-
F Aharonian,
A G Akhperjanian,
U Barres,
De Almeida,
A R Bazer-Bachi,
B Behera,
M Beilicke,
W Benbow,
K Bernï Ohr,
C Boisson, [......],
C Venter, J P Vialle,
P Vincent,
M Vivier,
H J Völk,
F Volpe,
S J Wagner,
M Ward,
A A Zdziarski,
A Zech
[show abstract]
[hide abstract]
ABSTRACT: We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes, which is sensitive to photons in the very high energy (VHE) γ -ray range (100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft γ -ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (HESS) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hr of observations were obtained during the night of 2006 June 2–3, and five additional hours were obtained over the next three nights. No VHE γ -ray signal was found during the period covered by the HESS observations. The 99% confidence level flux upper limit (> 1 TeV) for the prompt phase (9 s) of GRB 060602B is 2.9 × 10 −9 erg cm −2 s −1 . Due to the very soft BAT spectrum of the burst compared with other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.
The Astrophysical Journal. 08/2015; 690:1068-1073.
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: The observational coverage with HESS of the Carina region in VHE gamma-rays benefits from deep exposure (40 h) of the neighboring open cluster Westerlund 2. The observations have revealed a new extended region of VHE gamma-ray emission. The new VHE source HESS J1018-589 shows a bright, point-like emission region positionally coincident with SNR G284.3-1.8 and 1FGL J1018.6 - 5856 and
a diffuse extension towards the direction of PSR J1016-5857. A soft Gamma=2.7+-0.5 photon index, with a differential flux at 1TeV of N0=(4.2+-1.1)10^-13 TeV^-1 cm^-2 s^-1 is found for the point-like source, whereas the total emission region including the diffuse emission region is well fit by a power-law function with spectral index Gamma=2.9+-0.4 and differential flux at 1TeV of N0=(6.8+-1.6) 10^-13 TeV^-1 cm^-2 s^-1. This H.E.S.S. detection motivated follow-up X-ray observations with the XMM-Newton satellite to
investigate the origin of the VHE emission. The analysis of the XMM-Newton data resulted in the discovery of a bright, non-thermal point-like source (XMMU J101855.4-58564) with a photon index of Gamma=1.65+-0.08 in the center of SNRG284.3-1.8, and a thermal, extended emission region coincident with its bright northern filament. The characteristics of this thermal emission are used to estimate the plasma density in the region as n~0.5 cm^-3(2.9kpc/d)^2. The position of XMMUJ101855.4-58564 is compatible with the position reported by the Fermi-LAT collaboration for the binary system 1FGL J1018.6-5856 and the variable Swift XRT source identified with it. The new X-ray data are used
alongside archival multi-wavelength data to investigate the relationship between the VHE gamma-ray emission from HESSJ1018-589 and the various potential counterparts in the Carina arm region.
Astronomy and Astrophysics 03/2012; 541:5. · 4.59 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: The Fornax galaxy cluster was observed with the High Energy Stereoscopic System (H.E.S.S.) for a total live time of 14.5 hours, searching for very-high-energy (VHE, E>100 GeV) gamma-rays from dark matter (DM) annihilation. No significant signal was found in searches for point-like and extended emissions. Using several models of the DM density distribution, upper limits on the DM velocity-weighted annihilation cross-section as a function of the DM particle mass are derived. Constraints are derived for different DM particle models, such as those arising from Kaluza-Klein and supersymmetric models. Various annihilation final states are considered. Possible enhancements of the DM annihilation gamma-ray flux, due to DM substructures of the DM host halo, or from the Sommerfeld effect, are studied. Additional gamma-ray contributions from internal bremsstrahlung and inverse Compton radiation are also discussed. For a DM particle mass of 1 TeV, the exclusion limits at 95% of confidence level reach values of ~10^-23cm^3s^-1, depending on the DM particle model and halo properties. Additional contribution from DM substructures can improve the upper limits on <\sigma v> by more than two orders of magnitude. At masses around 4.5 TeV, the enhancement by substructures and the Sommerfeld resonance effect results in a velocity-weighted annihilation cross-section upper limit at the level of <\sigma v> ~ 10^-26cm^3s^-1.
The Astrophysical Journal 02/2012; 750:123. · 6.02 Impact Factor
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini,
J. Becker, [......],
N. P. Lee,
C. Ly,
J. Madrid,
F. Massaro,
C. G. Mundell,
H. Nagai,
E. S. Perlman,
I. A. Steele,
R. C. Walker,
and D. L. Wood
[show abstract]
[hide abstract]
ABSTRACT: The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 – 6) × 109 M ☉) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE γ-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of τrise d = (1.69 ± 0.30) days and τdecay d = (0.611 ± 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Φ>0.35 TeV (1-3) × 10–11 photons cm–2 s–1), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE γ-ray emission reveal an enhanced flux from the core (flux increased by factor ~2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE γ-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE γ-ray emission from M 87 are reviewed in the light of the new data.
The Astrophysical Journal 02/2012; 746(2):151. · 6.02 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July and August 2006, the period when two exceptional flares at very high energies (VHE, E>= 100 GeV) occurred, provide a detailed picture of the evolution of its emission. The complete data set from this campaign is presented, including observations in VHE gamma-rays (H.E.S.S.), X-rays (RXTE, CHANDRA, SWIFT XRT), optical (SWIFT UVOT, Bronberg, Watcher, ROTSE), and in the radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to 2008 are compared to the available VHE data from this period, to put the 2006
campaign into the context of the long-term evolution of the source. The X-ray and VHE gamma-ray emission are correlated during the observed high state of the source, but show no direct connection with longer wavelengths. The long-term flux evolution in the optical and radio bands is found to be correlated and shows that the source reaches a high state at long wavelengths after the occurrence of the VHE flares. Spectral hardening is seen in the SWIFT XRT data. The nightly averaged high-energy spectra of the non-flaring nights can be reproduced by a stationary one-zone SSC model, with only small variations in
the parameters. The spectral and flux evolution in the high-energy band during the night of the second VHE flare is modelled with multi-zone SSC models, which can provide relatively simple interpretations for the hour time-scale evolution of the high-energy emission, even for such a complex data set. For the first
time in this type of source, a clear indication is found for a relation between high activity at high energies and a long-term increase in the low frequency fluxes.
Astronomy and Astrophysics 01/2012; 539:149. · 4.59 Impact Factor
-
The HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin,
L. Costamante,
S. Fegan,
M. Ajello
[show abstract]
[hide abstract]
ABSTRACT: 1ES 0414+009 (z = 0.287) is a distant high-frequency-peaked BL Lac object, and has long been considered a likely emitter of very-high energy (VHE, E>100 GeV) gamma-rays due to its high X-ray and radio flux. Observations in the VHE gamma-ray band and across the electromagnetic spectrum can provide insights into the origin of highly energetic particles present in the source and the radiation processes at work. Because of the distance of the source, the gamma-ray spectrum might provide further limits on the level of the Extragalactic Background Light (EBL). We report observations made between
October 2005 and December 2009 with H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV - 100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation are also reported. To complete the multi-wavelength picture, archival UV and
X-ray observations with the Swift satellite and optical observations with the ATOM telescope are also used. Based on the observations with H.E.S.S., 1ES 0414+009 is detected for the first time in the VHE band. An excess of 224 events is measured, corresponding to a significance of 7.8 sigma. The photon spectrum of the source is well described by a power law, with photon index of 3.45 \pm 0.25stat \pm 0.20syst. The integral flux above 200 GeV is (1.88 \pm 0.20stat \pm 0.38syst) \times10-12 cm-2 s-1. Observations with the Fermi-LAT in the first 20 months of operation show a flux between 200 MeV and 100 GeV of
(2.3 \pm 0.2stat) \times 10-9 erg cm-2 s-1, and a spectrum well described by a power-law function with a photon index 1.85 \pm 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8 - 1) \times 10-11 erg cm-2 s-1, and a steep spectrum (2.2 - 2.3). Combining X-ray with optical-UV data, a fit
with a log-parabolic function locates the synchrotron peak around 0.1 keV.
Astronomy and Astrophysics 01/2012; 538:103. · 4.59 Impact Factor
-
The H. E. S. S. Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini,
J. Becker, [......],
N. P. Lee,
C. Ly,
J. Madrid,
F. Massaro,
C. G. Mundell,
H. Nagai,
E. S. Perlman,
I. A. Steele,
R. C. Walker,
D. L. Wood
[show abstract]
[hide abstract]
ABSTRACT: Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and
very massive black hole provides a unique opportunity to investigate the origin
of very high energy (VHE; E>100 GeV) gamma-ray emission generated in
relativistic outflows and the surroundings of super-massive black holes. M 87
has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray
emission displays strong variability on timescales as short as a day. In this
paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and
VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE
was detected triggering further observations at VHE (H.E.S.S.), X-rays
(Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray
light curve enables one to derive a precise temporal characterization of the
flare: the single, isolated flare is well described by a two-sided exponential
function with significantly different flux rise and decay times. While the
overall variability pattern of the 2010 flare appears somewhat different from
that of previous VHE flares in 2005 and 2008, they share very similar
timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2
s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions
indicate no enhanced flux in 2010 in contrast to observations in 2008, where an
increase of the radio flux of the innermost core regions coincided with a VHE
flare. On the other hand, Chandra X-ray observations taken ~3 days after the
peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The
long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio
to VHE and including data from HST, LT, VLA and EVN, is used to further
investigate the origin of the VHE gamma-ray emission. No unique, common MWL
signature of the three VHE flares has been identified.
The Astrophysical Journal 11/2011; 746:151. · 6.02 Impact Factor
-
The H.E.S.S. Collaboration,
the Fermi LAT Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini, [......],
G. Vianello,
N. Vilchez,
V Vitale,
A P Waite,
P Wang,
B. L. Winer,
K. S. Wood,
Z Yang,
T. Ylinen,
M Ziegler
[show abstract]
[hide abstract]
ABSTRACT: The high-frequency peaked BL Lac object PKS 2005-489 was the target of a
multi-wavelength campaign with simultaneous observations in the TeV gamma-ray
(H.E.S.S.), GeV gamma-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and
optical (ATOM, Swift) bands. This campaign was carried out during a high flux
state in the synchrotron regime. The flux in the optical and X-ray bands
reached the level of the historical maxima. The hard GeV spectrum observed with
Fermi/LAT connects well to the very high energy (VHE, E>100GeV) spectrum
measured with H.E.S.S. with a peak energy between ~5 and 500 GeV. Compared to
observations with contemporaneous coverage in the VHE and X-ray bands in 2004,
the X-ray flux was ~50 times higher during the 2009 campaign while the TeV
gamma-ray flux shows marginal variation over the years. The spectral energy
distribution during this multi-wavelength campaign was fit by a one zone
synchrotron self-Compton model with a well determined cutoff in X-rays. The
parameters of a one zone SSC model are inconsistent with variability time
scales. The variability behaviour over years with the large changes in
synchrotron emission and small changes in the inverse Compton emission does not
warrant an interpretation within a one-zone SSC model despite an apparently
satisfying fit to the broadband data in 2009.
Astronomy and Astrophysics 11/2011; 533:110. · 4.59 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Results obtained in very-high-energy (VHE; E > 100 GeV) \gamma-ray
observations performed with the H.E.S.S. telescope array are used to
investigate particle acceleration processes in the vicinity of the young
massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from
\gamma-ray induced particle cascades in the Earth's atmosphere is used to
search for VHE \gamma\ rays from the region around Wd 1. Possible catalogued
counterparts are searched for and discussed in terms of morphology and
energetics of the H.E.S.S. source. The detection of the degree-scale extended
VHE \gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S.
observations performed between 2004 and 2008. The VHE \gamma-ray source is
centred on the nominal position of Wd 1 and detected with a total statistical
significance of ~20\sigma. The emission region clearly extends beyond the
H.E.S.S. point-spread function (PSF). The differential energy spectrum follows
a power law in energy with an index of \Gamma=2.19 \pm 0.08_{stat} \pm
0.20_{sys} and a flux normalisation at 1 TeV of \Phi_0 = (9.0 \pm 1.4_{stat}
\pm 1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2
TeV amounts to (5.2 \pm 0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident
with HESS J1646-458 are discussed in the search of a counterpart, namely the
magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR
J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario,
Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent
population would be accelerated within the stellar cluster. Beside this, there
is evidence for a multi-source origin, where a scenario involving PSR
J1648-4611 could be viable to explain parts of the VHE \gamma-ray emission of
HESS J1646-458.
11/2011;
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M Ward,
R White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Several newly discovered very-high-energy (VHE; E > 100 GeV) gamma-ray
sources in the Galaxy are thought to be associated with energetic pulsars.
Among them, middle-aged (> 1E+4 yr) systems exhibit large centre-filled VHE
nebulae, offset from the pulsar position, which result from the complex
relationship between the pulsar wind and the surrounding medium, and reflect
the past evolution of the pulsar. Imaging Atmospheric Cherenkov Telescopes
(IACTs) have been successful in revealing extended emission from these sources
in the VHE regime. Together with radio and X-ray observations, this
observational window allows one to probe the energetics and magnetic field
inside these large-scale nebulae. H.E.S.S., with its large field of view,
angular resolution of < 0.1deg and unprecedented sensitivity, has been used to
discover a large population of such VHE sources. In this paper, the H.E.S.S.
data from the continuation of the Galactic Plane Survey (-80deg < l < 60deg,
|b| < 3deg), together with the existing multi-wavelength observations, are
used. A new VHE gamma-ray source was discovered at R.A. (J2000) = 13h56m00s,
Dec. (J2000) = -64d30m00s with a 2' statistical error in each coordinate,
namely HESS J1356-645. The source is extended, with an intrinsic Gaussian width
of (0.20 +/- 0.02)deg. Its integrated energy flux between 1 and 10 TeV of 8E-12
erg cm-2 s-1 represents ~ 11% of the Crab Nebula flux in the same energy band.
The energy spectrum between 1 and 20 TeV is well described by a power law dN/dE
~ E-Gamma with photon index Gamma = 2.2 +/- 0.2stat +/- 0.2sys. The inspection
of archival radio images at three frequencies and the analysis of X-ray data
from ROSAT/PSPC and XMM-Newton/MOS reveal the presence of faint non-thermal
diffuse emission coincident with HESS J1356-645. HESS J1356-645 is most likely
associated with the young and energetic pulsar PSR J1357-6429 (Abridged)
08/2011;
-
S. Orsi,
D. Haas,
W. Hajdas,
V. Honkimäki,
G. Lamanna,
C. Lechanoine-Leluc,
R. Marcinkowski,
M. Pohl,
N. Produit,
D. Rapin,
E. Suarez-Garcia,
D. Rybka, J.-P. Vialle
[show abstract]
[hide abstract]
ABSTRACT: POLAR is a novel compact space-borne Compton polarimeter conceived for a
precise measurement of hard X-ray polarization and optimized for the
detection of Gamma-Ray Burst (GRB) photons in the energy range
50-500 keV. In December 2009 we have performed a systematic
calibration of one modular unit of POLAR at four energy levels (200,
288, 356 and 511 keV) with a 100% polarized synchrotron radiation source
at the beam line ID15A at ESRF. The detector was displaced several times
on the beam line in order to achieve a uniform illumination, which
mimics the flux from a GRB placed on the zenith of the experiment.
Several rotations of the detector on the beam axis allowed us to test
the response of POLAR to several polarization angles. Two different
analysis methods to reconstruct the polarization angle of the beam and
the modulation factor μ100 are presented; the first relies
on the existence of a unpolarized sample, produced by merging two data
sets with orthogonal polarization directions, and is less dependent on
systematic effects due to asymmetries in the detector; the second is
independent from unpolarized measurements, and will likely be used to
analyze the polarization of GRB during the flight. Both methods
reconstruct the input polarization angle within 2° and produce
modulation factors μ100 between 30% and 50% depending on
the beam energy. Monte Carlo simulations performed with GEANT4 confirm
the experimental results.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 07/2011; 648:139-154. · 1.21 Impact Factor
-
M. Aguilar,
J. Alcaraz,
J. Allaby,
B. Alpat,
G. Ambrosi,
H. Anderhub,
L. Ao,
A. Arefiev,
L. Arruda,
P. Azzarello, [......],
Z. P. Zhang,
D. X. Zhao,
F. Zhou,
Y. Zhou,
G. Y. Zhu,
W. Z. Zhu,
H. L. Zhuang,
A. Zichichi,
B. Zimmermann,
and P. Zuccon
[show abstract]
[hide abstract]
ABSTRACT: The variety of isotopes in cosmic rays allows us to study different aspects of the processes that cosmic rays undergo between the time they are produced and the time of their arrival in the heliosphere. In this paper, we present measurements of the isotopic ratios 2H/4He, 3He/4He, 6Li/7Li, 7Be/(9Be+10Be), and 10B/11B in the range 0.2-1.4 GeV of kinetic energy per nucleon. The measurements are based on the data collected by the Alpha Magnetic Spectrometer, AMS-01, during the STS-91 flight in 1998 June.
The Astrophysical Journal 07/2011; 736(2):105. · 6.02 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M Ward,
R White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: The H.E.S.S. very-high-energy (VHE, E > 0.1 TeV) gamma-ray telescope system
has discovered a new source, HESS J1747-248. The measured integral flux is (1.2
+/- 0.3) \times 10^-12 cm-2 s-1 above 440 GeV for a power-law photon spectral
index of 2.5 +/- 0.3 stat +/- 0.2 sys. The VHE gamma-ray source is located in
the close vicinity of the Galactic globular cluster Terzan 5 and extends beyond
the H.E.S.S. point spread function (0.07 degree). The probability of a chance
coincidence with Terzan 5 and an unrelated VHE source is quite low (~ 10^-4).
With the largest population of identified millisecond pulsars (msPSRs), a very
high core stellar density and the brightest GeV range flux as measured by
Fermi-LAT, Terzan 5 stands out among Galactic globular clusters. The properties
of the VHE source are briefly discussed in the context of potential emission
mechanisms, notably in relation to msPSRs. Interpretation of the available data
accommodates several possible origins for this VHE gamma-ray source, although
none of them offers a satisfying explanation of its peculiar morphology.
06/2011;
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini, [......],
S. Vorobiov,
M. Vorster,
S. J. Wagner,
M. Ward,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H.-S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Observations of the globular clusters (GCs) NGC 6388 and M15 were carried out by the High Energy Stereoscopic System array of Cherenkov telescopes for a live time of 27.2 and 15.2 hr, respectively. No gamma-ray signal is found at the nominal target position of NGC 6388 and M15. In the primordial formation scenario, GCs are formed in a dark matter (DM) halo and DM could still be present in the baryon-dominated environment of GCs. This opens the possibility of observing a DM self-annihilation signal. The DM content of the GCs NGC 6388 and M15 is modeled taking into account the astrophysical processes that can be expected to influence the DM distribution during the evolution of the GC: the adiabatic contraction of DM by baryons, the adiabatic growth of a black hole in the DM halo, and the kinetic heating of DM by stars. Ninety-five percent confidence level exclusion limits on the DM particle velocity-weighted annihilation cross section are derived for these DM halos. In the TeV range, the limits on the velocity-weighted annihilation cross section are derived at the 10–25 cm3 s–1 level and a few 10–24 cm3 s–1 for NGC 6388 and M15, respectively.
The Astrophysical Journal 06/2011; 735(1):12. · 6.02 Impact Factor
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HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi, [......],
S. Vorobiov,
M. Vorster,
S. J. Wagner,
M Ward,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: The recent discovery of the radio shell-type supernova remnant (SNR),
G353.6-0.7, in spatial coincidence with the unidentified TeV source HESS
J1731-347 has motivated further observations of the source with the High Energy
Stereoscopic System (H.E.S.S.) Cherenkov telescope array to test a possible
association of the gamma-ray emission with the SNR. With a total of 59 hours of
observation, representing about four times the initial exposure available in
the discovery paper of HESS J1731-347, the gamma-ray morphology is investigated
and compared with the radio morphology. An estimate of the distance is derived
by comparing the interstellar absorption derived from X-rays and the one
obtained from 12CO and HI observations. The deeper gamma-ray observation of the
source has revealed a large shell-type structure with similar position and
extension (r~0.25{\deg}) as the radio SNR, thus confirming their association.
By accounting for the H.E.S.S. angular resolution and projection effects within
a simple shell model, the radial profile is compatible with a thin, spatially
unresolved, rim. Together with RX J1713.7-3946, RX J0852.0-4622 and SN 1006,
HESS J1731-347 is now the fourth SNR with a significant shell morphology at TeV
energies. The derived lower limit on the distance of the SNR of 3.2 kpc is used
together with radio and X-ray data to discuss the possible origin of the
gamma-ray emission, either via inverse Compton scattering of electrons or the
decay of neutral pions resulting from proton-proton interaction.
05/2011;
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A Abramowski,
F Acero,
F Aharonian,
A G Akhperjanian,
G Anton,
A Barnacka,
U Barres de Almeida,
A R Bazer-Bachi,
Y Becherini,
J Becker, [......],
F Volpe,
S Vorobiov,
M Vorster,
S J Wagner,
M Ward,
A Wierzcholska,
A Zajczyk,
A A Zdziarski,
A Zech,
H-S Zechlin
[show abstract]
[hide abstract]
ABSTRACT: A search for a very-high-energy (VHE; ≥100 GeV) γ-ray signal from self-annihilating particle dark matter (DM) is performed towards a region of projected distance r∼45-150 pc from the Galactic center. The background-subtracted γ-ray spectrum measured with the High Energy Stereoscopic System (H.E.S.S.) γ-ray instrument in the energy range between 300 GeV and 30 TeV shows no hint of a residual γ-ray flux. Assuming conventional Navarro-Frenk-White and Einasto density profiles, limits are derived on the velocity-weighted annihilation cross section (σv) as a function of the DM particle mass. These are among the best reported so far for this energy range and in particular differ only little between the chosen density profile parametrizations. In particular, for the DM particle mass of ∼1 TeV, values for (σv) above 3×10(-25) cm(3) s(-1) are excluded for the Einasto density profile.
Physical Review Letters 04/2011; 106(16):161301. · 7.37 Impact Factor
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HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi, [......],
S. Vorobiov,
M. Vorster,
S. J. Wagner,
M Ward,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Observations of the globular clusters NGC 6388 and M 15 were carried out by
the H.E.S.S. array of Cherenkov telescopes for a live time of 27.2 and 15.2
hours respectively. No gamma-ray signal is found at the nominal target position
of NGC 6388 and M 15. In the primordial formation scenario, globular clusters
are formed in a dark matter halo and dark matter could still be present in the
baryon-dominated environment of globular clusters. This opens the possibility
of observing a dark matter self-annihilation signal. The dark matter content of
the globular clusters NGC 6388 and M 15 is modelled taking into account the
astrophysical processes that can be expected to influence the dark matter
distribution during the evolution of the globular cluster: the adiabatic
contraction of dark matter by baryons, the adiabatic growth of a black hole in
the dark matter halo and the kinetic heating of dark matter by stars. 95%
confidence level exclusion limits on the dark matter particle velocity-weighted
annihilation cross section are derived for these dark matter haloes. In the TeV
range, the limits on the velocity-weighted annihilation cross section are
derived at the 10-25 cm3 s-1 level and a few 10-24 cm3 s-1 for NGC 6388 and M
15 respectively.
04/2011;
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H. E. S. S. Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini,
J Becker, [......],
F. Volpe,
S. Vorobiov,
M. Vorster,
S. J. Wagner,
M Ward,
A. Wierzcholska,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: A search for a very-high-energy (VHE; >= 100 GeV) gamma-ray signal from
self-annihilating particle Dark Matter (DM) is performed towards a region of
projected distance r ~ 45-150 pc from the Galactic Center. The
background-subtracted gamma-ray spectrum measured with the High Energy
Stereoscopic System (H.E.S.S.) gamma-ray instrument in the energy range between
300 GeV and 30 TeV shows no hint of a residual gamma-ray flux. Assuming
conventional Navarro-Frenk-White (NFW) and Einasto density profiles, limits are
derived on the velocity-weighted annihilation cross section < \sigma v> as a
function of the DM particle mass. These are among the best reported so far for
this energy range. In particular, for the DM particle mass of ~1 TeV, values
for <\sigma v> above 3 * 10^(-25) cm^3 s^(-1) are excluded for the Einasto
density profile. The limits derived here differ much less for the chosen
density profile parametrizations, as opposed to limits from gamma-ray
observations of dwarf galaxies or the very center of the Milky Way, where the
discrepancy is significantly larger.
03/2011;
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H. E. S. S. Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini, [......],
M. Vorster,
S. J. Wagner,
M Ward,
A. Wierzcholska,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin,
T H Burnett,
A. B. Hill
[show abstract]
[hide abstract]
ABSTRACT: We report on a newly detected point-like source, HESS J1943+213 located in
the Galactic plane. This source coincides with an unidentified hard X-ray
source IGR J19443+2117, which was proposed to have radio and infrared
counterparts. HESS J1943+213 is detected at the significance level of 7.9
\sigma (post-trials) at RA(J2000)=19h 43m 55s +- 1s (stat) +- 1s (sys),
DEC(J2000) = +21deg 18' 8" +- 17" (stat) +- 20" (sys). The source has a soft
spectrum with photon index Gamma = 3.1 +- 0.3 (stat) +- 0.2 (sys) and a flux
above 470 GeV of 1.3 +- 0.2 (stat) +- 0.3 (sys) x 10^{-12} cm^{-2} s^{-1}.
There is no Fermi/LAT counterpart down to a flux limit of 6 x 10^{-9} cm^{-2}
s^{-1} in the 0.1-100 GeV energy range (95% confidence upper limit calculated
for an assumed power-law model with a photon index Gamma=2.0). The data from
radio to VHE gamma-rays do not show any significant variability. We combine new
H.E.S.S., Fermi/LAT and Nancay Radio Telescope observations with pre-existing
non-simultaneous multi-wavelength observations of IGR J19443+2117 and discuss
the likely source associations as well as the interpretation as an active
galactic nucleus, a gamma-ray binary or a pulsar wind nebula. The lack of a
massive stellar counterpart disfavors the binary hypothesis, while the soft VHE
spectrum would be very unusual in case of a pulsar wind nebula. In addition,
the distance estimates for Galactic counterparts places them outside of the
Milky Way. All available observations favor an interpretation as an extreme,
high-frequency peaked BL Lac object with a redshift z>0.14. This would be the
first time a blazar is detected serendipitously from ground-based VHE
observations, and the first VHE AGN detected in the Galactic Plane.
03/2011;
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H. E. S. S. Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Barnacka,
U. Barres de Almeida,
A. R. Bazer-Bachi,
Y. Becherini,
J Becker, [......],
S. Vorobiov,
S. J. Wagner,
M Ward,
A. Wierzcholska,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin,
G. Dubner,
E. Giacani
[show abstract]
[hide abstract]
ABSTRACT: The gamma-ray pulsar PSR B1706-44 and the adjacent supernova remnant (SNR)
candidate G343.1-2.3 were observed by H.E.S.S. during a dedicated observation
campaign in 2007. As a result of this observation campaign, a new source of
very-high-energy (VHE; E > 100 GeV) gamma-ray emission, HESS J1708-443, was
detected with a statistical significance of 7 sigma, although no significant
point-like emission was detected at the position of the energetic pulsar
itself. In this paper, the morphological and spectral analyses of the
newly-discovered TeV source are presented. The centroid of HESS J1708-443 is
considerably offset from the pulsar and located near the apparent center of the
SNR, at RA(J2000) = 17h08m11s +/- 17s and Dec(J2000) = -44d20' +/- 4'. The
source is found to be significantly more extended than the H.E.S.S. point
spread function (~0.1 deg), with an intrinsic Gaussian width of 0.29 deg +/-
0.04 deg. Its integral flux between 1 and 10 TeV is ~ 3.8 x 10^-12 ph cm^-2
s^-1, equivalent to 17% of the Crab Nebula flux in the same energy range. The
measured energy spectrum is well-fit by a power law with a relatively hard
photon index Gamma = 2.0 +/- 0.1 (stat) +/- 0.2 (sys). Additional
multi-wavelength data, including 330 MHz VLA observations, were used to
investigate the VHE gamma-ray source's possible associations with the pulsar
wind nebula of PSR B1706-44 and/or with the complex radio structure of the
partial shell-type SNR G343.1-2.3.
02/2011;
