229 research outputs found
Active Galactic Nuclei under the scrutiny of CTA
Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer
excellent conditions for efficient particle acceleration in internal and
external shocks, turbulence, and magnetic reconnection events. The jets as well
as particle accelerating regions close to the supermassive black holes
(hereafter SMBH) at the intersection of plasma inflows and outflows, can
produce readily detectable very high energy gamma-ray emission. As of now, more
than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the
present ground-based gamma-ray telescopes, which represents more than one third
of the cosmic sources detected so far in the VHE gamma-ray regime. The future
Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the
VHE range by about one order of magnitude, shedding new light on AGN population
studies, and AGN classification and unification schemes. CTA will be a unique
tool to scrutinize the extreme high-energy tail of accelerated particles in
SMBH environments, to revisit the central engines and their associated
relativistic jets, and to study the particle acceleration and emission
mechanisms, particularly exploring the missing link between accretion physics,
SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an
extremely rewarding observing program which will inform us about the inner
workings and evolution of AGN. Furthermore these AGN are bright beacons of
gamma-rays which will allow us to constrain the extragalactic infrared and
optical backgrounds as well as the intergalactic magnetic field, and will
enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure
Mrk 421, Mrk 501, and 1ES 1426+428 at 100 GeV with the CELESTE Cherenkov Telescope
We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in
the energy range between 50 and 350 GeV (1.2 to 8.3 x 10^25 Hz). The detector,
called CELESTE, used first 40, then 53 heliostats of the former solar facility
"Themis" in the French Pyrenees to collect Cherenkov light generated in
atmospheric particle cascades. The signal from Mrk 421 is often strong. We
compare its flux with previously published multi-wavelength studies and infer
that we are straddling the high energy peak of the spectral energy
distribution. The signal from Mrk 501 in 2000 was weak (3.4 sigma). We obtain
an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab
flux near 100 GeV. The data analysis and understanding of systematic biases
have improved compared to previous work, increasing the detector's sensitivity.Comment: 15 pages, 14 figures, accepted to A&A (July 2006) August 19 --
corrected error in author lis
High redshift Fermi blazars observed by GROND and Swift
We observed 5 gamma-ray loud blazars at redshift greater than 2 with the
X-Ray Telescope (XRT) and the UltraViolet and Optical Telescope (UVOT) onboard
the Swift satellite, and the Gamma-Ray burst Optical Near-Infrared Detector
(GROND) instrument. These observations were quasi simultaneous, usually within
a few hours. For 4 of these blazars the near-IR to UV data show the presence of
an accretion disc, and we could reliably estimate its accretion rate and black
hole mass. One of them, PKS 1348+007, was found in an extraordinarily high
IR-optical state, almost two orders of magnitude brighter than at the epoch of
the Sloan Digital Sky Survey observations. For all the 5 quasars the physical
parameters of the jet emitting zone, derived by applying a one-zone emission
model, are similar to that found for the bulk of other gamma-ray loud quasars.
With our observations we have X-ray data for the full sample of blazars at z>2
present in the Fermi 2-yrs (2LAC) catalog. This allows to have a rather
complete view of the spectral energy distribution of all high-redshift Fermi
blazars, and to draw some conclusions about their properties, and especially
about the relation between the accretion rate and the jet power.Comment: 11 pages, 6 figures, submitted to MNRA
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
Here we investigate some aspects of stochastic acceleration of
ultrarelativistic electrons by magnetic turbulence. In particular, we discuss
the steady-state energy spectra of particles undergoing momentum diffusion due
to resonant interactions with turbulent MHD modes, taking rigorously into
account direct energy losses connected with different radiative cooling
processes. For the magnetic turbulence we assume a given power spectrum of the
type . In contrast to the previous approaches, however, we
assume a finite range of turbulent wavevectors , consider a variety of
turbulence spectral indexes , and concentrate on the case of a
very inefficient particle escape from the acceleration site. We find that for
different cooling and injection conditions, stochastic acceleration processes
tend to establish a modified ultrarelativistic Maxwellian distribution of
radiating particles, with the high-energy exponential cut-off shaped by the
interplay between cooling and acceleration rates. For example, if the timescale
for the dominant radiative process scales with the electron momentum as
, the resulting electron energy distribution is of the form
, where , and
is the equilibrium momentum defined by the balance between stochastic
acceleration and energy losses timescales. We also discuss in more detail the
synchrotron and inverse-Compton emission spectra produced by such an electron
energy distribution, taking into account Klein-Nishina effects. We point out
that the curvature of the high frequency segments of these spectra, even though
being produced by the same population of electrons, may be substantially
different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted
for publication in Ap
Discovery of Very High Energy gamma-rays from 1ES 1011+496 at z=0.212
We report on the discovery of Very High Energy (VHE) gamma-ray emission from
the BL Lacertae object 1ES1011+496. The observation was triggered by an optical
outburst in March 2007 and the source was observed with the MAGIC telescope
from March to May 2007. Observing for 18.7 hr we find an excess of 6.2 sigma
with an integrated flux above 200 GeV of (1.58 photons
cm s. The VHE gamma-ray flux is >40% higher than in March-April
2006 (reported elsewhere), indicating that the VHE emission state may be
related to the optical emission state. We have also determined the redshift of
1ES1011+496 based on an optical spectrum that reveals the absorption lines of
the host galaxy. The redshift of z=0.212 makes 1ES1011+496 the most distant
source observed to emit VHE gamma-rays up to date.Comment: 4 pages, 6 figures, minor changes to fit the ApJ versio
Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies
provides unique insights into the extreme physical processes occurring in
relativistic jets and in the vicinity of super-massive black holes. To this
end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in
the high (HE, 100 MeV 200 GeV)
gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE
light curve in the quiescent state is consistent with a log-normal behavior.
The VHE variability in this state is well described by flicker noise
(power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales
larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives
consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10
days) compatible with the VHE findings. The HE and VHE power spectral densities
show a scale invariance across the probed time ranges. A direct linear
correlation between the VHE and HE fluxes could neither be excluded nor firmly
established. These long-term-variability properties are discussed and compared
to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during
VHE-flaring states. The difference in power spectral noise behavior at VHE
energies during quiescent and flaring states provides evidence that these
states are influenced by different physical processes, while the compatibility
of the HE and VHE long-term results is suggestive of a common physical link as
it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
Detailed spectral and morphological analysis of the shell type SNR RCW 86
Aims: We aim for an understanding of the morphological and spectral
properties of the supernova remnant RCW~86 and for insights into the production
mechanism leading to the RCW~86 very high-energy gamma-ray emission. Methods:
We analyzed High Energy Spectroscopic System data that had increased
sensitivity compared to the observations presented in the RCW~86 H.E.S.S.
discovery publication. Studies of the morphological correlation between the
0.5-1~keV X-ray band, the 2-5~keV X-ray band, radio, and gamma-ray emissions
have been performed as well as broadband modeling of the spectral energy
distribution with two different emission models. Results:We present the first
conclusive evidence that the TeV gamma-ray emission region is shell-like based
on our morphological studies. The comparison with 2-5~keV X-ray data reveals a
correlation with the 0.4-50~TeV gamma-ray emission.The spectrum of RCW~86 is
best described by a power law with an exponential cutoff at TeV and a spectral index of ~. A static
leptonic one-zone model adequately describes the measured spectral energy
distribution of RCW~86, with the resultant total kinetic energy of the
electrons above 1 GeV being equivalent to 0.1\% of the initial kinetic
energy of a Type I a supernova explosion. When using a hadronic model, a
magnetic field of ~100G is needed to represent the measured data.
Although this is comparable to formerly published estimates, a standard
E spectrum for the proton distribution cannot describe the gamma-ray
data. Instead, a spectral index of ~1.7 would be required, which
implies that ~erg has been transferred into
high-energy protons with the effective density cm^-3. This
is about 10\% of the kinetic energy of a typical Type Ia supernova under the
assumption of a density of 1~cm^-3.Comment: accepted for publication by A&
Future Exoplanet Research: Science Questions and How to Address Them
Started approximately in the late 1980s, exoplanetology has up to now
unveiled the main gross bulk characteristics of planets and planetary systems.
In the future it will benefit from more and more large telescopes and advanced
space missions. These instruments will dramatically improve their performance
in terms of photometric precision, detection speed, multipixel imaging,
high-resolution spectroscopy, allowing to go much deeper in the knowledge of
planets. Here we outline some science questions which should go beyond these
standard improvements and how to address them. Our prejudice is that one is
never too speculative: experience shows that the speculative predictions
initially not accepted by the community have been confirmed several years later
(like spectrophotometry of transits or circumbinary planets).Comment: Invited review, accepte
A deep spectromorphological study of the -ray emission surrounding the young massive stellar cluster Westerlund 1
Young massive stellar clusters are extreme environments and potentially
provide the means for efficient particle acceleration. Indeed, they are
increasingly considered as being responsible for a significant fraction of
cosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the most
massive known young stellar cluster in our Galaxy is a prime candidate for
studying this hypothesis. While the very-high-energy -ray source HESS
J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its
association could not be firmly identified. We aim to identify the physical
processes responsible for the -ray emission around Westerlund 1 and
thus to better understand the role of massive stellar clusters in the
acceleration of Galactic CRs. Using 164 hours of data recorded with the High
Energy Stereoscopic System (H.E.S.S.), we carried out a deep
spectromorphological study of the -ray emission of HESS J1646-458. We
furthermore employed H I and CO observations of the region to infer the
presence of gas that could serve as target material for interactions of
accelerated CRs. We detected large-scale ( diameter) -ray
emission with a complex morphology, exhibiting a shell-like structure and
showing no significant variation with -ray energy. The combined energy
spectrum of the emission extends to several tens of TeV, and is uniform across
the entire source region. We did not find a clear correlation of the
-ray emission with gas clouds as identified through H I and CO
observations. We conclude that, of the known objects within the region, only
Westerlund 1 can explain the bulk of the -ray emission. Several CR
acceleration sites and mechanisms are conceivable, and discussed in detail.
(abridged)Comment: 15 pages, 9 figures. Corresponding authors: L. Mohrmann, S. Ohm, R.
Rauth, A. Specoviu
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