480 research outputs found
Rapid X-ray Variability of the BL Lacertae Object PKS 2155-304
(Abridged) We present a detailed power density spectrum and cross-correlation
analysis of the X-ray light curves of the BL Lac object PKS 2155-304, observed
with BeppoSAX in 1997 and 1996, aimed at exploring the rapid variability
properties and the inter-band cross correlations in the X-rays. We also perform
the same analysis on the (archival) X-ray light curve obtained with ASCA in
1994.Comment: 47 pages, 11 figures, AAS Latex macros V4.0, accepted for publication
in the Astrophysical Journa
NuSTAR and multifrequency study of the two high-redshift blazars S5 0836+710 and PKS 2149-306
The most powerful blazars are the flat spectrum radio quasars whose emission
is dominated by a Compton component peaking between a few hundred keV and a few
hundred MeV. We selected two bright blazars, PKS 2149-306 at redshift z=2.345
and S5 0836+710 at z=2.172, in order to observe them in the hard X-ray band
with the NuSTAR satellite. In this band the Compton component is rapidly rising
almost up to the peak of the emission. Simultaneous soft-X-rays and UV-optical
observations were performed with the Swift satellite, while near-infrared (NIR)
data were obtained with the REM telescope. To study their variability, we
repeated these observations for both sources on a timescale of a few months.
While no fast variability was detected during a single observation, both
sources were found to be variable in the X-ray band, up to 50%, between the two
observations, with larger variability at higher energies. No variability was
detected in the optical/NIR band. These data together with Fermi-LAT, WISE and
other literature data are then used to study the overall spectral energy
distributions (SEDs) of these blazars. Although the jet non-thermal emission
dominates the SED, it leaves the UV band unhidden, allowing us to detect the
thermal emission of the disc and to estimate the mass of the black hole. The
non-thermal emission is well reproduced by a one-zone leptonic model. The
non-thermal radiative processes are synchrotron, self-Compton and external
Compton using seed photons from both the broad-line region (BLR) and the torus.
We find that our data are better reproduced if we assume that the location of
the dissipation region of the jet, R_diss, is in-between the torus, (at
R_torus), and the BLR (R_torus>R_diss>R_BLR). The observed variability is
explained by changing a minimum number of model parameters by a very small
amount.Comment: 11 pages, 5 figures, accepted for publication in Ap
Liquefied natural gas for the UK: a life cycle assessment
PURPOSE: Liquefied natural gas (LNG) is expected to become an important component of the UKâs energy supply because the national hydrocarbon reserves on the continental shelf have started diminishing. However, use of any carbon-based fuel runs counter to mitigation of greenhouse gas emissions (GHGs). Hence, a broad environmental assessment to analyse the import of LNG to the UK is required.
METHODS: A cradle to gate life cycle assessment has been carried out of a specific but representative case: LNG imported to the UK from Qatar. The analysis covers the supply chain, from gas extraction through to distribution to the end-user, assuming state-of-the-art facilities and ships. A sensitivity analysis was also conducted on key parameters including the energy requirements of the liquefaction and vaporisation processes, fuel for propulsion, shipping distance, tanker volume and composition of raw gas.
RESULTS AND DISCUSSION: All environmental indicators of the CML methodology were analysed. The processes of liquefaction, LNG transport and evaporation determine more than 50% of the cradle to gate global warming potential (GWP). When 1% of the total gas delivered is vented as methane emissions leakage throughout the supply chain, the GWP increases by 15% compared to the GWP of the base scenario. The variation of the GWP increases to 78% compared to the base scenario when 5% of the delivered gas is considered to be lost as vented emissions. For all the scenarios analysed, more than 75% of the total acidification potential (AP) is due to the sweetening of the natural gas before liquefaction. Direct emissions from transport always determine between 25 and 49% of the total eutrophication potential (EP) whereas the operation and maintenance of the sending ports strongly influences the fresh water aquatic ecotoxicity potential (FAETP).
CONCLUSIONS: The study highlights long-distance transport of LNG and natural gas processing, including sweetening, liquefaction and vaporisation, as the key operations that strongly affect the life cycle impacts. Those cannot be considered negligible when the environmental burdens of the LNG supply chain are considered. Furthermore, the effect of possible fugitive methane emissions along the supply chain are critical for the impact of operations such as extraction, liquefaction, storage before transport, transport itself and evaporation
Optical afterglows of Gamma-Ray Bursts: peaks, plateaus, and possibilities
The optical light-curves of GRB afterglows display either peaks or plateaus.
We identify 16 afterglows of the former type, 17 of the latter, and 4 with
broad peaks, that could be of either type. The optical energy release of these
two classes is similar and is correlated with the GRB output, the correlation
being stronger for peaky afterglows, which suggests that the burst and
afterglow emissions of peaky afterglows are from the same relativistic ejecta
and that the optical emission of afterglows with plateaus arises more often
from ejecta that did not produce the burst emission.
Consequently, we propose that peaky optical afterglows are from impulsive
ejecta releases and that plateau optical afterglows originate from long-lived
engines, the break in the optical light-curve (peak or plateau end) marking the
onset of the entire outflow deceleration.
In the peak luminosity--peak time plane, the distribution of peaky afterglows
displays an edge with L_p \propto t_p^{-3}, which we attribute to variations
(among afterglows) in the ambient medium density. The fluxes and epochs of
optical plateau breaks follow a L_b \propto t_b^{-1} anticorrelation.
Sixty percent of 25 afterglows that were well-monitored in the optical and
X-rays show light-curves with comparable power-law decays indices and
achromatic breaks. The other 40 percent display three types of decoupled
behaviours: i) chromatic optical light-curve breaks (perhaps due to the peak of
the synchrotron spectrum crossing the optical), ii) X-ray flux decays faster
than in the optical (suggesting that the X-ray emission is from local
inverse-Compton scattering), and iii) chromatic X-ray light-curve breaks
(indicating that the X-ray emission is from external up-scattering).Comment: 11 pages, table with afterglows added, to appear in MNRA
Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase
We present the results of BeppoSAX observations of PKS 2155-304 during an
intense gamma-ray flare. The source was in a high X-ray state. A temporal
analysis of the data reveals a tendency of the amplitude of variations to
increase with energy, and the presence of a soft lag with a timescale of the
order 10^3 s. A curved continuum spectrum, with no evidence of spectral
features, extends up to ~50 keV, while there is indication of a flatter
component emerging at higher energies, consistent with the interpretation of
the broad band spectral energy distribution (SED) as due to synchrotron
self-Compton (SSC) emission from a single region. Notably, the fitting of the
SED with such a model is consistent with an interpretation of the detected soft
lag as due to radiative cooling, supporting the idea that radiation losses play
an important role in variability. The observed shifts of the SED peaks between
the lowest and highest flux levels can be accounted for by an increase of the
break energy in the relativistic particle spectrum. The model predicts emission
at TeV energies in good agreement with the recently reported detection.Comment: 36 pages (8 figures), Latex with AAS macros, etc), accepted for
publication on Astrophysical Journa
Ballistic supercurrent discretization and micrometer-long Josephson coupling in germanium
We fabricate Josephson field-effect-transistors in germanium quantum wells
contacted by superconducting aluminum and demonstrate supercurrents carried by
holes that extend over junction lengths of several micrometers. In
superconducting quantum point contacts we observe discretization of
supercurrent, as well as Fabry-Perot resonances, demonstrating ballistic
transport. The magnetic field dependence of the supercurrent follows a clear
Fraunhofer-like pattern and Shapiro steps appear upon microwave irradiation.
Multiple Andreev reflections give rise to conductance enhancement and evidence
a transparent interface, confirmed by analyzing the excess current. These
demonstrations of ballistic superconducting transport are promising for hybrid
quantum technology in germanium
Extremes of the jet-accretion power relation of blazars, as explored by NuSTAR
Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z > 2 flat-spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z = 3.366) and B0222+185 (at z = 2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/X-ray Telescope, showing different variability behaviours. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no Îł-ray emission is detected. The two sources have proven to have respectively the most luminous accretion disc and the most powerful jet among known blazars. Thanks to these properties, they are located at the extreme end of the jetâaccretion disc relation previously found for Îłâray detected blazars, to which they are consistent
The July 2010 outburst of the NLS1 PMN J0948+0022
We report about the multiwavelength campaign on the Narrow-Line Seyfert 1
(NLS1) Galaxy PMN J0948+0022 (z = 0.5846) performed in 2010 July-September and
triggered by high activity as measured by Fermi/LAT. The peak luminosity in the
0.1-100 GeV energy band exceeded, for the first time in this type of source,
the value of 10^48 erg/s, a level comparable to the most powerful blazars. The
comparison of the spectral energy distribution of the NLS1 PMN J0948+0022 with
that of a typical blazar - like 3C 273 - shows that the power emitted at gamma
rays is extreme.Comment: 2011 Fermi Symposium proceedings - eConf C11050
Simbol-X Hard X-ray Focusing Mirrors: Results Obtained During the Phase A Study
Simbol-X will push grazing incidence imaging up to 80 keV, providing a strong
improvement both in sensitivity and angular resolution compared to all
instruments that have operated so far above 10 keV. The superb hard X-ray
imaging capability will be guaranteed by a mirror module of 100 electroformed
Nickel shells with a multilayer reflecting coating. Here we will describe the
technogical development and solutions adopted for the fabrication of the mirror
module, that must guarantee an Half Energy Width (HEW) better than 20 arcsec
from 0.5 up to 30 keV and a goal of 40 arcsec at 60 keV. During the phase A,
terminated at the end of 2008, we have developed three engineering models with
two, two and three shells, respectively. The most critical aspects in the
development of the Simbol-X mirrors are i) the production of the 100 mandrels
with very good surface quality within the timeline of the mission; ii) the
replication of shells that must be very thin (a factor of 2 thinner than those
of XMM-Newton) and still have very good image quality up to 80 keV; iii) the
development of an integration process that allows us to integrate these very
thin mirrors maintaining their intrinsic good image quality. The Phase A study
has shown that we can fabricate the mandrels with the needed quality and that
we have developed a valid integration process. The shells that we have produced
so far have a quite good image quality, e.g. HEW <~30 arcsec at 30 keV, and
effective area. However, we still need to make some improvements to reach the
requirements. We will briefly present these results and discuss the possible
improvements that we will investigate during phase B.Comment: 6 pages, 3 figures, invited talk at the conference "2nd International
Simbol-X Symposium", Paris, 2-5 december, 200
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