TACTIC Telescope Research Articles

Generative Adversarial Network based method for generation of synthetic image parameters for TACTIC [formula omitted]-ray telescope

Very High Energy (VHE) γ and cosmic rays interact with the atmosphere and generate an Extensive Air Shower (EAS) of particles comprising mainly electron–positron pairs. These energetic secondary charged particles move down the atmosphere and upon de-excitation, the medium generates a flash of Cherenkov light. The subtle differences in the shape of Cherenkov flash images generated by γ/cosmic rays at the top of the atmosphere can be explored by the ground-based Imaging Atmospheric Cherenkov technique (IACT) telescopes. To characterize and calibrate such IACT telescopes, an extensive library of EAS for γ-rays and cosmic rays of various energies are required. However, the main difficulty in generating a simulation database is that the process of shower generation is computationally extensive. To overcome the challenge of creating a huge database of simulated shower images, we propose a method based on the generation of synthetic data by employing the unsupervised machine learning (ML)-based method, namely the Generative Adversarial Networks (GAN). The present study discusses the methodology for generating synthetic image parameters using GAN for the TACTIC telescope. A comparison between Monte Carlo simulation-generated image parameters for the TACTIC and GAN-generated parameters is presented. Results are also presented from the independent Random Forest-based ML classifier to identify similarities between the simulation and synthetic data species. The potential applications of GAN-generated data towards IACT data analysis for TACTIC are also discussed.

Long-term multi-wavelength study of temporal and spectral properties of 3C 279

We present a long-term multi-wavelength study of the flat spectrum radio quasar 3C 279 using near simultaneous observations available from July 01, 2012 and June 31, 2018 (MJD 56109-58301). The data set includes measurements from the TACTIC, Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, Steward-SPOL, and OVRO. The TACTIC telescope has monitored the source 3C 279 for ∼ 70 h between 2013 and 2018 in the energy range of > 0.85 TeV and yields null result. We have analysed the Fermi-LAT data to obtain the 5-day binned light curve in the energy range 0.1–300 GeV. One low and six high emission states are identified in the Fermi-LAT light curve. The high emission states are modelled with an exponential rise and decay time profile using 1-day averaged flux points and have been characterized as the asymmetric flares. The spectral analysis of near simultaneous X-ray and γ-ray data from the Swift-XRT and Fermi-LAT respectively indicates harder when brighter trend during the high activity states. The time averaged broadband spectral energy distributions corresponding to the seven states are successfully reproduced under the framework of a simple one zone leptonic emission model with IR photons originating from the dusty torus as the dominant target for the inverse Compton scattering by the relativistic electrons. Results obtained from the broadband modelling suggest that the non-thermal emission region in the jet is above the broadline region. This is also supported by the average value of seed factor estimated using the derived model parameters. A positive correlation between the bulk Lorentz factor of the emission zone and average γ-ray flux measurements for each state indicates that an increase in the bulk Lorentz factor can be the dominant cause for the observed flares from 3C 279. We also discuss the implications of the obtained model parameters in detail to understand the long-term behaviour of the source.

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

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 86 ± 17 γ- ray like events from Mrk 421 with a statistical significance of 5.17σ in a livetime of ∼ 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 ∼ 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 2.99 ± 0.38 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 ph cm−2 s−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 ph cm−2 s−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.

Search for very high energy gamma-ray emission from the peculiar radio galaxy IC 310 with TACTIC during 2012 to 2015

Non-blazar active galactic nuclei like radio galaxies have emerged as a new class of γ-ray sources in the sky. Observations of very high energy (VHE) γ-rays from radio galaxies with misaligned jets offer a unique tool to understand the physical processes involved in these type of objects. In this work, we present the results of our observations of the nearby peculiar radio galaxy IC 310 (z = 0.0189) with TACTIC telescope for nearly 95.5 hours from 03 December, 2012 to 19 January, 2015 (MJD 56265 - 57041). Detailed analysis of the data reveals absence of a statistically significant γ-ray signal from the source direction (both on the overall period and on yearly basis). Our results suggest that the source was possibly in a low-TeV emission state (below the TACTIC sensitivity level) during the above mentioned observation period and the resulting 3σ upper limit on the integral flux above 850 GeV has been estimated to be 4.99×10−12phcm−2s−1 (23% of the Crab Nebula flux). Analysis of the contemporaneous data collected by Fermi-LAT in the 30 – 300 GeV energy range, also indicate the absence of a statistically significant γ-ray signal, therefore 2σ upper limit on the integral flux above 30 GeV has been estimated on yearly basis. We also report the results from dedicated optical observations in B, V and R bands from ARIES observatory carried out from December, 2014 to March, 2015.

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

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 850 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 850 GeV - 17.24 TeV which fits well with a power law function of the form dF/dE=f0E−Γ with f0=(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.

VHE observation of Mrk 421 with TACTIC during 2012; multiwavelength study

The nearby (z=0.031) TeV γ-ray emitter BL Lac object Mrk 421 has been observed with the TACTIC telescope for nearly 118 hours during 01 February to 26 April, 2012 (MJD 55958–56044). Detailed analysis reveals the detection of 529 ± 76 γ-rays with 6.9σ significance above 850 GeV energy threshold during the period of 15–26 March, 2012 (MJD 56001–56013). Nearly simultaneous multiwavelength observations of the source also indicate a comparatively higher flux state during this period (MJD 56001–56013). In this paper, we present the study of Mrk 421 during this period in very high energy (VHE) domain along with the results from high energy (HE), X-ray, optical and radio observations. We reproduce the spectral energy distribution (SED) of the source during this period using the synchrotron and synchrotron self-Compton (SSC) process. The model parameters of the emission region of the source are found to be consistent with the values reported in the literature.

New method of analysis for TeV gamma-ray data collected by the TACTIC telescope

We describe a new fractal and wavelet based framework for analysing atmospheric Cerenkov images of extensive air showers. The methodology developed using simulated data has been tested with the data collected from Mrk421 and Crab nebula directions during 10 October 2009–16 May 2010 by the TACTIC telescope. The telescope detected flaring activity in TeV gamma-rays from Mrk421 during 15th-17th February 2010. Hillas parameter analysis of Cerenkov images showed presence of TeV γ-ray signal with statistical significance of 12.5 σ beyond 1TeV energy for the 265 hours data set. Based on a Monte-Carlo simulated database of Cerenkov images recorded by the TACTIC telescope, the wavelet dimension B6 can be exploited as a useful parameter to segregate gamma-ray initiated events from hadron initiated events with better efficiency. We derive a preliminary value of the relevant significance for gamma initiated events using wavelet parameter B6 for the 265 hours long observation spell of Mrk421 including the flaring episode during 15th–17th February 2010 and 66 hours data of observations on Crab nebula. Wavelet analysis approach has also been tested on Crab OFF- source data of 27 hours. Application of wavelet parameter B6 on the recorded Cerenkov images of the TACTIC telescope reveals better results in terms of segregation efficiency and enhancement of the determined value of statistical significance as compared to the Hillas parameters approach.

Multi-wavelength study of Mrk 421 TeV flare observed with TACTIC telescope in February 2010

We present results from multi-wavelength study of intense flaring activity from a high frequency peaked BL Lac object Mrk 421. The source was observed in its flaring state on February 16, 2010 with the TACTIC at energies above 1.5TeV. Near simultaneous multi-wavelength data were obtained from high energy (MeV–GeV) γ-ray observations with Fermi–LAT, X-ray observations by the Swift and MAXI satellites, optical V-band observation by SPOL at Steward Observatory and radio 15GHz observation at OVRO 40 meter-telescope. We have performed a detailed spectral and temporal analysis of TACTIC, Fermi–LAT and Swift–XRT observations of Mrk 421 during February 10–23, 2010 (MJD 55237–55250). The flaring activity of the source is studied by investigating the properties of daily light curves from radio to TeV energy range and we present the correlation and variability analysis in each energy band. The TeV flare detected by TACTIC on February 16, 2010 is well correlated with the activity in lower energy bands. The differential energy spectrum of the source, in the energy range 1.5–11TeV, as measured by TACTIC on this night is described by a power law (dN/dE∝E-Γ) with spectral index Γ=2.6±0.3. After accounting for absorption of TeV photons by low energy extragalactic background light photons via pair production, the intrinsic TeV spectrum reveals a power law index of 2.3±0.3. Finally the broad band spectral energy distribution of the source in flaring state is reproduced using a simple emission model involving synchrotron and synchrotron self Compton processes. The obtained parameters are then used to understand the energetics of the source during the flaring episode.

Artificial Neural Network based [formula omitted] segregation methodology for TACTIC telescope

The sensitivity of a Cherenkov imaging telescope is strongly dependent on the rejection of the cosmic-ray background events. The methods which have been used to achieve the segregation between the γ-rays from the source and the background cosmic-rays, include methods like Supercuts/Dynamic Supercuts, Maximum likelihood classifier, Kernel methods, Fractals, Wavelets and random forest. While the segregation potential of the neural network classifier has been investigated in the past with modest results, the main purpose of this paper is to study the γ/hadron segregation potential of various ANN algorithms, some of which are supposed to be more powerful in terms of better convergence and lower error compared to the commonly used Backpropagation algorithm. The results obtained suggest that Levenberg–Marquardt method outperforms all other methods in the ANN domain. Applying this ANN algorithm to ∼101.44h of Crab Nebula data collected by the TACTIC telescope, during November 10, 2005–January 30, 2006, yields an excess of ∼(1141±106) with a statistical significance of ∼11.07σ, as against an excess of ∼(928±100) with a statistical significance of ∼9.40σ obtained with Dynamic Supercuts selection methodology. The main advantage accruing from the ANN methodology is that it is more effective at higher energies and this has allowed us to re-determine the Crab Nebula energy spectrum in the energy range ∼1–24TeV.

Cosmic-ray composition with TACTIC telescope using Fractal and Wavelet analysis

A preliminary flux estimate of various cosmic-ray constituents based on the atmospheric Cerenkov light flux of extensive air showers using fractal and wavelet analysis approach is proposed. Using a Monte-Carlo simulated database of Cerenkov images recorded by the TACTIC telescope, we show that one of the wavelet parameters (wavelet dimension B6) provides ⩾90% segregation of the simulated events in terms of the primary mass. We use these results to get a preliminary estimate of primary flux for various cosmic-ray primaries above 5TeV energy. The simulation based flux estimates of the primary mass as recorded by the TACTIC telescope are in good agreement with the experimentally determined values.

Simulation studies for optimizing the trigger generation criteria for the TACTIC telescope

In this paper, we present the results of Monte Carlo simulations of γ ‐ ray and cosmic-ray proton induced extensive air showers as detected by the TACTIC atmospheric Cherenkov imaging telescope for optimizing its trigger field of view and topological trigger generation scheme. The simulation study has been carried out at several zenith angles. The topological trigger generation uses a coincidence of two or three nearest neighbor pixels for producing an event trigger. The results of this study suggest that a trigger field of 11×11 pixels ( ∼ 3 . 4 ∘ × 3 . 4 ∘ ) is quite optimum for achieving maximum effective collection area for γ ‐ rays from a point source. With regard to optimization of topological trigger generation, it is found that both two and three nearest neighbor pixels yield nearly similar results up to a zenith angle of 25 ∘ with a threshold energy of ∼ 1.5 TeV for γ ‐ rays . Beyond zenith angle of 25 ∘ , the results suggest that a two-pixel nearest neighbor trigger should be preferred. Comparison of the simulated integral rates has also been made with corresponding measured values for validating the predictions of the Monte Carlo simulations, especially the effective collection area, so that energy spectra of sources (or flux upper limits in case of no detection) can be determined reliably. Reasonably good matching of the measured trigger rates (on the basis of ∼ 207 h of data collected with the telescope in NN-2 and NN-3 trigger configurations) with that obtained from simulations reassures that the procedure followed by us in estimating the threshold energy and detection rates is quite reliable.

Search for diffuse cosmic gamma-ray flux using fractal and wavelet analysis from galactic region using single imaging Cerenkov telescopes

We show from a simulations-based study of the TACTIC telescope that fractal and wavelet analysis of Cerenkov images, recorded in a single imaging Cerenkov telescope, enables almost complete segregation of isotropic gamma-ray initiated events from the overwhelming background of cosmic ray hadron-initiated events. This presents a new method for measuring galactic and extragalactic gamma-ray background above 1 TeV energy. Preliminary results based on this method are reported here. Primary aim is to explore the possibility of using data recorded by a single imaging atmospheric Cerenkov telescope ( IACT) for making accurate measurements of diffuse galactic and extragalactic gamma-ray flux above ∼1 TeV energy. Using simulated data of atmospheric Cerenkov images recorded in an IACT, initiated, both, by cosmic ray protons and diffuse gamma-rays with energies above 4 TeV and 2 TeV respectively, we identify the most efficient fractal/wavelet parameters of the recorded images for primary identification. The method is based on the pattern recognition technique and employs fractal and wavelet analysis of the recorded Cerenkov images for gamma-hadron segregation. We show that the value of wavelet dimension parameter B 6 can segregate Cerenkov images of hadronic origin from those of diffuse gamma-ray origin with almost 100% accuracy. We use the results to get a preliminary upper limit estimate of the diffuse galactic gamma-ray flux within galactic range of ∣ b∣ ⩽ −5° and ∣ l∣ ⩽ 200° above 2 TeV from a 36 h data set recorded by the TACTIC telescope.

Search for TeV γ-rays from H1426+428 during 2004–2007 with the TACTIC telescope

The BL Lac object H1426+428 (z ≡ 0.129) is an established source of TeV γ-rays and detections of these photons from this object also have important implications for estimating the extragalactic background light in addition to the understanding of the particle acceleration and γ-ray production mechanisms in the AGN jets. We have observed this source for about 244 h in 2004, 2006 and 2007 with the TACTIC γ-ray telescope located at Mt Abu, India. Detailed analysis of these data do not indicate the presence of any statistically significant TeV γ-ray signal from the source direction. Accordingly, we have placed an upper limit of ⩽1.18 × 10−12 photons cm−2 s−1 on the integrated γ-ray flux at 3σ significance level.

ANN-based energy reconstruction procedure for TACTIC [formula omitted]-ray telescope and its comparison with other conventional methods

The energy estimation procedures employed by different groups, for determining the energy of the primary γ-ray using a single atmospheric Cherenkov imaging telescope, include methods like polynomial fitting in SIZE and DISTANCE, general least square fitting and look-up table based interpolation. A novel energy reconstruction procedure, based on the utilization of Artificial Neural Network (ANN), has been developed for the TACTIC telescope. The procedure uses a 3:30:1 ANN configuration with resilient backpropagation algorithm to estimate the energy of a γ-ray like event on the basis of its image SIZE, DISTANCE and zenith angle. The new ANN-based energy reconstruction method, apart from yielding an energy resolution of ∼26%, which is comparable to that of other single imaging telescopes, has the added advantage that it considers zenith angle dependence as well. Details of the ANN-based energy estimation procedure along with its comparative performance with other conventional energy reconstruction methods are presented in the paper and the results indicate that amongst all the methods considered in this work, ANN method yields the best results. The performance of the ANN-based energy reconstruction has also been validated by determining the energy spectrum of the Crab Nebula in the energy range 1–16TeV, as measured by the TACTIC telescope.

Observations of TeV γ-rays from Mrk 421 during December 2005 to April 2006 with the TACTIC telescope

The TACTIC γ-ray telescope has observed Mrk 421 on 66 clear nights from December 07, 2005 to April 30, 2006, totalling ∼202 h of on-source observations. Here, we report the detection of flaring activity from the source at ⩾1 TeV energy and the time-averaged differential γ-ray spectrum in the energy range 1–11 TeV for the data taken between December 27, 2005 and February 07, 2006 when the source was in a relatively higher state as compared to the rest of the observation period. Analysis of this data spell, comprising ∼97 h reveals the presence of a ∼12.0 σ γ-ray signal with daily flux of >1 Crab unit on several days. A pure power law spectrum with exponent −3.11 ± 0.11 as well as a power law spectrum with an exponential cutoff ( Γ = −2.51 ± 0.26 and E 0 = (4.7 ± 2.1) TeV) are found to provide reasonable fits to the inferred differential spectrum within statistical uncertainties. We believe that the TeV light curve presented here, for nearly 5 months of extensive coverage, as well as the spectral information at γ-ray energies of >5 TeV provide a useful input for other groups working in the field of γ-ray astronomy.

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.