style file included. Poster
contribution to the symposium "The Active X-ray Sky: Results from BepppoSAX
and Rossi-XTE", Rome, October 199
AGN's UV and X-ray luminosities in clumpy accretion flows
We consider the fuelling of the central massive black hole in Active Galactic
Nuclei, through an inhomogeneous accretion flow. Performing simple analytical
treatments, we show that shocks between elements (clumps) forming the accretion
flow may account for the UV and X-ray emission in AGNs. In this picture, a
cascade of shocks is expected, where optically thick shocks give rise to
optical/UV emission, while optically thin shocks give rise to X-ray emission.
The resulting blue bump temperature is found to be quite similar in different
AGNs. We obtain that the ratio of X-ray luminosity to UV luminosity is smaller
than unity, and that this ratio is smaller in massive objects compared to less
massive sources. This is in agreement with the observed ratio
and suggests a possible interpretation of the
anticorrelation.Comment: 8 pages, 1 figure, accepted for publication in A&
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