960 research outputs found
Multiband optical polarimetry of BL Lac objects with the Nordic Optical Telescope
Optical polarization of seven selected BL Lac objects in UBVRI bands was
studied with the Nordic Optical Telescope from December 10-14, 1999. Two of
them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total
integration time of 4.75 and 5.5 hours, respectively. Other objects (1Jy
0138-097, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely
during the run. Apart from PKS 0823-223 (more polarized than observed in the
past), the sources show levels of flux and polarization consistent with results
at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during
December 11 and 12 and exhibited variability of polarization, both on
internight and intranight time scales. Wavelength dependence of polarization
has been investigated, as well as circular polarization. The results are
discussed within the standard model for BL Lacs.Comment: 11 pages, 7 figures, accepted to Astronomy & Astrophysic
Redshift constraints for RGB 0136+391 and PKS 0735+178 from deep optical imaging
We present the results of deep I-band imaging of two BL Lacerate objects, RGB
0136+391 and PKS 0735+178, during an epoch when the optical nucleus was in a
faint state in both targets. In PKS 0735+178 we find a significant excess over
a point source, which, if fitted by the de Vaucouleurs model, corresponds to a
galaxy with I = 18.64 +- 0.11 and r_eff = 1.8 +- 0.4 arcsec. Interpreting this
galaxy as the host galaxy of PKS 0735+178 we derive z = 0.45 +- 0.06 using the
host galaxy as a "standard candle". We also discuss the immediate optical
environment of PKS 0735+178 and the identity of the MgII absorber at z = 0.424.
Despite of the optimally chosen epoch and deep imaging we find the surface
brightness profile of RGB 0136+391 to be consistent with a point source. By
determining a lower limit for the host galaxy brightness by simulations, we
derive z > 0.40 for this target.Comment: 6 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Multiband optical polarimetry of the BL Lac object PKS 2155--304. Intranight and long term variability
The polarized and total flux of the BL Lac object PKS 2155-304 were monitored
intensively and simultaneously in the optical UBVRI bands with the Turin
photopolarimeter at the CASLEO 2.15 m telescope during 4 campaigns in June,
August, November 1998 and August 1999. The effective observation time amounted
to ~47 hours. PKS 2155-304 showed a linear polarization percentage (P) usually
ranging between 3% and 7% and a polarization position angle (PA) mainly between
70 and 120 deg. The highest temporal resolution of our observations, 15
minutes, is unprecedented for polarimetric monitoring of this source, and has
allowed us to detect amplitude variations of the linear polarization percentage
from 6 to 7.5% in time scales of hours. In some nights the polarization
percentage seems to increase toward shorter wavelengths, however the polarized
spectrum does not vary significantly with time. The most remarkable variability
event occurred on 1998 June 18, when the degree of linear polarization
decreased by more than a factor 2 in one day in all bands, while the PA rotated
by 90 deg. This is consistent with the presence of two emission components, of
different polarization degree and position angle. Intranight variability of P
and PA can be interpreted with small amplitude physical or geometrical changes
within the jet. Measurements of the circular polarization over time intervals
of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin
Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did
not show light variations correlated with those of the linearly polarized flux.Comment: 21 pages, 6 eps figures. Accepted for The Astrophysical Journal
Supplement Serie
Optical morphology of distant RATAN-600 radio galaxies from subarcsecond resolution NOT images
We present direct imaging data of 22 ultra steep spectrum radio sources
obtained at (or near) a subarcsecond seeing. The basic sample of 40 double
radio sources was selected from the RATAN-600 catalogue. The FRII-structure has
been confirmed with VLA and preliminary optical identifications which come from
the 6 m-telescope. As the RATAN-600 flux limit at 3.9 GHz ( 10mJy) is
fainter than that of major surveys, the sample may have high- contents. This
is also suggested by the faint magnitudes in the Hubble diagram. The final aim
is to create a homogeneous sample of high- radio galaxies in a well defined
strip around the sky, with faint radio limit and subarcsecond morphology down
to .
We could confirm 16 identifications down to 24. Most of the
extended objects have multicomponent structures as expected from other surveys
of high-redshift radio galaxies. We found five unresolved objects even with a
subarcsecond seeing. Of the remaining six objects, three are extremely faint
and the other three have such a complex environment that further observations
are needed to confirm the optical identification.Comment: A&AS in pres
Discovery of an Optical Jet in the BL Lac Object 3C 371
We have detected an optical jet in the BL Lac object 3C 371 that coincides
with the radio jet in this object in the central few kpc. The most notable
feature is a bright optical knot 3 arcsec (4 kpc) from the nucleus that occurs
at the location where the jet apparently changes its direction by ~30 degrees.
The radio, near-infrared and optical observations of this knot are consistent
with a single power-law spectrum with a radio-optical spectral index alpha =
-0.81. One possible scenario for the observed turn is that the jet is
interacting with the material in the bridge connecting 3C 371 to nearby
galaxies and the pressure gradient is deflecting the jet significantly.Comment: 11 pages, LaTeX, 4 figures (1 eps, 3 gifs), accepted for publication
in ApJ Letter
- …