25 research outputs found
Ultrafast Suppression of the Ferroelectric Instability in KTaO
We use an x-ray free-electron laser to study the ultrafast lattice dynamics
following above band-gap photoexcitation of the incipient ferroelectric
potassium-tantalate, \kto. %
We use ultrafast near-UV (central wavelength 266\,nm and 50 fs pulse
duration) laser light to photoexcite charge carriers across the gap and probe
the ultrafast lattice dynamics by recording the x-ray diffuse intensity
throughout multiple Brillouin zones using pulses from the Linac Coherent Light
Source (LCLS) (central wavelength 1.3\,\AA\, and ~fs pulse duration). We
observe changes in the diffuse intensity that we conclude are associated with a
hardening of the soft transverse optical and transverse acoustic phonon
branches along to and to . Using ground- and
excited-state interatomic force constants from density functional theory (DFT)
and assuming the phonon populations can be described by a time-dependent
temperature, we fit the quasi-equilibrium thermal diffuse intensity to the
experimental time-dependent intensity. We obtain the instantaneous lattice
temperature and density of photoexcited charge carriers as a function of time
delay. The DFT calculations demonstrate that photoexcitation transfers charge
from oxygen derived -bonding orbitals to Ta derived antibonding
orbitals, further suppressing the ferroelectric instability and increasing the
stability of the cubic, paraelectric structure.Comment: 8 pages, 4 figure
The X-ray spectral properties of the AGN population in the XMM-Newton bright serendipitous survey
We present here a detailed X-ray spectral analysis of the AGN belonging to
the XMM-Newton bright survey (XBS) that comprises more than 300 AGN up to
redshift ~ 2.4. We performed an X-ray analysis following two different
approaches: by analyzing individually each AGN X-ray spectrum and by
constructing average spectra for different AGN types. From the individual
analysis, we find that there seems to be an anti correlation between the
spectral index and the sources' hard X-ray luminosity, such that the average
photon index for the higher luminosity sources (> 10E44 erg/s) is significantly
flatter than the average for the lower luminosity sources. We also find that
the intrinsic column density distribution agrees with AGN unified schemes,
although a number of exceptions are found (3% of the whole sample), which are
much more common among optically classified type 2 AGN. We also find that the
so-called "soft-excess", apart from the intrinsic absorption, constitutes the
principal deviation from a power-law shape in AGN X-ray spectra and it clearly
displays different characteristics, and likely a different origin, for
unabsorbed and absorbed AGN. Regarding the shape of the average spectra, we
find that it is best reproduced by a combination of an unabsorbed (absorbed)
power law, a narrow Fe Kalpha emission line and a small (large) amount of
reflection for unabsorbed (absorbed) sources. We do not significantly detect
any relativistic contribution to the line emission and we compute an upper
limit for its equivalent width (EW) of 230 eV at the 3 sigma confidence level.
Finally, by dividing the type 1 AGN sample into high- and low-luminosity
sources, we marginally detect a decrease in the narrow Fe Kalpha line EW and in
the amount of reflection as the luminosity increases, the "so-called"
Iwasawa-Taniguchi effect.Comment: 42 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that
has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of
1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in
the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this
paper we report the i, K and 3.6micron identifications of the 1761 X-ray point
sources. We use the likelihood ratio technique to derive the association of
optical/infrared counterparts for 97% of the X-ray sources. For most of the
remaining 3%, the presence of multiple counterparts or the faintness of the
possible counterpart prevented a unique association. For only 10 X-ray sources
we were not able to associate a counterpart, mostly due to the presence of a
very bright field source close by. Only 2 sources are truly empty fields.
Making use of the large number of X-ray sources, we update the "classic locus"
of AGN and define a new locus containing 90% of the AGN in the survey with full
band luminosity >10^42 erg/s. We present the linear fit between the total i
band magnitude and the X-ray flux in the soft and hard band, drawn over 2
orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and
XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we
test its known correlation with redshift and luminosity, and a recently
introduced anti-correlation with the concentration index (C). We find a strong
anti-correlation (though the dispersion is of the order of 0.5 dex) between C
and X/O, computed in the hard band, and that 90% of the obscured AGN in the
sample with morphological information live in galaxies with regular morphology
(bulgy and disky/spiral), suggesting that secular processes govern a
significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5
erg/s.Comment: 21 pages, 17 figures, 4 tables; accepted for publication in ApJS. The
catalog is available at the urls listed in the pape
Contribution of the accretion disk, hot corona, and obscuring torus to the luminosity of Seyfert galaxies: INTEGRAL and Spitzer observations
We estimate the relative contributions of the supermassive black hole (SMBH)
accretion disk, corona, and obscuring torus to the bolometric luminosity of
Seyfert galaxies, using Spizter mid-infrared (MIR) observations of a complete
sample of 68 nearby active galactic nuclei from the INTEGRAL all-sky hard X-ray
(HX) survey. This is the first HX-selected (above 15 keV) sample of AGNs with
complementary high angular resolution, high signal to noise, MIR data.
Correcting for the host galaxy contribution, we find a correlation between HX
and MIR luminosities: L_MIR L_HX^(0.74+/-0.06). Assuming that the observed MIR
emission is radiation from an accretion disk reprocessed in a surrounding dusty
torus that subtends a solid angle decreasing with increasing luminosity (as
inferred from the declining fraction of obscured AGNs), the intrinsic disk
luminosity, L_D, is approximately proportional to the luminosity of the corona
in the 2-300 keV energy band, L_C, with the L_D/L_C ratio varying by a factor
of 2.1 around a mean value of 1.6. This ratio is a factor of ~2 smaller than
for typical quasars producing the cosmic X-ray background (CXB). Therefore,
over three orders of magnitude in luminosity, HX radiation carries a large, and
roughly comparable, fraction of the bolometric output of AGNs. We estimate the
cumulative bolometric luminosity density of local AGNs at ~(1-3)x10^40
erg/s/Mpc^3. Finally, the Compton temperature ranges between kT_c~2 and ~6 keV
for nearby AGNs, compared to kT_c~2 keV for typical quasars, confirming that
radiative heating of interstellar gas can play an important role in regulating
SMBH growth.Comment: 22 pages, 14 figures, accepted for publication in Ap
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha
Rapidly growing black holes and host galaxies in the distant universe from the Herschel Radio Galaxy Evolution Project
We present results from a comprehensive survey of 70 radio galaxies at redshifts 1 2.5 are higher than the sSFR of typical star forming galaxies over the same redshift range, but are similar or perhaps lower than the galaxy population for radio galaxies at z< 2.5. By comparing the sSFR and the specific ṀBH (sṀBH), we conclude that black holes in radio loud AGN are already, or soon will be, overly massive compared to their host galaxies in terms of expectations from the local MBH–MGal relation. In order to catch up with the black hole, the galaxies require about an order of magnitude more time to grow in mass at the observed SFRs compared to the time the black hole is actively accreting. However, during the current cycle of activity, we argue that this catching up is likely to be difficult because of the short gas depletion times. Finally, we speculate on how the host galaxies might grow sufficiently in stellar mass to ultimately fall onto the local MBH–MGal relation
A curriculum for the 21st century? Towards a new basis for overcoming academic/vocational divisions
X-ray Spectral Constraints for z~2 Massive Galaxies: The Identification of Reflection-Dominated Active Galactic Nuclei
We use the 4 Ms Chandra Deep Field-South (CDF-S) survey to place direct constraints on the ubiquity of z ≈ 2 heavily obscured active galactic nuclei (AGNs) in K ~ 3 × 10^23 cm-2). The other 36 X-ray detected BzK galaxies appear to be relatively unobscured AGNs and starburst galaxies; we use X-ray variability analyses over a rest-frame baseline of ≈3 years to further confirm the presence of AGN activity in many of these systems. The majority (7 out of 11) of the heavily obscured AGNs have excess infrared emission over that expected from star formation (termed "infrared-excess galaxies"). However, we find that X-ray detected heavily obscured AGNs only comprise ≈25% of the infrared-excess galaxy population, which is otherwise composed of relatively unobscured AGNs and starburst galaxies. We find that the typical X-ray spectrum of the heavily obscured AGNs is better characterized by a pure reflection model than an absorbed power-law model, suggesting extreme Compton-thick absorption (N H >~ 1024 cm-2) in some systems. We verify this result by producing a composite rest-frame 2-20 keV spectrum, which has a similar shape as a reflection-dominated X-ray spectrum and reveals an emission feature at rest-frame energy ≈6.4 keV, likely to be due to Fe K. These heavily obscured AGNs are likely to be the distant analogs of the reflection-dominated AGNs recently identified at z ≈ 0 with >10 keV observatories. On the basis of these analyses, we estimate the space density for typical (intrinsic X-ray luminosities of L 2-10 keV >~ 1043 erg s-1) heavily obscured and Compton-thick AGNs at z ≈ 2. Our space-density constraints are conservative lower limits but they are already consistent with the range of predictions from X-ray background models