40,886 research outputs found
Origin of the X-ray Emission in the Nuclei of FR Is
We investigate the X-ray origin in FRIs using the multi-waveband high
resolution data of eight FR I sources, which have very low Eddington ratios. We
fit their multi-waveband spectrum using a coupled accretion-jet model. We find
that X-ray emission in the source with the highest L_X (~1.8*10^-4 L_Edd) is
from the advection-dominated accretion flow (ADAF). Four sources with moderate
L_X(~several*10^-6 L_Edd) are complicated. The X-ray emission of one FR I is
from the jet, and the other three is from the sum of the jet and ADAF. The
X-ray emission in the three least luminous sources (L_X<1.0*10^-6L_Edd) is
dominated by the jet. These results roughly support the predictions of Yuan and
Cui(2005) where they predict that when the X-ray luminosity of the system is
below a critical value, the X-radiation will not be dominated by the emission
from the ADAF any longer, but by the jet. We also find that the accretion rates
in four sources must be higher than the Bondi rates, which implies that other
fuel supply (e.g., stellar winds) inside the Bondi radius should be important.Comment: 6 pages. To published in Journal of Physics, in proceedings of "The
Universe under the Microscope - Astrophysics at High Angular Resolution" (Bad
Honnef, Germany, April 2008), eds. R. Schoedel, A. Eckart, S. Pfalzner, and
E. Ro
Star Formation and Dust Extinction Properties of Local Galaxies as seen from AKARI and GALEX
An accurate estimation of the star formation-related properties of galaxies
is crucial for understanding the evolution of galaxies. In galaxies,
ultraviolet (UV) light emitted by recently formed massive stars is attenuated
by dust, which is also produced by star formation (SF) activity, and is
reemitted at mid- and far- infrared (IR) wavelengths. In this study, we
investigate the star formation rate (SFR) and dust extinction using UV and IR
data. We selected local galaxies which are detected at AKARI FIS 90 um and
matched the IRAS IIFSCz 60 um select catalog. We measured FUV and NUV flux
densities from GALEX images. We examined the SF and extinction of Local
galaxies using four bands of AKARI. Then, we calculated FUV and total IR
luminosities, and obtained the SF luminosity, L_{SF}, the total luminosity
related to star formation activity, and the SFR. We find that in most galaxies,
L_{SF} is dominated by L_{dust}. We also find that galaxies with higher SF
activity have a higher fraction of their SF hidden by dust. In fact, the SF of
galaxies with SFRs >20 M_{sun}/yr is almost completely hidden by dust. Our
results boast a significantly higher precision with respect to previously
published works, due to the use of much larger object samples from the AKARI
and GALEX all sky surveys.Comment: 9 pages, 12 figures, accepted for publication in Earth, Planets, and
Space, A few minor corrections, and a reference adde
An investigation of star formation and dust attenuation in major mergers using ultraviolet and infrared data
Merger processes play an important role in galaxy formation and evolution. To
study the influence of merger processes on the evolution of dust properties and
cosmic star formation rate, we investigate a local sample of major merger
galaxies and a control sample of isolated galaxies using GALEX ultraviolet (UV)
and Spitzer infrared (IR) images. Through a statistical study, we find that
dust attenuation in merger galaxies is enhanced with respect to isolated
galaxies. We find this enhancement is contributed mainly by spiral galaxies in
spiral-spiral (S-S) pairs, and increases with the increasing stellar mass of a
galaxy. Combining the IR and UV parts of star formation rates (SFRs), we then
calculated the total SFRs and specific star formation rates (SSFRs). We find
the SSFRs to be enhanced in merger galaxies. This enhancement depends on galaxy
stellar mass and the companion's morphology, but depends little on whether the
galaxy is a primary or secondary component or on the separation between two
components. These results are consistent with a previous study based only on IR
images. In addition, we investigate the nuclear contributions to SFRs. SFRs in
paired galaxies are more concentrated in the central part of the galaxies than
in isolate galaxies. Our studies of dust attenuation show that the nuclear
parts of pairs most resemble ULIRGs. Including UV data in the present work not
only provides reliable information on dust attenuation, but also refines
analyses of SFRs.Comment: 21 pages, 21 figure
Low frequency noise peak near magnon emission energy in magnetic tunnel junctions
We report on the low frequency (LF) noise measurements in magnetic tunnel
junctions (MTJs) below 4 K and at low bias, where the transport is strongly
affected by scattering with magnons emitted by hot tunnelling electrons, as
thermal activation of magnons from the environment is suppressed. For both
CoFeB/MgO/CoFeB and CoFeB/AlO/CoFeB MTJs, enhanced LF noise is observed
at bias voltage around magnon emission energy, forming a peak in the bias
dependence of noise power spectra density, independent of magnetic
configurations. The noise peak is much higher and broader for unannealed
AlO-based MTJ, and besides Lorentzian shape noise spectra in the
frequency domain, random telegraph noise (RTN) is visible in the time traces.
During repeated measurements the noise peak reduces and the RTN becomes
difficult to resolve, suggesting defects being annealed. The Lorentzian shape
noise spectra can be fitted with bias-dependent activation of RTN, with the
attempt frequency in the MHz range, consistent with magnon dynamics. These
findings suggest magnon-assisted activation of defects as the origin of the
enhanced LF noise.Comment: 6 pages, 5 figure
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