396 research outputs found
The New RF Control System for the CERN SPS Accelerator
The old SPS RF control system designed in 1972 has been replaced completely, i.e. both hardware and software. The new system has to control both RF equipment conceived during the last 23 years and future (modern) equipment. Using information analysis methods, we derived a model of an RF command and designed a data base accordingly (ORACLE®). Information from this data base is used for command generation and processing and also for archiving settings. The advantage is purely generic software, i.e. the same computer code is used for switching on an RF amplifier, as for setting a frequency synthesizer. New equipment is added very simply by entering new records in the data base. Additional features include a reservation scheme whereby a user can take private control of any piece of equipment, a reporting facility notifying the user of the simultaneous control activity by other users on RF equipment, and a capability scheme assigning a level of expertise to each user restricting action on the equipment
VSOP and Ground-based VLBI Imaging of the TeV Blazar Markarian 421 at Multiple Epochs
We present thirty VLBI images of the TeV blazar Markarian 421 (1101+384) at
fifteen epochs spanning the time range from 1994 to 1997, and at six different
frequencies from 2.3 to 43 GHz. The imaged observations include a
high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with
the HALCA satellite on 1997 November 14; full-track VLBA observations from 1994
April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six
epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from
Radio Reference Frame studies; and five geodetic VLBI observations at 2 and 8
GHz from the archive of the Washington VLBI Correlator Facility located at the
U.S. Naval Observatory. The dense time coverage of the images allows us to
unambiguously track components in the parsec-scale jet over the observed time
range. We measure the speeds of three inner jet components located between 0.5
and 5 mas from the core (0.3 to 3 pc projected linear distance) to be 0.19 +/-
0.27, 0.30 +/- 0.07, and -0.07 +/- 0.07 c (H_{0}=65 km s^{-1} Mpc^{-1}). If the
sole 43 GHz image is excluded, all measured speeds are consistent with no
motion. These speeds differ from tentative superluminal speeds measured by
Zhang & B\aa\aa th from three epochs of data from the early 1980's. Possible
interpretations of these subluminal speeds in terms of the high Doppler factor
demanded by the TeV variability of this source are discussed.Comment: 18 pages, including 7 figures, emulateapj.sty, accepted by The
Astrophysical Journal; modified text describing Radio Reference Frame
observation
Investigations of primary and secondary particulate matter of different wood combustion appliances with a high-resolution time-of-flight aerosol mass spectrometer
A series of photo-oxidation smog chamber experiments were performed to investigate the primary emissions and secondary aerosol formation from two different log wood burners and a residential pellet burner under different burning conditions: starting and flaming phase. Emissions were sampled from the chimney and injected into the smog chamber leading to primary organic aerosol (POA) concentrations comparable to ambient levels. The composition of the aerosol was measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS) and black carbon (BC) instrumentation. The primary emissions were then exposed to xenon light to initiate photo-chemistry and subsequent secondary organic aerosol (SOA) production. After correcting for wall losses, the average increase in organic matter (OM) concentrations by SOA formation for the starting and flaming phase experiments with the two log wood burners was found to be a factor of 4.1&plusmn;1.4 after five hours of aging. No SOA formation was observed for the stable burning phase of the pellet burner. The startup emissions of the pellet burner showed an increase in OM concentration by a factor of 3.3. Including the measured SOA formation potential, average emission factors of BC+POA+SOA, calculated from CO<sub>2</sub> emission, were found to be in the range of 0.04 to 3.9 g/kg wood for the stable burning pellet burner and an old log wood burner during startup respectively. SOA contributed significantly to the ion C<sub>2</sub>H<sub>4</sub>O<sub>2</sub><sup>+</sup> at mass to charge ratio <i>m/z</i> 60, a commonly used marker for primary emissions of wood burning. This contribution at <i>m/z</i> 60 can overcompensate for the degradation of levoglucosan leading to an overestimation of the contribution of wood burning or biomass burning to the total OM. The primary organic emissions from the three different burners showed a wide range in O:C atomic ratio (0.19&minus;0.60) for the starting and flaming conditions, which also increased during aging. Primary wood burning emissions have a rather low relative contribution at <i>m/z</i> 43 (<i>f</i> 43) to the total organic mass spectrum. The non-oxidized fragment C<sub>3</sub>H<sub>7</sub><sup>+</sup> has a considerable contribution at <i>m/z</i> 43 for the fresh OA with an increasing contribution of the oxygenated ion C<sub>2</sub>H<sub>3</sub>O<sup>+</sup> during aging. After five hours of aging, the OA has a rather low C<sub>2</sub>H<sub>3</sub>O<sup>+</sup> signal for a given CO<sub>2</sub><sup>+</sup> fraction, possibly indicating a higher ratio of acid to non-acid oxygenated compounds in wood burning OA compared to other oxygenated organic aerosol (OOA)
Simultaneous radio-interferometric and high-energy TeV observations of the gamma-ray blazar Mkn 421
The TeV-emitting BL Lac object Mkn 421 was observed with very long baseline
interferometry (VLBI) at three closely-spaced epochs one-month apart in
March-April 1998. The source was also monitored at very-high gamma-ray energies
(TeV measurements) during the same period in an attempt to search for
correlations between TeV variability and the evolution of the radio morphology
on parsec scales. While the VLBI maps show no temporal changes in the Mkn 421
VLBI jet, there is strong evidence of complex variability in both the total and
polarized fluxes of the VLBI core of Mkn 421 and in its spectrum over the
two-month span of our data. The high-energy measurements indicate that the
overall TeV activity of the source was rising during this period, with a
gamma-ray flare detected just three days prior to our second VLBI observing
run. Although no firm correlation can be established, our data suggest that the
two phenomena (TeV activity and VLBI core variability) are connected, with the
VLBI core at 22 GHz being the self-absorbed radio counterpart of synchrotron
self-Compton (SSC) emission at high energies. Based on the size of the VLBI
core, we could derive an upper limit of 0.1 pc (3 x 10**17 cm) for the
projected size of the SSC zone. This determination is the first model-free
estimate of the size of the gamma-ray emitting region in a blazar.Comment: 12 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Impact of aftertreatment devices on primary emissions and secondary organic aerosol formation potential from in-use diesel vehicles: results from smog chamber experiments
Diesel particulate matter (DPM) is a significant source of aerosol in urban
areas and has been linked to adverse health effects. Although newer European
directives have introduced increasingly stringent standards for primary PM
emissions, gaseous organics emitted from diesel cars can still lead to large
amounts of secondary organic aerosol (SOA) in the atmosphere. Here we
present results from smog chamber investigations characterizing the primary
organic aerosol (POA) and the corresponding SOA formation at atmospherically
relevant concentrations for three in-use diesel vehicles with different
exhaust aftertreatment systems. One vehicle lacked exhaust aftertreatment
devices, one vehicle was equipped with a diesel oxidation catalyst (DOC) and
the third vehicle used both a DOC and diesel particulate filter (DPF). The
experiments presented here were obtained from the vehicles at conditions
representative of idle mode, and for one car in addition at a speed of 60 km/h.
An Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) was used to measure the organic aerosol (OA) concentration and
to obtain information on the chemical composition. For the conditions
explored in this paper, primary aerosols from vehicles without a particulate
filter consisted mainly of black carbon (BC) with a low fraction of organic
matter (OM, OM/BC < 0.5), while the subsequent aging by photooxidation
resulted in a consistent production of SOA only for the vehicles without a
DOC and with a deactivated DOC. After 5 h of aging ~80% of the
total organic aerosol was on average secondary and the estimated "emission
factor" for SOA was 0.23–0.56 g/kg fuel burned. In presence of both a DOC
and a DPF, only 0.01 g SOA per kg fuel burned was produced within 5 h
after lights on. The mass spectra indicate that POA was mostly a
non-oxidized OA with an oxygen to carbon atomic ratio (O/C) ranging from
0.10 to 0.19. Five hours of oxidation led to a more oxidized OA with an O/C
range of 0.21 to 0.37
Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission
The gamma-ray blazar 3C 279 was monitored on a nearly daily basis with IUE,
ROSAT and EGRET for three weeks between December 1992 and January 1993. During
this period, the blazar was at a historical minimum at all wavelengths. Here we
present the UV data obtained during the above multiwavelength campaign. A
maximum UV variation of ~50% is detected, while during the same period the
X-ray flux varied by no more than 13%. At the lowest UV flux level the average
spectrum in the 1230-2700 A interval is unusually flat for this object
(~1). The flattening could represent the lowest energy tail of the
inverse Compton component responsible for the X-ray emission, or could be due
to the presence of a thermal component at ~20000 K possibly associated with an
accretion disk. The presence of an accretion disk in this blazar object, likely
observable only in very low states and otherwise hidden by the beamed, variable
synchrotron component, would be consistent with the scenario in which the seed
photons for the inverse Compton mechanism producing the gamma-rays are external
to the relativistic jet. We further discuss the long term correlation of the UV
flux with the X-ray and gamma-ray fluxes obtained at various epochs. All UV
archival data are included in the analysis. Both the X- and gamma-ray fluxes
are generally well correlated with the UV flux, approximately with square root
and quadratic dependences, respectively.Comment: 22 pages, Latex, 7 PostScript figures, to appear in The Astrophysical
Journa
Unusual Flaring Activity in the Blazar PKS 1424-418 during 2008-2011
Context. Blazars are a subset of active galactic nuclei (AGN) with jets that
are oriented along our line of sight. Variability and spectral energy
distribution (SED) studies are crucial tools for understanding the physical
processes responsible for observed AGN emission.
Aims. We report peculiar behaviour in the bright gamma-ray blazar PKS
1424-418 and use its strong variability to reveal information about the
particle acceleration and interactions in the jet. Methods. Correlation
analysis of the extensive optical coverage by the ATOM telescope and nearly
continuous gamma-ray coverage by the Fermi Large Area Telescope is combined
with broadband, time-dependent modeling of the SED incorporating supplemental
information from radio and X-ray observations of this blazar.
Results. We analyse in detail four bright phases at optical-GeV energies.
These flares of PKS 1424-418 show high correlation between these energy ranges,
with the exception of one large optical flare that coincides with relatively
low gamma-ray activity. Although the optical/gamma-ray behaviour of PKS
1424-418 shows variety, the multiwavelength modeling indicates that these
differences can largely be explained by changes in the flux and energy spectrum
of the electrons in the jet that are radiating. We find that for all flares the
SED is adequately represented by a leptonic model that includes inverse Compton
emission from external radiation fields with similar parameters.
Conclusions. Detailed studies of individual blazars like PKS 1424-418 during
periods of enhanced activity in different wavebands are helping us identify
underlying patterns in the physical parameters in this class of AGN.Comment: accepted for publication in A&
First Beam Commissioning of the 400 MHz LHC RF System
Hardware commissioning of the LHC RF system was successfully completed in time for first beams in LHC in September 2008. All cavities ware conditioned to nominal field, power systems tested and all Low level synchronization systems, cavity controllers and beam control electronics were tested and calibrated. Beam was successfully captured in ring 2, cavities phased, and a number of initial measurements made. These results are presented and tests and preparation for colliding beams in 2009 are outlined
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