1,429 research outputs found
Discovery of VHE and HE emission from the blazar 1ES 0414+009 with H.E.S.S and Fermi-LAT
The high energy peaked BL Lac (HBL) object 1ES 0414+009 (z=0.287) is a
distant very high-energy (VHE, E > 100 GeV) blazars with well-determined
redshift. This source was detected with the High Energy Stereoscopic System
(H.E.S.S.) between October 2005 and September 2009. It was also detected with
the Fermi Large Area Telescope (LAT) in 21 months of data. The combined high
energy (HE) and VHE spectra, once corrected for gamma-gamma absorption on the
extragalactic background light (EBL), indicate a Compton peak located above few
TeV, among the highest in the BL Lac class.Comment: proceeding from the 25th Texas Symposium on Relativistic Astrophysics
(Heidelberg, Germany, 2010
Compton-thick AGN in the NuSTAR era II: A deep NuSTAR and XMM-Newton view of the candidate Compton thick AGN in NGC 1358
We present the combined NuSTATR and XMM-Newton 0.6-79 keV spectral analysis
of a Seyfert 2 galaxy, NGC 1358, which we selected as a candidate Compton thick
(CT-) active galactic nucleus (AGN) on the basis of previous Swift/BAT and
Chandra studies. According to our analysis, NGC 1358 is confirmed to be a
CT-AGN using physical motivated models, at >3 confidence level. Our
best-fit shows that the column density along the 'line-of-sight' of the
obscuring material surrounding the accreting super-massive black hole is N = [1.96--2.80] 10 cm. The high-quality data from
NuSTAR gives the best constraints on the spectral shape above 10 keV to
date on NGC 1358. Moreover, by combining NuSTAR and XMM-Newton data, we find
that the obscuring torus has a low covering factor ( <0.17), and the
obscuring material is distributed in clumps, rather than uniformly. We also
derive an estimate of NGC 1358's Eddington ratio, finding it to be
10, which is in
acceptable agreement with previous measurements. Finally, we find no evidence
of short-term variability, over a 100 ks time-span, in terms of both
'line-of-sight' column density and flux.Comment: 12 pages, 6 figure
Atomic carbon in the upper atmosphere of Titan
The atomic carbon emission C I line feature at 1657 Ã… (^(3)P^(0)_(J) -^(3)P_J ) in the upper atmosphere of Titan is first identified from the airglow spectra obtained by the Cassini Ultra-violet Imaging Spectrograph. A one-dimensional photochemical model of Titan is used to study the photochemistry of atomic carbon on Titan. Reaction between CH and atomic hydrogen is the major source of atomic carbon, and reactions with hydrocarbons (C_2H_2 and C_2H_4) are the most important loss processes. Resonance scattering of sunlight by atomic carbon is the dominant emission mechanism. The emission intensity calculations based on model results show good agreement with the observations
Studies of extreme-ultraviolet emission from Rydberg series of H_2 by electron impact
Electron excitation cross sections have been measured for the following two Rydberg series of H_2: ^1Σ_u^+ 1sσnpσ (B, B′, and B", states with principal quantum numbers n=2, 3, and 4, respectively) and ^1Π_u 1sσnpπ (C, D, and D′ states with principal quantum numbers n=2, 3, and 4, respectively) over the energy range from threshold to 350 eV. The cross sections for these six states account for all (>99%) of the vacuum-ultraviolet emission (78-170 nm) of the singlet states of H_2. The estimated total direct-excitation cross sections for these six states at 100 eV in decreasing value are (4.02±0.60)×10^(−17) cm^2 for B^1Σ_u^+ (3.86±0.60)×10^(−17) cm^2 for C^1Π_u, (0.76±0.11)×10^(−17) cm^2 for D^1Π_u, (0.76±0.11)×10^(−17) cm^2 for B' ^1Σ_u^+, (0.30±0.06)×10^(−17) cm^2 for D′^1Π_u, and (0.23±0.05)×10^(−17) cm^2 for B"^1Σ_u^+ and, additionally, (0.43±0.10)×10^(−17) cm^2 for E,F^1Σ_g^+ which populates the B^1Σ_u^+ state through radiative cascade transitions. We estimate the predissociation (autoionization is weak) and emission yields of the vibrational levels of the D, D′, and B" states whose band systems exhibit strong "breaking off in emission" for wavelengths below 85 nm. Furthermore, we report the first direct measurement of the dissociative excitation cross section for production of Lyman-β of (8.9±3.0)×10^(−19) cm^2 at 100 eV. In particular, it is shown that the high-lying Rydberg states (n=3 and 4) make a substantial contribution to the observed emission below 110 nm while above 110 nm the Lyman bands (B^1Σ_u^+→X^1Σ_g^+) and Werner bands (C^1Π_u→X^1Σ_g^+), the first members of the Rydberg series, dominate the spectrum. As a result of these measurements and spectroscopic models the ultraviolet (UV) spectrum from H_2 by electron impact can serve as an intensity calibration standard from 80 to 170 nm
Laboratory studies of uv emissions of H_2 by electron impact. The Werner- and Lyman-band systems
We report a laboratory measurement of absolute emission cross sections of both the Lyman bands (B^1Σ_u^+→X^1Σ_g^+) and Werner bands (C^1Π_u→X^1Π_g^+) of H_2 by electron impact over the energy range from threshold to 400 eV with the same optical system. We find the emission cross section for the B^1Σ_u^+→X^1Σ_g^+ transition at 100 eV to be (3.55±0.8) × 10^(−17) cm^2 (2.7 × 10^(−17) cm^2, direct excitation, 0.85 × 10^(−17) cm^2, cascading) and the emission cross section for the C^1Π_u→X^1Σ_g^+ transition at 100 eV to be (3.1±0.6) × 10^(−17) cm^2 (cascading is estimated to be not present). The cross-section ratio Qc/Qb for direct excitation is 1.21±0.30 at 300 eV in excellent agreement with published values for this ratio from theoretical calculations and experimental data of the optical oscillator strengths. We measure the cross section for cascading to the B state to be 24±10% of the total emission cross section both at 100 and 300 eV. We show that cascading increases to 51±20% of the total cross section of the B state at 20 eV. The vibrational population distribution of the B state is found to be a function of electron-impact energy as the importance of cascading relative to direct excitation changes with electron-impact energy
Modeling the flaring activity of the high z, hard X-ray selected blazar IGR J22517+2217
We present new Suzaku and Fermi data, and re-analyzed archival hard X-ray
data from INTEGRAL and Swift-BAT survey, to investigate the physical properties
of the luminous, high-redshift, hard X-ray selected blazar IGR J22517+2217,
through the modelization of its broad band spectral energy distribution (SED)
in two different activity states. Through the analysis of the new Suzaku data
and the flux selected data from archival hard X-ray observations, we build the
source SED in two different states, one for the newly discovered flare occurred
in 2005 and one for the following quiescent period. Both SEDs are strongly
dominated by the high energy hump peaked at 10^20 -10^22 Hz, that is at least
two orders of magnitude higher than the low energy (synchrotron) one at 10^11
-10^14 Hz, and varies by a factor of 10 between the two states. In both states
the high energy hump is modeled as inverse Compton emission between
relativistic electrons and seed photons produced externally to the jet, while
the synchrotron self-Compton component is found to be negligible. In our model
the observed variability can be accounted for by a variation of the total
number of emitting electrons, and by a dissipation region radius changing from
within to outside the broad line region as the luminosity increases. In its
flaring activity, IGR J22517+2217 shows one of the most powerful jet among the
population of extreme, hard X-ray selected, high redshift blazar observed so
far.Comment: Accepted for publication in MNRA
Diffuse -ray emission from misaligned active galactic nuclei
Active galactic nuclei (AGN) with jets seen at small viewing angles are the
most luminous and abundant objects in the -ray sky. AGN with jets
misaligned along the line-of-sight appear fainter in the sky, but are more
numerous than the brighter blazars. We calculate the diffuse -ray
emission due to the population of misaligned AGN (MAGN) unresolved by the Large
Area Telescope (LAT) on the {\it Fermi} Gamma-ray Space Telescope ({\it
Fermi}). A correlation between the -ray luminosity and the radio-core
luminosity is established and demonstrated to be physical by statistical tests,
as well as compatible with upper limits based on {\it Fermi}-LAT data for a
large sample of radio-loud MAGN. We constrain the derived -ray
luminosity function by means of the source count distribution of the radio
galaxies (RGs) detected by the {\it Fermi}-LAT. We finally calculate the
diffuse -ray flux due to the whole MAGN population. Our results
demonstrate that the MAGN can contribute from 10% up to nearly the entire
measured Isotropic Gamma-Ray Background (IGRB). We evaluate a theoretical
uncertainty on the flux of almost an order of magnitude.Comment: Accepted for publication in Ap
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