413 research outputs found
A brief (blazar oriented) overview on topics for multi-wavelength observations with TeV photons
Multi-wavelength observations with TeV photons are an essential diagnostic
tool to study the physics of TeV sources. The complex variability of blazars,
however (timescales from years down to minutes, with different patterns and SED
behaviours), requires a great effort on simultaneous campaigns, which should be
performed possibly over several days. Spectral information is essential, and
now with the new TeV and X-ray telescopes it can be obtained on timescales less
than one hour. The insights from such observations can be tremendous, since
recent results have shown that the X-ray and TeV emissions do not always follow
the same behaviour, and flares can have different relations between rise and
decay times. Unfortunately, the strong pointing constraints of XMM do not allow
the full use of this satellite simultaneously with ground telescopes.Comment: 8 pages, 6 figures. Invited talk at the 2nd Veritas Symposium: "TeV
Astrophysics of extragalactic sources". April 2003, Chicag
Blazar properties: an update from recent results
After a brief critical overview of the main properties of blazars and their
classification, some significant results from recent multiwavelength
observations are summarized, in the context of the jet physics.Comment: 5 pages, 2 figures. Invited talk at the 2nd Heidelberg workshop:
"High-Energy Gamma-rays and Neutrinos from Extra-Galactic Sources", January
13-16, 2009, to be published in Int. J. Mod. Phys. D. Updated reference
Constraints on the Optical-IR extragalactic background from gamma-ray absorption studies
Very high energy (VHE) gamma-rays from extragalactic sources, interacting by
gamma-gamma collisions with diffuse intergalactic radiation fields, provide an
alternative way to constrain the diffuse background light, completely
independent of direct measurements. The limits depend however on our knowledge
of the physics of the gamma-ray sources. After clarifying the interplay between
background light and VHE spectra, I summarize the extent and validity of the
obtainable limits, and where future improvements can be expected.Comment: 8 pages, 8 figures. To appear in Proceedings of IAU Symposium No.
284, The Spectral Energy Distribution of Galaxies (SED2011), Preston, UK, 5-9
sep. 2011 (corrected typos and updated affiliations
The SED of the TeV BLLac 1ES 1426+428 after correction for the TeV--IR absorption
The recent HEGRA detection and spectrum of 1ES 1426+428 at TeV energies, once
corrected for absorption using present estimates of the diffuse extragalactic
IR background, suggest that the high energy peak of the Spectral Energy
Distribution (SED) could be much higher than the synchrotron one
(), and lie at energies above 8-10 TeV. To see if such an SED could
be accounted for, we have applied a "finite injection time" SSC model, and
present here some preliminary results. Within this model, we found the need of
an external ("ambient") contribution to the energy density of seed photons, in
order to account for both the high Compton dominance and the hard spectrum.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Relativistic jets in the Chandra and XMM era", Bologna, 23-27/9/02 (New
Astr. Rev.
Constraining the Cosmic Background Light with four BL Lac TeV spectra
The intrinsic BL Lac spectra above few hundreds GeV can be very different
from the observed ones due to the absorption effects by the diffuse
Extragalactic Background Light (EBL), at present poorly known. With the recent
results, there are now 4 sources with good spectral information: Mkn 421
(z=0.031), Mkn 501 (z=0.034), 1ES 1426+428 (z=0.129) and 1ES 1959+650
(z=0.047). Making simple assumptions on the shape of the intrinsic spectra
(according to the present blazar knowledge), we have considered the effects of
different EBL spectral energy distributions (SED) for the first time on all 4
objects together, deriving constraints for the EBL fluxes. These resulted
significantly lower than many direct estimates.Comment: 4 pages, 8 figures; to appear in the proceedings of the 2nd Veritas
Symposium: "TeV Astrophysics of extragalactic sources". April 2003, Chicag
BeppoSAX Observations of 1-Jy BL Lacertae Objects - II
We present new BeppoSAX LECS and MECS observations, covering the energy range
0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the
1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which
we interpret as inverse Compton emission. One object shows evidence for a
low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the
synchrotron component merging into the inverse Compton one around ~ 2 keV. A
variable synchrotron tail would explain why the ROSAT spectra of our sources
are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The
broad-band spectral energy distributions fully confirm this picture and model
fits using a synchrotron inverse Compton model allow us to derive the physical
parameters (intrinsic power, magnetic field, etc.) of our sources. By combining
the results of this paper with those previously obtained on other sources we
present a detailed study of the BeppoSAX properties of a well-defined
sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s)
1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous
radio and X-ray powers for the 1-Jy sources in which the X-ray band is
dominated by inverse Compton emission, which points to a strong link between
X-ray and radio emission components in these objects.Comment: 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.htm
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