52 research outputs found
On the dependence of the spectral parameters on the observational conditions in homogeneous time dependent models of the TeV blazars
Most of current models of TeV blazars emission assume a Synchrotron
Self-Compton mechanism where relativistic particles emit both synchrotron
radiation and Inverse Compton photons. For sake of simplicity, these models
usually consider only steady state emission. The spectral features are thus
only related to the shape of the particle distribution, and do not depend on
the timing of observations. In this letter, we study the effect of, firstly,
the lag between the beginning of the injection of the fresh particles and the
trigger of the observation, and secondly, of a finite injection duration. We
illustrate these effects considering an analytical time-dependent model of the
synchrotron emission by a monoenergetic distribution of leptons. We point out
that the spectral shape can be in fact very dependent on observational
conditions if the particle injection term is time-dependent, particularly
taking into account the effect of the time averaging procedure on the final
shape of the SED. Consequences on the acceleration process are also discussed.Comment: Letter to Editor, accepted for publication in A&
A unified accretion-ejection paradigm for Black Hole X-ray Binaries
We present a new picture for the central regions of Black Hole X-ray
Binaries. In our view, these central regions have a multi-flow configuration
which consists in (1) an outer standard accretion disc down to a transition
radius r_J, (2) an inner magnetized accretion disc below r_J driving (3) a non
relativistic self-collimated electron-proton jet surrounding, when adequate
conditions for pair creation are met, (4) a ultra relativistic
electron-positron beam. This accretion-ejection paradigm provides a simple
explanation to the canonical spectral states by varying the transition radius
r_J and disc accretion rate independently. Large values of r_J correspond to
the Quiescent state for low and the Hard state for larger .
These states are characterized by the presence of a steady electron-proton MHD
jet emitted by the disc below r_J. The hard X-ray component is expected to form
at the jet basis. When r_j becomes smaller than the marginally stable orbit
r_i, the whole disc resembles a standard accretion disc, characteristic of the
Soft state. Intermediate states correspond to situations where r_J ~ r_i. At
large , an unsteady pair cascade process is triggered within the jet
axis, giving birth to ejection of relativistic pair blobs. This would
correspond to the luminous intermediate state, with its associated superluminal
motions. The variation of r_J independently of is a necessary
ingredient in this picture, arising from the presence of a large scale vertical
magnetic field threading the disc.Comment: 15 pages, 2 figures. Accepted by A&A main journa
Absorption of 10 Gev--1 Tev Gamma Rays from 3C 279
In this paper, we revisit gamma-ray--emitting region for 10 GeV--1 TeV gamma
rays from 3C 279 through studying the photon-photon absorption optical depth
due to the diffuse radiation of the broad-line region (BLR) and the
extragalactic background light (EBL). Based on the power-law spectrum detected
by MAGIC, the preabsorbed spectra are inferred by correcting the photon-photon
absorption on the diffuse photons of the BLR (internal absorption) and the EBL
(external absorption). Position of gamma-ray emitting region
determines the relative contributions of this two diffuse radiation to the
total absorption. Our results indicate that may be within the
BLR shell for 3C 279, likely closer to the inner radius, which is consistent
with our previous results. This is neither consistent with the suggestions of
B\"ottcher et al. (2008b), that VHE gamma-ray emission is produced far outside
the BLR, nor with the assumptions of Tavecchio & Mazin (2008), that VHE
gamma-ray--emitting region is inside the BLR cavity. is a key
physical quantity that could set some constraints on emission mechanisms that
produce the VHE gamma rays from 3C 279. Observations of -LAT, MAGIC,
HESS, and VERITAS in the near future could give more constraints on the
position of gamma-ray emitting region relative to the BLR.Comment: 4 Figures, accepted in Ap
First detection of a VHE gamma-ray spectral maximum from a Cosmic source: H.E.S.S. discovery of the Vela X nebula
The Vela supernova remnant (SNR) is a complex region containing a number of
sources of non-thermal radiation. The inner section of this SNR, within 2
degrees of the pulsar PSR B0833-45, has been observed by the H.E.S.S. gamma-ray
atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from
an extended region to the south of the pulsar, within an integration region of
radius 0.8 deg. around the position (RA = 08h 35m 00s, dec = -45 deg. 36'
J2000.0). The excess coincides with a region of hard X-ray emission seen by the
ROSAT and ASCA satellites. The observed energy spectrum of the source between
550 GeV and 65 TeV is well fit by a power law function with photon index = 1.45
+/- 0.09(stat) +/- 0.2(sys) and an exponential cutoff at an energy of 13.8 +/-
2.3(stat) +/- 4.1(sys) TeV. The integral flux above 1 TeV is (1.28 +/- 0.17
(stat) +/- 0.38(sys)) x 10^{-11} cm^{-2} s^{-1}. This result is the first clear
measurement of a peak in the spectral energy distribution from a VHE gamma-ray
source, likely related to inverse Compton emission. A fit of an Inverse Compton
model to the H.E.S.S. spectral energy distribution gives a total energy in
non-thermal electrons of ~2 x 10^{45} erg between 5 TeV and 100 TeV, assuming a
distance of 290 parsec to the pulsar. The best fit electron power law index is
2.0, with a spectral break at 67 TeV.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics letter
Observations of Mkn 421 in 2004 with H.E.S.S. at large zenith angles
Mkn 421 was observed during a high flux state for nine nights in April and
May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.)
in Namibia. The observations were carried out at zenith angles of
60--65, which result in an average energy threshold of 1.5 TeV
and a collection area reaching 2~km at 10~TeV. Roughly 7000 photons from
Mkn~421 were accumulated with an average gamma-ray rate of 8 photons/min. The
overall significance of the detection exceeds 100 standard deviations. The
light-curve of integrated fluxes above 2~TeV shows changes of the diurnal flux
up to a factor of 4.3. For nights of high flux, intra-night variability is
detected with a decay time of less than 1 hour. The time averaged energy
spectrum is curved and is well described by a power-law with a photon index
\egamm and an exponential cutoff at \ecut~TeV and an average integral flux
above 2~TeV of 3 Crab flux units. Significant variations of the spectral shape
are detected with a spectral hardening as the flux increases. Contemporaneous
multi-wavelength observations at lower energies (X-rays and gamma-rays above
~GeV) indicate smaller relative variability amplitudes than seen
above 2~TeV during high flux state observed in April 2004.Comment: 5 pages, 4 figures, published in A&
Very high energy gamma rays from the direction of Sagittarius A*.
We report the detection of a point-like source of very high energy (VHE) -rays coincident within 1' of Sgr A *, obtained with the HESS array of Cherenkov telescopes. The -rays exhibit a power-law energy spectrum with a spectral index of and a flux above the 165 GeV threshold of m -2 s -1. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration
A possible association of the new VHE gamma-ray source HESS J1825--137 with the pulsar wind nebula G18.0--0.7
We report on a possible association of the recently discovered very
high-energy -ray source HESS J1825--137 with the pulsar wind nebula
(commonly referred to as G 18.0--0.7) of the year old
Vela-like pulsar PSR B1823--13. HESS J1825--137 was detected with a
significance of 8.1 in the Galactic Plane survey conducted with the
H.E.S.S. instrument in 2004. The centroid position of HESS J1825--137 is offset
by 11\arcmin south of the pulsar position. \emph{XMM-Newton} observations have
revealed X-ray synchrotron emission of an asymmetric pulsar wind nebula
extending to the south of the pulsar. We argue that the observed morphology and
TeV spectral index suggest that HESS J1825--137 and G 18.0--0.7 may be
associated: the lifetime of TeV emitting electrons is expected to be longer
compared to the {\it XMM-Newton} X-ray emitting electrons, resulting in
electrons from earlier epochs (when the spin-down power was larger)
contributing to the present TeV flux. These electrons are expected to be
synchrotron cooled, which explains the observed photon index of , and
the longer lifetime of TeV emitting electrons naturally explains why the TeV
nebula is larger than the X-ray size. Finally, supernova remnant expansion into
an inhomogeneous medium is expected to create reverse shocks interacting at
different times with the pulsar wind nebula, resulting in the offset X-ray and
TeV -ray morphology.Comment: 5 pages, 3 figures, to appear in Astronomy and Astrophysics Letter
3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039
New observations of LS 5039, a High Mass X-ray Binary comprising a massive
star and compact object, were carried out with the High Energy Stereoscopic
System of Cherenkov Telescopes (H.E.S.S.) in 2005 at very high energy (VHE)
gamma-ray energies. These observations reveal that its flux and energy spectrum
are modulated with the 3.9 day orbital period of the binary system. This is the
first time in gamma-ray astronomy that orbital modulation has been observed,
and periodicity clearly established using ground-based gamma-ray detectors. The
VHE gamma-ray emission is largely confined to half of the orbit, peaking around
the inferior conjunction epoch of the compact object. For this epoch, there is
also a hardening of the energy spectrum in the energy range between 0.2 TeV and
a few TeV. The flux vs. orbital phase profile provides the first clear
indication of gamma-ray absorption via pair production within an astrophysical
source, a process which is expected to occur if the gamma-ray production site
is situated within ~1 AU of the compact object. Moreover the production region
size must be not significantly greater than the binary separation (~0.15 AU).
Notably, these constraints are also considerably smaller than the collimated
outflows or jets (extending out to ~1000 AU) observed in LS 5039. The spectral
hardening could arise from variations with phase in the maximum electron
energies, and/or the dominant VHE gamma-ray production mechanism.Comment: 8 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Discovery of extended VHE gamma-ray emission from the asymmetric pulsar wind nebula in MSH 15-52 with H.E.S.S
The Supernova Remnant MSH 15-52 has been observed in very high energy (VHE)
gamma-rays using the H.E.S.S. 4-telescope array located in Namibia. A gamma-ray
signal is detected at the 25 sigma level during an exposure of 22.1 hours live
time. The image reveals an elliptically shaped emission region around the
pulsar PSR B1509-58, with semi-major axis 6' in the NW-SE direction and
semi-minor axis 2' approximately. This morphology coincides with the diffuse
pulsar wind nebula as observed at X-ray energies by ROSAT. The overall energy
spectrum from 280 GeV up to 40 TeV can be fitted by a power law with photon
index Gamma = 2.27 +/- 0.03(stat.) +/- 0.20(syst.). The detected emission can
be plausibly explained by inverse Compton scattering of accelerated
relativistic electrons with soft photons.Comment: 5 pages, 3 figures, accepted by A&A letter
Detection of TeV gamma-ray Emission from the Shell-Type Supernova Remnant RX J0852.0-4622 with H.E.S.S
We report the detection of TeV gamma-rays from the shell-type supernova
remnant RX J0852.0-4622 with data of 3.2 h of live time recorded with H.E.S.S.
in February 2004. An excess of (700 +/- 60) events from the whole remnant with
a significance of 12 sigma was found. The observed emission region is clearly
extended with a radius of the order of 1 degree and the spatial distribution of
the signal correlates with X-ray observations. The spectrum in the energy range
between 500 GeV and 15 TeV is well described by a power law with a photon index
of 2.1 +/- 0.1(stat) +/- 0.2(syst) and a differential flux at 1 TeV of (2.1 +/-
0.2(stat) +/- 0.6(syst)) 10^{-11} cm^{-2} s^{-1} TeV^{-1}. The integral flux
above 1 TeV was measured to be (1.9 +/- 0.3(stat) +/- 0.6(syst)) 10^{-11}
cm^{-2} s^{-1}, which is at the level of the flux of the Crab nebula at these
energies. More data are needed to draw firm conclusions on the magnetic field
in the remnant and the type of the particle population creating the TeV
gamma-rays.Comment: 5 pages, 3 figures, accepted for publication in A&A letter
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