31 research outputs found
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
BeppoSAX Observations of 1 Jy BL Lacertae Objects. I
We present new BeppoSAX observations of seven BL Lacertae objects selected
from the 1 Jy sample plus one additional source. The collected data cover the
energy range 0.1 - 10 keV (observer's frame), reaching ~ 50 keV for one source
(BL Lac). All sources characterized by a peak in their multifrequency spectra
at infrared/optical energies (i.e., of the LBL type) display a relatively flat
(alpha_x ~ 0.9) X-ray spectrum, which we interpret as inverse Compton emission.
Four objects (2/3 of the LBL) show some evidence for a low-energy steepening
which is likely due to the synchrotron tail merging into the inverse Compton
component around ~ 1 - 3 keV. If this were generally the case with LBL, it
would explain why the 0.1 - 2.4 keV ROSAT spectra of our sources are
systematically steeper than the BeppoSAX ones (by ~ 0.5 in alpha_x). The
broad-band spectral energy distributions fully confirm this picture and a
synchrotron inverse Compton model allows us to derive the physical parameters
(intrinsic power, magnetic field, etc.) of our sources. Combining our results
with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron
peak frequencies, we confirm and clarify the dependence of the X-ray spectral
index on synchrotron peak frequency originally found in ROSAT data.Comment: 14 pages, 7 figures. Accepted for publication in MNRAS. Postscript
file also available at
http://icarus.stsci.edu/~padovani/xrayspectra_papers.htm
BeppoSAX Observations of Synchrotron X-ray Emission from Radio Quasars
We present new BeppoSAX LECS, MECS, and PDS observations of four
flat-spectrum radio quasars (FSRQ) having effective spectral indices alpha_ro
and alpha_ox typical of high-energy peaked BL Lacs. Our sources have
X-ray-to-radio flux ratios on average ~ 70 times larger than ``classical'' FSRQ
and lie at the extreme end of the FSRQ X-ray-to-radio flux ratio distribution.
The collected data cover the energy range 0.1 - 10 keV (observer's frame),
reaching ~ 100 keV for one object. The BeppoSAX band in one of our sources, RGB
J1629+4008, is dominated by synchrotron emission peaking at ~ 2 x 10^16 Hz, as
also shown by its steep (energy index alpha_x ~ 1.5) spectrum. This makes this
object the FIRST known FSRQ whose X-ray emission is not due to inverse Compton
radiation. Two other sources display a flat BeppoSAX spectrum (alpha_x ~ 0.7),
with weak indications of steepening at low X-ray energies. The combination of
BeppoSAX and ROSAT observations, (non-simultaneous) multifrequency data, and a
synchrotron inverse Compton model suggest synchrotron peak frequencies ~ 10^15
Hz, although a better coverage of their spectral energy distributions is needed
to provide firmer values. If confirmed, these values would be typical of
``intermediate'' BL Lacs for which the synchrotron and inverse Compton
components overlap in the BeppoSAX band. Our sources, although firmly in the
radio-loud regime, have powers more typical of high-energy peaked BL Lacs than
of FSRQ, and indeed their radio powers put them near the low-luminosity end of
the FSRQ luminosity function. We discuss this in terms of an anti-correlation
between synchrotron peak frequency and total power, based on physical
arguments, and also as possibly due to a selection effect.Comment: 18 pages, 9 figures. Accepted for publication in The Astrophysical
Journal. Postscript file also available at
http://www.stsci.edu/~padovani/xrayspectra_papers.htm
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye
The Compton Spectrometer and Imager
The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX)
satellite mission in development with a planned launch in 2027. COSI is a
wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV.
It provides imaging, spectroscopy, and polarimetry of astrophysical sources,
and its germanium detectors provide excellent energy resolution for emission
line measurements. Science goals for COSI include studies of 0.511 MeV emission
from antimatter annihilation in the Galaxy, mapping radioactive elements from
nucleosynthesis, determining emission mechanisms and source geometries with
polarization measurements, and detecting and localizing multimessenger sources.
The instantaneous field of view for the germanium detectors is >25% of the sky,
and they are surrounded on the sides and bottom by active shields, providing
background rejection as well as allowing for detection of gamma-ray bursts and
other gamma-ray flares over most of the sky. In the following, we provide an
overview of the COSI mission, including the science, the technical design, and
the project status.Comment: 8 page
The cosipy library: COSI's high-level analysis software
The Compton Spectrometer and Imager (COSI) is a selected Small Explorer
(SMEX) mission launching in 2027. It consists of a large field-of-view Compton
telescope that will probe with increased sensitivity the under-explored MeV
gamma-ray sky (0.2-5 MeV). We will present the current status of cosipy, a
Python library that will perform spectral and polarization fits, image
deconvolution, and all high-level analysis tasks required by COSI's broad
science goals: uncovering the origin of the Galactic positrons, mapping the
sites of Galactic nucleosynthesis, improving our models of the jet and emission
mechanism of gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), and
detecting and localizing gravitational wave and neutrino sources. The cosipy
library builds on the experience gained during the COSI balloon campaigns and
will bring the analysis of data in the Compton regime to a modern open-source
likelihood-based code, capable of performing coherent joint fits with other
instruments using the Multi-Mission Maximum Likelihood framework (3ML). In this
contribution, we will also discuss our plans to receive feedback from the
community by having yearly software releases accompanied by publicly-available
data challenges
Discovery of X-ray polarization angle rotation in active galaxy Mrk 421
The magnetic field conditions in astrophysical relativistic jets can be
probed by multiwavelength polarimetry, which has been recently extended to
X-rays. For example, one can track how the magnetic field changes in the flow
of the radiating particles by observing rotations of the electric vector
position angle . Here we report the discovery of a
rotation in the X-ray band in the blazar Mrk 421 at an average flux state.
Across the 5 days of Imaging X-ray Polarimetry Explorer (IXPE) observations of
4-6 and 7-9 June 2022, rotated in total by .
Over the two respective date ranges, we find constant, within uncertainties,
rotation rates ( and ) and polarization
degrees (). Simulations of a random walk of the
polarization vector indicate that it is unlikely that such rotation(s) are
produced by a stochastic process. The X-ray emitting site does not completely
overlap the radio/infrared/optical emission sites, as no similar rotation of
was observed in quasi-simultaneous data at longer wavelengths. We
propose that the observed rotation was caused by a helical magnetic structure
in the jet, illuminated in the X-rays by a localized shock propagating along
this helix. The optically emitting region likely lies in a sheath surrounding
an inner spine where the X-ray radiation is released