573 research outputs found
Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for GLAST
A study of the statistics of cosmological black-hole jet sources is applied
to EGRET blazar data, and predictions are made for GLAST. Black-hole jet
sources are modeled as collimated relativistic plasma outflows with radiation
beamed along the jet axis due to strong Doppler boosting. The comoving rate
density of blazar flares is assumed to follow a blazar formation rate (BFR),
modeled by analytic functions based on astronomical observations and fits to
EGRET data. The redshift and size distributions of gamma-ray blazars observed
with EGRET, separated into BL Lac object (BL) and flat spectrum radio quasar
(FSRQ) distributions, are fit with monoparametric functions for the
distributions of the jet Lorentz factor \Gamma, comoving directional power
l'_e, and spectral slope. A BFR factor ~10 x greater at z ~ 1 than at present
is found to fit the FSRQ data. A smaller comoving rate density and greater
luminosity of BL flares at early times compared to the present epoch fits the
BL data. Based on the EGRET observations, ~1000 blazars consisting of ~800
FSRQs and FR2 radio galaxies and ~200 BL Lacs and FR1 radio galaxies will be
detected with GLAST during the first year of the mission. Additional AGN
classes, such as hard-spectrum BL Lacs that were mostly missed with EGRET,
could add more GLAST sources. The FSRQ and BL contributions to the EGRET
gamma-ray background at 1 GeV are estimated at the level of ~10 - 15% and ~2 -
4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered
in the optimal case, and a GLAST analysis method for blazar detection is
outlined.Comment: 17 pages, 9 figures, ApJ, in press, v.660, May 1, 2007 (minor changes
from previous version
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
Chronic subdural haematoma: A retrospective 232 cases review - a comparison of a single centre between a population aged under 75 and above 75 years old
Chronic subdural haematoma represents a major problem in the neurosurgical field concerning population both under and above 75 years old. It is not only a matter of prevalence or pure statistics but it embodies true understanding of how the pathology a?ects older patients, their life expectancy and quality when reliable factors are implied such as comorbidities, relation with the moment of surgery, the existence of anticoagulation therapy, and many more. We target these challenges in a study on 232 cases of treated chronic subdural haematomas with impact on two major categories - those under and above 75 years old in the Neurosurgery Department, County Hospital âPius Branzeuâ Timisoara, Romania
Chandra X-ray Observations of the Quadruply Lensed Quasar RX J0911.4+0551
We present results from X-ray observations of the quadruply lensed quasar RX
J0911.4+0551 using data obtained with the Advanced CCD Imaging Spectrometer
(ACIS) on board the Chandra X-ray Observatory. The 29 ks observation detects a
total of ~404 X-ray photons (0.3 to 7.0 keV) from the four images of the lensed
quasar. Deconvolution of the aspect corrected data resolves all four lensed
images, with relative positions in good agreement with optical measurements.
When compared to contemporaneous optical data, one of the lensed images
(component A3) is dimmer by a factor of ~6 in X-rays with respect to the 2
brighter images (components A1 and A2). Spectral fitting for the combined
images shows significant intrinsic absorption in the soft (0.2 to 2.4 keV)
energy band, consistent with the mini-BAL nature of this quasar, while a
comparison with ROSAT PSPC observations from 1990 shows a drop of ~6.5 in the
total soft bandpass flux. The observations also detect ~157 X-ray photons
arising from extended emission of the nearby cluster (peaked ~42" SW of
RXJ0911.4+0551) responsible for the large external shear present in the system.
The Chandra observation reveals the cluster emission to be complex and
non-spherical, and yields a cluster temperature of kT = 2.3^{+1.8}_{-0.8} keV
and a 2.0 to 10 keV cluster luminosity within a 1 Mpc radius of L_X =
7.6_{-0.2}^{+0.6} x 10^{43} ergs/s (error bars denote 90% confidence limits).
Our mass estimate of the cluster within its virial radius is 2.3^{+1.8}_{-0.7}
x 10^{14} solar, and is a factor of 2 smaller than, although consistent with,
previous mass estimates based on the observed cluster velocity dispersion.Comment: 16 pages, 3 figures (figure 1 is color ps). Accepted by Ap
ROSAT Blank Field Sources I: Sample Selection and Archival Data
We have identified a population of blank field sources (or `blanks') among
the ROSAT bright unidentified X-ray sources with faint optical counterparts.
The extreme X-ray over optical flux ratio of blanks is not compatible with the
main classes of X-ray emitters except for extreme BL Lacertae objects. From the
analysis of ROSAT archival data we found no indication of variability and
evidence for only three sources, out of 16, needing absorption in excess of the
Galactic value. We also found evidence for an extended nature for only one of
the 5 blanks with a serendipitous HRI detection; this source (1WGAJ1226.9+3332)
was confirmed as a z=0.89 cluster of galaxies. Palomar images reveal the
presence of a red (O-E~2) counterpart in the X-ray error circle for 6 blanks.
The identification process brought to the discovery of another high z cluster
of galaxies, one (possibly extreme) BL Lac, two ultraluminous X-ray sources in
nearby galaxies and two apparently normal type1 AGNs. These AGNs, together with
4 more AGN-like objects seem to form a well defined group: they present
unabsorbed X-ray spectra but red Palomar counterparts. We discuss the possible
explanations for the discrepancy between the X-ray and optical data, among
which: a suppressed big blue bump emission, an extreme dust to gas (~40-60 the
Galactic ratio), a high redshift (z>3.5) QSO nature, an atypical dust grain
size distribution and a dusty warm absorber. These AGN-like blanks seem to be
the bright (and easier to study) analogs of the sources which are found in deep
Chandra observations. Three more blanks have a still unknown nature.Comment: 23 pages, 8 figures, accepted by ApJ main journa
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Last updated: 5/21/201
The Quasar Pair Q 1634+267 A, B and the Binary QSO vs. Dark Lens Hypotheses
Deep HST/NICMOS H (F160W) band observations of the z=1.96 quasar pair Q
1634+267A,B reveal no signs of a lens galaxy to a 1 sigma threshold of
approximately 22.5 mag. The minimum luminosity for a normal lens galaxy would
be a 6L_* galaxy at z > 0.5, which is 650 times greater than our detection
threshold. Our observation constrains the infrared mass-to-light ratio of any
putative, early-type, lens galaxy to (M/L)_H > 690h_65 (1200h_65) for
Omega_0=0.1 (1.0) and H_0=65h_65 km/s/Mpc. We would expect to detect a galaxy
somewhere in the field because of the very strong Mg II absorption lines at
z=1.1262 in the Q 1634+267 A spectrum, but the HST H-band, I-band (F785LP) and
V-band (F555W) images require that any associated galaxy be very under-luminous
less than 0.1 L^*_H (1.0 L^*_I) if it lies within less than 40 h^{-1} (100
h^{-1}) kpc from Q 1634+267 A,B.
While the large image separation (3.86 arcsec) and the lack of a lens galaxy
strongly favor interpreting Q 1634+267A,B as a binary quasar system, the
spectral similarity remains a puzzle. We estimate that at most 0.06% of
randomly selected quasar pairs would have spectra as similar to each other as
the spectra of Q 1634+267 A and B. Moreover, spectral similarities observed for
the 14 quasar pairs are significantly greater than would be expected for an
equivalent sample of randomly selected field quasars. Depending on how strictly
we define similarity, we estimate that only 0.01--3% of randomly drawn samples
of 14 quasar pairs would have as many similar pairs as the observational
sample.Comment: 24 pages, including 4 figures, LaTex, ApJ accepted, comments from the
editor included, minor editorial change
The Evolution of a Mass-Selected Sample of Early-Type Field Galaxies
We investigate the evolution of mass-selected early-type field galaxies using
a sample of 28 gravitational lenses spanning the redshift range 0 < z < 1.
Based on the redshift-dependent intercept of the fundamental plane in the rest
frame B band, we measure an evolution rate of d log (M/L)_B / dz = -0.56 +/-
0.04 (all errors are 1 sigma unless noted) if we directly compare to the local
intercept measured from the Coma cluster. Re-fitting the local intercept helps
minimize potential systematic errors, and yields an evolution rate of d log
(M/L)_B / dz = -0.54 +/- 0.09. An evolution analysis of properly-corrected
aperture mass-to-light ratios (defined by the lensed image separations) is
closely related to the Faber-Jackson relation. In rest frame B band we find an
evolution rate of d log (M/L)_B / dz = -0.41 +/- 0.21, a present-day
characteristic magnitude of M_{*0} = -19.70 + 5 log h +/- 0.29 (assuming a
characteristic velocity dispersion of sigma_{DM*} = 225 km/s), and a
Faber-Jackson slope of gamma_{FJ} = 3.29 +/- 0.58. The measured evolution rates
favor old stellar populations (mean formation redshift z_f > 1.8 at 2 sigma
confidence for a Salpeter initial mass function and a flat Omega_m =0.3
cosmology) among early-type field galaxies, and argue against significant
episodes of star formation at z < 1.Comment: 38 pages; 9 figs; ApJ accepted; REVISION: erroneous image separation
corrected for one lens, another lens removed; results recalculated and
slightly modifie
A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption
We point out that the tidal disruption of a giant may leave a luminous
(10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source
detected by Chandra at the center of M31 may be such a core; whether or not it
is, the observations have shown that such a core is detectable, even in the
center of a galaxy. We therefore explore the range of expected observational
signatures and how they may be used to (1) test the hypothesis that the M31
source is a remnant of tidal stripping and (2) discover evidence of black holes
and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37
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