1,447 research outputs found
Analysis of X-ray spectral variability and black hole mass determination of the NLS1 galaxy Mrk 766
We present an XMM-Newton time-resolved spectral analysis of the NLS1 galaxy
Mrk 766. We analyse eight available observations of the EPIC-pn camera taken
between May 2000 and June 2005 to investigate the X-ray spectral variability as
produced by changes in the mass accretion rate. The 0.2-10 keV spectra are
extracted in time bins longer than 3 ks to accurately trace the variations of
the best fit parameters of our adopted Comptonisation spectral model. We test a
bulk-motion Comptonisation (BMC) model which is in general applicable to any
physical system powered by accretion onto a compact object, and assumes that
soft seed photons are efficiently up-scattered via inverse Compton scattering
in a hot and dense electron corona. The Comptonised spectrum has a
characteristic power-law shape, whose slope was found to increase for large
values of the normalisation of the seed component, that is proportional to the
mass accretion rate (in Eddington units). Our baseline spectral model also
includes a warm absorber lying on the line of sight and radiation reprocessing
from the accretion disk or from outflowing matter in proximity of the central
compact object. Our study reveals that the normalisation-slope correlation,
observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766:
variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to
be related to the variations of m-dot, as observed in X-ray binary systems. We
finally applied a scaling technique based on the observed correlation to
estimate the BH mass in Mrk 766. This technique is commonly and successfully
applied to measure masses of GBHs, and this is the first time it is applied in
detail to estimate the BH mass in an AGN. We obtain a value of
M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with
that estimated by the reverberation mappingComment: 26 pages, 7 figures, 4 tables to be published in Astronomy and
Astrophysic
The Diversity of Extrasolar Terrestrial Planets
Extrasolar planetary host stars are enriched in key planet-building elements.
These enrichments have the potential to drastically alter the building blocks
available for terrestrial planet formation. Here we report on the combination
of dynamical models of late-stage terrestrial planet formation within known
extrasolar planetary systems with chemical equilibrium models of the
composition of solid material within the disk. This allows us to constrain the
bulk elemental composition of extrasolar terrestrial planets. A wide variety of
resulting planetary compositions exist, ranging from those that are essentially
"Earth-like", containing metallic Fe and Mg-silicates, to those that are
dominated by graphite and SiC. This implies that a diverse range of terrestrial
planets are likely to exist within extrasolar planetary systems.Comment: 4 pages, 1 figure. Submitted to the proceedings of IAU symposium 265
Chemical Abundances in the Universe: Connecting First Stars to Planet
An extreme [O III] emitter at z = 3.2: A low metallicity Lyman continuum source
Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z > 6, it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues (z ∼ 2?4) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 ?m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S/N > 5. Non-zero Lyaα flux at the systemic redshift and high Lyman-aα ESCape fraction (f(Lyaα) ≥ 0.78) suggest a low H i column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The [O iii] iiiλλ4959, 5007+Hβ equivalent width (EW) is one of the largest reported for a galaxy at z > 3 (EW([O iii] λλ4959, 5007 + Hβ) ≈1600 Å, rest-frame; 6700 Å observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong [O iii] emission. The large observed [O iii]/[O ii] ratio (>10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW([O iiiλλ4959, 5007+Hβ) is comparable to recent measurements reported at z ∼ 7?9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high-z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage. © 2015 ESO.We acknowledge the financial contribution from PRIN-INAF 2012.Peer Reviewe
The XMM deep survey in the CDF-S. X. X-ray variability of bright sources
We aim to study the variability properties of bright hard X-ray selected
Active Galactic Nuclei (AGN) with redshift between 0.3 and 1.6 detected in the
Chandra Deep Field South (XMM-CDFS) by a long XMM observation. Taking advantage
of the good count statistics in the XMM CDFS we search for flux and spectral
variability using the hardness ratio techniques. We also investigated spectral
variability of different spectral components. The spectra were merged in six
epochs (defined as adjacent observations) and in high and low flux states to
understand whether the flux transitions are accompanied by spectral changes.
The flux variability is significant in all the sources investigated. The
hardness ratios in general are not as variable as the fluxes. Only one source
displays a variable HR, anti-correlated with the flux (source 337). The
spectral analysis in the available epochs confirms the steeper when brighter
trend consistent with Comptonisation models only in this source. Finding this
trend in one out of seven unabsorbed sources is consistent, within the
statistical limits, with the 15 % of unabsorbed AGN in previous deep surveys.
No significant variability in the column densities, nor in the Compton
reflection component, has been detected across the epochs considered. The high
and low states display in general different normalisations but consistent
spectral properties. X-ray flux fluctuations are ubiquitous in AGN. In general,
the significant flux variations are not associated with a spectral variability:
photon index and column densities are not significantly variable in nine out of
the ten AGN over long timescales (from 3 to 6.5 years). The photon index
variability is found only in one source (which is steeper when brighter) out of
seven unabsorbed AGN. These results are consistent with previous deep samples.Comment: 14 pages, 11 figures. Accepted in A&
Probing black hole accretion in quasar pairs at high redshift
Models and observations suggest that luminous quasar activity is triggered by
mergers, so it should preferentially occur in the most massive primordial dark
matter haloes, where the frequency of mergers is expected to be the highest.
Since the importance of galaxy mergers increases with redshift, we identify the
high-redshift Universe as the ideal laboratory for studying dual AGN. Here we
present the X-ray properties of two systems of dual quasars at z=3.0-3.3
selected from the SDSS-DR6 at separations of 6-8 arcsec (43-65kpc) and observed
by Chandra for 65ks each. Both members of each pair are detected with good
photon statistics to allow us to constrain the column density, spectral slope
and intrinsic X-ray luminosity. We also include a recently discovered dual
quasar at z=5 (separation of 21 arcsec, 136kpc) for which XMM-Newton archival
data allow us to detect the two components separately. Using optical spectra we
derived bolometric luminosities, BH masses and Eddington ratios that were
compared to those of luminous SDSS quasars in the same redshift ranges. We find
that the brighter component of both pairs at z=3.0-3.3 has high luminosities
compared to the distribution of SDSS quasars at similar redshift, with J1622A
having an order magnitude higher luminosity than the median. This source lies
at the luminous end of the z~3.3 quasar luminosity function. While we cannot
conclusively state that the unusually high luminosities of our sources are
related to their having a close companion, for J1622A there is only a 3%
probability that it is by chance.Comment: MNRAS, in pres
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
The primordial environment of super massive black holes: large scale galaxy overdensities around QSOs with LBT
We investigated the presence of galaxy overdensities around four
QSOs, namely SDSS J1030+0524 (z = 6.28), SDSS J1148+5251 (z = 6.41), SDSS
J1048+4637 (z = 6.20) and SDSS J1411+1217 (z = 5.95), through deep -, -
and - band imaging obtained with the wide-field () Large
Binocular Camera (LBC) at the Large Binocular Telescope (LBT). We adopted
color-color selections within the vs plane to identify samples of
-band dropouts at the QSO redshift and measure their relative abundance and
spatial distribution in the four LBC fields, each covering
physical Mpc at . The same selection criteria were then applied to
-band selected sources in the 1 deg Subaru-XMM Newton Deep Survey
to derive the expected number of dropouts over a blank LBC-sized field
(0.14 deg). The four observed QSO fields host a number of candidates
larger than what is expected in a blank field. By defining as -band dropouts
objects with and undetected in the -band, we found
16, 10, 9, 12 dropouts in SDSS J1030+0524, SDSS J1148+5251, SDSS J1048+4637,
and SDSS J1411+1217, respectively, whereas only 4.3 such objects are expected
over a 0.14 deg blank field. This corresponds to overdensity significances
of 3.3, 1.9, 1.7, 2.5, respectively. By considering the total number of
dropouts in the four LBC fields and comparing it with what is expected in four
blank fields of 0.14 deg each, we find that high-z QSOs reside in overdense
environments at the level. This is the first direct and unambiguous
measurement of the large scale structures around QSOs. [shortened]Comment: 12 pages, 8 figures. Accepted for publication in A&
Simulating the WFXT sky
We investigate the scientific impact of the Wide Field X-ray Telescope
mission. We present simulated images and spectra of X-ray sources as observed
from the three surveys planned for the nominal 5-year WFXT lifetime. The goal
of these simulations is to provide WFXT images of the extragalactic sky in
different energy bands based on accurate description of AGN populations, normal
and star forming galaxies, groups and clusters of galaxies. The images are
realized using a detailed PSF model, instrumental and physical
backgrounds/foregrounds, accurate model of the effective area and the related
vignetting effect. Thanks to this comprehensive modelization of the WFXT
properties, the simulated images can be used to evaluate the flux limits for
detection of point and extended sources, the effect of source confusion at very
faint fluxes, and in general the efficiency of detection algorithms. We also
simulate the spectra of the detected sources, in order to address specific
science topics which are unique to WFXT. Among them, we focus on the
characterization of the Intra Cluster Medium (ICM) of high-z clusters, and in
particular on the measurement of the redshift from the ICM spectrum in order to
build a cosmological sample of galaxy clusters. The end-to-end simulation
procedure presented here, is a valuable tool in optimizing the mission design.
Therefore, these simulations can be used to reliably characterize the WFXT
discovery space and to verify the connection between mission requirements and
scientific goals. Thanks to this effort, we can conclude on firm basis that an
X-ray mission optimized for surveys like WFXT is necessary to bring X-ray
astronomy at the level of the optical, IR, submm and radio wavebands as
foreseen in the coming decade.Comment: "Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societa
Astronomica Italiana 2010 (arXiv:1010.5889)
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