2,260 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
The discovery of high power - high synchrotron peak blazars
We study the quasi-simultaneous near-IR, optical, UV, and X-ray photometry of
eleven gamma-ray selected blazars for which redshift estimates larger than 1.2
have been recently provided. Four of these objects turn out to be high-power
blazars with the peak of their synchrotron emission between ~ 3 x 10^15 and ~
10^16 Hz, and therefore of a kind predicted to exist but never seen before.
This discovery has important implications for our understanding of physical
processes in blazars, including the so-called "blazar sequence", and might also
help constraining the extragalactic background light through gamma-ray
absorption since two sources are strongly detected even in the 10 - 100 GeV
Fermi-LAT band. Based on our previous work and their high powers, these sources
are very likely high-redshift flat-spectrum radio quasars with their emission
lines swamped by the non-thermal continuum.Comment: 5 pages, 6 colour figures, accepted for publication in the Monthly
Notices of the Royal Astronomical Societ
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
Synchrotron and Compton Components and their Variability in BL Lac Objects
BL Lacertae objects are extreme extragalactic sources characterized by the
emission of strong and rapidly variable nonthermal radiation over the entire
electromagnetic spectrum. Synchrotron emission followed by inverse Compton
scattering in a relativistic beaming scenario is generally thought to be the
mechanism powering these objects. ...Comment: 4 pages, TeX plus 3 figures. Proceedings of the conference "X-ray
Astronomy 1999", September 6-10,1999, Bologn
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