599 research outputs found
The Deep X-ray Radio Blazar Survey (DXRBS). II. New Identifications
We have searched the archived, pointed ROSAT Position Sensitive Proportional
Counter data for blazars by correlating the WGACAT X-ray database with several
publicly available radio catalogs, restricting our candidate list to
serendipitous X-ray sources with a flat radio spectrum (alpha_r <= 0.70). This
makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new
identifications and spectra for 106 sources, including 86 radio-loud quasars,
11 BL Lacertae objects, and 9 narrow-line radio galaxies. Together with our
previously published objects and already known sources, our sample now contains
298 identified objects: 234 radio-loud quasars (181 flat-spectrum quasars: FSRQ
[alpha_r <= 0.50] and 53 steep-spectrum quasars: SSRQ), 36 BL Lacs, and 28
narrow-line radio galaxies. Redshift information is available for 96% of these.
Thus our selection technique is ~ 90% efficient at finding radio-loud quasars
and BL Lacs. Reaching 5 GHz radio fluxes ~ 50 mJy and 0.1-2.0 keV X-ray fluxes
a few x 10^-14 erg/cm^2/s, DXRBS is the faintest and largest flat-spectrum
radio sample with nearly complete (~ 85%) identification. We review the
properties of the DXRBS blazar sample, including redshift distribution and
coverage of the X-ray-radio power plane for quasars and BL Lacs. Additionally,
we touch upon the expanded multiwavelength view of blazars provided by DXRBS.
By sampling for the first time the faint end of the radio and X-ray luminosity
functions, this sample will allow us to investigate the blazar phenomenon and
the validity of unified schemes down to relatively low powers.Comment: 33 pages, 5 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/survey.htm
What Types of Jets does Nature Make: A New Population of Radio Quasars
We use statistical results from a large sample of about 500 blazars, based on
two surveys, the Deep X-ray Radio Blazar Survey (DXRBS), nearly complete, and
the RASS-Green Bank survey (RGB), to provide new constraints on the spectral
energy distribution of blazars, particularly flat-spectrum radio quasars
(FSRQ). This reassessment is prompted by the discovery of a population of FSRQ
with spectral energy distribution similar to that of high-energy peaked BL
Lacs. The fraction of these sources is sample dependent, being ~ 10% in DXRBS
and ~ 30% in RGB (and reaching ~ 80% for the Einstein Medium Sensitivity
Survey). We show that these ``X-ray strong'' radio quasars, which had gone
undetected or unnoticed in previous surveys, indeed are the strong-lined
counterparts of high-energy peaked BL Lacs and have synchrotron peak
frequencies, nu_peak, much higher than ``classical'' FSRQ, typically in the UV
band for DXRBS. Some of these objects may be 100 GeV - TeV emitters, as are
several known BL Lacs with similar broadband spectra. Our large, deep, and
homogeneous DXRBS sample does not show anti-correlations between nu_peak and
radio, broad line region, or jet power, as expected in the so-called ``blazar
sequence'' scenario. However, the fact that FSRQ do not reach X-ray-to-radio
flux ratios and nu_peak values as extreme as BL Lacs and the elusiveness of
high nu_peak - high-power blazars suggest that there might be an intrinsic,
physical limit to the synchrotron peak frequency that can be reached by
strong-lined, powerful blazars. Our findings have important implications for
the study of jet formation and physics and its relationship to other properties
of active galactic nuclei.Comment: 15 pages, 12 figures. Accepted for publication in The Astrophysical
Journal (May 1 2003 issue). Postscript file also available at
http://www.stsci.edu/~padovani/unif_papers.htm
The Sedentary Survey of Extreme High Energy Peaked BL Lacs III. Results from Optical Spectroscopy
The multi-frequency Sedentary Survey is a flux limited, statistically
well-defined sample of highly X-ray dominated BL Lacertae objects (HBLs) which
includes 150 sources. In this paper, the third of the series, we report the
results of a dedicated optical spectroscopy campaign that, together with
results from other independent optical follow up programs, led to the
spectroscopic identification of all sources in the sample. We carried out a
systematic spectroscopic campaign for the observation of all unidentified
objects of the sample using the ESO 3.6m, the KPNO 4m, and the TNG optical
telescopes. We present new identifications and optical spectra for 76 sources,
50 of which are new BL Lac objects, 18 are sources previously referred as BL
Lacs but for which no redshift information was available, and 8 are broad
emission lines AGNs. We find that the multi-frequency selection technique used
to build the survey is highly efficient (about 90%) in selecting BL Lacs
objects. We present positional and spectroscopic information for all confirmed
BL Lac objects. Our data allowed us to determined 36 redshifts out of the 50
new BL Lacs and 5 new redshifts for the previously known objects. The redshift
distribution of the complete sample is presented and compared with that of
other BL Lacs samples. For 26 sources without recognizable absorption features,
we calculated lower limits to the redshift using a method based on simulated
optical spectra with different ratios between jet and galaxy emission. For a
subsample of 38 object with high-quality spectra, we find a correlation between
the optical spectral slope, the 1.4 GHz radio luminosity, and the Ca H&K break
value, indicating that for powerful/beamed sources the optical light is
dominated by the non-thermal emission from the jet.Comment: 23 pages, accepted by A&
Optical variability of the strong-lined and X-ray bright source 1WGA J0447.9-0322
We present the historic light curve of 1WGA J0447.9-0322, spanning the time
interval from 1962 to 1991, built using the Asiago archive plates. The source
shows small fluctuations of about 0.3 mag around B=16 until 1986 and a fast
dimming of its average level by about 0.5 mag after that date, again with small
short term variations. The variability pattern is within the values shown by
other QSOs with long term monitoring, notwithstanding its high X-ray/optical
ratio. We present also its overall SED using literature data and recent
UV-optical SWIFT observations.Comment: 17 pages, 4 figures, accepted by The Astronomical Journal. Table 2
available upon reques
The classification of BL Lacertae objects: the Ca H&K break
We investigate why BL Lacertae objects (BL Lacs) have values of the Ca H&K
break (a stellar absorption feature) lower than low-power radio galaxies and if
its use is justified to separate the two classes. For this purpose we relate
this parameter to the radio and optical core emissions, as well as to the X-ray
powers, for a sample of ~90 radio sources. We find that the Ca H&K break value
decreases with increasing jet powers, and that it also anti-correlates with the
radio core dominance parameter but not with extended radio emission. Based on
this we conclude that the Ca H&K break value of BL Lacs and radio galaxies is a
suitable indicator of orientation. From the luminosity ratios between objects
with low and high Ca H&K break values we constrain the average Lorentz factors
for BL Lacs and low-power radio galaxies in the radio and X-ray band to Gamma ~
2 -- 4 and derive average viewing angles for the galaxies. Our values are in
agreement with results from independent methods. We find that the correlations
between Ca H&K break and radio core and X-ray luminosity hold independently for
low- (LBL) and high-energy peaked BL Lacs (HBL). We derive average viewing
angles for their parent populations, which turn out to be similar to the ones
for our entire sample, and compare for the first time the luminosities of LBL
and HBL at different orientations.Comment: 14 pages, 12 figures. Accepted for publication in MNRA
EVN & MERLIN studies of a new sample of BL Lac objects
The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac objects spanning the intermediate range of spectral energy distributions between “classical” X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evolution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available
The Blazar Sequence: Validity and Predictions
The "blazar sequence" posits that the most powerful BL Lacertae objects and
flat-spectrum radio quasars should have relatively small synchrotron peak
frequencies, nu_peak, and that the least powerful such objects should have the
highest nu_peak values. This would have strong implications for our
understanding of jet formation and physics and the possible detection of
powerful, moderately high-redshift TeV blazars. I review the validity of the
blazar sequence by using the results of very recent surveys and compare its
detailed predictions against observational data. I find that the blazar
sequence in its simplest form is ruled out. However, powerful flat-spectrum
radio quasars appear not to reach the nu_peak typical of BL Lacs. This could
indeed be related to some sort of sequence, although it cannot be excluded that
it is instead due to a selection effect.Comment: 9 pages, 4 figures, invited talk at the Workshop "The Multi-messenger
approach to high energy gamma-ray sources", Barcelona, Spain, July 4-7, 2006,
to appear in the proceeding
A Flare in the Jet of Pictor A
A Chandra X-ray imaging observation of the jet in Pictor A showed a feature
that appears to be a flare that faded between 2000 and 2002. The feature was
not detected in a follow-up observation in 2009. The jet itself is over 150 kpc
long and a kpc wide, so finding year-long variability is surprising. Assuming a
synchrotron origin of the observed high-energy photons and a minimum energy
condition for the outflow, the synchrotron loss time of the X-ray emitting
electrons is of order 1200 yr, which is much longer than the observed
variability timescale. This leads to the possibility that the variable X-ray
emission arises from a very small sub-volume of the jet, characterized by
magnetic field that is substantially larger than the average over the jet.Comment: 12 pages, 3 figures, to appear in Ap. J. Letter
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