1,689 research outputs found
High Energy Gamma-ray Absorption and Cascade Emission in Nearby Starburst Galaxies
High energy gamma-ray emission from two nearby bright starburst galaxies, M82
and NGC 253, have recently been detected by Fermi, H.E.S.S., and VERITAS. Since
starburst galaxies have a high star formation rate and plenty of dust in the
central starburst region, infrared emissions are strong there. Gamma-ray
photons are absorbed by the interstellar radiation field photons via electron
and positron pair creation. The generated electron and positron pairs up
scatter the interstellar photons to very high energy gamma-ray photons via
cascade emission through inverse Compton scattering. In this paper, we evaluate
the contribution of this cascade emission to the gamma-ray spectra of M82 and
NGC 253. Although it would be difficult to see direct gamma- ray evidence of
cosmic-rays with an energy > 10 TeV due to the gamma-ray attenuation, the
resulting cascade emission would be indirect evidence. By including the cascade
component, we find that the total flux above 1 TeV increases ~18% and ~45%
compared with the absorbed flux assuming the maximum kinetic proton energy as
45.3 TeV and 512 TeV, respectively. Future gamma-ray observatories such as CTA
would be able to see the indirect evidence of cosmic-ray with an energy > 10
TeV by comparing with theoretical emission models including this cascade
effect.Comment: 5 pages, Accepted for publication in Ap
Glass Formation, Chemical Properties and Surface Analysis of Cu-Based Bulk Metallic Glasses
This paper reviews the influence of alloying elements Mo, Nb, Ta and Ni on glass formation and corrosion resistance of Cu-based bulk metallic glasses (BMGs). In order to obtain basic knowledge for application to the industry, corrosion resistance of the CuâHfâTiâ(Mo, Nb, Ta, Ni) and CuâZrâAgâAlâ(Nb) bulk glassy alloy systems in various solutions are reported in this work. Moreover, X-ray photoelectron spectroscopy (XPS) analysis is performed to clarify the surface-related chemical characteristics of the alloy before and after immersion in the solutions; this has lead to a better understanding of the correlation between the surface composition and the corrosion resistance
The Integrated Relativistic Iron Line from Active Galactic Nuclei: Chasing the Spin Evolution of Supermassive Black Holes
The spin of a supermassive black hole (SMBH) is directly related to the
radiative efficiency of accretion on to the hole, and therefore impacts the
amount of fuel required for the black hole to reach a certain mass. Thus, a
knowledge of the SMBH spin distribution and evolution is necessary to develop a
comprehensive theory of the growth of SMBHs and their impact on galaxy
formation. Currently, the only direct measurement of SMBH spin is through
fitting the broad Fe K line in AGNs. The evolution of spins could be determined
by fitting the broad line in the integrated spectra of AGNs over different
redshift intervals. The accuracy of these measurements will depend on the
observed integrated line strength. Here, we present theoretical predictions of
the integrated relativistic Fe K line strength as a function of redshift and
AGN luminosity. The equivalent widths of the integrated lines are much less
than 300 eV. Searches for the integrated line will be easiest for unobscured
AGNs with 2-10 keV luminosities between 44 < log L_{X} <= 45. The total
integrated line makes up less than 4% of the X-ray background, but its shape is
sensitive to the average SMBH spin. By following these recommendations, future
International X-ray Observatory surveys of broad Fe K lines should be able to
determine the spin evolution of SMBHs.Comment: 13 pages, 3 figures, accepted by ApJ Letter
Probing The Dust-To-Gas Ratio of z > 0 Galaxies Through Gravitational Lenses
We report the detection of differential gas column densities in three
gravitational lenses, MG0414+0534, HE1104-1805, and PKS1830-211. Combined with
the previous differential column density measurements in B1600+434 and
Q2237+0305 and the differential extinction measurements of these lenses, we
probe the dust-to-gas ratio of a small sample of cosmologically distant normal
galaxies. We obtain an average dust-to-gas ratio of E(B-V)/NH =(1.4\pm0.5) e-22
mag cm^2/atoms with an estimated intrinsic dispersion in the ratio of ~40%.
This average dust-to-gas ratio is consistent with the average Galactic value of
1.7e-22 mag cm^2/atoms and the estimated intrinsic dispersion is also
consistent with the 30% observed in the Galaxy.Comment: 14 pages, 4 figures, Accepted by Ap
Probing Intergalactic Magnetic Fields in the GLAST Era through Pair Echo Emission from TeV Blazars
More than a dozen blazars are known to be emitters of multi-TeV gamma rays,
often with strong and rapid flaring activity. By interacting with photons of
the cosmic microwave and infrared backgrounds, these gamma rays inevitably
produce electron-positron pairs, which in turn radiate secondary inverse
Compton gamma rays in the GeV-TeV range with a characteristic time delay that
depends on the properties of the intergalactic magnetic field (IGMF). For
sufficiently weak IGMF, such "pair echo" emission may be detectable by the
Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on
the IGMF. We perform detailed calculations of the time-dependent spectra of
pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking
proper account of the echo geometry and other crucial effects. In some cases,
the presence of a weak but non-zero IGMF may enhance the detectability of
echos. We discuss the quantitative constraints that can be imposed on the IGMF
from GLAST observations, including the case of non-detections.Comment: 4 pages, 3 figures, minor revisions, accepted for publication in APJ
Contribution of the Gamma-ray Loud Radio Galaxies Core Emissions to the Cosmic MeV and GeV Gamma-Ray Background Radiation
The Fermi gamma-ray satellite has recently detected gamma-ray emissions from
radio galaxy cores. From these samples, we first examine the correlation
between the luminosities at 5 GHz, L_{5GHz}, and at 0.1-10 GeV, L_{gamma}, of
these gamma-ray loud radio galaxies. We find that the correlation is
significant with L_{gamma} \propto L_{5GHz}^{1.16} based on a partial
correlation analysis. Using this correlation and the radio luminosity function
(RLF) of radio galaxies, we further explore the contribution of gamma-ray loud
radio galaxies to the unresolved extragalactic gamma-ray background (EGRB). The
gamma-ray luminosity function is obtained by normalizing the RLF to reproduce
the source count distribution of the Fermi gamma-ray loud radio galaxies. We
find that gamma-ray loud radio galaxies will explain ~25% of the unresolved
Fermi EGRB flux above 100 MeV and will also make a significant contribution to
the EGRB in the 1-30 MeV energy band. Since blazars explain 22% of the EGRB
above 100 MeV, radio loud active galactic nuclei (AGNs) population explains
~47% of the unresolved EGRB. We further make an interpretation on the origin of
the EGRB. The observed EGRB spectrum at 0.2-100 GeV does not show an absorption
signature by the extragalactic background light. Thus, the dominant population
of the origin of EGRB at very high energy (>30 GeV) might be nearby gamma-ray
emitting sources or sources with very hard gamma-ray spectrum.Comment: 9 pages, accepted for publication in Ap
Differential X-ray Absorption and Dust-To-Gas Ratios of the Lens Galaxies SBS0909+523, FBQS0951+2635, and B1152+199
We analyzed Chandra observations of three gravitational lenses, SBS0909+523,
FBQS0951+2635, and B1152+199, to measure the differential X-ray absorption and
the dust-to-gas ratio of the lens galaxies. We successfully detected the
differential X-ray absorption in SBS0909+523 and B1152+199, and failed to
detect it in FBQS0951+2635 due to the dramatic drop in its flux from the ROSAT
epoch. These measurements significantly increase the sample of dust-to-gas
ratio measurements in cosmologically-distant, normal galaxies. Using the larger
sample, we obtain an average dust-to-gas ratio of E(B-V)/NH = (1.5\pm0.5)e-22
mag cm^2/atoms with an estimated intrinsic dispersion in the ratio of \simeq
40%. This average dust-to-gas ratio is consistent with our previous
measurement, and the average Galactic value of 1.7e-22 mag cm^2/atoms and the
estimated intrinsic dispersion is also consistent with the 30% observed in the
Galaxy. A larger sample size is still needed to improve the measurements and to
begin studying the evolution in the ratio with cosmic time. We also detected
X-ray microlensing in SBS0909+523 and significant X-ray variability in
FBQS0951+2635.Comment: 20 pages, 6 figures, Accepted by Ap
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