Abstract. In this paper we report on the Markarian 501 results obtained during our TeV γ-ray observations from March 11 to May 12, 2005 and February 28 to May 7, 2006 for 112.5 hours with the TACTIC γ-ray telescope. During 2005 observations for 45.7 hours, the source was found to be in a low state and we have placed an upper limit of 4.62 × 10 −12 photons cm −2 s −1 at 3σ level on the integrated TeV γ-ray flux above 1 TeV from the source direction. However, during the 2006 observations for 66.8h, detailed data analysis revealed the presence of a TeV γ-ray signal from the source with a statistical significance of 7.5σ above E γ ≥ 1 TeV. The time averaged differential energy spectrum of the source in the energy range 1-11 TeV is found to match well with the power law function of the form (dΦ/dE = f 0 E −Γ ) with f 0 = (1.66 ± 0.52) × 10 −11 cm −2 s −1 T eV −1 and Γ = 2.80 ± 0.27.
The TACTIC γ-ray telescope, equipped with a light collector of area ∼9.5m 2 and a medium resolution imaging camera of 349-pixels, has been in operation at Mt.Abu, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11×11 pixels (∼ 3.4 • ×3.4 • ), the telescope records a cosmic ray event rate of ∼2.5 Hz at a typical zenith angle of 15 • . Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0σ in ∼25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong γ-ray signals from Mrk 501 (during 1997(during and 2006(during observations) and Mrk 421 (during 2001(during and 2005(during -2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV γ-ray emission activity from other active galactic nuclei on a long term basis.
Abstract. We have observed the blazar Markarian 421 with the TACTIC γ-ray telescope at Mt. Abu, India, from 22 November 2009 to 16 May 2010 for 265 hours. Detailed analysis of the data so recorded revealed presence of a TeV γ-ray signal with a statistical significance of 12.12σ at E γ ≥ 1 TeV. We have estimated the time averaged differential energy spectrum of the source in the energy range 1.0 -16.44 TeV. The spectrum fits well with the power law function of the form (dF/dE = f 0 E −Γ ) with f 0 = (1.39 ± 0.239) × 10 −11 cm −2 s −1 T eV −1 and Γ = 2.31 ± 0.14.
We have observed the BL Lac object 1ES2344+514 (z = 0.044) in very high energy (VHE) gamma-ray and near-infrared wavelength bands with TACTIC and MIRO telescopes, respectively. The observations were made from 18th October to 9th December 2004 and 27th October 2005 to 1st January 2006. Detailed analysis of the TACTIC data indicates the absence of a statistically significant gamma-ray signal both in overall data and on a nightly basis from the source direction. We estimate an upper limit of I(⩾1.5 TeV) ⩽ 3.84 × 10−12 photons cm−2 s−1 at a 3σ confidence level on the integrated γ-ray flux. In addition, we have also compared TACTIC TeV light curves with those of the RXTE ASM (2–12 keV) for the contemporary period and find that there are no statistically significant increases in the signal strengths from the source in both these energy regions. During 2004 IR observations, 1ES2344+514 shows low level (0.06 magnitude) day-to-day variation in both, J and H bands. However, during the 2005 observation epoch, the source brightens up by about 0.41 magnitude from its October 2005 level J magnitude = 12.64 to J = 12.23 on December 6, 2005. It then fades by about 0.2 magnitude during 6 to 10 December, 2005. The variation is seen in both, J and H, bands simultaneously. The light travel time arguments suggest that the emission region size is of the order of 1017 cm.
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