10,479 research outputs found
Review of the measurements of the strong coupling constant at LEP 2
Since 1995, LEP has steadily increased the center of mass energy of the
colliding beams, from the M_Z resonance to 133, 161 and 172 GeV. New
measurements of the strong coupling constant, alpha_s, at these energies have
been performed by the LEP experiments, L3, ALEPH, OPAL and DELPHI. In this
article, the new results are summarized, and combined with the previous LEP
measurement of alpha_s(M_Z) in order to obtain an updated LEP average of
alpha_s(M_Z) = 0.120 +- 0.005.Comment: 5-6 pages, 3 figures. Talk given at the DIS 97 conference, Chicago,
April 1997.Replacement: update figure 1. Also available here
http://hep.ph.liv.ac.uk/~martis
Submillimeter polarization and variability of quasar PKS 1830-211
Polarization from active galactic nuclei is interpreted as a signpost of the
role of magnetic fields in the launch and collimation of their relativistic
radio jets. Here, we report the detection of a clear polarization signal from
ALMA observations of the gravitationally lensed quasar PKS 1830-211 at
submillimeter wavelengths (Band 9, 650 GHz). Applying a
differential-polarimetry technique to the two compact lensed images of the
quasar, we estimate a fractional polarization of ~5% for one lensed image,
while the other appears nearly unpolarized, which implies that the polarization
activity varies on a timescale of a few weeks. With additional ALMA Band 7 and
8 (between 300-500 GHz) concomitant data, we constrain a Faraday rotation of a
few rad m. We also observe flux-density variability of ~10%
within one hour in Band 9. This work illustrates that a differential analysis
can extract high-accuracy information (flux-density ratio and polarimetry) free
of calibration issues from resolved sources in the submillimeter domain.Comment: 7 pages, 6 figures, accepted for publications in A&
Using gravitational lensed images to investigate the intrinsic AGN variability
We discuss about how the relative flux densities among the images of
gravitationally-lensed active galactic nuclei, AGN, can be used to study the
intrinsic AGN variability with high accuracy. Multi-frequency monitoring
observations of resolved gravitational lenses can allow us to detect signals of
very weak variability and also provide information about the jet opacity and
structure. As an example, we investigate the variability of the flux-density
ratio between the two lensed images of the blazar B0218+357, using
dual-frequency cm-wave observations. Similar to our previously reported
submm-wave observations of the lensed blazar PKS1830-211, we observe a clear
chromatic variability, starting short before an increase in the flux-density of
the blazar. The evolution of the flux-density ratios between the blazar images
shows a more clear and rich structure than that of the mere lightcurves of each
individual image. The accuracy in the ratio measurements is allowing us to see
variability episodes in the blazar that are weaker than the natural scatter in
the absolute flux-density measurements. A simple opacity model in the jet is
used to consistently explain the difference between the flux-density-ratio
evolution at the two frequencies.Comment: 5 pages, 2 figures. Accepted for publication in A&A. Final versio
Deep Infrared Imaging of the Microquasars 1E1740-2942 and GRS 1758-258
We present deep infrared (m) imaging of the Galactic microquasars
1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998.
The observations were taken under excellent seeing conditions (\sim 0.45
\arcsec full-width half-maximum), making them exceptionally deep for these
crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the
infrared images (along with an optical CCD image in the case of GRS 1758-258),
providing independent frame ties to the known radio positions of the objects.
For 1E1740-2942, we confirm potential candidates for the microquasar previously
identified by Marti et al., and show that none of the objects near the
microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our
astrometry indicates a position shifted from previous reports of candidates for
the microquasar. We find no candidates inside our 90% confidence radius to a limiting magnitude of mag. We discuss the implications of
these results for the nature of the microquasar binary systems.Comment: To appear in the Astrophysical Journal; 15 pages, including 4 figure
UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data
The analysis of astronomical interferometric data is often performed on the
images obtained after deconvolution of the interferometer's point spread
function (PSF). This strategy can be understood (especially for cases of sparse
arrays) as fitting models to models, since the deconvolved images are already
non-unique model representations of the actual data (i.e., the visibilities).
Indeed, the interferometric images may be affected by visibility gridding,
weighting schemes (e.g., natural vs. uniform), and the particulars of the
(non-linear) deconvolution algorithms. Fitting models to the direct
interferometric observables (i.e., the visibilities) is preferable in the cases
of simple (analytical) sky intensity distributions. In this paper, we present
UVMULTIFIT, a versatile library for fitting visibility data, implemented in a
Python-based framework. Our software is currently based on the CASA package,
but can be easily adapted to other analysis packages, provided they have a
Python API. We have tested the software with synthetic data, as well as with
real observations. In some cases (e.g., sources with sizes smaller than the
diffraction limit of the interferometer), the results from the fit to the
visibilities (e.g., spectra of close by sources) are far superior to the output
obtained from the mere analysis of the deconvolved images. UVMULTIFIT is a
powerful improvement of existing tasks to extract the maximum amount of
information from visibility data, especially in cases close to the
sensitivity/resolution limits of interferometric observations.Comment: 10 pages, 4 figures. Accepted in A&A. Code available at
http://nordic-alma.se/support/software-tool
Non-thermal radiation from a runaway massive star
We present a study of the radio emission from a massive runaway star. The
star forms a bow shock that is clearly observed in the infrared. We have
performed VLA observations under the assumption that the reverse shock in the
stellar wind might accelerate charged particles up to relativistic energies.
Non-thermal radio emission of synchrotron origin has been detected, confirming
the hypothesis. We have then modeled the system and we predict a spectral
energy distribution that extends up to gamma-rays. Under some simplifying
assumptions, we find that the intensity at high energies is too low to be
detected by current instruments, but the future Cherenkov Telescope Array might
detect the source.Comment: Accepted for publication in the proceedings of the conference "The
multi-wavelength view of Hot, Massive Stars", held in Liege, July 12-16 201
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