1,338 research outputs found
X-ray Emission from Extragalactic Jets
This review focuses on the X-ray emission processes of extra-galactic jets on
scales resolvable by the sub arcsec resolution of the Chandra X-ray
Observatory. It is divided into 4 parts. The introductory chapter reviews the
classical problems for jets, as well as those associated directly with the
X-ray emission. Throughout this section, we deal with the dualisms of low
powered radio sources versus high powered radio galaxies and quasars;
synchrotron models versus inverse Compton models; and the distinction between
the relativistic plasma responsible for the received radiation and the medium
responsible for the transport of energy down the jet. The second part collects
the observational and inferred parameters for the currently detected X-ray jets
and attempts to put their relative sizes and luminosities in perspective. In
part 3, we first give the relevant radio and optical jet characteristics, and
then examine the details of the X-ray data and how they can be related to
various jet attributes. The last section is devoted to a critique of the two
non-thermal emission processes and to prospects for progress in our
understanding of jets.Comment: This is a version of a review article to be published (2006 Sep) in
the Annual Reviews of Astronomy and Astrophysics, vol. 44, p. 463. 8 of the
12 figures have been removed from the article and are provided as separate
jpg files to conserve space. There are 38 pages remaining in the text.
Complete postscript and pdf versions are available at:
http://hea-www.harvard.edu/~harris/Xjetreview
Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry
The dissipation of energy flux in blazar jets plays a key role in the
acceleration of relativistic particles. Two possibilities are commonly
considered for the dissipation processes, magnetic reconnection -- possibly
triggered by instabilities in magnetically-dominated jets -- , or shocks -- for
weakly magnetized flows. We consider the polarimetric features expected for the
two scenarios analyzing the results of state-of-the-art simulations. For the
magnetic reconnection scenario we conclude, using results from global
relativistic MHD simulations, that the emission likely occurs in turbulent
regions with unstructured magnetic fields, although the simulations do not
allow us to draw firm conclusions. On the other hand, with local
particle-in-cell simulations we show that, for shocks with a magnetic field
geometry suitable for particle acceleration, the self-generated magnetic field
at the shock front is predominantly orthogonal to the shock normal and becomes
quasi-parallel downstream. Based on this result we develop a simplified model
to calculate the frequency-dependent degree of polarization, assuming that
high-energy particles are injected at the shock and cool downstream. We apply
our results to HBLs, blazars with the maximum of their synchrotron output at
UV-soft X-ray energies. While in the optical band the predicted degree of
polarization is low, in the X-ray emission it can ideally reach 50\%,
especially during active/flaring states. The comparison between measurements in
the optical and in the X-ray band made during active states (feasible with the
planned {\it IXPE} satellite) are expected to provide valuable constraints on
the dissipation and acceleration processes.Comment: 9 pages, 6 figures, accepted for publication by MNRA
On the radio and GeV-TeV gamma-ray emission connection in Fermi blazars
The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by
blazars. Looking for a possible connection between radio and gamma-ray emission
is a central issue for understanding the blazar physics, and various works were
dedicated to this topic. However, while a strong and significant correlation
was found between radio and gamma-ray emission in the 0.1-100 GeV energy range,
the connection between radio and very high energy (VHE, E>0.1 TeV) emission is
still elusive. The main reason is the lack of a homogeneous VHE sky coverage,
due to the operational mode of the imaging atmospheric Cherenkov telescopes.
With the present work we aim to quantify and assess the significance of the
possible connection between high-resolution radio emission, on milliarcsecond
scale, and GeV-TeV gamma-ray emission in blazars. For achieving our goal we
extract two large and unbiased blazar samples from the 1FHL and 2FHL Fermi
catalogs, above 10 GeV and 50 GeV, respectively. To investigate how the
correlation evolves as the gamma-ray energy increases, we perform the same
analysis by using the 0.1-300 GeV 3FGL gamma-ray energy fluxes. When we
consider the 0.1-300 GeV gamma-ray energy range, we find a strong and
significant correlation for all of the blazar sub-classes. Conversely, when we
consider the gamma-ray emission above 10 GeV the correlation with the radio
emission vanishes, with the exception of the blazar sub-class of high
synchrotron peaked objects.Comment: 6 pages, 2 figures, 1 table. For the proceedings of the 7th
International Fermi Symposiu
No axions from the Sun
Preliminary evidence of solar axions in XMM-Newton observations has quite
recently been claimed by Fraser et al. as an interpretation of their detection
of a seasonally-modulated excess of the X-ray background. Within such an
interpretation, these authors also estimate the axion mass to be eV. Since an axion with this mass behaves as a cold dark
matter particle, according to the proposed interpretation the considered
detection directly concerns cold dark matter as well. So, the suggested
interpretation would lead to a revolutionary discovery if confirmed.
Unfortunately, we have identified three distinct problems in this
interpretation of the observed result of Fraser et al. which ultimately imply
that the detected signal - while extremely interesting in itself - cannot have
any relation with hypothetical axions produced by the Sun. Thus, a physically
consistent interpretation of the observed seasonally-modulated X-ray excess
still remains an exciting challenge.Comment: 3 pages, 1 figure, accepted for publication in MNRA
A kpc-scale X-ray jet in the BL Lac source S5 2007+777
X-ray jets in AGN are commonly observed in FRII and FRI radio-galaxies, but
rarely in BL Lacs, most probably due to their orientation close to the line of
sight and the ensuing foreshortening effects. Only three BL Lacs are known so
far to contain a kpc-scale X-ray jet. In this paper, we present the evidence
for the existence of a fourth extended X-ray jet in the classical
radio-selected source S5 2007+777, which for its hybrid FRI/II radio morphology
has been classified as a HYMOR (HYbrid MOrphology Radio source). Our Chandra
ACIS-S observations of this source revealed an X-ray counterpart to the
19"-long radio jet. Interestingly, the X-ray properties of the kpc-scale jet in
S5 2007+777 are very similar to those observed in FRII jets. First, the X-ray
morphology closely mirrors the radio one, with the X-rays being concentrated in
the discrete radio knots. Second, the X-ray continuum of the jet/brightest knot
is described by a very hard power law, with photon index Gamma_x~1, although
the uncertainties are large. Third, the optical upper limit from archival HST
data implies a concave radio-to-X-ray SED. If the X-ray emission is attributed
to IC/CMB with equipartition, strong beaming (delta=13) is required, implying a
very large scale (Mpc) jet. The beaming requirement can be somewhat relaxed
assuming a magnetic field lower than equipartition. Alternatively, synchrotron
emission from a second population of very high-energy electrons is viable.
Comparison to other HYMOR jets detected with Chandra is discussed, as well as
general implications for the origin of the FRI/II division.Comment: Accepted for publication in ApJ, 19 pages, 3 figure
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