The Crab supernova remnant has been observed regularly with the stereoscopic system of five imaging air Cerenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In ...total, close to 400 hr of useful data have been collected from 1997 to 2002. The differential energy spectrum of the combined data set can be approximated by a power law-type energy spectrum: d Phi /dE = Phi sub(0) (E/TeV) Gamma , Phi sub(0) = (2.83 plus or minus 0.04 sub(stat) plus or minus 0.6 sub(sys)) 10 super(-11) photons cm super(-2) s super(-1) TeV super(-1), and Gamma = -2.62 plus or minus 0.02 sub(stat) plus or minus 0.05 sub(sys). 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 millimeter 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 161.6 plus or minus 0.8 sub(stat) plus or minus 18 sub(sys) mu 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 to be as low as 20% of the total spin-down luminosity for bulk Lorentz factors of the wind in the range of 10 super(4)-10 super(6). The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13" to the west of the pulsar, with a systematic uncertainty of 25". No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2' at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3'. No indication of pulsed emission has been found, and upper limits in differential bins of energy have been calculated reaching typically 1%-3% of the unpulsed component.
The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data (1999 to 2002) we confirm this ...new source as steady in flux over the four years of data taking, extended with radius 6.2′ (±$1.2^\prime_{\rm stat}$ ± $0.9^\prime_{\rm sys}$) and exhibiting a hard spectrum with photon index -1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies $E>1$ TeV amounts to ~5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeV J2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.
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 $107.4\,\pm\,26.8$ events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or $N_\gamma(E > 730\,\mbox{GeV}) = (0.96 \pm 0.23) \times 10^{-12}$ 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.
Deep observation (~113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov telescopes has serendipitously revealed a signal positionally inside the core of ...the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ~$0.5^\circ$ north of Cyg X-3. The source centre of gravity is RA $\alpha_{\rm J2000}$: $20^{\rm hr} 32^{\rm m} 07^{\rm s}\pm 9.2^{\rm s}_{\rm stat} \pm2.2^{\rm s}_{\rm sys}$, Dec $\delta_{\rm J2000}$: $+41^\circ 30^\prime 30^{\prime\prime}\pm 2.0^\prime_{\rm stat} \pm 0.4^\prime_{\rm sys}$. The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius $5.6^\prime$ at the ~$3\sigma$ level, and has a differential power-law flux with hard photon index of $-1.9 \pm0.3_{\rm stat}\pm0.3_{\rm sys}$. The integral flux above 1 TeV amounts ~3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.
232 hours of data were accumulated from 1997 to 1999, using the HEGRA Stereoscopic Cherenkov Telescope System to observe the supernova remnant Cassiopeia A. TeV γ-ray emission was detected at the $5 ...\sigma$ level, and a flux of $(5.8 \pm 1.2_{\mathrm{stat}} \pm 1.2_{\mathrm{syst}}) 10^{-9}\ {ph} {m}^{-2} {s}^{-1}$ above 1 TeV was derived. The spectral distribution is consistent with a power law with a differential spectral index of $-2.5 \pm 0.4_{\mathrm{stat}} \pm 0.1_{\mathrm{syst}}$ between 1 and 10 TeV. As this is the first report of the detection of a TeV γ-ray source on the "centi-Crab"scale, we present the analysis in some detail. Implications for the acceleration of cosmic rays depend on the details of the source modeling. We discuss some important aspects in this paper.
H.E.S.S. observations of PKS 2155-304 Aharonian, F.; Akhperjanian, A. G.; Aye, K.-M. ...
Astronomy and astrophysics (Berlin),
02/2005, Letnik:
430, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The high-frequency peaked BL Lac PKS 2155-304 at redshift $z=0.117$ has been detected with high significance (~45σ) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging ...air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June–September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of $\Gamma=3.32\pm0.06$. An improved $\chi^2$ per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2σ). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.
Very high energy (VHE; E > or =, slanted E 100 GeV) and high-energy (HE; 100 MeV < or =, slanted E < or =, slanted 100 GeV) data from gamma-ray observations performed with the H.E.S.S. telescope ...array and the Fermi-LAT instrument, respectively, are analyzed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. A power-law fit to the differential HE gamma-ray spectrum reveals a photon index of Gamma = 2.24 + or - 0.14 sub(stat) + or - 0.03 sub(sys) and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 + or - 1.0 sub(stat) + or - 0.3 sub(sys)) x 10 super(-9) cm super(-2) s super(-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 + or - 0.03 with a p-value of 30%..
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 >$23\,\sigma$ 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 $2.83 \pm 0.14_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ or by a power law with an exponential cut-off at $(4.2^{+0.8}_{-0.6~{\mbox{\tiny stat}}} \pm 0.9_{\mbox{\tiny sys}})$ TeV and a spectral index of $1.83 \pm 0.15_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of $3.18 \pm 0.17_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$.
Galactic cosmic rays reach energies of at least a few petaelectronvolts (of the order of 10(15) electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators ('PeVatrons'), but ...all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 10(13) electronvolts) were inferred from recent γ-ray observations. However, none of the currently known accelerators--not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays--has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outburstsand an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 10(6)-10(7) years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.
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