The BL Lacertae (BL Lac) object 1ES 2344+514 (1ES 2344), at a redshift of 0.044, was discovered as a source of very high energy (VHE) gamma rays by the Whipple Collaboration in 1995 \citep{2344Catanese98}. This detection was recently confirmed by the HEGRA Collaboration \citep{2344Hegra03}. As is typical for high-frequency peaked blazars, the VHE gamma-ray emission is highly variable. On the night of 20 December, 1995, a gamma-ray flare of 5.3-sigma significance was detected, the brightest outburst from this object to-date. The emission region is compatible with a point source. The spectrum between 0.8 TeV and 12.6 TeV can be described by a power law $\frac{\ud^3 N}{\ud E \ud A \ud t}=(5.1\pm1.0_{st}\pm1.2_{sy})\times10^{-7} (E/ \mathrm{TeV})^{-2.54 \pm0.17_{st}\pm0.07_{sy}} \mathrm{\frac{1}{TeV m^2 s}}$. Comparing the spectral index with that of the other five confirmed TeV blazars, the spectrum of 1ES 2344 is similar to 1ES 1959+650, located at almost the same distance. The spectrum of 1ES 2344 is steeper than the brightest flare spectra of Markarian 421 (Mrk~421) and Markarian 501 (Mrk~501), both located at a distance about 2/3 that of 1ES 2344, and harder than the spectra of PKS 2155-304 and H~1426+428, which are located almost three times as far. This trend is consistent with attenuation caused by the infrared extragalactic background radiation.

/pdf/a-very-high-energy-gamma-ray-spectrum-of-1es-2344-514-14ka1iajbe.pdf

A Very High Energy Gamma-Ray Spectrum of 1ES 2344+514

Steady-state emission of blazars at very high energies

We have observed extragalactic sources Mrk421 (z=0.30) and H1426+428 (z=0.129) in very high energy gamma region using TACTIC telescope at Mount Abu, Rajasthan, India. Mrk421 was observed during 07 December 2005 to 18 April 2007 for a total of ~331hours while H1426+428 was observed for ~244 hours between 22 March 2004 and 11 June 2007. Detailed analysis of Mrk421 data revealed the presence of a gamma signal with the excess of (~951\pm 82) gamma like events corresponding to a statistical significance of ~12.0 \sigma in ~97 hours during 27 December 2005 to 07 February 2006. The combined average flux was measured to be ~(1.04\pm0.14) Crab units. Observed time averaged differential energy spectrum was found to be consistent with both a pure power law $d\Phi/dE=(4.66\pm0.46)\times 10^{-11}\,E^{(-3.11\pm0.11)}\,\,cm^{-2}\,s^{-1}\,TeV^{-1}$ and power law with an exponential cutoff functions $d\Phi/dE=f_0E^{-\Gamma}exp(-E/E_0)$ with $f_0=(4.88\pm0.38)\times10^{-11}\,cm^{-2}\,s^{-1}\,TeV^{-1}$, $\Gamma=2.51\pm0.26$ and $E_0=(4.7\pm2.1)TeV$. The later function fits the observed spectrum well with lower value of reduced $\chi^2$. However, during the observation period from 18 December 2006 to 18 April 2007, the source was found to be in a low emission state and we have placed an upper limit of $1.44\times\,10^{-12}\,photons\,cm^{-2}\,s^{-1}$ at $3\sigma$ level on the integrated \gam flux above $1\,TeV$. Detailed analysis of H1426+428 data does not indicate the presence of any statistically significant $TeV$ \gam signal. Accordingly we have placed an upper limit of $\leq1.18\times10^{-12}\,photons\,cm^{-2}\,s^{-1}$ on the integrated $\gamma$-ray flux above $1\,TeV$ at 3$\sigma$ significance level.

TeV Gamma-Ray Observations of Mrk421 and H1426+428 with TACTIC Imaging Telescope

The opacity of the Universe to gamma rays is still poorly measured, in particular anomalies may have been observed. Assuming that such anomalies find their origin in conventional physics like intrinsic source spectra or the density of the extragalactic background light, they would be evenly distributed over the sky. If they exist, axion-like particles (ALPs) would have a potential effect on the opacity of the Universe to gamma rays, possibly related to the anomalies in the spectral indices of distant gamma-ray sources. Under the assumption that distant sources offer suitable environments for their production, ALPs convert back to photons in the Galactic magnetic field and their effect on the opacity is expected to depend on the position of the sources. In that case the anomaly is expected to exhibit peculiar correlations on the sky. We propose a method to test the origin of the opacity anomaly, based on angular correlations of spectral softening anomalies. Such a diagnosis requires a wide-field survey of high-energy gamma-ray sources over a broad range of energy. The future Cherenkov Telescope Array (CTA) is perfectly suited to perform such a study. It is shown that while the current sample of sources is not large enough to base conclusions on, with this method CTA will be sensitive to ALP couplings to gamma rays of the order of 3 ? 10?11GeV?1 for ALP masses below 10?8 eV.

Anisotropy test of the axion-like particle Universe opacity effect: a case for the Cherenkov Telescope Array

The BL Lacertae (BL Lac) object 1ES 2344+514 (1ES 2344), at a redshift of 0.044, was discovered as a source of very high energy (VHE) gamma rays by the Whipple collaboration in 1995 (Catanese et al.). This detection was recently confirmed by the HEGRA collaboration (Tluczykont et al.). As is typical for high-frequency-peaked blazars, the VHE gamma-ray emission is highly variable. On the night of 1995 December 20, a gamma-ray flare of 5.3 σ significance was detected, the brightest outburst from this object to date. The emission region is compatible with a point source. The spectrum between 0.8 and 12.6 TeV can be described by a power law, If we compare the spectral index with that of the other five confirmed TeV blazars, the spectrum of 1ES 2344 is similar to that of 1ES 1959+650, which is located at almost the same distance. The spectrum of 1ES 2344 is steeper than the brightest flare spectra of Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501), both of which are located at a distance about two-thirds that of 1ES 2344, and harder than the spectra of PKS 2155-304 and H1426+428, which are located almost 3 times as far away. This trend is consistent with attenuation caused by the infrared extragalactic background radiation.

/pdf/a-very-high-energy-gamma-ray-spectrum-of-1es-2344-514-1i3ssbtadg.pdf

Detection of VHE Gamma-rays from Mkn 421 with the HEGRA Cherenkov Telescopes

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Detection of VHE Gamma-rays from Mkn 421 with the HEGRA Cherenkov Telescopes