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Featured researches published by V. Vitale.


The Astrophysical Journal | 2004

The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA Stereoscopic Air Cerenkov Telescopes

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; M. V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; D. Kranich; H. Lampeitl; M. López; E. Lorenz; F. Lucarelli; O. Mang

The Crab supernova remnant has been observed regularly with the stereoscopic system of 5 imaging air Cherenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hours of useful data have been collected from 1997 until 2002. The spectrum extends up to energies of 80 TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula including for the first time a recently detected compact emission region at mm-wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model.The average magnetic field in the emitting volume is determined to be


Astronomy and Astrophysics | 2003

Is the Giant Radio Galaxy M 87 a TeV Gamma-Ray Emitter?

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; M. V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; H. Kornmeyer; D. Kranich; H. Lampeitl; M. López; E. Lorenz; F. Lucarelli

(161.6\pm0.8mathrm{stat}\pm18_\mathrm{sys}) \mu


Astronomy and Astrophysics | 2003

Detection of TeV gamma-rays from the BL Lac 1ES 1959+650 in its low states and during a major outburst in 2002

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; M. V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; H. Kornmeyer; D. Kranich; H. Lampeitl; M. López; E. Lorenz; F. Lucarelli

G. The presence of protons in the nebula is not required to explain the observed flux and upper limits on the injected power of protons are calculated being as low as 20 % of the total spin down luminosity for bulk Lorentz factors of the wind in the range of


Astronomy and Astrophysics | 2004

Observations of 54 Active Galactic Nuclei with the HEGRA system of Cherenkov telescopes

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Bojahr; O. Bolz; H. Börst; T. Coarasa; J. Contreras; J. Cortina; S. Denninghoff; V. Fonseca; M. Girma; N. Gotting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; J. Kettler; A. Kohnle; A. Konopelko; H. Kornmeyer; D. Kranich; H. Krawczynski; H. Lampeitl; M. Lopez

10^4-10^6


Astroparticle Physics | 2003

The technical performance of the HEGRA system of imaging air Cherenkov telescopes

G. Pühlhofer; O. Bolz; N. Götting; A. Heusler; D. Horns; A. Kohnle; H. Lampeitl; M. Panter; M. Tluczykont; F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; T. Coarasa; J.L. Contreras; J. Cortina; S. Denninghoff; M.V. Fonseca; M. Girma; G. Heinzelmann; G. Hermann; W. Hofmann; I. Jung; R. Kankanyan; M. Kestel; A. Konopelko; H. Kornmeyer; D. Kranich

.The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13\arcsec to the west of the pulsar with a systematic uncertainty of 25\arcsec. No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2\arcmin at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3\arcmin.No indications for pulsed emission has been found and upper limits in differential bins of energy have been calculated reaching typically 1-3 % of the unpulsed component.


Astronomy and Astrophysics | 2003

TeV

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; J. A. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; V. Fonseca; M. Girma; F. Goebel; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; J. Kettler; A. Kohnle; A. Konopelko; D. Kranich; H. Krawczynski; H. Lampeitl; M. López

For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M 87 has been measured at a significance level above 4 σ. The data have been taken during the years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of 107.4 ± 26.8 events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or Nγ(E > 730 GeV) = (0.96 ± 0.23) × 10 −12 phot cm −2 s −1 . M 87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of z = 0.00436 and is a promising candidate among the class of giant radio galaxies for the emission of TeV γ-radiation. The detection of TeV γ-rays from M 87 - if confirmed - would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.


Astronomy and Astrophysics | 2005

\gamma

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; D. Kranich; H. Lampeitl; M. López; E. Lorenz; F. Lucarelli; O. Mang

TeV γ-rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 σ at a value of 5.3% of the Crab flux and in May 2002 during strong outbursts with >23 σ at a flux level of up to 2.2 Crab, making 1ES 1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES 1959+650 is located at a redshift of z = 0.047, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES 1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES 1959+650 during the flares can be fitted by a power law with a spectral index of 2.83 ± 0.14stat ± 0.08sys or by a power law with an exponential cut-off at (4.2 +0.8 −0.6 stat ± 0.9sys) TeV and a spectral index of 1.83 ± 0.15stat ± 0.08sys. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of 3.18 ± 0.17stat ± 0.08sys.


Astronomy and Astrophysics | 2004

-ray light curve and energy spectrum of Mkn 421 during its 2001 flare as measured with HEGRA CT1

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; M. V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; H. Kornmeyer; D. Kranich; H. Lampeitl; M. López; E. Lorenz; F. Lucarelli

A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in the TeV energy regime. The observations were motivated by the positive results obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary over a large range of redshifts between z = 0.004 and z = 0.7. Among the observed AGN are the now-established TeV-emitting BL Lac type objects H 1426+428 and 1ES 1959+650. Furthermore the BL Lac object 1ES 2344+514 and the radio galaxy M 87 show evidence for a signal on a4 σ level. The observation of 1ES 2344+514 together with the Whipple results firmly establishes this AGN as a TeV source. Several objects (PKS 2155-304, BL Lacertae, 3C 066A) that have been claimed as TeV γ-ray emitters by other groups are included in this data sample but could not be confirmed using data analysed here. The upper limits from several AGN included in this analysis are compared with predictions in the frame-work of SSC models.


Astronomy and Astrophysics | 2005

TeV gamma-ray observations of SS-433 and a survey of the surrounding field with the HEGRA IACT-System

F. Aharonian; A. G. Akhperjanian; M. Beilicke; K. Bernlöhr; H.-G. Börst; H. Bojahr; O. Bolz; T. Coarasa; J. L. Contreras; J. Cortina; S. Denninghoff; V. Fonseca; M. Girma; N. Götting; G. Heinzelmann; G. Hermann; A. Heusler; W. Hofmann; D. Horns; I. Jung; R. Kankanyan; M. Kestel; A. Kohnle; A. Konopelko; D. Kranich; H. Lampeiti; M. Lopez; E. Lorenz; F. Lucarelli; O. Mang

Abstract Between early 1997 and late 2002, the HEGRA collaboration operated a stereoscopic system of four (later five) imaging atmospheric Cherenkov telescopes. In this paper we present the calibration schemes which were developed for the system, and report on the performance of the detector over the years. In general, the telescope system was very well understood, regarding both the absolute calibration and the slight changes in performance over the years. The system had an energy threshold of 500 GeV for observations at zenith and under optimum detector conditions. With the corresponding calibration schemes, a systematic accuracy of 15% on the absolute energy scale has been achieved. The continuous sensitivity monitoring provided a relative accuracy of a few percent, and showed that the threshold did not exceed 600 GeV throughout the entire operation time. The readout electronics and the imaging quality of the dishes were well monitored and stable. The absolute pointing had an accuracy of at least 25″; this number was guaranteed throughout the whole lifetime of the experiment.


Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment | 2004

Observation of the Monoceros Loop SNR region with the HEGRA system of IACTs

C. Baixeras; D. Bastieri; C. Bigongiari; O. Blanch; G. Blanchot; R. K. Bock; T. Bretz; A. Chilingarian; J. A. Coarasa; E. Colombo; Jl Contreras; D. Corti; J. Cortina; C. Domingo; E. Domingo; D. Ferenc; E. Fernández; J. Flix; V. Fonseca; L. Font; N. Galante; M. Gaug; M. Garczarczyk; J. Gebauer; M. Giller; F. Goebel; T. Hengstebeck; P. Jacone; O. C. de Jager; O. Kalekin

In the first months of 2001 the AGN Mkn 421 showed highly variable, strong TeV activity at flux levels frequently exceeding 1 Crab. Here we present the light curve and energy spectrum of Mkn 421 as measured with the HEGRA stand alone telescope CT1. Around 30% of the data were taken under moonlight conditions. The spectrum shows a significant exponential energy cuto at around 3.4 TeV. The results from the dark night- and the moon data are in excellent agreement with each other. A significant spectral shape variation depending on the flux level has been found. The TeV light curve is also found to be highly correlated with the X-ray light curve of the RXTE/ ASM satellite, showing no significant time lag larger than 0.2 d. The derived correlation coecient of 0.83 corresponds to a 5:2 significance.

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D. Horns

University of Hamburg

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