2,520 research outputs found
Quasar optical variability: searching for interband time delays
Aims. The main purpose of this paper is to study time delays between the
light variations in different wavebands for a sample of quasars. Measuring a
reliable time delay for a large number of quasars may help constraint the
models of their central engines. The standard accretion disk irradiation model
predicts a delay of the longer wavelengths behind the shorter ones, a delay
that depends on the fundamental quasar parameters. Since the black hole masses
and the accretion rates are approximately known for the sample we use, one can
compare the observed time delays with the expected ones.
Methods. We applied the interpolation cross-correlation function (ICCF)
method to the Giveon et al. sample of 42 quasars, monitored in two (B and R)
colors, to find the time lags represented by the ICCF peaks. Different tests
were performed to assess the influence of photometric errors, sampling, etc.,
on the final result.
Results. We found that most of the objects show a delay in the red light
curve behind the blue one (a positive lag), which on average for the sample is
about +4 days (+3 for the median), although the scatter is significant. These
results are broadly consistent with the reprocessing model, especially for the
well-sampled objects. The normalized time-lag deviations do not seem to
correlate significantly with other quasar properties, including optical, radio,
or X-ray measurables. On the other hand, many objects show a clear negative
lag, which, if real, may have important consequences for the variability
models.Comment: 5 pages, 4 figures, accepted in A&
VLBI imaging of extremely high redshift quasars at 5 GHz
We present very long baseline interferometry (VLBI) images of ten very high
redshift (z>3) quasars at 5 GHz. The sources 0004+139, 0830+101, 0906+041,
0938+119 and 1500+045 were observed in September 1992 using a global VLBI
array, while 0046+063, 0243+181, 1338+381, 1428+423 and 1557+032 were observed
in October 1996 with the European VLBI Network and Hartebeesthoek, South
Africa. Most of the sources are resolved and show asymmetric structure. The
sample includes 1428+423, the most distant radio loud quasar known to date
(z=4.72). It is barely resolved with an angular resolution of about 2.0*1.4
mas.Comment: Astronomy and Astrophysics, in press, Latex2e, 10 pages, 3 figures
(and lots of sub-figures
A complete sample of quasars from the 7C redshift survey
We present details of a new sample of radio-loud quasars drawn from 0.013 sr
of the 7C Redshift Survey. This sample is small (21 quasars) but complete in
that every object with an unresolved nucleus and/or broad emission lines with
S(151MHz) > 0.5 Jy has been discovered. The dependence of the quasar fraction
with redshift and radio luminosity is investigated, providing new evidence
supporting the unification of radio-loud quasars and powerful radio galaxies.
This 7C sample is compared with optically-selected quasars, in order to
determine whether there are systematic biases in the different selection
techniques. There are no lightly reddened (Av approx. 1) quasars in our sample
amongst the 14 with z < 2. The discovery of a reddened quasar at z = 2.034 and
its implications are discussed. A tight correlation between radio luminosity
and optical/near infrared continuum luminosity for a subset of the sample is
also found.Comment: 6 pages Latex, To appear in the "Cosmology with the New Radio
Surveys" Conference - Tenerife 13-15 January 199
Optically Faint Microjansky Radio Sources
We report on the identifications of radio sources from our survey of the
Hubble Deep Field and the SSA13 fields, both of which comprise the deepest
radio surveys to date at 1.4 GHz and 8.5 GHz respectively. About 80% of the
microjansky radio sources are associated with moderate redshift starburst
galaxies or AGNs within the I magnitude range of 17 to 24 with a median of I =
22 mag. Thirty-one (20%) of the radio sources are: 1) fainter than 25 mag,
with two objects in the HDF 28.5, 2) often identified with very red
objects 4, and 3) not significantly different in radio properties than
the brighter objects. We suggest that most of these objects are associated with
heavily obscured starburst galaxies with redshifts between 1 and 3. However,
other mechanisms are discussed and cannot be ruled out with the present
observations.Comment: to appear in Astrophysical Journal Letters, 3 figures, 1 tabl
The Shroud Around the Twin Radio Jets in NGC 1052
(Abridged) We discuss multiple VLBI continuum and spectral line observations
and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at
approximately 0.26c in bi-symmetric jets, most likely oriented near the plane
of the sky. Absorption and emission signatures reveal ionised, atomic, and
molecular components of the surrounding medium.
Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption
in the inner parsec, probably together with synchrotron self-absorption. There
is apparently a geometrically thick but patchy structure oriented roughly
orthogonal to the jets. The western jet is receding: it is covered more deeply
and extensively. HI spectral line VLBI reveals atomic gas in front of both
jets. There appear to be three velocity systems. The deepest, at "high
velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2
pc away from the core, within which this gas might be largely ionised. WSRT
spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio
varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity
system the OH profiles are similar to HI, suggesting co-location of that atomic
and molecular gas, and leaving unclear the connection to the H2O masing gas
seen elsewhere. We have also detected both 18cm OH satellite lines in the high
velocity system. They have conjugate profiles: 1612 MHz is in absorption, and
1720 MHz in emission.Comment: 16 pages, 14 figures, LaTeX, includes aa.cls, accepted for
publication in Astronomy and Astrophysic
Variability and Velocity of Superluminal Sources
We investigate the relation between the Doppler factor determined from
variations in total flux at 22 and 37 GHz, and the apparent transverse velocity
determined from VLBA observations at 2 cm. The data are consistent with the
relativistic beaming theory for compact radio sources, in that the distribution
of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte
Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close
to the "equipartition value" calculated by Readhead (1994). We deduce the
distribution of Lorentz factors for a group of 48 sources; the values range up
to about gamma=40.Comment: To be published in "Radio Astronomy at the Fringe", ASP Conf. Ser.
Vol. 300, J. A. Zensus, M. H. Cohen, & E. Ros (eds.), 8 pages, 3 figures,
needs rafringe.st
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