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

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 $d\mathrm{\Phi }\mathrm{/}dE=f_0{E}^{-\mathrm{\Gamma }}exp(-E\mathrm{/}{E}_0)$ with $f_0=(4.88\pm 0.38)\times {10}^{-11}c{m}^{-2}{s}^{-1}Te{V}^{-1}$, $\mathrm{\Gamma }=2.51\pm 0.26$ and $E_0=(4.7\pm 2.1)TeV$. 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 $1.44\times {10}^{-12}photonsc{m}^{-2}{s}^{-1}$ at $3\sigma$ level on the integrated \gam flux above $1TeV$. 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 $\le 1.18\times {10}^{-12}photonsc{m}^{-2}{s}^{-1}$ on the integrated $\gamma$-ray flux above $1TeV$ at 3$\sigma$ significance level.