336 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
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
The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279
Recent VHE observations have unveiled a surprising behaviour in two
well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have
shown for the first time in an HBL a large Compton dominance, high gamma-ray
luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the
first FSRQ detected at VHE. The high luminosity required to overcome the
significant absorption caused by the BLR emission cannot be easily reconciled
with the historical and quasi-simultaneous SED properties. Both cases shed a
new light on the structure and ambient fields of blazars. Contrary to previous
claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide
robust constraints on the EBL.Comment: Proceedings of "4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy 2008" (Gamma 2008), July 7-11, 2008. Slightly refined
text with updated reference
The NuSTAR view on Hard-TeV BL Lacs
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic
TeV spectrum, locating the peak of their gamma-ray emission in the spectral
energy distribution (SED) above 2-10 TeV. Such high energies are problematic
for the Compton emission, using a standard one-zone leptonic model. We study
six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR
satellite. Together with simultaneous observations with the SWIFT satellite, we
fully constrain the peak of the synchrotron emission in their SED, and test the
leptonic synchrotron self-Compton (SSC) model. We confirm the extreme nature of
5 objects also in the synchrotron emission. We do not find evidence of
additional emission components in the hard X-ray band. We find that a one-zone
SSC model can in principle reproduce the extreme properties of both peaks in
the SED, from X-ray up to TeV energies, but at the cost of i) extreme electron
energies with very low radiative efficiency, ii) conditions heavily out of
equipartition (by 3 to 5 orders of magnitude), and iii) not accounting for the
simultaneous UV data, which then should belong to a different emission
component, possibly the same as the far-IR (WISE) data. We find evidence of
this separation of the UV and X-ray emission in at least two objects. In any
case, the TeV electrons must not "see" the UV or lower-energy photons, even if
coming from different zones/populations, or the increased radiative cooling
would steepen the VHE spectrum.Comment: 13 pages, 2 figures. Version accepted for publication in MNRAS. Fig.
2 corrected for a small plotting erro
Discovery of VHE and HE emission from the blazar 1ES 0414+009 with H.E.S.S and Fermi-LAT
The high energy peaked BL Lac (HBL) object 1ES 0414+009 (z=0.287) is a
distant very high-energy (VHE, E > 100 GeV) blazars with well-determined
redshift. This source was detected with the High Energy Stereoscopic System
(H.E.S.S.) between October 2005 and September 2009. It was also detected with
the Fermi Large Area Telescope (LAT) in 21 months of data. The combined high
energy (HE) and VHE spectra, once corrected for gamma-gamma absorption on the
extragalactic background light (EBL), indicate a Compton peak located above few
TeV, among the highest in the BL Lac class.Comment: proceeding from the 25th Texas Symposium on Relativistic Astrophysics
(Heidelberg, Germany, 2010
- âŠ