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 $d\Phi/dE=f_0E^{-\Gamma}exp(-E/E_0)$ 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.
