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Register a new userDetection of high-energy gamma rays from winter thunderclouds
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A report is made on a comprehensive observation of a burst-like $gamma$-ray emission from thunderclouds on the Sea of Japan, during strong thunderstorms on 2007 January 6. The detected emission, lasting for $sim$40 seconds, preceded cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung photons originating from relativistic electrons. This ground-based observation provides first clear evidence that strong electric fields in thunderclouds can continuously accelerate electrons beyond 10 MeV prior to lightning discharges.
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In this work we study how the cosmological parameter, the Hubble constant $H_0$, can be constrained by observation of very high energy (VHE) $gamma$-rays at the TeV scale. The VHE $gamma$-rays experience attenuation by background radiation field through $e^+e^-$ pair production during the propagation in the intergalactic space. This effect is proportional to the distance that the VHE $gamma$-rays go through. Therefore the absorption of TeV $gamma$-rays can be taken as cosmological distance indicator to constrain the cosmological parameters. Two blazars Mrk 501 and 1ES 1101-232, which have relatively good spectra measurements by the atmospheric Cerenkov telescope, are studied to constrain $H_0$. The mechanism constraining the Hubble constant adopted here is very different from the previous methods such as the observations of type Ia supernovae and the cosmic microwave background. However, at $2sigma$ level, our result is consistent with other methods.
The Very High Energy Gamma Ray Astronomy (VHE) is a rapidly evolving branch of modern astronomy, which covers the range from about 50 GeV to several tens of TeV from the ground. In the past years, the second generation instruments firmly established a growing and varied list of sources including plerions, supernova remnants and active galactic nuclei, and started to study some fundamental questions such as the origin of cosmic rays or the emission mechanisms of the active galactic nuclei. New results now include the first VHE unidentified sources as well as more puzzling sources such as the Galactic center. The arrival of new generation instruments (HESS, CANGAROO III, VERITAS, MAGIC) already gives a impressive look at the near future. Here we attempt to summarize the current status of the field. We briefly describe the instruments and analysis techniques, and give an outlook on the sources detected sofar.
The BL Lacertae object 1ES 1440+122 was observed in the energy range from 85 GeV to 30 TeV by the VERITAS array of imaging atmospheric Cherenkov telescopes. The observations, taken between 2008 May and 2010 June and totalling 53 hours, resulted in the discovery of $gamma$-ray emission from the blazar, which has a redshift $z$=0.163. 1ES 1440+122 is detected at a statistical significance of 5.5 standard deviations above the background with an integral flux of (2.8$pm0.7_{mathrm{stat}}pm0.8_{mathrm{sys}}$) $times$ 10$^{-12}$ cm$^{-2}$ s$^{-1}$ (1.2% of the Crab Nebulas flux) above 200 GeV. The measured spectrum is described well by a power law from 0.2 TeV to 1.3 TeV with a photon index of 3.1 $pm$ 0.4$_{mathrm{stat}}$ $pm$ 0.2$_{mathrm{sys}}$. Quasi-simultaneous multi-wavelength data from the Fermi Large Area Telescope (0.3--300 GeV) and the Swift X-ray Telescope (0.2--10 keV) are additionally used to model the properties of the emission region. A synchrotron self-Compton model produces a good representation of the multi-wavelength data. Adding an external-Compton or a hadronic component also adequately describes the data.
We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia every year since 1992. This is the first concerted search for pulsed and unpulsed emission from the Vela region, and the imaging technique also allows the location of the emission within the field of view to be examined. A significant excess of gamma-ray-like events is found offset from the Vela pulsar to the southeast by about 0.13deg. The excess shows the behavior expected of gamma-ray images when the asymmetry cut is applied to the data. There is no evidence for the emission being modulated with the pulsar period -- in contrast to earlier claims of signals from the Vela pulsar direction.
We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia between January 1993 and March 1995. Evidence of an unpulsed gamma-ray signal has been detected at the 5.8 sigma level. The detected gamma-ray flux is (2.9 +/- 0.5 +/- 0.4) x 10^-12 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV and the signal is consistent with steady emission over the two years. The gamma-ray emission region is offset from the Vela pulsar position to the southeast by about 0.13 deg. No pulsed emission modulated with the pulsar period has been detected and the 95 % confidence flux upper limit to the pulsed emission from the pulsar is (3.7 +/- 0.7) x 10^-13 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV.
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