511 research outputs found
Lifting the Veil on Obscured Accretion: Active Galactic Nuclei Number Counts and Survey Strategies for Imaging Hard X-Ray Missions
Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope which, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV, and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a greater than tenfold increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population shows enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions
Measuring the Fraction of Obscured Quasars by the Infrared Luminosity of Unobscured Quasars
Recent work has suggested that the fraction of obscured AGN declines with
increasing luminosity, but it has been difficult to quantify this trend. Here,
we attempt to measure this fraction as a function of luminosity by studying the
ratio of mid-infrared to intrinsic nuclear bolometric luminosity in unobscured
AGN. Because the mid-infrared is created by dust reprocessing of shorter
wavelength nuclear light, this ratio is a diagnostic of f_obsc, the fraction of
solid angle around the nucleus covered by obscuring matter. In order to
eliminate possible redshift-dependences while also achieving a large dynamic
range in luminosity, we have collected archival 24 micron MIPS photometry from
objects with z~1 in the Sloan Digital Sky Survey (SDSS), the Great
Observatories Origins Deep Survey (GOODS) and the Cosmic Evolution Survey
(COSMOS). To measure the bolometric luminosity for each object, we used
archival optical data supplemented by GALEX data. We find that the mean ratio
of 24 microns to bolometric luminosity decreases by a factor of ~3 in the
L_bol=10^44-3x10^47 ergs s^-1 range, but there is also a large scatter at
constant L_bol. Using radiation transfer solutions for model geometries, we
show how the IR/bolometric ratio relates to f_obsc and compare these values
with those obtained obtained from samples of X-ray selected AGN. Although we
find approximate agreement, our method indicates somewhat higher values of
f_obsc, particularly in the middle range of luminosities, suggesting that there
may be a significant number of heavily obscured AGN missed by X-ray surveys.Comment: ApJ, in press. 10 pages in emulateapj style, 4 figures, 3 table
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Certainty in ascending sensory signals - The unexplored driver of analgesic placebo response.
Previous frameworks have failed to adequately explain the observed correlation between within-subject variability in pain reporting and analgesic placebo response. These relationships have been observed in both clinical and experimental setups. Within-subject variability of clinical pain scores is traditionally assessed based on daily pain diaries collected during the pre-intervention stage. Experimental variability can be assessed by the Focused Analgesia Selection Test (FAST), which calculates the relationship between noxious stimuli administrated at various intensities and pain reports. The variability, either clinical or experimental, has been shown to predict the placebo response. In explaining the placebo response, Bayesian Brain Hypothesis (BBH) posits that pain perception (posterior), is composed of certainty (precision) of expectations (priors due to belief or conditioning) and incoming sensory information (likelihood), with the bulk of research focused on the precision of priors. Virtually all placebo analgesia research has focused on the priors and their certainty, rather than on the certainty of the likelihood, mainly because it cannot be assessed directly. We propose that the within-subject variability, as encapsulated by the FAST, is a proxy for certainty in (or, precision of) ascending sensory signals, and our results suggest that it could not only be assessed, but also manipulated. If true, our hypothesis will facilitate new lines of research and could potentially promote precision analgesic medicine by use of variability of pain scores as a diagnostic method to identify pain patients who will benefit from specific treatments
Suzaku observations of hard X-ray selected Seyfert 2 galaxies
We present Suzaku observations of five hard X-ray selected nearby Seyfert 2
galaxies. All the sources were clearly detected with the pin Hard X-ray
Detector up to several tens of keV, allowing for a fairly good characterization
of the broad-band X-ray continuum. We find that a unique model, even including
multiple components, fails to represent the spectra of all the sources. Heavy
obscuration manifests itself in different flavours. For two sources there is
evidence for a reflection dominated continuum; among the other three, one is
"mildly" Compton thick (N_H ~ 10^24 cm-2), while the remaining two are heavily
obscured (N_H ~ 10^23.5 cm-2), but Compton thin. Strong, narrow, iron Kalpha
lines (EW ~ 1-2 keV) due to neutral or mildly ionized gas, are detected in
Compton thick AGN. In all of them the Kalpha line is accompanied by the Kbeta.
The intensity and shape of the soft X-ray spectrum are different from object to
object. Soft X--rays may originate from a nuclear component scattered off, or
leaking through, the X-ray absorber, plus thermal X-rays from the host galaxy.
Emission from circumnuclear gas photoionized by the active nucleus,
parameterized with a power law plus individual narrow Gaussian lines, also
provides an acceptable description of the soft X-ray spectra. The limited
Suzaku XIS CCD energy resolution does not allow us to draw firm conclusions on
the origin of the soft X--ray emission. We briefly discuss our findings in the
light of AGN Unified model and the geometry of the obscuring gas.Comment: 22 pages, 3 Figures; ApJ in pres
INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission
We derive the spectra of the cosmic X-ray background (CXB) and of the
Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of
the IBIS instrument aboard the INTEGRAL satellite obtained during the four
dedicated Earth-occultation observations of early 2006. We analyse the
modulation of the IBIS/ISGRI detector counts induced by the passage of the
Earth through the field of view of the instrument. Unlike previous studies, we
do not fix the spectral shape of the various contributions, but model instead
their spatial distribution and derive for each of them the expected modulation
of the detector counts. The spectra of the diffuse emission components are
obtained by fitting the normalizations of the model lightcurves to the observed
modulation in different energy bins. The obtained CXB spectrum is consistent
with the historic HEAO-1 results and falls slightly below the spectrum derived
with Swift/BAT. A 10% higher normalization of the CXB cannot be completely
excluded, but it would imply an unrealistically high albedo of the Earth. The
derived spectrum of the GRXE confirms the presence of a minimum around 80 keV
with improved statistics and yields an estimate of ~0.6 M_Sun for the average
mass of white dwarfs in the Galaxy. The analysis also provides updated
normalizations for the spectra of the Earth's albedo and the cosmic-ray induced
atmospheric emission.Comment: 13 pages, 13 figures, minor changes to text, A&A in pres
Quantifying the anisotropy in the infrared emission of powerful AGN
We use restframe near- and mid-IR data of an isotropically selected sample of
quasars and radio galaxies at 1.0 \leq z \leq 1.4, which have been published
previously, to study the wavelength-dependent anisotropy of the IR emission.
For that we build average SEDs of the quasar subsample (= type 1 AGN) and radio
galaxies (= type 2 AGN) from ~1-17 {\mu}m and plot the ratio of both average
samples. From 2 to 8 {\mu}m restframe wavelength the ratio gradually decreases
from 20 to 2 with values around 3 in the 10{\mu}m silicate feature. Longward of
12{\mu}m the ratio decreases further and shows some high degree of isotropy at
15 {\mu}m (ratio ~1.4). The results are consistent with upper limits derived
from the X-ray/mid-IR correlation of local Seyfert galaxies. We find that the
anisotropy in our high-luminosity radio-loud sample is smaller than in
radio-quiet lower-luminosity AGN which may be interpreted in the framework of a
receding torus model with luminosity-dependent obscuration properties. It is
also shown that the relatively small degree of anisotropy is consistent with
clumpy torus models.Comment: 6 pages, 2 figures; accepted by Ap
High precision X-ray logN-logS distributions: implications for the obscured AGN population
We have constrained the extragalactic source count distributions over a broad
range of X-ray fluxes and in various energy bands to test whether the
predictions from X-ray background synthesis models agree with the observational
constraints provided by our measurements. We have used 1129 XMM-Newton
observations at |b|>20 deg covering a sky area of 132.3 deg^2 to compile the
largest complete samples of X-ray objects to date in the 0.5-1 keV, 1-2 keV,
2-4.5 keV, 4.5-10 keV, 0.5-2 keV and 2-10 keV energy bands. Our survey includes
in excess of 30,000 sources down to ~10^-15 erg/cm^2/s below 2 keV and down to
~10^{-14} erg/cm^2/s above 2 keV. A break in the source count distributions was
detected in all energy bands except the 4.5-10 keV band. An analytical model
comprising 2 power-law components cannot adequately describe the curvature seen
in the source count distributions. The shape of the logN(>S)-logS is strongly
dependent on the energy band with a general steepening apparent as we move to
higher energies. This is due to non-AGN populations, comprised mainly of stars
and clusters of galaxies, contribute up to 30% of the source population at
energies 10^{-13} erg/cm^2/s, and these populations of
objects have significantly flatter source count distributions than AGN. We find
a substantial increase in the relative fraction of hard X-ray sources at higher
energies, from >55% below 2 keV to >77% above 2 keV. However the majority of
sources detected above 4.5 keV still have significant flux below 2 keV.
Comparison with predictions from the synthesis models suggest that the models
might be overpredicting the number of faint absorbed AGN, which would call for
fine adjustment of some model parameters such as the obscured to unobscured AGN
ratio and/or the distribution of column densities at intermediate obscuration.Comment: Accepted for publication in Astronomy and Astrophysics. Abridged
Abstract. 23 pages, 47 figures, 8 table
The Young, the Old, and the Dusty: Stellar Populations of AGN Hosts
Studying the average properties of active galactic nuclei (AGN) host stellar
populations is an important step in understanding the role of AGN in galaxy
evolution and the processes which trigger and fuel AGN activity. Here we
calculate model spectral energy distributions (SEDs) that include emission from
the AGN, the host galaxy stellar population, and dust enshrouded star
formation. Using the framework of cosmic X-ray background population synthesis
modeling, the model AGN hosts are constrained using optical (B band) and near
infrared (J band, 3.6 um, 5.7 um, 8.0 um, and 24 um) luminosity functions and
number counts. It is found that at z < 1, type 1 and type 2 AGN hosts have
similar stellar populations, in agreement with the orientation based unified
model and indicative of secular evolution. At z > 1, type 2 AGN hosts are
intrinsically different from type 1 AGN hosts, suggesting that the simple
orientation based unified model does not hold at z > 1. Also, it is found that
if Compton thick (CT) AGN evolve like less obscured type 2 AGN, then, on
average, CT AGN hosts are similar to type 2 AGN hosts; however, if CT
obscuration is connected to an evolutionary stage of black hole growth, then CT
AGN hosts will also be in specific evolutionary stages. Multi-wavelength
selection criteria of CT AGN are discussed.Comment: accepted Ap
The Contribution of Active Galactic Nuclei to the Microjansky Radio Population
A X-ray background synthesis model is used to calculate the contribution of
Active Galactic Nuclei (AGNs) to the 1.4 GHz number counts between 100 nJy and
10 mJy. The number counts are broken down into contributions from radio-quiet
and radio-loud AGNs, obscured and unobscured AGNs, and for different ranges in
redshift and 2-10 keV X-ray luminosity, L_X. Compton-thick AGNs are included,
but only to the level required to fit the peak of the X-ray background. The
predicted radio counts show that the microJy AGN population will be dominated
by obscured, radio-quiet Seyfert galaxies with log L_X < 43, and spanning 0 < z
<~ 3. However, depending on the exact relationship between the radio and X-ray
luminosities in radio-quiet AGNs, additional radio flux due to star-formation
within AGN host galaxies may be necessary in order to match the observed AGN
counts at a flux density of ~50 microJy. The star-formation rates (SFR)
required are modest, only ~3 Msun per year, assuming a constant rate with z and
L_X. A more observationally and theoretically motivated relationship, where the
SFR \propto (1+z)^{1.76}(log L_X-40)^{3.5}, will also account for the observed
counts. The microJy AGN population will provide a very clean sample to trace
the accretion and galactic star-formation histories of Seyfert galaxies over a
significant fraction of cosmic time.Comment: 30 pages, 11 figures, accepted by the Ap
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