Results 1 - 10 of 1133
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[en] The brightness distribution, position angles and degree of polarization in the Crab nebula are calculated. The space distribution of magnetic field strength H is adopted to be the same as for magnetic dipole radiation but H-dependence on distance to pulsar is assumed to be H approximately tausup(0.35). From comparison between the calculation results for hard X-ray brightness distribution and the observation it was found that the projection of pulsar's spin to the sky plane is directed along the small axis of the Crab nebula and the angle theta0 between pulsar's spin and the line of sight is within 650-800
[en] We report the discovery of TeV gamma-ray emission from the Type Ia supernova remnant (SNR) G120.1+1.4, known as Tycho's SNR. Observations performed in the period 2008-2010 with the VERITAS ground-based gamma-ray observatory reveal weak emission coming from the direction of the remnant, compatible with a point source located at 00h25m27.s0, + 64010'50'' (J2000). The TeV photon spectrum measured by VERITAS can be described with a power law dN/dE = C(E/3.42 TeV)-Γ with Γ = 1.95 ± 0.51stat ± 0.30sys and C = (1.55 ± 0.43stat ± 0.47sys) x 10-14 cm-2 s-1 TeV-1. The integral flux above 1 TeV corresponds to ∼0.9% of the steady Crab Nebula emission above the same energy, making it one of the weakest sources yet detected in TeV gamma rays. We present both leptonic and hadronic models that can describe the data. The lowest magnetic field allowed in these models is ∼80 μG, which may be interpreted as evidence for magnetic field amplification.
[en] The Crab and Vela pulsars (PSR0531+21 and PSR0833-45 respectively) were among the earliest identified sources of high-energy gamma radiation and as such were among the earliest targets selected for detailed study by Satellite COS-B. Data are now available from six separate epochs when the Crab was within 250 of the centre of the field of view and seven in the case of the Vela pulsar. The observations from which data were used in the present analysis are listed in the full paper
[en] We present results of 2 mm observations of the Crab Nebula, obtained using the Goddard-IRAM Superconducting 2 Millimeter Observer (GISMO) bolometer camera on the IRAM 30 m telescope. Additional 3.3 mm observations with the MUSTANG bolometer array on the Green Bank Telescope are also presented. The integrated 2 mm flux density of the Crab Nebula provides no evidence for the emergence of a second synchrotron component that has been proposed. It is consistent with the radio power-law spectrum, extrapolated up to a break frequency of log (νb[GHz]) = 2.84 ± 0.29 or νb = 695+651-336 GHz. The Crab Nebula is well resolved by the ∼16.''7 beam (FWHM) of GISMO. Comparison to radio data at comparable spatial resolution enables us to confirm significant spatial variation of the spectral index between 21 cm and 2 mm. The main effect is a spectral flattening in the inner region of the Crab Nebula, correlated with the toroidal structure at the center of the nebula that is prominent in the near-IR through X-ray regime.
[en] Because the Vela supernova remnant (SNR) is so much closer and older (distance approx. 500 pc, age approx. 10,000 yr) than the Crab SNR (distance approx. 2 kpc, age approx. 1900 yr), the Vela SNR has a much larger angular extent than does the Crab: nearly 5 degrees as compared to approx. 3 arcmin. This made study of the Vela SNR with the 1-degree field-of-view Einstein telescope tedious, requiring nearly 40 separate pointings to cover the entire extent of the X-ray shell. This paper centers mainly on the data for the two pulsars, and also includes brief discussions of the nebular structure surrounding each of them. (Auth.)
[en] During a balloon flight of the MISO telescope on 1980 May 17, the Crab Nebula and the Seyfert galaxy NGC 4151 were studied over the photon energy range 0.03-16 MeV. The photon spectrum of the Crab Nebula was measured up to approximately 2 MeV. No gamma-ray emission from NGC 4151 was detected on this occasion
[en] Over a past decade very high energy (VHE) gamma ray astronomy has emerged as a major astronomical discipline. In India, we have a long tradition of experiments in this field. Few years ago, multi-institutional Himalayan Gamma Ray Observatory (HiGRO) collaboration was formed to set up VHE gamma rays experiments at Hanle, a high altitude location in Himalayas. HAGAR, the first phase of this collaboration is operational since 2008. HAGAR has successfully detected VHE gamma ray emission from some of the extragalactic objects like Mrk 421, Mrk 501 as well as galactic sources including Crab nebula/pulsar. Details of HAGAR telescope system and results obtained will be discussed. HiGRO is now gearing up for the next phase, i.e. 21 m diameter MACE telescope, which is being installed at Hanle at present. Details of MACE telescope system and future plans will be discussed. (author)
[en] This paper presents the results of measurements of the Crab nebula in the decametre range with an interferometer whose baseline is 2.4-3.5 X 103 of the wavelength. Visibility function values, which in these observations determine the contribution by the compact source to the total nebula flux, have been measured at frequencies 16.7, 20 and 25 MHz to be 0.64 +- 0.07, 0.43 +- 0.04 and 0.31 +- 0.03, respectively. The spectral index of the spectrum obtained for the compact source in the range 16.7-122 MHz is 2.09 +- 0.04. Flattering of the nebula spectrum without the compact source has been confirmed for the decametre range. (orig.)
[en] The existence of excess showers from Crab nebula and Cyg X-3 is examined with about 100,000 showers of electron sizes larger than 3 105 observed during 1978 and 1981 at Akeno. No excess showers are observed from Crab or Cyg X-3. In order to check whether the source direction is enriched by the gamma-primary, the ratio of muon to electron size of each shower is also studied as a function of the arrival direction of EAS
[en] We present the results of observations of the TeV binary LS I +610 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation (5.6σ) post-trials significance. In total, between 2008 October and 2010 December a total exposure of 64.5 hr was accumulated with VERITAS on LS I +610 303, resulting in an excess at the 3.3σ significance level for constant emission over the entire integrated data set. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e., close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at 10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +610 303 by the Swift X-ray Telescope and the Rossi X-ray Timing Explorer Proportional Counter Array. We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the Fermi Large Area Telescope.