-
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.
-
H. E. S. S. Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
S. Balenderan,
A. Balzer,
A. Barnacka,
Y. Becherini, [......],
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
D. Wouters,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: The quasar PKS 1510-089 (z=0.361) was observed with the H.E.S.S. array of
imaging atmospheric Cherenkov telescopes during high states in the optical and
GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV)
emission. VHE \gamma-rays were detected with a statistical significance of 9.2
standard deviations in 15.8 hours of H.E.S.S. data taken during March and April
2009. A VHE integral flux of I(0.15 TeV < E < 1.0 TeV) = (1.0 +- 0.2 (stat) +-
0.2 (sys) x 10^{-11} cm^{-2}s^{-1} is measured. The best-fit power law to the
VHE data has a photon index of \Gamma=5.4 +- 0.7 (stat) +- 0.3 (sys). The GeV
and optical light curves show pronounced variability during the period of
H.E.S.S. observations. However, there is insufficient evidence to claim
statistically significant variability in the VHE data. Because of its
relatively high redshift, the VHE flux from PKS 1510-089 should suffer
considerable attenuation in the intergalactic space due to the extragalactic
background light (EBL). Hence, the measured \gamma-ray spectrum is used to
derive upper limits on the opacity due to EBL, which are found to be comparable
with the previously derived limits from relatively-nearby BL Lac objects.
Unlike typical VHE-detected blazars where the broadband spectrum is dominated
by non-thermal radiation at all wavelengths, the quasar PKS 1510-089 has a
bright thermal component in the optical to UV frequency band. Among all VHE
detected blazars, PKS 1510-089 has the most luminous broad line region (BLR).
The detection of VHE emission from this quasar indicates a low level of
\gamma-\gamma absorption on the internal optical to UV photon field.
Astronomy and Astrophysics 04/2013; · 4.59 Impact Factor
-
H. Sol,
A. Zech, C. Boisson,
U. Barres de Almeida,
J. Biteau,
J. -L. Contreras,
B. Giebels,
T. Hassan,
Y. Inoue,
K. Katarzynski, [......],
L. Fallon,
E. de Gouveia Dal Pino,
J. -A. Hinton,
S. Inoue,
J. -P. Lenain,
A. Neronov,
K. Takahashi,
H. Takami,
R. White,
on behalf of the CTA collaboration
[show abstract]
[hide abstract]
ABSTRACT: 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.
04/2013;
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
S. Balenderan,
A. Balzer,
A. Barnacka,
Y. Becherini,
J. Becker Tjus, [......],
M. deNaurois,
D. Nedbal,
N. Nguyen,
J. Niemiec,
S. J. Nolan,
S. Ohm,
E. deOñ aWilhelmi,
B. Opitz,
M. Ostrowski,
I. Oya
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
S. Balenderan,
A. Balzer,
A. Barnacka,
Y. Becherini,
J. Becker Tjus, [......],
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
D. Wouters,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin
[show abstract]
[hide abstract]
ABSTRACT: Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae (PWNe), and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) $\gamma$-ray emission (HESS J0835-455}) was discovered using the H.E.S.S. experiment in 2004. The VHE $\gamma$-ray emission was found to be coincident with a region of X-ray emission discovered with ${\it ROSAT}$ above 1.5 keV (the so-called \textit{Vela X cocoon}): a filamentary structure extending southwest from the pulsar to the centre of Vela X. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. In order to increase the sensitivity to the faint $\gamma$-ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of $\gamma$-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of $\gamma$-ray sources but with comparable observation conditions. The $\gamma$-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE $\gamma$-ray emission from the PWN HESS J0835-455 is statistically significant over a region of radius 1.2$^{\circ}$ around the position $\alpha$ = 08h 35m 00s,
$\delta$ = -45$^{\circ}$ 36' 00'' (J2000).
Astronomy and Astrophysics 12/2012; 548:38. · 4.59 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
S. Balenderan,
A. Balzer,
A. Barnacka,
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: We present the significant detection of the first extragalactic pulsar wind
nebula (PWN) detected in gamma rays, N157B, located in the large Magellanic
Cloud (LMC). Pulsars with high spin-down luminosity are found to power
energised nebulae that emit gamma rays up to energies of several tens of TeV.
N157B is associated with PSRJ0537-6910, which is the pulsar with the highest
known spin-down luminosity. The High Energy Stereoscopic System telescope array
observed this nebula on a yearly basis from 2004 to 2009 with a dead-time
corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV
is well-described by a pure power-law with a photon index of 2.8 \pm 0.2(stat)
\pm 0.3(syst) and a normalisation at 1 TeV of (8.2 \pm 0.8(stat) \pm 2.5(syst))
\times 10^-13 cm^-2s^-1TeV^-1. A leptonic multi-wavelength model shows that an
energy of about 4 \times 10^49erg is stored in electrons and positrons. The
apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the
pulsar's spindown luminosity, 0.08% \pm 0.01%, is comparable to those of PWNe
found in the Milky Way. The detection of a PWN at such a large distance is
possible due to the pulsar's favourable spin-down luminosity and a bright
infrared photon-field serving as an inverse-Compton-scattering target for
accelerated leptons. By applying a calorimetric technique to these
observations, the pulsar's birth period is estimated to be shorter than 10 ms.
Astronomy and Astrophysics 09/2012; 545:2. · 4.59 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
S. Balenderan,
A. Balzer,
A. Barnacka,
Y. Becherini, [......],
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin,
M. O. Ali
[show abstract]
[hide abstract]
ABSTRACT: In some galaxy clusters powerful AGN have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, but cosmic rays are a viable possibility. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from
these systems. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Data obtained in 20.2 hours of
dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, that involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 muG in the inner lobes.
Astronomy and Astrophysics 08/2012; 545:103. · 4.59 Impact Factor
-
H. E. S. S. Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
Y. Becherini,
J. Becker, [......],
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: Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100 GeV) data from \gamma-ray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE
\gamma-ray data can be described by a power law in energy with differential photon index \Gamma=2.14 \pm 0.18_stat \pm 0.30_sys and differential flux normalisation at 1 TeV of F_0 = (9.6 \pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14} TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE \gamma-ray
spectrum reveals a photon index of \Gamma=2.24 \pm 0.14_stat \pm 0.03_sys and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 \pm 1.0_stat \pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument
and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE \gamma-ray data results in a differential photon index \Gamma=2.34 \pm 0.03 with a p-value of 30%. The \gamma-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in
the HE and VHE \gamma-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the \gamma-ray production, the single
overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.
The Astrophysical Journal 05/2012; 757:158. · 6.02 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
Y. Becherini,
J. Becker, [......],
S. J. Wagner,
M. Ward,
R. White,
A. Wierzcholska,
M. Zacharias,
A. Zajczyk,
A. A. Zdziarski,
A. Zech,
H. -S. Zechlin,
T. Montmerle
[show abstract]
[hide abstract]
ABSTRACT: The massive binary system Eta Carinae and the surrounding HII complex, the Carina Nebula, are potential particle acceleration sites from which very-high-energy (VHE; E > 100 GeV) \gamma-ray emission could be expected. This paper presents data collected during VHE \gamma-ray observations with the H.E.S.S. telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-hour data set no hint of significant \gamma-ray emission from Eta Carinae has been found and an upper limit on the \gamma-ray flux of 7.7 x 10-13 ph cm-2 s-1 (99% confidence level) is derived above the energy
threshold of 470 GeV. Together with the detection of high-energy (HE; 0.1 GeV >
E > 100 GeV) \gamma-ray emission by the Fermi-LAT up to 100 GeV, and assuming a continuation of the average HE spectral index into the VHE domain, these results imply a cut-off in the \gamma-ray spectrum between the HE and VHE \gamma-ray range. This could be caused either by a cut-off in the accelerated
particle distribution or by severe \gamma-\gamma\ absorption losses in the wind collision region. Furthermore, the search for extended \gamma-ray emission from the Carina Nebula resulted in an upper limit on the \gamma-ray flux of 4.2 x 10-12 ph cm-2 s-1 (99% confidence level). The derived upper limit of ~23 on the cosmic-ray enhancement factor is compared with results found for the old-age
mixed-morphology supernova remnant W 28.
Monthly Notices of the Royal Astronomical Society 04/2012; 424:128. · 4.90 Impact Factor
-
HESS Collaboration,
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
Y. Becherini,
J. Becker, [......],
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: 1RXS J101015.9-311909 is a galaxy located at a redshift of z=0.14 hosting an active nucleus belonging to the class of bright BL Lac objects. Observations at high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide insights into the origin of very energetic particles present in such sources
and the radiation processes at work. We report on results from VHE observations performed between 2006-10 with H.E.S.S. H.E.S.S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9-311909 is detected with H.E.S.S. for the first time. In a total good-quality livetime of about 49 h, we measure 263 excess counts, corresponding to a significance of 7.1\sigma. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of \Gamma\ = 3.08\pm0.42_{stat}\pm0.20_{sys}. The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no
significant variability over the time reported. In addition, public Fermi/LAT data are analysed to search for high energy emission from the source. The Fermi/LAT HE emission is significant at 8.3\sigma\ in the chosen 25-month dataset. UV and X-ray contemporaneous observations with the Swift satellite in May 2007 are also reported, together with optical observations performed with the ATOM telescope located at the H.E.S.S. site. Swift observations reveal an absorbed X-ray flux of F_{0.3-7 keV} = 1.04^{+0.04}_{-0.05} \times 10^{-11} erg.cm^{-2}.s^{-1} in the 0.3-7 keV range. Finally, all the available data are used to study the source's multi-wavelength properties. The SED can be reproduced using a simple one-zone SSC model with emission from a region with a Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These parameters are similar to those obtained for other sources of this type.
Astronomy and Astrophysics 04/2012; 542:94. · 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 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;
-
[show abstract]
[hide abstract]
ABSTRACT: We report an analysis and modelling of new Suzaku observations of the Seyfert
1 galaxies Mrk509 and Mrk841, taken between April and November 2006,for
Mrk509,and between January and July 2007,for Mrk841,for a total exposure time
of ~100 ks each.Data from XIS and HXD/PIN instruments,going from 0.5 to 60
keV,represent the highest spectral resolution simultaneous broad-band X-ray
spectrum for these objects.We fitted the broad-band spectrum of both sources
with a double Comptonisation model,adding neutral reflection from distant
material and a two-phase warm absorber.We then studied the two competing models
developed to explain the soft excess in terms of atomic processes:a blurred
ionised disc reflection and an ionised absorption by a high velocity
material.When fitting the data in the 3-10 keV range with a power law
spectrum,and extrapolating this result to low energies,a soft excess is clearly
observed below 2 keV:its strength is however weak compared to previous
observations of both sources.A moderate hard excess is seen at energies greater
than 10 keV,together with a neutral Fe K_a narrow emission line at E~6.4 keV
and a broad Fe emission line.For Mrk509,the broad Fe emission line is required
in all the three physical models in order to have a good fit to the data:this
finding suggests that the blurred reflection model correctly describes the soft
excess,but that it underestimates the broad Fe emission line.For the smeared
absorption model,this suggests instead that the continuum spectrum absorbed by
the outflowing gas should contain indeed a reflected component.For Mrk841,all
three models that we tested provide a good fit to the data,and we cannot
invalidate any of them.A broad emission line is required in the double
Comptonisation and smeared absorption models,while the blurred reflection model
consistently fits the broad-band spectrum,without adding any extra
emission-line component.
09/2011;
-
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;
