75 research outputs found
The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution
We present new GMRT observations of HDF 130, an inverse-Compton (IC) ghost of
a giant radio source that is no longer being powered by jets. We compare the
properties of HDF 130 with the new and important constraint of the upper limit
of the radio flux density at 240 MHz to an analytic model. We learn what values
of physical parameters in the model for the dynamics and evolution of the radio
luminosity and X-ray luminosity (due to IC scattering of the cosmic microwave
background (CMB)) of a Fanaroff-Riley II (FR II) source are able to describe a
source with features (lobe length, axial ratio, X-ray luminosity, photon index
and upper limit of radio luminosity) similar to the observations. HDF 130 is
found to agree with the interpretation that it is an IC ghost of a powerful
double-lobed radio source, and we are observing it at least a few Myr after jet
activity (which lasted 5--100 Myr) has ceased. The minimum Lorentz factor of
injected particles into the lobes from the hotspot is preferred to be
for the model to describe the observed quantities well,
assuming that the magnetic energy density, electron energy density, and lobe
pressure at time of injection into the lobe are linked by constant factors
according to a minimum energy argument, so that the minimum Lorentz factor is
constrained by the lobe pressure. We also apply the model to match the features
of 6C 0905+3955, a classical double FR II galaxy thought to have a low-energy
cutoff of in the hotspot due to a lack of hotspot
inverse-Compton X-ray emission. The models suggest that the low-energy cutoff
in the hotspots of 6C 0905+3955 is , just slightly above
the particles required for X-ray emission.Comment: 9 pages, 3 figure
Antiplatelet Therapy Changes for Patients With Myocardial Infarction With Recurrent Ischemic Events: Insights Into Contemporary Practice From the TRANSLATE-ACS (Treatment With ADP Receptor Inhibitors: Longitudinal Assessment of Treatment Patterns and Events After Acute Coronary Syndrome) Study
BACKGROUND: Guidelines recommend P2Y12 inhibitor therapy for 1 year after myocardial infarction (MI), yet little guidance is provided on antiplatelet management for patients with recurrent ischemic events during that year. We describe changes in P2Y12 inhibitor type among patients with recurrent ischemic events in the first year after MI.
METHODS AND RESULTS: The TRANSLATE-ACS (Treatment With ADP Receptor Inhibitors: Longitudinal Assessment of Treatment Patterns and Events After Acute Coronary Syndrome) study enrolled 12 365 patients with MI treated with percutaneous coronary intervention. We examined whether P2Y12 inhibitor choice changed among patients with recurrent MI, stent thrombosis, and/or unplanned revascularization during the first year after MI, and modeled factors associated with P2Y12 inhibitor intensification (changing clopidogrel to prasugrel or ticagrelor). In the first year after MI, 1414 patients (11%) had a total of 1740 recurrent ischemic events (771 recurrent MIs, 969 unplanned revascularizations, and 165 stent thromboses). Median time to the first recurrent ischemic event was 154 days (25th-75th percentiles, 55-287 days). Of those with recurrent ischemic events, 101 of 1092 (9.3%) occurring in clopidogrel-treated patients led to P2Y12 inhibitor intensification. Recurrent events involving stent thrombosis or MI were the strongest factors associated with P2Y12 inhibitor intensification, yet only 40% of patients with stent thrombosis and 14% of patients with recurrent MI had P2Y12 inhibitor intensification. Increasing age and longer time from the index MI were associated with lower likelihood for intensification.
CONCLUSIONS: Few patients after MI with a recurrent ischemic event who were taking clopidogrel switched to a more potent P2Y12 inhibitor, even after stent thrombosis events. Specific guidance is needed for patients who have recurrent ischemic events, particularly when closely spaced.
CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov. Unique identifier: NCT01088503
A multifrequency study of giant radio sources-II. Spectral ageing analysis of the lobes of selected sources
Multifrequency observations with the GMRT and the VLA are used to determine
the spectral breaks in consecutive strips along the lobes of a sample of
selected giant radio sources (GRSs) in order to estimate their spectral ages.
The maximum spectral ages estimated for the detected radio emission in the
lobes of our sources range from 6 to 36 Myr with a median value of
20 Myr using the classical equipartition fields. Using the magnetic field
estimates from the Beck & Krause formalism the spectral ages range from 5
to 38 Myr with a median value of 22 Myr. These ages are significantly
older than smaller sources. In all but one source (J1313+6937) the spectral age
gradually increases with distance from the hotspot regions, confirming that
acceleration of the particles mainly occurs in the hotspots. Most of the GRSs
do not exhibit zero spectral ages in the hotspots, as is the case in earlier
studies of smaller sources. This is likely to be largely due to contamination
by more extended emission due to relatively modest resolutions. The injection
spectral indices range from 0.55 to 0.88 with a median value of
0.6. We discuss these values in the light of theoretical expectations,
and show that the injection spectral index appears to be correlated with
luminosity and/or redshift as well as with linear size.Comment: 12 Pages, 13 Figures, 9 Tables, Accepted for publication in MNRA
Physical Properties of Very Powerful FRII Radio Galaxies
We estimate ages and physical properties of powerful radio galaxies. An
analysis of new multi-wavelength VLA observations of eleven very powerful
classical double (FRIIb) radio galaxies with redshifts between 0.4 and 1.3 is
presented. We estimate ages and velocities for each side of each source. The
eleven new sources are combined with previously studied samples and the
characteristics of the full sample of 31 sources are studied; the full sample
includes sources with redshifts between 0.056 and 1.79, and core-hot spot sizes
of about 30 to 400 kpc. The velocities are independent of core-hotspot
separation, suggesting the rate of growth of a given source is roughly constant
over the source lifetime. We combine the rate of growth, width, and pressure of
a source to study the beam power, lifetime, energy, and ambient gas density
using standard methods previously applied to smaller samples. Typical beam
powers are in the range from 1E44 to 1E46 erg/s; we show that this quantity is
insensitive to assumptions regarding minimum energy conditions. The beam powers
are independent of core-hotspot separation suggesting that the beam power of a
given source is roughly constant over the source lifetime. Typical total source
lifetimes are found to be about a few E6 to E7 years, and typical total outflow
energies (E/c^2) are found to be about 5(E5 - E6) solar masses. Ambient gas
densities are found to decrease with increasing core-hotspot distance, but have
no redshift dependence. Overall, the results obtained with the sample of 31
sources studied here are consistent with those obtained earlier with smaller
samples.Comment: Astronomy and Astrophysics, accepte
Cosmological Studies with Radio Galaxies and Supernovae
Physical sizes of extended radio galaxies can be employed as a cosmological
"standard ruler", using a previously developed method. Eleven new radio
galaxies are added to our previous sample of nineteen sources, forming a sample
of thirty objects with redshifts between 0 and 1.8. This sample of radio
galaxies are used to obtain the best fit cosmological parameters in a
quintessence model in a spatially flat universe, a cosmological constant model
that allows for non-zero space curvature, and a rolling scalar field model in a
spatially flat universe. Results obtained with radio galaxies are compared with
those obtained with different supernova samples, and with combined radio galaxy
and supernova samples. Results obtained with different samples are consistent,
suggesting that neither method is seriously affected by systematic errors. Best
fit radio galaxy and supernovae model parameters determined in the different
cosmological models are nearly identical, and are used to determine
dimensionless coordinate distances to supernovae and radio galaxies, and
distance moduli to the radio galaxies. The distance moduli to the radio
galaxies can be combined with supernovae samples to increase the number of
sources, particularly high-redshift sources, in the samples. The constraints
obtained here with the combined radio galaxy plus supernovae data set in the
rolling scalar field model are quite strong. The best fit parameter values
suggest a value of omega is less than about 0.35, and the model parameter alpha
is close to zero; that is, a cosmological constant provides a good description
of the data. We also obtain new constraints on the physics of engines that
power the large-scale radio emission.Comment: 32 pages. Accepted for publication in the Astrophysical Journa
Chandra Observations of 3C Radio Sources with z<0.3: Nuclei, Diffuse Emission, Jets and Hotspots
We report on our Chandra Cycle 9 program to observe half of the 60
(unobserved by Chandra) 3C radio sources at z<0.3 for 8 ksec each. Here we give
the basic data: the X-ray intensity of the nuclei and any features associated
with radio structures such as hot spots and knots in jets. We have measured
fluxes in soft, medium and hard bands and are thus able to isolate sources with
significant intrinsic column density. For the stronger nuclei, we have applied
the standard spectral analysis which provides the best fit values of X-ray
spectral index and column density. We find evidence for intrinsic absorption
exceeding a column density of 10^{22} cm^{-2} for one third of our sources.Comment: 12 pages, 37 figures (the complete version of the paper with all
figures is available on line, see appendix for details), ApJ accepte
The Lifetime of FRIIs in Groups and Clusters: Implications for Radio-Mode Feedback
We determine the maximum lifetime t_max of 52 FRII radio sources found in 26
central group galaxies from cross correlation of the Berlind SDSS group catalog
with the VLA FIRST survey. Mock catalogs of FRII sources were produced to match
the selection criteria of FIRST and the redshift distribution of our parent
sample, while an analytical model was used to calculate source sizes and
luminosities. The maximum lifetime of FRII sources was then determined via a
comparison of the observed and model projected length distributions. We
estimate the average FRII lifetime is 1.5x10^7 years and the duty cycle is
~8x10^8 years. Degeneracies between t_max and the model parameters: jet power
distribution, axial ratio, energy injection index, and ambient density
introduce at most a factor of two uncertainty in our lifetime estimate. In
addition, we calculate the radio active galactic nuclei (AGN) fraction in
central group galaxies as a function of several group and host galaxy
properties. The lifetime of radio sources recorded here is consistent with the
quasar lifetime, even though these FRIIs have substantially sub-Eddington
accretion. These results suggest a fiducial time frame for energy injection
from AGN in feedback models. If the morphology of a given extended radio source
is set by large-scale environment, while the lifetime is determined by the
details of the accretion physics, this FRII lifetime is relevant for all
extended radio sources.Comment: 18 pages, 7 figures. Accepted for publication in ApJ. High resolution
paper available at http://www.astronomy.ohio-state.edu/~bird/BMK07.pd
The spectra and energies of classical double radio lobes
We compare two temporal properties of classical double radio sources: i)
radiative lifetimes of synchrotron-emitting particles and ii) dynamical source
ages. We discuss how these can be quite discrepant from one another, rendering
use of the traditional spectral ageing method inappropriate: we contend that
spectral ages give meaningful estimates of dynamical ages only when these ages
are << 10^7 years. In juxtaposing the fleeting radiative lifetimes with source
ages which are significantly longer, a refinement of the paradigm for radio
source evolution is required. The changing spectra along lobes are explained,
not predominantly by synchrotron ageing but, by gentle gradients in a magnetic
field mediated by a low-gamma matrix which illuminates an energy-distribution
of particles, controlled largely by classical synchrotron loss in the high
magnetic field of the hotspot. The energy in the particles is an order of
magnitude higher than that inferred from the minimum-energy estimate, implying
that the jet-power is of the same order as the accretion luminosity produced by
the quasar central engine. This refined paradigm points to a resolution of the
findings of Rudnick et al (1994) and Katz-Stone & Rudnick (1994) that both the
Jaffe-Perola and Kardashev-Pacholczyk model spectra are invariably poor
descriptions of the curved spectral shape of lobe emission, and indeed that for
Cygnus A all regions of the lobes are characterised by a `universal spectrum'.
[abridged]Comment: LaTeX, 4 figures. To appear in A
The Entrainment-Limited Evolution of FR II Sources: Maximum Sizes and A Possible Connection to FR Is
We construct a simple theoretical model to investigate how entrainment
gradually erodes high-speed FR II jets. This process is described by embedding
a mixing-layer model developed originally to describe FR I objects in a
self-similar model for the lobe structure of classical FR II sources. Following
the classical FR II models, we assume that the lobe is dominated by the
particles injected from the central jet. The entrainment produces a boundary
shear layer which acts at the interface between the dense central jet and the
less denser surrounding lobe, and the associated erosion of the jet places
interesting limits on the maximum size of FR II sources. The model shows that
this limit depends mainly on the initial bulk velocity of the relativistic jet
triggered. The bulk velocities of FR IIs suggested by our model are in good
agreement with that obtained from direct pc-scale observations on ordinary
radio galaxies and quasars. Finally, we discuss how FR IIs may evolve into FR
Is upon reaching their maximum, entrainment-limited sizes.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region
A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy,
resolved by MERLIN+VLA at 0".2-2".0 resolution. 55 have Chandra X-ray
counterparts including 18 with a hard X-ray photon index and high luminosity
characteristic of a type-II (obscured) AGN. >70% of the radio sources have been
classified as starbursts or AGN using radio morphologies, spectral indices and
comparisons with optical appearance and MIR emission. Starbursts outnumber
radio AGN 3:1. This study extends the VO methods previously used to identify
X-ray-selected obscured type-II AGN to investigate whether very luminous radio
and X-ray emission originates from different phenomena in the same galaxy. The
high-redshift starbursts have typical sizes of 5--10 kpc and star formation
rates of ~1000 Msun/yr. There is no correlation between radio and X-ray
luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected
AGN and the starburst samples were detected by Chandra. The X-ray luminosity
indicates the presence of an AGN in at least half of the 45 cross-matched radio
starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution
overlaps closely with the X-ray detected radio sources which were also detected
by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray
sources is correlated with X-ray hardness. Most extended radio starbursts at
z>1.3 host X-ray selected obscured AGN. Radio emission from most of these
ultra-luminous objects is dominated by star formation but it contributes less
than 1/3 of their X-ray luminosity. Our results support the inferences from
SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more
extended and more copious, it is closely linked to AGN activity and it is
triggered differently, compared with star formation at lower redshifts.Comment: 24 pages, 12 figures, uses graphicx, rotating, natbib, supertabular
packages and aa.cls. Accepted for publication in A&
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