301 research outputs found
Photon breeding mechanism in relativistic jets: astrophysical implications
Photon breeding in relativistic jets involves multiplication of high-energy
photons propagating from the jet to the external environment and back with the
conversion into electron-positron pairs. The exponential growth of the energy
density of these photons is a super-critical process powered by the bulk energy
of the jet. The efficient deceleration of the jet outer layers creates a
structured jet morphology with the fast spine and slow sheath. In initially
fast and high-power jets even the spine can be decelerated efficiently leading
to very high radiative efficiencies of conversion of the jet bulk energy into
radiation. The decelerating, structured jets have angular distribution of
radiation significantly broader than that predicted by a simple blob model with
a constant Lorentz factor. This reconciles the discrepancy between the high
Doppler factors determined by the fits to the spectra of TeV blazars and the
low apparent velocities observed at VLBI scales as well as the low jet Lorentz
factors required by the observed statistics and luminosity ratio of
Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a
population of high-energy leptons in agreement with the constraints on the
electron injection function required by spectral fits of the TeV blazars.
Relativistic pairs created outside the jet and emitting gamma-rays by inverse
Compton process might explain the relatively high level of the TeV emission
from the misaligned jet in the radio galaxies. The mechanism reproduces basic
spectral features observed in blazars including the blazar sequence (shift of
the spectral peaks towards lower energies with increasing luminosity). The
mechanism is very robust and can operate in various environments characterised
by the high photon density.Comment: 6 pages, 3 figures, to appear in the proceedings of the HEPRO
conference, September 24-28, 2007, Dublin, Irelan
What do HST and Chandra tell us about the jet and the nuclear region of the radio galaxy 3C270?
The HST/STIS ultraviolet image of the FR I radio galaxy 3C270 shows the
presence of a jet-like structure emerging from the position of the nucleus.
This feature, which represents the first jet-like component ever detected in
the UV in a radio galaxy with jets lying almost on the plane of the sky, has
the same position angle as the jet in the radio and X-ray images. We propose
two different scenarios for the origin of the emission: i) non-thermal
synchrotron from a mildly relativistic component of the jet; ii) scattered
light from the nucleus, where a BL Lac source may be hosted. Either of these
pictures would have important consequences for the AGN unification schemes and
for our knowledge of the jet structure. In the Chandra image a faint
counter-jet is also present. From a comparative analysis of the HST images and
Chandra X-ray spectrum, we find that the nucleus is only moderately obscured.
The obscuring structure might well reside in the geometrically thin dark disk
observed on large scales. This fits the scenario in which a standard
geometrically and optically thick torus is not present in FR I radio galaxies.Comment: 13 pages, 5 figures, in press on ApJ. Revised version, the discussion
has been improved according to the requests of the refere
The X-ray properties of high-z FRI candidates in the COSMOS field
We report the X-ray analysis of a sample of candidate high-redshift (1<z<2)
FRI sources from the sample of Chiaberge et al. (2009), observed in the Chandra
COSMOS field (C-COSMOS). Our main goals are to study their nuclear properties
by means of unresolved X-ray emission, and to constrain the presence of
clusters surrounding the FRI sources from the diffuse X-ray emission by the
associated hot plasma. Among 19 FRI candidates, 6 have an X-ray unresolved
counterpart in the C-COSMOS catalog. Two additional sources are not present in
the C-COSMOS catalog but are clearly detected in the Chandra images. X-ray
spectral analysis, when possible, or hardness ratio of the stacked emission
from X-ray detected sources, suggest that some of them have significant
intrinsic absorption (N_H~10^22 cm^-2), and high X-ray luminosities with
respect to local FRIs. From the stacking analysis of the 11 non-detected
sources, however, we find evidence for unresolved soft X-ray emission and no
detected hard emission, suggesting an unabsorbed spectrum. Therefore, the X-ray
properties vary significantly from source to source among these FRI candidates.
From the analysis of the stacked images of all 19 FRI candidates we can rule
out the presence of virialized haloes with temperatures larger than 2-3 keV;
however, the upper limit on the average extended emission is still consistent
with the presence of ~1-2 keV hot gas.Comment: 11 pages, 5 figures, MNRAS accepted; added co-author affiliatio
The X-ray variability and the near-IR to X-ray spectral energy distribution of four low luminosity Seyfert 1 galaxies
We present the results from a study of the X-ray variability and the near-IR
to X-ray spectral energy distribution of four low-luminosity, Seyfert 1
galaxies. We compared their variability amplitude and broad band spectrum with
those of more luminous AGN in order to investigate whether accretion in
low-luminosity AGN operates as in their luminous counterparts. We used archival
XMM-Newton and, in two cases, ASCA data to estimate their X-ray variability
amplitude and determine their X-ray spectral shape and luminosity. We also used
archival HST data to measure their optical nuclear luminosity, and near-IR
measurements from the literature, in order to construct their near-IR to X-ray
spectra. The X-ray variability amplitude of the four Seyferts is what one would
expect, given their black hole masses. Their near-IR to X-ray spectrum has the
same shape as the spectrum of quasars which are 10^2-10^5 times more luminous.
The objects in our sample are optically classified as Seyfert 1-1.5. This
implies that they host a relatively unobscured AGN-like nucleus. They are also
of low luminosity and accrete at a low rate. They are therefore good candidates
to detect radiation from an inefficient accretion process. However, our results
suggest that they are similar to AGN which are 10^2-10^5 times more luminous.
The combination of a "radiative efficient accretion disc plus an X-ray
producing hot corona" may persist at low accretion rates as well.Comment: 11 pages, 8 figures, accepted for publication in A&
High Energy gamma-rays From FR I Jets
Thanks to Hubble and Chandra telescopes, some of the large scale jets in
extragalactic radio sources are now being observed at optical and X-ray
frequencies. For the FR I objects the synchrotron nature of this emission is
surely established, although a lot of uncertainties - connected for example
with the particle acceleration processes involved - remain. In this paper we
study production of high energy gamma-rays in FR I kiloparsec-scale jets by
inverse-Compton emission of the synchrotron-emitting electrons. We consider
different origin of seed photons contributing to the inverse-Compton
scattering, including nuclear jet radiation as well as ambient, stellar and
circumstellar emission of the host galaxies. We discuss how future detections
or non-detections of the evaluated gamma-ray fluxes can provide constraints on
the unknown large scale jet parameters, i.e. the magnetic field intensity and
the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find
measurable fluxes of TeV photons resulting from synchrotron self-Compton
process and from comptonisation of the galactic photon fields, respectively. In
the case of Centaurus A, we also find a relatively strong emission component
due to comptonisation of the nuclear blazar photons, which could be easily
observed by GLAST at energy ~10 GeV, providing important test for the
unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for
publication in Astrophysical Journa
The MURALES survey II. Presentation of MUSE observations of 20 3C low-z radio galaxies and first results
We present observations of a complete sub-sample of 20 radio galaxies from
the Third Cambridge Catalog (3C) with redshift <0.3 obtained from VLT/MUSE
optical integral field spectrograph. These data have been obtained as part of
the survey MURALES (a MUse RAdio Loud Emission line Snapshot survey) with the
main goal of exploring the Active Galactic Nuclei (AGN) feedback process in a
sizeable sample of the most powerful radio sources at low redshift. We present
the data analysis and, for each source, the resulting emission line images and
the 2D gas velocity field. Thanks to their unprecedented depth (the median 3
sigma surface brightness limit in the emission line maps is 6X10^-18 erg s-1
cm-2 arcsec-2, these observations reveal emission line structures extending to
several tens of kiloparsec in most objects. In nine sources the gas velocity
shows ordered rotation, but in the other cases it is highly complex. 3C sources
show a connection between radio morphology and emission line properties.
Whereas, in three of the four Fanaroff and Riley Class I radio galaxies (FRIs),
the line emission regions are compact, ~1 kpc in size; in all but one of the
Class II radiogalaxies FRIIs, we detected large scale structures of ionized gas
with a median extent of 17 kpc. Among the FRIIs, those of high and low
excitation show extended gas structures with similar morphological properties,
suggesting that they both inhabit regions characterized by a rich gaseous
environment on kpc scale.Comment: Accepted for publication in A&
Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies
The clear sign of a hidden quasar inside a radio galaxy is the appearance of
quasar spectral features in its polarized (scattered) light. However that
observational test requires suitably placed scattering material to act as a
mirror, allowing us to see the nuclear light. A rather robust and more general
test for a hidden quasar is to look for the predicted high mid-IR luminosity
from the nuclear obscuring matter. The nuclear waste heat is detected and well
isolated in the nearest narrow line radio galaxy, Cen A. This confirms other
indications that Cen A does contain a modest quasar-like nucleus. However we
show here that M87 does not: at high spatial resolution, the mid-IR nucleus is
seen to be very weak, and consistent with simple synchrotron emission from the
base of the radio jet. This fairly robustly establishes that there are "real"
narrow line radio galaxies, without the putative accretion power, and with
essentially all the luminosity in kinetic form. Next we show the intriguing
mid-IR morphology of Cygnus A, reported previously by us and later discussed in
detail by Radomski et al. (2002). All of this mid-IR emission is consistent
with reprocessing by a hidden quasar, known to exist from spectropolarimetry by
Ogle et al. (1997) and other evidence.Comment: 21 pages, 5 figure
New Extreme Synchrotron BL Lac Objects
We report on the BeppoSAX observations of four "extreme" BL Lacs, selected to
have high synchrotron peak frequencies. All have been detected also in the PDS
band. For 1ES 0120+340, PKS 0548-322 and H 2356-309 the spectrum is well fitted
by a convex broken power-law, thus locating the synchrotron peak around 1 - 4
keV. 1ES 1426+428 presents a flat energy spectral index (alpha_x= 0.92) up to
100 keV, thus constraining the synchrotron peak to lie near or above that
value. For their extreme properties, all sources could be strong TeV emitters.Comment: To appear in the proceedings of the conference "X-Ray Astronomy '99",
Bologna, Italy, September 1999; 4 pages, 3 figures, uses bo99.sty. Corrected
authors' lis
Extended X-ray emission in radio galaxies: the peculiar case of 3C 305
Extended X-ray structures are common in Active Galactic Nuclei (AGNs). Here
we present the first case of a Compact Steep Spectrum (CSS) radio galaxy, 3C
305, in which the X-ray radiation appears to be associated with the optical
emission line region, dominated by the [O III]5007. On the basis of a
morphological study, performed using the comparison between the X-rays, the
optical and the radio band, we argue that the high energy emission has a
thermal nature and it is not directly linked to the radio jet and hotspots of
this source. Finally, we discuss the origin of the extended X-ray structure
connected with the optical emission line region following two different
interpretations: as due to the interaction between matter outflows and
shock-heated environment gas, or as due to gas photoionized by nuclear
emission.Comment: 5 pages, 2 figures, Accepted for publication in The ApJL Comments:
references and affilitations correcte
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