477 research outputs found
Hard X-ray detection of the high redshift quasar 4C 71.07
BATSE/OSSE observations of the high redshift quasar 4C 71.07 indicate that
this is the brightest and furthest AGN so far detected above 20 keV. BATSE
Earth occultation data have been used to search for emission from 4C 71.07 from
nearly 3 years of observation. The mean source flux over the whole period in
the BATSE energy range 20-100 keV is (13.2 +/- 1.06) x 10^(-11) erg cm^(-2)
s^(-1) corresponding to a luminosity of 2 x 10^(48) erg s^(-1). The BATSE light
curve over the 3 years of observations shows several flare-like events, one of
which (in January 1996) is associated with an optical flare (R=16.1) but with a
delay of 55 days. The OSSE/BATSE spectral analysis indicates that the source is
characterized by a flat power spectrum (Gamma about 1.1 - 1.3) when in a low
state; this spectral form is consistent within errors with the ASCA and ROSAT
spectra. This means that the power law observed from 0.1 to 10 keV extends up
to at least 1 MeV but steepens soon after to meet EGRET high energy data. BATSE
data taken around the January 1996 flare suggests that the spectrum could be
steeper when the source is in a bright state. The nuF-nu representation of the
source is typical of a low frequency peaked/gamma-ray dominated blazar, with
the synchrotron peak in the mm-FIR band and the Compton peak in the MeV band.
The BATSE and OSSE spectral data seem to favour a model in which the high
energy flux is due to the sum of the synchrotron self-Compton and the external
Compton contributions; this is also supported by the variability behaviour of
the source.Comment: 19 pages, LaTeX, plus 4 .ps figures. accepted by Astrophysical
Journa
Jet emission in NGC1052 at radio, optical, and X-ray frequencies
We present a combined radio, optical, and X-ray study of the nearby LINER
galaxy NGC 1052. Data from a short (2.3 ksec) {\it CHANDRA} observation of NGC
1052 reveal the presence of various jet-related X-ray emitting regions, a
bright compact core and unresolved knots in the jet structure as well as an
extended emitting region inside the galaxy well aligned with the radio
synchrotron jet-emission. The spectrum of the extended X-ray emission can best
be fitted with a thermal model with keV, while the compact
core exhibits a very flat spectrum, best approximated by an absorbed power-law
with . We compare the radio
structure to an optical ``structure map'' from a {\it Hubble Space Telescope}
({\it HST}) observation and find a good positional correlation between the
radio jet and the optical emission cone. Bright, compact knots in the jet
structure are visible in all three frequency bands whose spectrum is
inconsistent with synchrotron emission.Comment: 8 pages, 5 figures (figure 2 in color), image resolution degraded wrt
journal version, needs aa.cls. Accepted for publication in A&
The AGN nature of 11 out of 12 Swift/RXTE unidentified sources through optical and X-ray spectroscopy
The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude
survey in the 14-195 keV band at a flux limit of ~10^{-11} erg cm^{-2} s^{-1},
leading to the discovery of new high energy sources, most of which have not so
far been properly classified. A similar work has also been performed with the
RXTE slew survey leading to the discovery of 68 sources detected above 8 keV,
many of which are still unclassified. Follow-up observations with the Swift
X-ray Telescope (XRT) provide, for many of these objects, source localization
with a positional accuracy of few arcsec, thus allowing the search for optical
counterparts to be more efficient and reliable. We present the results of
optical/X-ray follow-up studies of 11 Swift BAT detections and one AGN detected
in the RXTE Slew Survey, aimed at identifying their counterparts and at
assessing their nature. These data allowed, for the first time, the optical
classification of 8 objects and a distance determination for 3 of them. For
another object, a more refined optical classification than that available in
the literature is also provided. For the remaining sources, optical
spectroscopy provides a characterization of the source near in time to the
X-ray measurement. The sample consists of 6 Seyfert 2 galaxies, 5 Seyferts of
intermediate type 1.2-1.8, and one object of Galactic nature - an Intermediate
Polar (i.e., magnetic) Cataclysmic Variable. Out of the 11 AGNs, 8 (~70%)
including 2 Seyferts of type 1.2 and 1.5, are absorbed with NH > 10^{22}
cm^{-2}. Up to 3 objects could be Compton thick (i.e. NH > 1.5 x 10^{24}
cm^{-2}), but only in one case (Swift J0609.1-8636) does all the observational
evidence strongly suggests this possibility.Comment: 50 pages, including 16 figures and 7 tables. Accepted for publication
in Ap
Comparative nanostructure analysis of gasoline turbocharged direct injection and diesel soot-in-oil with carbon black
Two gasoline turbocharged direct injection (GTDI) and two diesel soot-in-oil samples were compared with one flame-generated soot sample. High resolution transmission electron microscopy imaging was employed for the initial qualitative assessment of the soot morphology. Carbon black and diesel soot both exhibit core-shell structures, comprising an amorphous core surrounded by graphene layers; only diesel soot has particles with multiple cores. In addition to such particles, GTDI soot also exhibits entirely amorphous structures, of which some contain crystalline particles only a few nanometers in diameter. Subsequent quantification of the nanostructure by fringe analysis indicates differences between the samples in terms of length, tortuosity, and separation of the graphitic fringes. The shortest fringes are exhibited by the GTDI samples, whilst the diesel soot and carbon black fringes are 9.7% and 15.1% longer, respectively. Fringe tortuosity is similar across the internal combustion engine samples, but lower for the carbon black sample. In contrast, fringe separation varies continuously among the samples. Raman spectroscopy further confirms the observed differences. The GTDI soot samples contain the highest fraction of amorphous carbon and defective graphitic structures, followed by diesel soot and carbon black respectively. The AD1:AG ratios correlate linearly with both the fringe length and fringe separation
BATSE Observations of the Piccinotti Sample of AGN
BATSE Earth occultation data have been used to search for emission in the
20-100 keV band from all sources in the Piccinotti sample, which represents to
date the only complete 2-10 keV survey of the extragalactic sky down to a
limiting flux of 3.1 x 10^(-11) erg cm^(-2)$ s^(-1). Nearly four years of
observations have been analyzed to reach a 5sigma sensitivity level of about
7.8x 10^(-11) erg cm^(-2) s^(-1) in the band considered. Of the 36 sources in
the sample, 14 have been detected above 5sigma confidence level while marginal
detection (3<sigma<5) can be claimed for 13 sources; for 9 objects 2sigma upper
limits are reported. Comparison of BATSE results with data at higher energies
is used to estimate the robustness of our data analysis: while the detection
level of each source is reliable, the flux measurement maybe overestimated in
some sources by as much as 35%, probably due to incomplete data cleaning.
Comparison of BATSE fluxes with X-ray fluxes, obtained in the 2-10 keV range
and averaged over years, indicates that a canonical power law of photon index
1.7 gives a good description of the broad band spectra of bright AGNs and that
spectral breaks preferentially occur above 100 keV.Comment: 18 pages, 1 figure. Accepted for publication on Apj
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Braze Process Optimization Involving Conventional Metal/Ceramic Brazing with 50Au-50Cu Alloy
Numerous process variables can influence the robustness of conventional metal/ceramic brazing processes. Experience with brazing of hermetic vacuum components has identified the following parameters as influencing the outcome of hydrogen furnace brazed Kovar{trademark} to metallized alumina braze joints: (a) Mo-Mn metallization thickness, sinter fire temperature and porosity (b) Nil plate purity, thickness, and sinter firing conditions (c) peak process temperature, time above liquidus and (d) braze alloy washer thickness. ASTM F19 tensile buttons are being used to investigate the above parameters. The F19 geometry permits determination of both joint hermeticity and tensile strength. This presentation will focus on important lessons learned from the tensile button study: (A) the position of the Kovar{trademark} interlayer can influence the joint tensile strength achieved--namely, off-center interlayers can lead to residual stress development in the ceramic and degrade tensile strength values. Finite element analysis has been used to demonstrate the expected magnitude in strength degradation as a function of misalignment. (B) Time above liquidus (TAL) and peak temperature can influence the strength and alloying level of the resulting braze joint. Excessive TAL or peak temperatures can lead to overbraze conditions where all of the Ni plate is dissolved. (C) Metallize sinter fire processes can influence the morphology and strength obtained from the braze joints
X-ray Spectral Survey with XMM--Newton of a Complete Sample of Nearby Seyfert Galaxies
Results obtained from an X-ray spectral survey of nearby Seyfert galaxies
using XMM--Newton are reported. The sample was optically selected, well
defined, complete in B mag, and distance limited: it consists of the nearest
(D<22 Mpc) 27 Seyfert galaxies (9 of type 1, 18 of type 2) taken from the Ho et
al. (1997) sample. This is one of the largest atlases of hard X-ray spectra of
low-L active galaxies ever assembled. All nuclear sources except two Sey 2s are
detected between 2-10 keV, half for the first time ever, and average spectra
are obtained for all of them. Nuclear luminosities reach values down to 10**38
erg/s. The shape of the distribution of X-ray parameters is affected by the
presence of Compton-thick objects (> 30% among type 2s). The latter have been
identified either directly from their intense FeK line and flat X-ray spectra,
or indirectly with flux diagnostic diagrams which use isotropic indicators.
After taking into account these highly absorbed sources, we find that (i) the
intrinsic X-ray spectral properties (i.e., spectral shapes and luminosities
above 2 keV) are consistent between type 1 and type 2 Sey, as expected from
``unified models'', (ii) Sey galaxies as a whole are distributed fairly
continuously over the entire range of Nh, between 10**20 and 10**25 cm**-2, and
(iii) while Sey 1s tend to have lower Nh and Sey 2s tend to have the highest,
we find 30% and 10% exceptions, respectively. Overall the sample well
represents the average intrinsic X-ray spectral properties of nearby AGN,
including a proper estimate of the distribution of their absorbing columns.
Finally, we conclude that, with the exception of a few cases, the present study
agrees with predictions of unified models of Sey galaxies, and extends their
validity down to very low luminosities.Comment: 23 pages, 4 tables, 4 figures, 2 Appendices with 27 source spectra
and notes, to be published in the Astronomy & Astrophysics Journa
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Author Correction: Mature Andean forests as globally important carbon sinks and future carbon refuges
Mature Andean forests as globally important carbon sinks and future carbon refuges
It is largely unknown how South Americaâs Andean forests affect the global carbon cycle, and thus regulate climate change. Here, we measure aboveground carbon dynamics over the past two decades in 119 monitoring plots spanning a range of >3000âm elevation across the subtropical and tropical Andes. Our results show that Andean forests act as strong sinks for aboveground carbon (0.67â±â0.08 Mg C haâ1 yâ1) and have a high potential to serve as future carbon refuges. Aboveground carbon dynamics of Andean forests are driven by abiotic and biotic factors, such as climate and size-dependent mortality of trees. The increasing aboveground carbon stocks offset the estimated C emissions due to deforestation between 2003 and 2014, resulting in a net total uptake of 0.027 Pg C yâ1. Reducing deforestation will increase Andean aboveground carbon stocks, facilitate upward species migrations, and allow for recovery of biomass losses due to climate change
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