scholarly journals The Size of IDV Jet Cores

2002 ◽  
Vol 19 (1) ◽  
pp. 55-59 ◽  
Author(s):  
T. Beckert ◽  
T. P. Krichbaum ◽  
G. Cimò ◽  
L. Fuhrmann ◽  
A. Kraus ◽  
...  
Keyword(s):  
Function Analysis ◽  
Large Fraction ◽  
Electron Population ◽  
Interstellar Scintillation ◽  
Spectrum Radio ◽  
Shock Models ◽  
Cold Electrons ◽  
Time Argument ◽  
Local Cloud ◽  
Size Estimates

AbstractRadio variability on timescales from a few hours to several days in extragalactic flat spectrum radio sources is generally classified as intraday variability (IDV). The origin of this short term variability is still controversial and both extrinsic and intrinsic mechanisms must be considered and may both contribute to the observed variations. The measured linear and circular polarisation of IDV sources constrains the low energy end of the electron population. Any population of cold electrons within sources at or above the equipartition temperature of 1011 K depolarises the emission and can be ruled out. Intrinsic shock models are shown to either violate the large fraction of sources displaying IDV or they do not relax the light travel time argument for intrinsic variations. From structure function analysis, we further conclude that interstellar scintillation also leads to tight size estimates unless a very local cloud in the ISM is responsible for IDV.

scholarly journals CSS versus Large Size Sources

1996 ◽  
Vol 175 ◽  
pp. 63-66 ◽  
Author(s):  
R. Fanti ◽  
R. E. Spencer
Keyword(s):  
Flux Density ◽  
High Power ◽  
High Frequency ◽  
Large Fraction ◽  
Radio Sources ◽  
Frequency Spectra ◽  
Large Size ◽  
Spectrum Radio

A large fraction of the sources in flux density limited radio samples have angular sizes < 2 arcsec (and hence projected linear sizes ≤ 10–15 kpc for H0 = 100 Km/(sec Mpc), and steep (α > 0.5, S∝ v–α) high frequency spectra (Kapahi, 1981; Peacock and Wall 1982). The proportion of these Compact Steep–spectrum Sources (CSSs) is high (15–30% depending on the selection frequency) amongst distant (z > 0.2) radio sources of high power, both galaxies and quasars. We include in this class the GHz Peaked Spectrum Radio Sources (GPS), sub–kpc objects whose radio spectra are peaked at GHz frequencies (see, e.g., O'Dea et al, 1991).

scholarly journals Multifrequency Polarization Variations in 0917+624

2001 ◽  
Vol 182 ◽  
pp. 119-122
Author(s):  
S.J. Qian ◽  
A. Kraus ◽  
T.P. Krichbaum ◽  
A. Witzel ◽  
J.A. Zensus
Keyword(s):  
Radio Sources ◽  
Apparent Brightness ◽  
Brightness Temperatures ◽  
Interstellar Scintillation ◽  
Spectrum Radio ◽  
Bl Lac ◽  
Show Evidence ◽  
Intraday Variability

AbstractIntraday variability in compact flat-spectrum radio sources has been intensively studied in recent years. For most IDV events the apparent brightness temperatures derived from the observed timescales are in the range of Tb,app ~ 1016−18 K. For extremely rapid variations, Tb,app can reach up to ~ 1021 K (e.g. Kedziora-Chudczer et al., 1997). Refractive interstellar scintillation may be the most likely extrinsic mechanism (Rickett et al., 1995; Qian, 1994a; Qian, 1994b). Especially for the case of extreme Tb,app (> 1018 K) RISS may be dominant (Dennet-Thorpe and de Bruyn, 2000). However, some IDV events with Tapp ~ 1017−18 K show evidence for an intrinsic origin e.g. the correlated radio-optical intraday variations observed in the BL Lac object 0716+714 (Wagner and Witzel, 1995, Qian et al, 1996). It seems important to distinguish between IDV which is a phenomenon intrinsic to the compact radio sources and IDV which is primarily due to RISS. Multifrequency polarization and VLB I observations would be most helpful (Gabuzda and Kochanev, 1997).

scholarly journals Morphology of Steep Spectrum Radio Sources Showing Variability at Low Frequency

1988 ◽  
Vol 129 ◽  
pp. 125-126
Author(s):  
Franco Mantovani ◽  
Tom Muxlow ◽  
Lucia Padrielli
Keyword(s):  
Interstellar Medium ◽  
Flux Density ◽  
Large Fraction ◽  
Low Frequency ◽  
Line Of Sight ◽  
Radio Sources ◽  
Relativistic Motion ◽  
High Brightness ◽  
Spectrum Radio ◽  
Low Frequency Variability

The observed variability at low frequency of the radio sources can be explained within the framework of the generally accepted models either extrinsic (refractive scintillation in the interstellar medium) or intrinsic (bulk relativistic motion along direction near the line of sight) for variability. Both explanations require a large fraction of the source flux density to be contained in a small high brightness component, of tens of m.a.s. in size. Radio sources with steep straight spectral index are usually tens of Kpc sized, with weak central components and they do not generally show low frequency variability.

Electron cooling at a weakly outgassing comet

2021 ◽  
Author(s):  
Peter Stephenson ◽  
Marina Galand ◽  
Jan Deca ◽  
Pierre Henri ◽  
Gianluca Carnielli
Keyword(s):  
Pure Water ◽  
Particle Model ◽  
Cold Electron ◽  
Subsequent Formation ◽  
Electron Population ◽  
Particle In Cell ◽  
Neutral Gas ◽  
Test Particles ◽  
Impedance Probe ◽  
Cold Electrons

&lt;p&gt;The Rosetta spacecraft arrived at comet 67P in August 2014 and then escorted it for 2 years along its orbit. Throughout this escort phase, two plasma instruments (Mutual Impedance Probe, MIP; and Langmuir Probe, LAP) measured a population of cold electrons (&lt; 1 eV) within the coma of 67P (Engelhardt et al., 2018; Wattieaux et al, 2020; Gilet et al., 2020). These cold electrons are understood to be formed by cooling warm electrons through collisions with the neutral gas. The warm electrons are primarily newly-born and produced at roughly 10eV within the coma through ionisation. While it was no surprise that cold electrons would form near perihelion given the high density of the neutral coma, the persistence of the cold electrons up to a heliocentric distance of 3.8 au was highly unexpected. With the low outgassing rates observed at such large heliocentric distances (Q &lt; 10&lt;sup&gt;26&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;), there should not be enough neutral molecules to cool the warm electrons efficiently before they ballistically escape the coma.&lt;/p&gt;&lt;p&gt;We use a collisional test particle model to examine the formation of the cold electron population at a weakly outgassing comet. The electrons are subject to stochastic collisions with the neutral coma which can either scatter or cool the electrons. Multiple electron neutral collision processes are included such that the electrons can undergo elastic scattering as well as collisions inducing excitation and ionisation of the neutral species. The inputted electric and magnetic fields, which act on the test particles, are taken from a 3D fully-kinetic, collisionless Particle-in-Cell (PiC) model of the solar wind and cometary ionosphere (Deca et al., 2017; 2019), with the same neutral coma as used in our model. We use a pure water coma with spherical symmetry and a 1/r&lt;sup&gt;2&lt;/sup&gt; dependence in the neutral number density to drive the production of cometary electrons and the electron-neutral collisions.&lt;/p&gt;&lt;p&gt;We first demonstrate the trapping of electrons in a potential well around the comet nucleus, formed by an ambipolar field. We show how this electron-trapping process can lead to more efficient cooling of electrons and the subsequent formation of a cold electron population, even at low outgassing rates.&lt;/p&gt;

scholarly journals Searching for signs of jet-driven negative feedback in the nearby radio galaxy UGC 05771

2019 ◽  
Vol 489 (4) ◽  
pp. 4944-4961 ◽  
Author(s):  
Henry R M Zovaro ◽  
Nicole P H Nesvadba ◽  
Robert Sharp ◽  
Geoffrey V Bicknell ◽  
Brent Groves ◽  
...  
Keyword(s):  
Star Formation ◽  
Radio Source ◽  
Large Fraction ◽  
Host Galaxy ◽  
Radio Sources ◽  
Host Galaxies ◽  
Ionized Gas ◽  
Spectrum Radio ◽  
Hydrodynamical Simulations ◽  
Jet Plasma

ABSTRACT Hydrodynamical simulations predict that the jets of young radio sources can inhibit star formation in their host galaxies by injecting heat and turbulence into the interstellar medium (ISM). To investigate jet–ISM interactions in a galaxy with a young radio source, we have carried out a multiwavelength study of the z = 0.025 Compact Steep Spectrum radio source hosted by the early-type galaxy UGC 05771. Using Keck/OSIRIS observations, we detected H2 1–0 S(1) and [Fe ii] emission at radii of 100s of parsecs, which traces shocked molecular and ionized gas being accelerated outwards by the jets to low velocities, creating a ‘stalling wind’. At kpc radii, we detected shocked ionized gas using observations from the CALIFA survey, covering an area much larger than the pc-scale radio source. We found that existing interferometric radio observations fail to recover a large fraction of the source’s total flux, indicating the likely existence of jet plasma on kpc scales, which is consistent with the extent of shocked gas in the host galaxy. To investigate the star formation efficiency in UGC 05771, we obtained IRAM CO observations to analyse the molecular gas properties. We found that UGC 05771 sits below the Kennicutt–Schmidt relation, although we were unable to definitively conclude if direct interactions from the jets are inhibiting star formation. This result shows that jets may be important in regulating star formation in the host galaxies of compact radio sources.

Intraday Variability of Flat-Spectrum Radio Sources

1998 ◽  
Vol 164 ◽  
pp. 257-264
Author(s):  
S. J. Wagner
Keyword(s):  
High Frequency ◽  
Large Fraction ◽  
Small Scale ◽  
Electromagnetic Spectrum ◽  
Radio Sources ◽  
Rapid Variability ◽  
Spectrum Radio ◽  
Interstellar Scattering ◽  
Polarization Characteristics ◽  
Intraday Variability

AbstractThe characteristics of rapid variability of flat-spectrum radio sources are reviewed. A large fraction of the blazar population is found to show variability on timescales shorter than one day throughout the entire electromagnetic spectrum. The spectral indices and polarization characteristics change equally fast.Structure functions of the well-monitored sources show pronounced breaks on scales of about 10 to 50 hours, with flatter slopes towards the fast end. This illustrates that Intraday Variability (IDV), i.e. the high frequency end of the power spectrum is qualitatively different and requires different mechanisms than slower variations.While intrinsic IDV provides direct clues on small-scale structure over fifteen decades in frequency, extrinsic contributions from interstellar scattering contributes at the lowest frequencies, and remains difficult to disentangle from the intrinsic effects.

Frequency Redistribution Effects in the Formation of Lyman α in Prominences and Their Influence on the Ratio of Hα to Lα

1979 ◽  
Vol 44 ◽  
pp. 53-55
Author(s):  
R.W. Milkey ◽  
J.N. Heasley ◽  
E.J. Schmahl ◽  
O. Engvold
Keyword(s):  
Line Profile ◽  
Large Fraction ◽  
Partial Frequency ◽  
Frequency Redistribution ◽  
Partial Frequency Redistribution ◽  
Emergent Intensity

The effect of partial frequency redistribution in the formation of Lyman α in the chromosphere has been discussed by Milkey and Mihalas (1973) and others, and it has been shown that in this case the coherency of scattering in the wings of the line substantially influences the line profile. Although there are non-negligible sources for La photons within a prominence, a large fraction of the emergent line photons are due to scattering of photons incident on the surface of the prominence so that one expects that in a prominence the frequency redistribution processes will play an important role in determining the emergent intensity.

Glucose metabolism in cancer cells: immunogold localization of hexokinase to the outer mitochondrial membrane

1995 ◽  
Vol 53 ◽  
pp. 1044-1045
Author(s):  
Krishan K. Arora ◽  
Glenn L. Decker ◽  
Peter L. Pedersen
Keyword(s):  
Tumor Cells ◽  
Mitochondrial Membrane ◽  
Present Report ◽  
Large Fraction ◽  
Mitochondrial Fraction ◽  
Immunogold Labeling ◽  
Outer Mitochondrial Membrane ◽  
Immunogold Localization ◽  
Hexokinase Activity ◽  
Normal Tissues

Hexokinase (ATP: D-hexose 6-phophotransferase EC 2.7.1.1) is the first enzyme of the glycolytic pathway which commits glucose to catabolism by catalyzing the phosphorylation of glucose with ATP. Previous studies have shown diat hexokinase activity is markedly elevated in rapidly growing tumor cells exhibiting high glucose catabolic rates. A large fraction (50-80%) of this enzyme activity is bound to the mitochondrial fraction (1,2) where it has preferred access to ATP (3). In contrast,the hexokinase activity of normal tissues is quite low, with one exception being brain which is a glucose-utilizing tissue (4). Biochemical evidence involving rigorous subfractionation studies have revealed striking differences between the subcellular distribution of hexokinase in normal and tumor cells [See review by Arora et al (4)].In the present report, we have utilized immunogold labeling techniques to evaluate die subcellular localization of hexokinase in highly glycolytic AS-30D hepatoma cells and in the tissue of its origin, i.e., rat liver.

Sensory and Cognitive Components of Auditory Processing in Individuals With Tinnitus

2019 ◽  
Vol 28 (4) ◽  
pp. 834-842
Author(s):  
Harini Vasudevan ◽  
Hari Prakash Palaniswamy ◽  
Ramaswamy Balakrishnan
Keyword(s):  
Selective Attention ◽  
Auditory Processing ◽  
Function Analysis ◽  
Event Related Potential ◽  
Oscillatory Activity ◽  
Default Mode ◽  
Auditory Selective Attention ◽  
Cognitive Components ◽  
Mode Function ◽  
Related Potentials

Purpose The main purpose of the study is to explore the auditory selective attention abilities (using event-related potentials) and the neuronal oscillatory activity in the default mode network sites (using electroencephalogram [EEG]) in individuals with tinnitus. Method Auditory selective attention was measured using P300, and the resting state EEG was assessed using the default mode function analysis. Ten individuals with continuous and bothersome tinnitus along with 10 age- and gender-matched control participants underwent event-related potential testing and 5 min of EEG recording (at wakeful rest). Results Individuals with tinnitus were observed to have larger N1 and P3 amplitudes along with prolonged P3 latency. The default mode function analysis revealed no significant oscillatory differences between the groups. Conclusion The current study shows changes in both the early sensory and late cognitive components of auditory processing. The change in the P3 component is suggestive of selective auditory attention deficit, and the sensory component (N1) suggests an altered bottom-up processing in individuals with tinnitus.

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