130 research outputs found
Cosmic Evolution of Black Holes and Spheroids. IV. The BH Mass - Spheroid Luminosity Relation
From high-resolution images of 23 Seyfert-1 galaxies at z=0.36 and z=0.57
obtained with the Near Infrared Camera and Multi-Object Spectrometer on board
the Hubble Space Telescope (HST), we determine host-galaxy morphology, nuclear
luminosity, total host-galaxy luminosity and spheroid luminosity. Keck
spectroscopy is used to estimate black hole mass (M_BH). We study the cosmic
evolution of the M_BH-spheroid luminosity (L_sph) relation. In combination with
our previous work, totaling 40 Seyfert-1 galaxies, the covered range in BH mass
is substantially increased, allowing us to determine for the first time
intrinsic scatter and correct evolutionary trends for selection effects. We
re-analyze archival HST images of 19 local reverberation-mapped active galaxies
to match the procedure adopted at intermediate redshift. Correcting spheroid
luminosity for passive luminosity evolution and taking into account selection
effects, we determine that at fixed present-day V-band spheroid luminosity,
M_BH/L_sph \propto (1+z)^(2.8+/-1.2). When including a sample of 44 quasars out
to z=4.5 taken from the literature, with luminosity and BH mass corrected to a
self-consistent calibration, we extend the BH mass range to over two orders of
magnitude, resulting in M_BH/L_sph \propto (1+z)^(1.4+/-0.2). The intrinsic
scatter of the relation, assumed constant with redshift, is 0.3+/-0.1 dex (<0.6
dex at 95% CL). The evolutionary trend suggests that BH growth precedes
spheroid assembly. Interestingly, the M_BH-total host-galaxy luminosity
relation is apparently non-evolving. It hints at either a more fundamental
relation or that the spheroid grows by a redistribution of stars. However, the
high-z sample does not follow this relation, indicating that major mergers may
play the dominant role in growing spheroids above z~1.Comment: 39 pages, 11 figures. Accepted for publication in the Astrophysical
Journa
Particle Acceleration and the Production of Relativistic Outflows in Advection-Dominated Accretion Disks with Shocks
Relativistic outflows (jets) of matter are commonly observed from systems
containing black holes. The strongest outflows occur in the radio-loud systems,
in which the accretion disk is likely to have an advection-dominated structure.
In these systems, it is clear that the binding energy of the accreting gas is
emitted primarily in the form of particles rather than radiation. However, no
comprehensive model for the disk structure and the associated outflows has yet
been produced. In particular, none of the existing models establishes a direct
physical connection between the presence of the outflows and the action of a
microphysical acceleration mechanism operating in the disk. In this paper we
explore the possibility that the relativistic protons powering the jet are
accelerated at a standing, centrifugally-supported shock in the underlying
accretion disk via the first-order Fermi mechanism. The theoretical analysis
employed here parallels the early studies of cosmic-ray acceleration in
supernova shock waves, and the particle acceleration and disk structure are
treated in a coupled, self-consistent manner based on a rigorous mathematical
approach. We find that first-order Fermi acceleration at standing shocks in
advection-dominated disks proves to be a very efficient means for accelerating
the jet particles. Using physical parameters appropriate for M87 and SgrA*, we
verify that the jet kinetic luminosities computed using our model agree with
estimates based on observations of the sources.Comment: accepted for publication in the Astrophysical Journa
Age-related changes in three-dimensional foot motion during barefoot walking in children aged between 7 and 11 years old
Background: The biomechanical complexity of childrenâs feet changes throughout childhood, yet kinematic development of the feet is poorly understood. Further work exploring the kinematic profile of childrenâs feet would be beneficial to help inform our understanding of the typical development of childrenâs feet.
Research Question: Do three-dimensional segmental kinematics of the feet during gait relate to age in a sample of children age 7â11 years?
Methods: This study was a secondary analysis of an existing database representing one hundred and twenty-one children age 7 â 11 years (90 male, 31 female; mean ± SD: age 9.57 ± , 1.17 years, height 1.37 ± 0.08 m, body mass 35.61 ± 9.33 kg). Fifteen, 9 mm retroreflective markers were attached to the right shank and foot of each
participant in, line with the 3DFoot model. Multi-segmental joint kinematics were collected during barefoot walking. Sagittal, frontal, and transverse planar motion was described for the shank-calcaneus, calcaneus-midfoot, and midfoot-metatarsals segment of the right foot. Principal component analysis (PCA) was used to reduce
the major modes of variation in the data to fully explore foot segment motion over the entire gait cycle. Correlations and multiple regression between PCA outputs with age, and potential confounding factors are presented.
Results: Significant positive correlations were found between age and greater calcaneus, dorsiflexion, midfoot inversion and adduction, and metatarsal dorsiflexion, plantarflexion and abduction. There were no significant confounding effects of height, body mass, walking speed or gender on the relationships between age and PCA
outputs.
Significance: The findings from this study demonstrated a relationship between foot kinematics and age suggesting that the development of foot kinematics is ongoing until at least the age of 11 years. This work offers a comprehensive data set of inter-segmental kinematics which helps to advance understanding of the development of the pediatric foot
Supercollapsars and their X-ray Bursts
The very first stars in the Universe can be very massive, up to
. If born in large numbers, such massive stars can have strong
impact on the subsequent star formation, producing strong ionising radiation
and contaminating the primordial gas with heavy elements. They would leave
behind massive black holes that could act as seeds for growing supermassive
black holes of active galactic nuclei. Given the anticipated fast rotation,
such stars would end their live as supermassive collapsars and drive powerful
magnetically-dominated jets. In this letter we investigate the possibility of
observing the bursts of high-energy emission similar to the Long Gamma Ray
Bursts associated with normal collapsars. We show that during the collapse of
supercollapsars, the Blandford-Znajek mechanism can produce jets as powerful as
fewerg/s and release up to erg of the black hole
rotational energy. Due to the higher intrinsic time scale and higher redshift
the initial bright phase of the burst can last for about seconds,
whereas the central engine would remain active for about one day. Due to the
high redshift the burst spectrum is expected to be soft, with the spectral
energy distribution peaking at around 20-30keV. The peak total flux density is
relatively low, , but not prohibitive. If one
supercollapsar is produced per every mini-halo of dark matter arising from the
3- cosmological fluctuations then the whole sky frequency of such
bursts could reach several tens per year.Comment: Accepted for publication in MNRAS, New calculations based on the
Bethe's model of progenitor are give
Heavy Nuclei Synthesized in Gamma-Ray Burst Outflows as the Source of UHECRs
Recent measurements by the Pierre Auger Observatory suggest that the
composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by
heavy nuclei at high energies. However, until now there has been no
astrophysical motivation for considering a source highly enriched in heavy
elements. Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs)
may indeed be composed primarily of nuclei with masses A ~ 40-200, which are
synthesized as hot material expands away from the central engine. In
particular, if the jet is magnetically-dominated (rather than a
thermally-driven fireball) its low entropy enables heavy elements to form
efficiently. Adopting the millisecond proto-magnetar model for the GRB central
engine, we show that heavy nuclei are both synthesized in proto-magnetar winds
and can in principle be accelerated to energies >1e20 eV in the shocks or
regions of magnetic reconnection that are responsible for powering the GRB.
Similar results may apply to accretion-powered GRB models if the jet originates
from a magnetized disk wind. Depending on the precise distribution of nuclei
synthesized, we predict that the average primary mass may continue to increase
beyond Fe group elements at the highest energies, possibly reaching the A ~ 90
(Zirconium), A ~ 130 (Tellurium), or even A ~ 195 (Platinum) peaks. Future
measurements of the UHECR composition at energies >~ 1e20 eV can thus confirm
or constrain our model and, potentially, probe the nature of GRB outflows. The
longer attenuation length of ultra-heavy nuclei through the extragalactic
background light greatly expands the volume of accesible sources and alleviates
the energetic constraints on GRBs as the source of UHECRs.Comment: 10 pages, 3 figures, final version now accepted to MNRA
Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity
Context.
Diffusive shock acceleration in the solar corona can accelerate solar energetic particles to very high energies. Acceleration
efficiency is increased by entrapment through self-generated waves, which is highly dependent on the amount of accelerated particles. This, in turn, is determined by the efficiency of particle injection into the acceleration process.
Aims.
We present an analysis of the injection efficiency at coronal shocks of varying obliquity.We assessed injection through reflection and downstream scattering, including the effect of a cross-shock potential. Both quasi-thermal and suprathermal seed populations were analysed. We present results on the effect of cross-field diffusion downstream of the shock on the injection efficiency.
Methods.
Using analytical methods, we present applicable injection speed thresholds that were compared with both semi-analytical flux integration and Monte Carlo simulations, which do not resort to binary thresholds. Shock-normal angle ΞBn and shock-normal velocity Vs were varied to assess the injection efficiency with respect to these parameters.
Results.
We present evidence of a significant bias of thermal seed particle injection at small shock-normal angles. We show that downstream isotropisation methods affect the ΞBn-dependence of this result. We show a non-negligible effect caused by the crossshock potential, and that the effect of downstream cross-field diffusion is highly dependent on boundary definitions.
Conclusions.
Our results show that for Monte Carlo simulations of coronal shock acceleration a full distribution function assessment with downstream isotropisation through scatterings is necessary to realistically model particle injection. Based on our results, seed particle injection at quasi-parallel coronal shocks can result in significant acceleration efficiency, especially when combined with varying field-line geometry
GeV emission from Gamma Ray Bursts: a radiative fireball?
We study the emission observed at energies greater than 100 MeV of 11 Gamma
Ray Bursts (GRBs) detected by the Fermi/Large Area Telescope (LAT) until
October 2009. The GeV emission has three main properties: (i) its duration is
often longer than the duration of the softer emission detected by the Gamma
Burst Monitor (GBM) onboard Fermi [this confirms earlier results from the
Energetic Gamma-Ray Experiment Telescope (EGRET)]; (ii) its spectrum is
consistent with F(v) propto v^(-1) and does not show strong spectral evolution;
(iii) for the brightest bursts, the flux detected by the LAT decays as a power
law with a typical slope: t^(-1.5). We argue that the observed >0.1 GeV flux
can be interpreted as afterglow emission shortly following the start of the
prompt phase emission as seen at smaller frequencies. The decay slope is what
expected if the fireball emission is produced in the radiative regime, i.e. all
dissipated energy is radiated away. We also argue that the detectability in the
GeV energy range depends on the bulk Lorentz factor Gamma of the bursts, being
strongly favoured in the case of large Gamma. This implies that the fraction of
bursts detected at high energies corresponds to the fraction of bursts having
the largest Gamma. The radiative interpretation can help to explain why the
observed X-ray and optical afterglow energetics are much smaller than the
energetics emitted during the prompt phase, despite the fact that the collision
with the external medium should be more efficient than internal shocks in
producing the radiation we see.Comment: 12 pages, 9 figures, accepted for publication in MNRAS, minor
changes, added EGRET light-curve of GRB 94021
Mergers as triggers for nuclear activity : A near-IR study of the close environment of AGN in the VISTA-VIDEO survey
copyright 2014 The Authors; Published by Oxford University Press on behalf of the Royal Astronomical SocietyThere is an ongoing debate concerning the driver of nuclear activity in galaxies, with active galactic nuclei (AGN) either being triggered by major or minor galactic mergers or, alternatively, through secular processes like cold gas accretion and/or formation of bars. We investigate the close environment of active galaxies selected in the X-ray, the radio and the mid-IR. We utilize the first data release of the new near-IR VISTA Deep Extragalactic Observations (VIDEO) survey of the XMM-Large Scale Structure field. We use two measures of environment density, namely counts within a given aperture and a finite redshift slice (pseudo- 3D density) and closest neighbour density measures â2 and â5. We select both AGN and control samples, matching them in redshift and apparent Ks-band magnitude. We find that AGN are found in a range of environments, with a subset of the AGN samples residing in overdense environments. Seyfert-like X-ray AGN and flat-spectrum radio-AGN are found to inhabit significantly overdense environments compared to their control sample. The relation between overdensities and AGN luminosity does not however reveal any positive correlation. Given the absence of an environment density-AGN luminosity relation, we find no support for a scheme where high-luminosity AGN are preferentially triggered by mergers. On the contrary, we find that AGN likely trace over dense environments at high redshift due to the fact that they inhabit the most massive galaxies, rather than being an AGN.Peer reviewe
Simultaneous radio-interferometric and high-energy TeV observations of the gamma-ray blazar Mkn 421
The TeV-emitting BL Lac object Mkn 421 was observed with very long baseline
interferometry (VLBI) at three closely-spaced epochs one-month apart in
March-April 1998. The source was also monitored at very-high gamma-ray energies
(TeV measurements) during the same period in an attempt to search for
correlations between TeV variability and the evolution of the radio morphology
on parsec scales. While the VLBI maps show no temporal changes in the Mkn 421
VLBI jet, there is strong evidence of complex variability in both the total and
polarized fluxes of the VLBI core of Mkn 421 and in its spectrum over the
two-month span of our data. The high-energy measurements indicate that the
overall TeV activity of the source was rising during this period, with a
gamma-ray flare detected just three days prior to our second VLBI observing
run. Although no firm correlation can be established, our data suggest that the
two phenomena (TeV activity and VLBI core variability) are connected, with the
VLBI core at 22 GHz being the self-absorbed radio counterpart of synchrotron
self-Compton (SSC) emission at high energies. Based on the size of the VLBI
core, we could derive an upper limit of 0.1 pc (3 x 10**17 cm) for the
projected size of the SSC zone. This determination is the first model-free
estimate of the size of the gamma-ray emitting region in a blazar.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
The CFHTLS Strong Lensing Legacy Survey: I. Survey overview and T0002 release sample
AIMS: We present data from the CFHTLS Strong Lensing Legacy Survey (SL2S).
Due to the unsurpassed combined depth, area and image quality of the
Canada-France-Hawaii Legacy Survey it is becoming possible to uncover a large,
statistically well-defined sample of strong gravitational lenses which spans
the dark halo mass spectrum predicted by the concordance model from galaxy to
cluster haloes. METHODS: We describe the development of several automated
procedures to find strong lenses of various mass regimes in CFHTLS images.
RESULTS: The preliminary sample of about 40 strong lensing candidates
discovered in the CFHTLS T0002 release, covering an effective field of view of
28 deg is presented. These strong lensing systems were discovered using an
automated search and consist mainly of gravitational arc systems with splitting
angles between 2 and 15 arcsec. This sample shows for the first time that it is
possible to uncover a large population of strong lenses from galaxy groups with
typical halo masses of about . We discuss the future
evolution of the SL2S project and its main scientific aims for the next 3
years, in particular our observational strategy to extract the hundreds of
gravitational rings also present in these fields.Comment: 11 pages, 5 figures, A&A in pres
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