2,499 research outputs found
Multiwavelength perspective of AGN evolution
Discovering and studying obscured AGN at z>1-3 is important not only to
complete the AGN census, but also because they can pinpoint galaxies where
nuclear accretion and star-formation are coeval, and mark the onset of AGN
feedback. We present the latest results on the characterization of z=1-3
galaxies selected for their high mid-infrared to optical flux ratio, showing
that they are massive and strongly star-forming galaxies, and that many do host
highly obscured AGN. We present a pilot program to push the search of
moderately obscured AGN up to z=5-6 and discuss the perspectives of this line
of research.Comment: Invited talk at the conference: X-Ray Astronomy 2009, Present Status,
multiwavelength approach and future perspectives, September 2009, Bologn
Are the X-ray Spectra of Flat-spectrum Radio Quasars and BL Lacertae Objects Different?
We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) using
the hardness ratios as given in the WGA catalogue of ROSAT sources. This sample
includes all WGA FSRQ with high-quality data and comprises about 20 per cent of
presently known such objects, which makes this the largest FSRQ sample ever
studied in the X-ray band. We find that FSRQ have a distribution of energy
spectral indices ranging between 0 and 3 with a mean value alpha_x ~ 1. This is
consistent with that of low-energy cutoff BL Lacs (LBL; alpha_x ~ 1.1),
generally found in radio surveys, but significantly different from that of
high-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, which
display steeper X-ray spectra (alpha_x ~ 1.5). The shape of the
optical-to-X-ray continuum is concave (that is alpha_x < alpha_ox) for the
majority of FSRQ, as found for LBL, supporting a dominance of inverse Compton
emission in the X-ray band in most objects. Our results are at odds with
previous studies of the X-ray spectra of FSRQ, which were however plagued by
low spectral resolution and/or small number statistics and selection effects,
and have important implications for the proposed connections between FSRQ and
BL Lacs.Comment: 8 pages, 5 figures, TeX file, uses mn.tex and psfig.tex. To appear in
MNRAS. TeX and postscript files also available at
http://itovf2.roma2.infn.it/padovani/xray_spectra/diff.htm
A High Resolution Intergalactic Explorer for the Soft X-ray/FUV
We present a mission concept for high resolution X-ray spectroscopy with a
resolving power, R~6000, (c.f. R=<1000 for Chandra, XMM-Newton). This
resolution is physics-driven, since it allows the thermal widths of coronal
X-ray lines to be measured, and astrophysics-driven, since 50 km/s resolves
internal galaxy motions, and galaxy motions within larger structures.
Such a mission could be small and have a rapid response allowing us to `X-ray
the Universe' using the afterglows of Gamma-ray Bursts (GRBs) as strong
background sources of X-rays, and so illuminate the `Cosmic Web'. The Cosmic
Web is predicted to contain most of the normal matter (baryons) in the nearby
Universe.Comment: Paper presented at the August 2002 SPIE meeting "Astronomical
Telescopes and Instrumentation", Kona, Hawai
Narrow Line Seyfert 1s in the IBISCO sample
We present the broad-band soft and hard X-ray spectral analysis of 8 Narrow
Line Seyfert 1 galaxies extracted from the IBISCO Sample. The study also
focuses on the properties of the NLS1 in our sample in relation to those of the
IBISCO parent Seyfert population. The IBISCO sample comprises 57 AGN selected
from the INTEGRAL IBIS AGN catalogue (in the 20-100 keV band), with z0.05
and covering a wide range of luminosities, BH masses and absorption. All AGN
have also measurements of the molecular gas (H) content of their host
galaxies, through the detection of CO emission lines. The main goals of this
analysis are to accurately determine the X-ray continuum emission, investigate
the presence of absorption features around 7 keV (indicative of the presence of
outflows) and measure the bolometric luminosity in order to study the accretion
parameters of the eight IBISCO NLS1, and study the accretion mechanisms and
investigate the feeding and feedback cycle in these peculiar AGN. Preliminary
results show that NLS1 tend to have higher Eddington ratios and larger
molecular gas fractions than their parent Seyfert population in the IBISCO
sample. Nuclear (AGN) vs. host galaxy properties scaling relations of NLS1 in
relation to the parent Seyfert population are also explored.Comment: 4 pages, 2 figures, 1 table. Proceedings of the conference
"Revisiting narrow-line Seyfert 1 galaxies and their place in the Universe ",
Padova 9-13 April 201
The interaction-driven starburst contribution to the cosmic star formation rate density
An increasing amount of observational evidence supports the notion that there
are two modes of star formation: a quiescent mode in disk-like galaxies, and a
starburst mode, which is generally interpreted as driven by merging. Using a
semi-analytic model of galaxy formation, we derive the relative contribution to
the cosmic star formation rate density of quiescently starforming and starburst
galaxies, predicted under the assumption that starburst events are triggered by
galaxy encounters (merging and fly-by kind) during their merging histories. We
show that, within this framework, quiescently starforming galaxies dominate the
cosmic star formation rate density at all redshifts. The contribution of the
burst-dominated starforming galaxies increases with redshift, rising from <5%
at low redshift (z5. We estimated that the fraction of the
final (z=0) galaxy stellar mass which is formed through the burst component of
star formation is ~10% for 10^10 M_\odot<M_*<10^11.5 M_\odot. Starburst
galaxies, selected according to their distance from the galaxy main sequence,
account for ~10% of the star formation rate density in the redshift interval
1.5<z<2.5, i.e. at the cosmic peak of the star formation activity.Comment: 11 pages, 8 figures. Accepted for publication in A&
Warm Absorbers in the ROSAT Spectra of Quasars
We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an 'ionized absorber' in its X-ray spectrum. The factor 1.7 change in flux in the approx. 2 years between the observations allows a test of of models for this ionized absorber. The absorption feature at approx. 0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity - and hence the ionizing flux at the absorber - is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n(sub e) greater than 2 x 10(exp 4)cm(exp -3); and the distance of the ionized gas from the central source R less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor 2 flux change in approx. 1 week in this source must show non-equilibrium effects in the ionized absorber
Extended X-Ray Emission around Quasars at Intermediate Redshift
We compare the optical to soft X-ray spectral energy distribution (SED) of a sample of bright low-redshift (0.048 less than z less than 0.155), radio-quiet quasars, with a range of thermal models which have been proposed to explain the optical/UV/soft X-ray quasar emission: (a) optically thin emission from an ionized plasma, (b) optically thick emission from the innermost regions of an accretion disk in Schwarzschild and Kerr geometries. We presented ROSAT PSPC observations of these quasars in an earlier paper. Here our goals are to search for the signature of thermal emission in the quasar SED, and to investigate whether a single component is dominating at different frequencies. We find that isothermal optically thin plasma models can explain the observed soft X-ray color and the mean OUV color. However, they predict an ultraviolet (1325 Angstrom) luminosity a factor of 3 to 10 times lower than observed. Pure disk models, even in a Kerr geometry, do not have the necessary flexibility to account for the observed OUV and soft X-ray luminosities. Additional components are needed both in the optical and in the soft X-rays (e.g. a hot corona can explain the soft X-ray color). The most constrained modification of pure disk models, is the assumption of an underlying power law component extending from the infrared (3 micrometers) to the X-ray. This can explain both the OUV and soft X-ray colors and luminosities and does not exceed the 3 micrometers luminosity, where a contribution from hot dust is likely to be important. We also discuss the possibility that the observed soft X-ray color and luminosity are dominated by reflection from the ionized surface of the accretion disk. While modifications of both optically thin plasma models and pure disk models might account for the observed SED, we do not find any strong evidence that the OUV bump and soft X-ray emission are one and the same component. Likewise, we do not find any strong argument which definitely argues in favor of thermal models
Spectral changes during six years of Scorpius X-1 monitoring with BeppoSAX Wide Field Cameras
We analyse a sample of fifty-five observations of Scorpius X-1 available in
the BeppoSAX Wide Field Camera public archive and spanning over the six years
of BeppoSAX mission life. Spectral changes are initially analysed by inspection
of colour-colour and colour-intensity diagrams, we also discuss the shift of
the Z tracks in these diagrams. Then we select two long observations for
spectral fitting analysis, a secular shift is evident between the tracks in
these observations. We finally extract spectra along the tracks and discuss the
best fit model, the parameter variations along the track and between tracks,
and their link to the accretion rate.Comment: 6 pages, 11 postscrpt figures.To appear in the conference proceedings
of `Interacting Binaries: Accretion, Evolution & Outcomes' (Cefalu', July
4-10 2004
Quantum-spacetime scenarios and soft spectral lags of the remarkable GRB130427A
We process the Fermi LAT data on GRB130427A using the Fermi Science Tools,
and we summarize some of the key facts that render this observation truly
remarkable, especially concerning the quality of information on high-energy
emission by GRBs. We then exploit this richness for a search of spectral lags,
of the type that has been recently of interest for its relevance in
quantum-spacetime research. We do find some evidence of systematic soft
spectral lags: when confining the analysis to photons of energies greater than
5 GeV there is an early hard development of minibursts within this long burst.
The effect turns out to be well characterized quantitatively by a linear
dependence, within such a miniburst, of the detection time on energy. With the
guidance of our findings for GRB130427A we can then recognize that some support
for these features is noticeable also in earlier Fermi-LAT GRBs, particularly
for the presence of hard minibursts whose onset is marked by the highest-energy
photon observed for the GRB. A comparison of these features for GRBs at
different redshifts provides some encouragement for a redshift dependence of
the effects of the type expected for a quantum-spacetime interpretation, but
other aspects of the analysis appear to invite the interpretation as intrinsic
properties of GRBs
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