S R Kaul

The TACTIC atmospheric Cherenkov imaging telescope

The TACTIC (TeV Atomospheric Cherenkov Telescope with Imaging Camera) γγ-ray telescope, equipped with a light collector of area ∼9.5m2 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 two-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 11×1111×11 pixels (∼3.4∘×3.4∘)(∼3.4∘×3.4∘), the telescope records a cosmic ray event rate of ∼2.5Hz at a typical zenith angle of 15∘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 ∼1.2TeV energy, at a sensitivity level of ∼5.0σ∼5.0σ in ∼25h, 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 γγ-ray signals from Mrk 501 (during 1997 and 2006 observations) and Mrk 421 (during 2001 and 2005–2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV γγ-ray emission activity from other active galactic nuclei on a long-term basis.

Programmable topological trigger generator for the 349-pixel imaging camera of the TACTIC telescope

Abstract A CAMAC-based programmable trigger generator has been developed for the imaging camera of the TACTIC telescope. The paper describes the design and implementation details of the multi-module trigger generator which selects Cherenkov events on the basis of simultaneous detection by 3 non-collinear nearest-neighbour pixels. The details of its performance evaluation along with its expected behaviour in deciding the threshold energy of the TACTIC telescope are also discussed.

Drive-Control System for the TACTIC Gamma-Ray Telescope

A PC-based drive-control system has been developed for the altitude-azimuth mounted TACTIC γ-ray telescope to control the speed and direction of motion of its 2-axes. Details of various hardware components chosen for the telescope, including hybrid-stepper motor, 16-bit absolute encoder and CAMAC-based programmable stepper motor controller, are discussed in this paper. The telescope-control strategy, based on the position-loop with a proportional type control for the source-seek mode and the on/off type control for the tracking mode, is explained in detail. Some important performance features of the telescope, including its blind-spot size, drive-system backlash and encoder-error compensation, are also presented. The drive system has been extensively field-tested and has been operating satisfactorily during observation campaigns carried out since March 1997 with the TACTIC Imaging Element. A tracking accuracy of ±3 arc-minutes has been achieved. A test report of its performance, with regard to its tracking accuracy on the basis of successful detection of TeV γ-rays from the active galaxy Markarian 501 in April–May 1997, is also presented.

TeV gamma-ray observations of Markarian 421 using TACTIC during 2009–10

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 h. Detailed analysis of the data so recorded revealed the presence of a TeV γ-ray signal with a statistical significance of 12.12σ at Eγ ⩾ 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 = f0E−Γ) with f0 = (1.39 ± 0.239) × 10−11u2009cm−2u2009s−1u2009TeV−1 and Γ = 2.31 ± 0.14.

On a single-counts rate stabilization scheme employed in the imaging camera of the TACTIC γ-ray telescope

A cost-effective methodology for operating the 349-pixel Cerenkov light imaging camera of the imaging element of the TACTIC γ-ray telescope array at stable single-counts rate and safe anode current values has successfully been tried out despite variations in the light of the night sky experienced by these pixels from time to time. Important details of this unconventional scheme are discussed and reference is made to the methods evolved for in situ absolute and relative gain calibrations of the camera pixels, which are particularly required for this mode of operation.

A computer-programmable delay-generator scheme for the TACTIC gamma-ray telescope

Abstract A programmable multi-channel digital-delay generator, characterised by negligible dead-time count losses, has been developed for deployment in the high-sensitivity TeV γ-ray telescope, TACTIC. This telescope comprises an array of 4 × 10 m 2 of light collectors, of which three are placed at the corners of an equilateral triangle of side ≥ 20 m and are operated in a prompt-coincidence mode with a resolving time of ∼10 ns to detect atmospheric Cherenkov events produced by very high energy γ-ray and cosmic-ray proton primaries. The delay generator helps to line up these events in real time whenever they are detected at different relative times by these elements. The basic circuit used in this scheme can be readily adapted for utilisation in several other areas, including experimental nuclear physics.

Multiwavelength study of VHE emission from Markarian 501 using TACTIC observations during April–May, 2012

Abstract We have observed Markarian 501 in Very High Energy (VHE) gamma-ray wavelength band for 70.6 h from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of ∼66.3 h of clean data revealed the presence of a TeV γ-ray signal (686± 77 γ-ray events) from the source direction with a statistical significance of 8.89σ above 850xa0 GeV. Further, a total of 375 ± 47 γ-ray like events were detected in 25.2 h of observation from 22 – 27 May, 2012 with a statistical significance of 8.05σ indicating that the source has possibly switched over to a relatively high gamma-ray emission state. We have derived time-averaged differential energy spectrum of the state in the energy range 850xa0 GeV - 17.24xa0TeV which fits well with a power law function of the form d F / d E = f 0 E − Γ with f 0 = ( 2.27 ± 0.38 ) × 10 − 11 photons cm − 2 s − 1 TeV − 1 and Γ = 2.57 ± 0.15 . In order to investigate the source state, we have also used almost simultaneous multiwavelength observations viz: high energy data collected by Fermi -LAT, X-ray data collected by Swift -XRT and MAXI, optical and UV data collected by Swift -UVOT, and radio data collected by OVRO, and reconstructed broad-band Spectral Energy Distribution (SED). The obtained SED supports leptonic model (homogeneous single zone) for VHE gamma-ray emission involving synchrotron and synchrotron self Compton (SSC) processes.

Hybrid Zener-based VDN - a better choice for Cerenkov light imaging cameras

A Zener-based voltage divider network has been used for the photomultiplier tubes (PMTs) employed in the multi-pixel camera of a Cherenkov imaging telescope. This hybrid network ensures a linear photomultiplier response at a chain current nearly an order of magnitude less than that required for a purely resistive voltage divider network of conventional design. The network has been validated for the Thorn-EMI photomultiplier tube 9135B and the results discussed here are also representative of other PMT makes.

Search for TeV γ-ray emission from blazar 1ES1218+304 with TACTIC telescope during March–April 2013

Abstract In this paper, we present results of TeV γ-ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (zxa0=xa00.182) with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are primarily motivated by the unusually hard GeV–TeV spectrum of the source despite its relatively large redshift. The source is observed in the TeV energy range with the TACTIC from March 1, 2013 to April 15, 2013 (MJD 56352–56397) for a total observation time of 39.62xa0h and no evidence of TeV γ-ray activity is found from the source. The corresponding 99% confidence level upper limit on the integral flux above a threshold energy of 1.1xa0TeV is estimated to be 3.41xa0 × 10 - 12 xa0photonsxa0cm−2xa0s−1 (i.e.

Study of short term enhanced TeV γ- ray emission from Mrk 421 observed with TACTIC on December 28, 2014

Abstract In this work, we report on the detection of enhanced TeV γ- ray emission from the high synchrotron-peaked blazar Mrk 421 with the TACTIC telescope on the night of December 28, 2014 (MJD 57019). We use data from the TACTIC observations of Mrk 421 for one week during December 25–31, 2014 (MJD 57016–57022) in this study. The TACTIC observation on December 28, 2014 (MJD 57019) alone results in the detection of 86u202f±u202f17 γ- ray like events from Mrk 421 with a statistical significance of 5.17σ in a livetime of u202f∼u202f 2.2 hours above an energy threshold of 0.85 TeV. The high statistics (higher than three Crab Units) of TeV photons enables us to study the very high energy (VHE) γ- ray emission from the source at shorter timescales. A minimum variability timescale of u202f∼u202f 0.72 days is obtained for the TeV γ- ray emission from Mrk 421 during the above flaring activity of the source. The intrinsic VHE spectrum is described by a power law with spectral index of 2.99u202f±u202f0.38 in the energy range 0.85–8.5 TeV. The integral VHE γ- ray flux above 0.85 TeV is determined to be ( 3.68 ± 0.64 ) × 10 − 11 phxa0cm − 2 xa0s − 1 from the TACTIC observations of Mrk 421 on the night of December 28, 2014 (MJD 57019). Near simultaneous measurements by the HAWC observatory give an integral flux of ( 2.90 ± 0.40 ) × 10 − 11 phxa0cm − 2 xa0s − 1 above 2 TeV from Mrk 421 observations on December 29, 2014 (MJD 57020.33–57020.58). We have also analysed the contemporaneous data from Fermi-LAT to study the high energy (HE) γ–ray emission during the high activity state of the source. The HE γ–ray emission is observed to be increasing after the TeV flaring activity detected with the TACTIC. We also use other near simultaneous archival data available from the Swift-BAT in hard X-rays and from SPOL at Steward Observatory in optical V and R bands to characterize the multi-wavelength emission of Mrk 421 during the high activity state observed at TeV energies. The TeV γ- ray emission observed on December 28, 2014 (MJD 57019) is found to be more prominent than the emissions in lower energy bands during the same period. The TeV γ- ray observation of Mrk 421 in high activity state with the TACTIC telescope is also used to understand the physical mechanism for blazar emission under the frame work of the leptonic single zone synchrotron self Compton process.