2,278 research outputs found
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
Detection of Exceptional X-Ray Spectral Variability in the TeV BL Lac 1ES 2344+514
We present the results of six BeppoSAX observations of 1ES 2344+514, five of
which were taken within a week. 1ES 2344+514, one of the few known TeV BL Lac
objects, was detected by the BeppoSAX Narrow Field Instruments between 0.1 to
\~50 keV. During the first five closely spaced observations 1ES 2344+514 showed
large amplitude luminosity variability, associated with spectacular spectral
changes, particularly when compared to the last observation when the source was
found to be several times fainter, with a much steeper X-ray spectrum. The
energy dependent shape of the lightcurve and the spectral changes imply a large
shift (factor of 30 or more in frequency) of the peak of the synchrotron
emission. At maximum flux the peak was located at or above 10 keV, making 1ES
2344+514 the second blazar (after MKN501) with the synchrotron peak in the hard
X-ray band. The shift, and the corresponding increase in luminosity, might be
due to the onset of a second synchrotron component extending from the soft to
the hard X-ray band where most of the power is emitted. Rapid variability on a
timescale of approximately 5000 seconds has also been detected when the source
was brightest.Comment: 7 pages, LaTeX, 6 PostScript figures. Accepted by MNRA
Blazar sequence - an artefact of Doppler boosting
The blazar sequence is a scenario in which the bolometric luminosity of the
blazar governs the appearance of its spectral energy distribution. The most
prominent result is the significant negative correlation between the
synchrotron peak frequencies and the synchrotron peak luminosities of the
blazar population. Observational studies of the blazar sequence have, in
general, neglected the effect of Doppler boosting. We study the dependence of
both the synchrotron peak frequency and luminosity with Doppler-corrected
quantities. We determine the spectral energy distributions of 135 radio-bright
AGN and find the best-fit parabolic function for the distribution to quantify
their synchrotron emission. The corresponding measurements of synchrotron peak
luminosities and frequencies are Doppler-corrected with a new set of Doppler
factors calculated from variability data. The relevant correlations for the
blazar sequence are determined for these intrinsic quantities. The Doppler
factor depends strongly on the synchrotron peak frequency, the lower energy
sources being more boosted. Applying the Doppler correction to the peak
frequencies and luminosities annuls the negative correlation between the two
quantities, which becomes positive. For BL Lacertae objects, the positive
correlation is particularly strong. The blazar sequence, when defined as the
anticorrelation between the peak frequency and luminosity of the synchrotron
component of the spectral energy distribution, disappears when the intrinsic,
Doppler-corrected values are used. It is an observational phenomenon created by
variable Doppler boosting across the synchrotron peak frequency range.Comment: 9 pages, 5 figures + 2 tables. The published version with minor
changes, the main conclusions are unchange
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 VLA Survey of the Chandra Deep Field South. IV. Source Population
We present a detailed analysis of 256 radio sources from our deep (flux
density limit of 42 microJy at the field centre at 1.4 GHz) Chandra Deep Field
South 1.4 and 5 GHz VLA survey. The radio population is studied by using a
wealth of multi-wavelength information in the radio, optical, and X-ray bands.
The availability of redshifts for ~ 80% of the sources in our complete sample
allows us to derive reliable luminosity estimates for the majority of the
objects. X-ray data, including upper limits, for all our sources turn out to be
a key factor in establishing the nature of faint radio sources. Due to the
faint optical levels probed by this study, we have uncovered a population of
distant Active Galactic Nuclei (AGN) systematically missing from many previous
studies of sub-millijansky radio source identifications. We find that, while
the well-known flattening of the radio number counts below 1 mJy is mostly due
to star forming galaxies, these sources and AGN make up an approximately equal
fraction of the sub-millijansky sky, contrary to some previous results. The AGN
include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a
significant radio-quiet component, which amounts to approximately one fifth of
the total sample. The ratio of radio to optical luminosity depends more on
radio luminosity, rather than being due to optical absorption.Comment: 13 pages, 8 figures, accepted for publication in the Astrophysical
Journa
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
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