901 research outputs found
BAT X-ray Survey - III: X-ray Spectra and Statistical Properties
In this concluding part of the series of three papers dedicated to the
Swift/BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and
statistical properties of the source sample. Using a dedicated method to
extract time-averaged spectra of BAT sources we show that Galactic sources
have, generally, softer spectra than extragalactic objects and that Seyfert 2
galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert
galaxies is consistent with a power-law with photon index of 2.00 (+/-0.07).
The cumulative flux-number relation for the extragalactic sources in the 14-170
keV band is best described by a power-law with a slope alpha=1.55 (+/-0.20) and
a normalization of 9.6 AGN deg (or 396(+/-80) AGN
all-sky) above a flux level of 2erg cm s (~0.85
mCrab). The integration of the cumulative flux per unit area indicates that BAT
resolves 1-2% of the X-ray background emission in the 14-170 keV band. A
sub-sample of 24 extragalactic sources above the 4.5 sigma detection limit is
used to study the statistical properties of AGN. This sample comprises local
Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of
the Seyfert galaxies are absorbed by column densities of Log(N_H)>22, but that
none is a bona fide Compton-thick. This study shows the capabilities of BAT to
probe the hard X-ray sky to the mCrab level.Comment: Accepted for publication in The Astrophysical Journal; 42 pages, 4
tables, 51 figure
Cosmic X-ray background and Earth albedo Spectra with Swift/BAT
We use Swift/BAT Earth occultation data at different geomagnetic latitudes to
derive a sensitive measurement of the Cosmic X-ray background (CXB) and of the
Earth albedo emission in the 15--200 keV band. We compare our CXB spectrum with
recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement.
Using an independent measurement of the CXB spectrum we are able to confirm our
results. This study shows that the BAT CXB spectrum has a normalization
~8(+/-3)% larger than the HEAO-1 measurement. The BAT accurate Earth albedo
spectrum can be used to predict the level of photon background for satellites
in low Earth and mid inclination orbits.Comment: Accepted for publication in the Astrophysical Journal. 38 Pages, 16
Figures, 2 Table
The Origin of the Extragalactic Gamma-Ray Background and Implications for Dark-Matter Annihilation
The origin of the extragalactic -ray background (EGB) has been
debated for some time. { The EGB comprises the -ray emission from
resolved and unresolved extragalactic sources, such as blazars, star-forming
galaxies and radio galaxies, as well as radiation from truly diffuse
processes.} This letter focuses on the blazar source class, the most numerous
detected population, and presents an updated luminosity function and spectral
energy distribution model consistent with the blazar observations performed by
the {\it Fermi} Large Area Telescope (LAT). We show that blazars account for
50\,\% of the EGB photons (0.1\,GeV), and that {\it Fermi}-LAT
has already resolved 70\,\% of this contribution. Blazars, and in
particular low-luminosity hard-spectrum nearby sources like BL Lacs, are
responsible for most of the EGB emission above 100\,GeV. We find that the
extragalactic background light, which attenuates blazars' high-energy emission,
is responsible for the high-energy cut-off observed in the EGB spectrum.
Finally, we show that blazars, star-forming galaxies and radio galaxies can
naturally account for the amplitude and spectral shape of the background in the
0.1--820\,GeV range, leaving only modest room for other contributions. This
allows us to set competitive constraints on the dark-matter annihilation cross
section.Comment: On behalf of the Fermi-LAT collaboration. Contact authors: M. Ajello,
D. Gasparrini, M. Sanchez-Conde, G. Zaharijas, M. Gustafsson. Accepted for
publication on ApJ
INTEGRAL observations of the blazar Mrk 421 in outburst (Results of a multi-wavelength campaign)
We report the results of a multi-wavelength campaign on the blazar Mrk 421
during outburst. We observed four strong flares at X-ray energies that were not
seen at other wavelengths (partially because of missing data). From the fastest
rise in the X-rays, an upper limit could be derived on the extension of the
emission region. A time lag between high-energy and low-energy X-rays was
observed, which allowed an estimation of the magnetic-field strength. The
spectral analysis of the X-rays revealed a slight spectral hardening of the
low-energy (3 - 43 keV) spectral index. The hardness-ratio analysis of the
Swift-XRT (0.2 - 10 keV) data indicated a small correlation with the intensity;
i. e., a hard-to-soft evolution was observed. At the energies of IBIS/ISGRI (20
- 150 keV), such correlations are less obvious. A multiwavelength spectrum was
composed and the X-ray and bolometric luminosities are calculated.Comment: 15 pages, 18 figures; accepted by Astronomy & Astrophysic
The Luminosity Function of Fermi-detected Flat-Spectrum Radio Quasars
Fermi has provided the largest sample of {\gamma}-ray selected blazars to
date. In this work we use a complete sample of FSRQs detected during the first
year of operation to determine the luminosity function (LF) and its evolution
with cosmic time. The number density of FSRQs grows dramatically up to redshift
\sim0.5-2.0 and declines thereafter. The redshift of the peak in the density is
luminosity dependent, with more luminous sources peaking at earlier times; thus
the LF of {\gamma}-ray FSRQs follows a luminosity-dependent density evolution
similarly to that of radio-quiet AGN. Also using data from the Swift Burst
Alert Telescope we derive the average spectral energy distribution of FSRQs in
the 10 keV-100 GeV band and show that there is no correlation of the peak
{\gamma}-ray luminosity with {\gamma}-ray peak frequency. The coupling of the
SED and LF allows us to predict that the contribution of FSRQs to the Fermi
isotropic {\gamma}-ray background is 9.3(+1.6/-1.0) (\pm3% systematic
uncertainty) in the 0.1-100GeV band. Finally we determine the LF of unbeamed
FSRQs, finding that FSRQs have an average Lorentz factor of {\gamma} =
11.7(+3.3/-2.2), that most are seen within 5\circ of the jet axis, and that
they represent only ~0.1 % of the parent population.Comment: Submitted to the Astrophysical Journa
BAT X-ray Survey - I: Methodology and X-ray Identification
We applied the Maximum Likelihood method, as an image reconstruction
algorithm, to the BAT X-ray Survey (BXS). This method was specifically designed
to preserve the full statistical information in the data and to avoid
mosaicking of many exposures with different pointing directions, thus reducing
systematic errors when co-adding images. We reconstructed, in the 14-170 keV
energy band, the image of a 90x90 deg sky region, centered on
(RA,DEC)=105,-25, which BAT surveyed with an exposure time
of Ms (in Nov. 2005). The best sensitivity in our image is
mCrab or erg cm. We detect 49 hard X-ray sources
above the 4.5 level; of these, only 12 were previously known as hard
X-ray sources (15 keV). Swift/XRT observations allowed us to firmly identify
the counterparts for 15 objects, while 2 objects have Einstein IPC counterparts
\citep{harris90}; in addition to those, we found a likely counterpart for 13
objects by correlating our sample with the ROSAT All-Sky Survey Bright Source
Catalog \citep{voges99}. 7 objects remain unidentified. Analysis of the noise
properties of our image shows that % of the area is surveyed to a flux
limit of 1 mCrab. This study shows that the coupling of the Maximum
Likelihood method to the most sensitive, all-sky surveying, hard X-ray
instrument, BAT, is able to probe for the first time the hard X-ray sky to the
mCrab flux level. The successful application of this method to BAT demonstrates
that it could also be applied with advantage to similar instruments like
INTEGRAL-IBIS.Comment: 35 pages, 10 figures, Accepted for publication in ApJ. 1st paper in a
series of thre
IMAPS Observations of Interstellar Neutral Argon and the Implications for Partially Ionized Gas
We use the absorption features from neutral argon at 1048 and 1066 A to
determine interstellar abundances or their lower limits toward nine early-type
stars. These features were observed with the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) along sight lines with low reddening and low
fractional abundances of molecular hydrogen. We find that the interstellar Ar I
is below its solar and B-star abundance with respect to hydrogen toward zeta
Pup, gamma2 Vel and beta Cen A with (logarithmic) reduction factors
-0.37+/-0.09, -0.18+/-0.10, and -0.61+/-0.12 dex, respectively.
While Ar can condense onto the surfaces of dust grains in the interiors of
dense clouds, it is unlikely that argon atoms are depleted by this process in
the low-density lines of sight considered in this study. Instead, we propose
that the relatively large photoionization cross section of Ar makes it much
easier to hide in its ionized form than H. In regions that are about half
ionized, this effect can lower Ar I/H I by -0.11 to -0.96 dex, depending on the
energy of the photoionizing radiation and its intensity divided by the local
electron density. We apply this interpretation for the condition of the gas in
front of beta Cen A, which shows the largest deficiency of Ar. Also, we
determine the expected magnitudes of the differential ionizations for He, N, O,
Ne and Ar in the partly ionized, warm gas in the local cloud around our solar
system. For the local cloud and others that can be probed by UV studies, the
observed Ar I to H I ratio may be a good discriminant between two possible
alternatives, collisional ionization or photoionization, for explaining the
existence of partly ionized regions.Comment: 36 pages, 3 figure
Fermi-LAT Observations of the Gamma-Ray Burst GRB 130427A
The observations of the exceptionally bright gamma-ray burst (GRB) 130427A by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope provide constraints on the nature of these unique astrophysical sources. GRB 130427A had the largest fluence, highest-energy photon (95 GeV), longest gamma-ray duration (20 hours), and one of the largest isotropic energy releases ever observed from a GRB. Temporal and spectral analyses of GRB 130427A challenge the widely accepted model that the nonthermal high-energy emission in the afterglow phase of GRBs is synchrotron emission radiated by electrons accelerated at an external shock
Influence of the Earth on the background and the sensitivity of the GRM and ECLAIRs instruments aboard the Chinese-French mission SVOM
SVOM (Space-based multi-band astronomical Variable Object Monitor) is a
future Chinese-French satellite mission which is dedicated to Gamma-Ray Burst
(GRB) studies. Its anti-solar pointing strategy makes the Earth cross the field
of view of its payload every orbit. In this paper, we present the variations of
the gamma-ray background of the two high energy instruments aboard SVOM, the
Gamma-Ray Monitor (GRM) and ECLAIRs, as a function of the Earth position. We
conclude with an estimate of the Earth influence on their sensitivity and their
GRB detection capability.Comment: 24 pages, 15 figures, accepted for publication in Experimental
Astronom
Atomic, Molecular, and Optical Physics: Optical Excitation Function of H(1s-2p) Produced by electron Impact from Threshold to 1.8 keV
The optical excitation function of prompt Lyman-Alpha radiation, produced by electron impact on atomic hydrogen, has been measured over the extended energy range from threshold to 1.8 keV. Measurements were obtained in a crossed-beams experiment using both magnetically confined and electrostatically focused electrons in collision with atomic hydrogen produced by an intense discharge source. A vacuum-ultraviolet mono- chromator system was used to measure the emitted Lyman-Alpha radiation. The absolute H(1s-2p) electron impact excitation cross section was obtained from the experimental optical excitation function by normalizing to the accepted optical oscillator strength, with corrections for polarization and cascade. Statistical and known systematic uncertainties in our data range from +/- 4% near threshold to +/- 2% at 1.8 keV. Multistate coupling affecting the shape of the excitation function up to 1 keV impact energy is apparent in both the present experimental data and present theoretical results obtained with convergent close- coupling (CCC) theory. This shape function effect leads to an uncertainty in absolute cross sections at the 10% level in the analysis of the experimental data. The derived optimized absolute cross sections are within 7% of the CCC calculations over the 14 eV-1.8 keV range. The present CCC calculations converge on the Bethe- Fano profile for H(1s-2p) excitation at high energy. For this reason agreement with the CCC values to within 3% is achieved in a nonoptimal normalization of the experimental data to the Bethe-Fano profile. The fundamental H(1s-2p) electron impact cross section is thereby determined to an unprecedented accuracy over the 14 eV - 1.8 keV energy range
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