623 research outputs found
Radio Spectra of Giant Radio Galaxies from RATAN-600 Data
Measurements of the flux densities of the extended components of seven giant
radio galaxies obtained using the RATAN-600 radio telescope at wavelengths of
6.25 and 13 cm ar e presented. The spectra of components of these radio
galaxies are constructed using these new RA TAN-600 data together with data
from the WENSS, NVSS, and GB6 surveys. The spectral indices in the stu died
frequency range are calculated, and the need for detailed estimates of the
integrated contributi on of such objects to the background emission is
demonstrated.Comment: 7 pages, 2 figures, 5 table
On the Evolution of and High-Energy Emission from GHz-Peaked-Spectrum Sources
Here we discuss evolution and broad-band emission of compact (< kpc) lobes in
young radio sources. We propose a simple dynamical description for these
objects, consisting of a relativistic jet propagating into a uniform gaseous
medium in the central parts of an elliptical host. In the framework of the
proposed model, we follow the evolution of ultrarelativistic electrons injected
from a terminal hotspot of a jet to expanding lobes, taking into account their
adiabatic energy losses as well as radiative cooling. This allows us to discuss
the broad-band lobe emission of young radio sources. In particular, we argue
that the observed spectral turnover in the radio synchrotron spectra of these
objects cannot originate from the synchrotron self-absorption process but is
most likely due to free-free absorption effects connected with neutral clouds
of interstellar medium engulfed by the expanding lobes and photoionized by
active centers. We also find a relatively strong and complex high-energy
emission component produced by inverse-Compton up-scattering of various
surrounding photon fields by the lobes' electrons. We argue that such high
energy radiation is strong enough to account for several observed properties of
GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In
addition, this emission is expected to extend up to GeV (or possibly even TeV)
photon energies and can thus be probed by several modern gamma-ray instruments.
In particular, we suggest that GPS radio galaxies should constitute a
relatively numerous class of extragalactic sources detected by GLAST.Comment: 32 pages, 3 figures included. Revised version, accepted for
publication in Ap
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
Swift BAT Survey of AGN
We present the results of the analysis of the first 9 months of data of the
Swift BAT survey of AGN in the 14-195 keV band. Using archival X-ray data or
follow-up Swift XRT observations, we have identified 129 (103 AGN) of 130
objects detected at |b|> 15 deg and with significance >4.8 sigma. One source
remains unidentified. These same X-ray data have allowed measurement of the
X-ray properties of the objects. We fit a power law to the log N - log S
distribution, and find the slope to be 1.42+/-0.14. Characterizing the
differential luminosity function data as a broken power law, we find a break
luminosity log L_*(erg/s) = 43.85+/-0.26, a low luminosity power law slope
a=0.84^{+0.16}_{-0.22}, and a high luminosity power law slope
b=2.55^{+0.43}_{-0.30}, similar to the values that have been reported based on
INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with
an rms spread of 0.27. Integration of our luminosity function gives a local
volume density of AGN above 10^{41} erg/s of 2.4x10^{-3}/Mpc^3, which is about
10% of the total luminous local galaxy density above M_*=-19.75. We have
obtained X-ray spectra from the literature and from Swift XRT follow-up
observations. These show that the distribution of log n_H is essentially flat
from n_H=10^{20}/cm^{2} to 10^{24}/cm^2, with 50% of the objects having column
densities of less than 10^{22}/cm^{2}. BAT Seyfert galaxies have a median
redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have
log n_H > 22.Comment: 15 pages, 15 figures, 2 tables; to appear in the Astrophysical
Journal, July 10, 2008, v. 68
Rejuvenated radio galaxies J0041+3224 and J1835+6204 : how long can the quiescent phase of nuclear activity last?
We present radio observations of two well-known doubledouble radio galaxies, J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using both the Giant Metrewave Radio Telescope and the Very Large Array. These observations, over a large radio frequency range, enable us to determine the spectra of the inner and outer lobes. Our detailed spectral ageing analysis of their inner and outer lobes demonstrates that the outer doubles of doubledouble radio galaxies are created by the previous cycle of activity, while the inner doubles are due to the present cycle of activity. The (core subtracted) spectra of the inner doubles of both sources are power laws over a large frequency range. We found that the duration of the quiescent phase of J0041+3224 is between 4 and 28 per cent of the active phase of the previous activity. The outer north-western lobe of J1835+6204 has a compact hotspot and the regions of both the outer hotspots have close to power-law (rather than curved) spectra, which indicates that the outer lobes are still fed by jet material ejected in the previous episode just before the central engine stopped powering the jet. We estimate that the duration of the quiescent phase of J1835+6204 is ?5 per cent of the duration of the active phase of the previous activity. Therefore, we conclude that the duration of the quiescent phase can be as short as a few per cent of the active phase in radio galaxies of this type.Peer reviewe
On the turbulent -disks and the intermittent activity in AGN
We consider effects of the MHD turbulence on the viscosity during the
evolution of the thermal-viscous ionization instability in the standard
-accretion disks. We consider the possibility that the accretion onto a
supermassive black hole proceeds through an outer standard accretion disk and
inner, radiatively inefficient and advection dominated flow. In this scenario
we follow the time evolution of the accretion disk in which the viscosity
parameter is constant throughout the whole instability cycle, as
implied by the strength of MHD turbulence. We conclude that the hydrogen
ionization instability is a promising mechanism to explain the intermittent
activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte
Insights into the Kinetics of Supramolecular Comonomer Incorporation in Water
Multicomponent supramolecular polymers are a versatile platform to prepare functional architectures, but a few studies have been devoted to investigate their noncovalent synthesis. Here, we study supramolecular copolymerizations by examining the mechanism and time scales associated with the incorporation of new monomers in benzene-1,3,5-tricarboxamide (BTA)-based supramolecular polymers. The BTA molecules in this study all contain three tetra(ethylene glycol) chains at the periphery for water solubility but differ in their alkyl chains that feature either 10, 12 or 13 methylene units. C(10)BTA does not form ordered supramolecular assemblies, whereas C(12)BTA and C(13)BTA both form high aspect ratio supramolecular polymers. First, we illustrate that C(10)BTA can mix into the supramolecular polymers based on either C(12)BTA or C(13)BTA by comparing the temperature response of the equilibrated mixtures to the temperature response of the individual components in water. Subsequently, we mix C(10)BTA with the polymers and follow the copolymerization over time with UV spectroscopy and hydrogen/deuterium exchange mass spectrometry experiments. Interestingly, the time scales obtained in both experiments reveal significant differences in the rates of copolymerization. Coarse-grained simulations are used to study the incorporation pathway and kinetics of the C(10)BTA monomers into the different polymers. The results demonstrate that the kinetic stability of the host supramolecular polymer controls the rate at which new monomers can enter the existing supramolecular polymers
A multifrequency study of the large radio galaxies 3C46 and 3C452
We present low-frequency observations starting from ~150 MHz with the Giant
Metrewave Radio Telescope (GMRT), and high-frequency observations with the Very
Large Array (VLA) of two large radio galaxies 3C46 and 3C452. These
observations were made with the objectives of estimating their spectral ages
and examining any evidence of diffuse extended emission at low radio
frequencies due to an earlier cycle of activity. While no evidence of extended
emission due to an earlier cycle of activity has been found, the spectral ages
have been estimated to be ~15 and 27 Myr for the oldest relativistic plasma
seen in the regions close to the cores for 3C46 and 3C452 respectively. The
spectra in the vicinity of the hotspots are consistent with a straight spectrum
with injection spectral indices of ~1.0 and 0.78 respectively, somewhat steeper
than theoretical expectations.Comment: Accepted for publication in MNRAS, 10 pages, 8 figures and 4 table
Optically Faint Radio Sources: Reborn AGN?
We have discovered a number of relatively strong radio sources in the
field-of-view of SDSS galaxy clusters which present no optical counterparts
down to the magnitude limits of the SDSS. The optically faint radio sources
appear as double-lobed or core-jet objects on the FIRST radio images and have
projected angular sizes ranging from 0.5 to 1.0 arcmin. We have followed-up
these sources with near-infrared imaging using the wide-field imager HAWK-I on
the VLT. K_s-band emitting regions, about 1.5 arcsec in size and coincident
with the centers of the radio structures, were detected in all the sources,
with magnitudes in the range 17-20 mag. We have used spectral modelling to
characterize the sample sources. In general, the radio properties are similar
to those observed in 3CRR sources but the optical-radio slopes are consistent
with moderate to high redshift (z<4) gigahertz-peaked spectrum sources. Our
results suggest that these unusual objects are galaxies whose black hole has
been recently re-ignited but retain large-scale radio structures, signatures of
previous AGN activity.Comment: Accepted for publication in A&
The Photometric and Kinematic Structure of Face-On Disk Galaxies. I. Sample Definition, H-alpha Integral Field Spectroscopy, and HI Line-Widths
We present a survey of the photometric and kinematic properties of 39 nearby,
nearly face-on disk galaxies. Our approach exploits echelle-resolution
integral-field spectroscopy of the H-alpha regions, obtained with DensePak on
the WIYN 3.5m telescope Bench Spectrograph. This data is complemented by HI
line-profiles observed with the Nancay radio telescope for 25 of these sample
galaxies. Twelve additional line-widths are available for sample galaxies from
the literature. In this paper, we introduce the goals of this survey, define
the sample selection algorithm, and amass the integral field spectroscopic data
and HI line-widths. We establish spatially-integrated H-alpha line-widths for
the sample. We test the veracity of these spatially-integrated line profiles by
convolving narrow-band imaging data with velocity field information for one of
the sample galaxies, PGC 38268, and also by comparing to HI line profiles. We
find HI and H-alpha line profiles to be similar in width but different in
shape, indicating we are observing different spatial distributions of ionized
and neutral gas in largely axisymmetric systems with flat outer
rotation-curves. We also find vertical velocity dispersions of the ionized disk
gas within several disk scale-lengths have a median value of 18 km/s and an 80%
range of 12-26 km/s. This is only a factor of ~2 larger than what is observed
for neutral atomic and molecular gas. With standard assumptions for intrinsic
and thermal broadening for H-alpha, this translates into a factor of three
range in turbulent velocities, between 8 and 25 km/s.Comment: 29 pages, 20 figures; accepted for publication in ApJ Supplement
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