Commissioning of the I-LHC RF low level with beam

During the machine development session of 2007-10-03, the energy matching between PS and SPS was completed for the 208Pb82+ beam. The I-LHC RF low level, including the phase loop and the synchronisation loop, was commissioned to capture a single bunch of 208Pb82+. After commissioning, the beam was used for several non RF related machine developments

Space VLBI Observations of 3C 279 at 1.6 and 5 GHz

We present the first VLBI Space Observatory Programme (VSOP) observations of the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and VLBA-only images at these two frequencies maps the jet structure on scales from 1 to 100 mas. On small angular scales the structure is dominated by the quasar core and the bright secondary component `C4' located 3 milliarcseconds from the core (at this epoch). On larger angular scales the structure is dominated by a jet extending to the southwest, which at the largest scale seen in these images connects with the smallest scale structure seen in VLA images. We have exploited two of the main strengths of VSOP: the ability to obtain matched-resolution images to ground-based images at higher frequencies and the ability to measure high brightness temperatures. A spectral index map was made by combining the VSOP 1.6 GHz image with a matched-resolution VLBA-only image at 5 GHz from our VSOP observation on the following day. The spectral index map shows the core to have a highly inverted spectrum, with some areas having a spectral index approaching the limiting value for synchrotron self-absorbed radiation of 2.5. Gaussian model fits to the VSOP visibilities revealed high brightness temperatures (>10^{12} K) that are difficult to measure with ground-only arrays. An extensive error analysis was performed on the brightness temperature measurements. Most components did not have measurable brightness temperature upper limits, but lower limits were measured as high as 5x10^{12} K. This lower limit is significantly above both the nominal inverse Compton and equipartition brightness temperature limits. The derived Doppler factor, Lorentz factor, and angle to the line-of-sight in the case of the equipartition limit are at the upper end of the range of expected values for EGRET blazars.Comment: 11 pages, 6 figures, emulateapj.sty, To be published in The Astrophysical Journal, v537, Jul 1, 200

Physical properties of the jet in 0836+710 revealed by its transversal structure

Studying the internal structure of extragalactic jets is crucial for understanding their physics. The Japanese-led space VLBI project VSOP has presented an opportunity for such studies, by reaching baseline lengths of up to 36,000 km and resolving structures down to an angular size of $\approx 0.3$ mas at 5 GHz. VSOP observations of the jet in 0836+710 at 1.6 and 5 GHz have enabled tracing of the radial structure of the flow on scales from 2 mas to 200 mas along the jet and determination of the wavelengths of individual oscillatory modes responsible for the formation of the structure observed. We apply linear stability analysis to identify the oscillatory modes with modes of Kelvin-Helmholtz instability that match the wavelengths of the structures observed. We find that the jet structure in 0836+710 can be reproduced by the helical surface mode and a combination of the helical and elliptic body modes of Kelvin-Helmholtz instability. Our results indicate that the jet is substantially stratified and different modes of the instability grow inside the jet at different distances to the jet axis. The helical surface mode can be driven externally, and we discuss the implications of the driving frequency on the physics of the active nucleus in 0836+710.Comment: Accepted for publication in Astronomy & Astrophysics Letter

Collaboration in a multi-user game: impacts of an awareness tool on mutual modeling

This paper presents an experimental research that focuses on collaboration in a multi-player game. The aim of the project is to study the cognitive impacts of awareness tools, i.e., artifacts that allow users of a collaborative system to be aware of what is going on in the joint virtual environment. The focus is on finding an effect on performance as well as on the representation an individual builds of what his partner knows, plans and intends to do (i.e., mutual modeling). We find that using awareness tools has a significant effect by improving task performance. However, the players who were provided with this tool did not show any improvement of their mutual modeling. Further analysis on contrasted groups revealed that there was an effect of the awareness tool on mutual modeling for players who spent a large amount of time using the too

SPS Beams for LHC: RF Beam Control to Minimize Rephasing in the SPS

After acceleration to 450 GeV/c in the SPS, the proton beams for the LHC must be rephased in order to be injected into the correct place in the collider. If the position before rephasing is random, th e beam must be rotated by up to 1/14th of an SPS turn. This procedure will take about 330 ms on the flat top. Besides lengthening the cycle, the beam may deteriorate if instabilities develop during th is long process. We present a beam control system that reduces the rotation angle needed and which has been successfully tested in the SPS

Variability in the X-ray Flux of Quasar 3C345: Inverse-Compton Emission from the Parsec-Scale Jet?

We present the results of the first systematic study of variability in the X-ray emission from the 'superluminal' quasar 3C 345. Its power-law 1-keV X-ray emission varies by a factor of two on a timescale of years, but with no change in spectral index, closely following the high-frequency radio flux. Using VLBI images, we show that one of the superluminal 'knots' in the jet (at a distance of ≈ 15 pc from the nucleus), rather than the nucleus, produces most of the observed X-rays, via the synchrotron self-Compton process. We show that this knot accelerates as it moves away from the nucleus, along along a path at ≈ 10° from the line of sight

Estudio del proceso de cambio conceptual y la construcción del modelo científico precursor de ser vivo en niños de pre-escolar

La presente investigación estudia el cambio conceptual y la construcción de modelos científicos precursores en un contexto socioconstructivista con niños pequeños, utilizando una metodología cualitativa. Se analizan los cambios epistemológicos y ontológicos en las concepciones de los niños acerca de los seres vivos y cómo una estrategia didáctica basada en la construcción de un modelo científico precursor basado en propiedades biológicas los promueven. Los cambios en la comprensión de los niños fueron significativos en las dimensiones ontológicas y epistemológicas, presentando diferentes patones. Los segundos proporcionaron una mejor coherencia explicativa en su sistema conceptual contribuyendo, de esta forma, a los cambios ontológicos y promoviendo, a su vez, la construcción del modelo científico precursor de ser vivo

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

What Happened to the NGC 6251 Counterjet?

We have used the VLBA to produce a high dynamic range image of the nucleus of NGC 6251 at 1.6 GHz and snapshot images at 5.0, 8.4, and 15.3 GHz to search for emission from a parsec-scale counterjet. Previous VLBI images at 1.6 GHz have set a lower limit for the jet/counterjet brightness ratio near the core at about 80:1, which is larger than expected given the evidence that the radio axis is fairly close to the plane of the sky. A possible explanation is that the inner few pc of the counterjet is hidden by free-free absorption by ionized gas associated with an accretion disk or torus. This would be consistent with the nearly edge-on appearance of the arcsecond-scale dust disk seen in the center of NGC 6251 by HST. We detect counterjet emission close to the core at 1.6 GHz, but not at the higher frequencies. Given that the optical depth of free-free absorption falls off more rapidly with increasing frequency than the optically thin synchrotron emission from a typical radio jet, this result implies that the absence of a detectable parsec-scale counterjet at high frequencies is not due to free-free absorption unless the density of ionized gas is extremely high and we have misidentified the core at 1.6 GHz. The most likely alternative is a large jet/counterjet brightness ratio caused by relativistic beaming, which in turn requires the inner radio axis to be closer to our line of sight than the orientation of the HST dust disk would suggest.Comment: Accepted for publication in ApJ. Includes 10 figure