640 research outputs found
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Qualification of electron-beam welded joints between copper and stainless steel for cryogenic application
Joints between copper and stainless steel are commonly applied in cryogenic systems. A relatively new and increasingly important method to combine these materials is electron-beam (EB) welding. Typically, welds in cryogenic applications need to withstand a temperature range from 300K down to 4 K, and pressures of several MPa. However, few data are available for classifying EB welds between OFHC copper and 316L stainless steel. A broad test program was conducted in order to qualify this kind of weld. The experiments started with the measurement of the hardness in the weld area. To verify the leak-tightness of the joints, integral helium leak tests at operating pressures of 16MPa were carried out at roomand at liquid nitrogen temperature. The tests were followed by destructive tensile tests at room temperature, at liquid nitrogen and at liquid helium temperatures, yielding information on the yield strength and the ultimate tensile strength of the welds at these temperatures. Moreover, nondestructive tensile tests up to the yield strength, i.e. the range in which the weld can be stressed during operation, were performed. Also, the behavior of the weld upon temperature fluctuations between room- and liquid nitrogen temperature was tested. The results of the qualification indicate that EB welded joints between OFHC copper and 316L stainless steel are reliable and present an interesting alternative to other technologies such as vacuum brazing or friction welding
The HST Survey of BL Lac Objects: Gravitational Lens Candidates and Other Unusual Sources
We present HST observations of seven unusual objects from the HST ``snapshot
survey'' of BL Lac objects, of which four are gravitational lens candidates. In
three cases a double point sources is observed: 0033+595, with 1.58 arcsec
separation, and 0502+675 and 1440+122, each with arcsec separation.
The last two also show one or more galaxies, which could be either host or
lensing galaxies. If any are confirmed as lenses, these BL Lac objects are
excellent candidates for measuring H via gravitational time delay because
of their characteristic rapid, high amplitude variability. An additional
advantage is that, like other blazars, they are likely superluminal radio
sources, in which case the source plane is mapped out over a period of years,
providing strong additional constraints on the lensing mass distribution. The
fourth gravitational lens candidate is 1517+656, which is surrounded by three
arclets forming an almost perfect ring of radius 2.4 arcsec. If this is indeed
an Einstein ring, it is most likely a background source gravitationally lensed
by the BL Lac object host galaxy and possibly a surrounding group or cluster.
In the extreme case that all four candidates are true lenses, the derived
frequency of gravitational lensing in this BL Lac sample would be an order of
magnitude higher than in comparable quasar samples.
We also report on three other remarkable BL Lac objects: 0138-097, which is
surrounded by a large number of close companion galaxies; 0806+524, whose host
galaxy contains an uncommon arc-like structure; and 1959+650, which is hosted
by a gas rich elliptical galaxy with a prominent dust lane of .Comment: 29 pages in total, 12 figure
Optical Monitoring of BL Lacertae Object S5 0716+714 with a Novel Multi-Peak Interference Filter
We at first introduce a novel photometric system, which consists of a Schmidt
telescope, an objective prism, a CCD camera, and, especially, a multi-peak
interference filter. The multi-peak interference filter enables light in multi
passbands to pass through it simultaneously. The light in different passbands
is differentially refracted by the objective prism and is focused on the CCD
separately, so we have multi "images" for each object on the CCD frames. This
system enables us to monitor blazars exactly simultaneously in multi wavebands
on a single telescope, and to accurately trace the color change during the
variation. We used this novel system to monitor the BL Lacertae object S5
0716+714 during 2006 January and February and achieved a very high temporal
resolution. The object was very bright and very active during this period. Two
strong flares were observed, with variation amplitudes of about 0.8 and 0.6
mags in the band, respectively. Strong bluer-when-brighter correlations
were found for both internight and intranight variations. No apparent time lag
was observed between the - and -band variations, and the observed
bluer-when-brighter chromatism may be mainly attributed to the larger variation
amplitude at shorter wavelength. In addition to the bluer-when-brighter trend,
the object also showed a bluer color when it was more active. The observed
variability and its color behaviors are consistent with the shock-in-jet model.Comment: 30 pages, 22 figures, accepted by A
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
The nature of the intranight variability of radio-quiet quasars
We select a sample of 10 radio-quiet quasars with confirmed intranight
optical variability and with available X-ray data. We compare the variability
properties and the broad band spectral constraints to the predictions of
intranight variability by three models: (i) irradiation of an accretion disk by
a variable X-ray flux (ii) an accretion disk instability (iii) the presence of
a weak blazar component. We concluded that the third model, e.g. the blazar
component model, is the most promising if we adopt a cannonball model for the
jet variable emission. In this case, the probability of detecting the
intranight variability is within 20-80%, depending on the ratio of the disk to
the jet optical luminosity. Variable X-ray irradiation mechanism is also
possible but only under additional requirement: either the source should have a
very narrow Hbeta line or occasional extremely strong flares should appear at
very large disk radii.Comment: MNRAS (in press
The FORS Deep Field: Field selection, photometric observations and photometric catalog
The FORS Deep Field project is a multi-colour, multi-object spectroscopic
investigation of an approx. 7 times 7 region near the south galactic pole based
mostly on observations carried out with the FORS instruments attached to the
VLT telescopes. It includes the QSO Q 0103-260 (z = 3.36). The goal of this
study is to improve our understanding of the formation and evolution of
galaxies in the young Universe. In this paper the field selection, the
photometric observations, and the data reduction are described. The source
detection and photometry of objects in the FORS Deep Field is discussed in
detail. A combined B and I selected UBgRIJKs photometric catalog of 8753
objects in the FDF is presented and its properties are briefly discussed. The
formal 50% completeness limits for point sources, derived from the co-added
images, are 25.64, 27.69, 26.86, 26.68, 26.37, 23.60 and 21.57 in U, B, g, R,
I, J and Ks (Vega-system), respectively. A comparison of the number counts in
the FORS Deep Field to those derived in other deep field surveys shows very
good agreement.Comment: 15 pages, 11 figures (included), accepted for publication in A&
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