Blazars, a subclass of face-on jetted active galactic nuclei, are the most persistent and powerful sources of cosmic gamma-rays in the Universe. They represent a dominant population of gamma-ray sources, spread all over the sky with known and unknown redshifts up to z similar to 6 (e.g. Belladitta et al. (2020)). Detection of more than 3500 blazars by the space-based Fermi-Large Area Telescope (LAT) in the high energy (HE, above 100 MeV) band reveals that a significant amount of power carried by the relativistic plasma jets in blazars is dissipated at gamma-ray energies. However, a small fraction of only less than 3% are detected at very high energies (VHE, above 30 GeV) by the ground-based Cherenkov telescopes. Therefore, their detection at the highest possible energies is extremely important to understand the blazar-phenomenon in the Universe. In this paper, we report observational results obtained from the recent monitoring of the six high redshift (z > 0.3) blazars with the Major Atmospheric Cherenkov Experiment (MACE) in the energy range above similar to 80 GeV. No statistically significant detection of the sources is found. Thus, for the null detections, we estimate 99% confidence level upper limit on the integral flux for the individual sources and use the Fermi-LAT measurements to constrain their gamma-ray emission behavior.

The radio galaxy NGC 1275, located at the central region of Perseus cluster, is a well-known very high-energy (VHE) gamma-ray emitter. The Major Atmospheric Cherenkov Experiment Telescope has detected two distinct episodes of VHE (E > 80 GeV) gamma-ray emission from NGC 1275 during 2022 December and 2023 January. The second outburst, observed on 2023 January 10, was the more intense of the two, with flux reaching 58% of the Crab Nebula flux above 80 GeV. The differential energy spectrum measured between 80 GeV and 1.5TeV can be described by a power law with a spectral index of Gamma=-2.90 +/- 0.16(stat) for both flaring events. The broadband spectral energy distribution derived from these flares, along with quasisimultaneous low-energy counterparts, suggests that the observed gamma-ray emission can be explained using a homogeneous single-zone synchrotron self-Compton model. The physical parameters derived from this model for both flaring states are similar. The intermediate state observed between two flaring episodes is explained by a lower Doppler factor or magnetic field, which subsequently returned to its previous value during the high-activity state observed on 2023 January 10.

The Major Atmospheric Cherenkov Experiment (MACE) is a large size (21m) Imaging Atmospheric Cherenkov Telescope (IACT) installed at an altitude of 4270m above sea level at Hanle, Ladakh in northern India. Here we report the detection of Very High Energy (VHE) gamma-ray emission from Crab Nebula above 80 GeV. We analysed ~15 hours of data collected at low zenith angle between November 2022 and February 2023. The energy spectrum is well described by a log-parabola function with a flux of ~(3.46 +/- 0.26stat) x 10-10 TeV-1 cm-2 s-1, at 400 GeV with spectral index of 2.09 +/- 0.06stat and a curvature parameter of 0.08 +/- 0.07stat. The gamma-rays are detected in an energy range spanning from 80 GeV to ~5 TeV. The energy resolution improves from ~34% at an analysis energy threshold of 80 GeV to ~21% above 1 TeV. The daily light curve and the spectral energy distribution obtained for the Crab Nebula is in agreement with previous measurements, considering statistical and systematic uncertainties.

ABSTRACT 1ES 1959+650, a high-energy peaked BL Lac object (HBL) with redshift z = 0.047, is known to exhibit flux and spectral variability in different energy bands. In this work, we primarily considered the simultaneous Fermi-LAT and Swift-XRT data of 1ES 1959+650 to study the flux and spectral variability in the energy ranges (0.1–300 GeV) and (0.6–10 keV), respectively. Using the Fermi-LAT light curve, a stretch of quiescent state and three high flux states of the source were identified. The flux and the spectral variability were studied using flare profiles and flux-index loops. In the γ-ray energies, 1ES 1959+650 clearly showed the evolution ‘softer-when-brighter’ whereas in the X-rays it showed the trend ‘harder-when-brighter’. 1ES 1959+650 also showed quick changes in spectral shape in the γ-ray band when the γ-ray flux was changing during the flares. The spectral energy distributions (SEDs) of the source were constructed using simultaneous data from Fermi-LAT, Swift-XRT, Swift-UVOT during its evolution. The TeV data from TACTIC imaging telescope were also used to construct the SED in the quiescent state. The spectral modelling of the SEDs revealed that due to the spectral variability of 1ES 1959+650, the standard leptonic one zone model cannot be used to explain the emission from this source. Instead a two zone leptonic model is needed to explain the observed SEDs from this blazar.

MACE (Major Atmospheric Cherenkov Experiment), an imaging atmospheric Cherenkov telescope, has recently been installed by the HiGRO (Himalayan Gamma-Ray Observatory) collaboration at Hanle (32.8$^\circ$N, 78.9$^\circ$E, 4270m asl) in Ladakh region of North India. The telescope has a 21m diameter large light collector consisting of indigenously developed 1424 square-shaped diamond turned spherical aluminum mirror facets of size $\sim$ 0.5m$\times$0.5m. MACE is the second largest Cherenkov telescope at the highest altitude in the northern hemisphere. The imaging camera of the telescope consists of 1088 photo-multiplier tubes with a uniform pixel resolution of $\sim 0.125^\circ$ covering a field of view of $\sim$ 4.0$^\circ$ $\times$ 4.0$^\circ$. The main objective of the MACE telescope is to study gamma-ray sources mainly in the unexplored energy region 20 -100 GeV and beyond with high sensitivity. In this paper, we describe the key design features and current status of MACE including results from the trial observations of the telescope.

The MACE telescope has recently been commissioned at Hanle, Ladakh, India. It had its first light in April 2021 with a successful detection of very high energy gamma-ray photons from the standard candle Crab Nebula. Equipped with a large light collector of 21 m diameter and situated at an alti-tude of similar to 4.3 km amsl, the MACE telescope is expected to explore the mysteries of the non-thermal Universe in the energy range above 20 GeV with very high sensitivity. It can also play an important role in carrying out multi-messenger astronomy in India.

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.

Located in the Perseus galaxy cluster, NGC 1275 (z = 0.0179), is the brightest radio galaxy detected in both high energy (HE; E > 100 MeV) and very high energy (VHE; E > 100 GeV) regimes. Here we present the results from the observations of NGC 1275 with TACTIC for 37.1 h between December, 2016 and February, 2017. In the absence of a statistically significant VHE gamma-ray detection from the TACTIC observations, we estimate the 3 sigma upper limit on the integral flux above 0.85 TeV to be 2.85 x 10(-11) erg cm(-2) s(-1). We also report the results of near simultaneous multiwavelength observations of the source from the Fermi-LAT, Swift-XRT/UVOT and OVRO. We find the source to be in flaring state on the night of 31 December, 2016 - 1 January, 2017 when the HE gamma-ray flux above 0.1 GeV went up to 2.0 x 10(-6) ph cm(-2) s(-1) which is approximately 4 times the flux level during its low state. We present the broad-band spectral energy distributions (SED) of the source during its flaring and low states. It is evident from the SED that the VHE upper limit obtained from the TACTIC observations is consistent with the low emission state of the source.

In this work, we report on the detection of enhanced TeV gamma- 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 gamma- ray like events from Mrk 421 with a statistical significance of 5.17 sigma in a live time of similar to 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) gamma- ray emission from the source at shorter timescales. A minimum variability timescale of similar to 0.72 days is obtained for the TeV gamma- 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 gamma- ray flux above 0.85 TeV is determined to be (3.68 +/- 0.64) x 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) x 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) gamma- ray emission during the high activity state of the source. The HE gamma- 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 gamma- 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 gamma- 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. (C) 2018 Elsevier B.V. All rights reserved.

•Long term observation (three years) with TACTIC telescope in search for TeV gamma-rays from IC 310.•Nearly simultaneous observation with Fermi-LAT and from ARIES observatory to understand emission mechanism.•Upper limits on integral flux in GeV and TeV energies indicate the source to be at low- state of emission.•Optical fluxes from the source, possibilily contaminated with host-galaxy emission, show variability.

We observed the TeV blazar Mrk421 in the very high energy (VHE) region with TACTIC from 21 December, 2014 to 21 February, 2015. The VHE gamma-ray signal is detected from the source at a statistical significance of similar to 9 sigma with 648 +/- 74 gamma-ray like events using a total observation time of 65.6 h of clean data. To denote the active period of the source, the entire observation has been divided into two spells: Spell I (21-30 December, 2014; MJD: 57012-57021) and Spell II (15 January-21 February, 2015; MJD: 57037-57074). The Spell I corresponds to relatively higher activity state wherein 337 +/- 47 gamma-rays are detected at a statistical significance of 7.39 sigma in 21.7 h. During Spell II, 311 +/- 57 gamma-ray events have been detected in similar to 44c at a statistical significance of 5.6 sigma. The analysis of high energy (HE) gamma-ray data from Fermi-LAT has been carried out for the period of TACTIC observations. In addition, near simultaneous multi-wavelength data from Swift-BAT and MAXI in the X-ray region, SPOL in optical V-band and OVRO at 15 GHz radio band have also been considered. The HE gamma-ray and optical observations indicate variability during both the spells whereas hard X-ray observations are consistent with the average emission through out the TACTIC observations. The broad-band spectral energy distributions of the source are reproduced by single zone homogeneous synchrotron self Compton process for the two spells. The model parameters estimated for the two spells are similar except for the particle energy density which is obtained to be slightly higher during Spell I.

In this work, we have reported the results of multiwavelength study of the Fermi hard spectrum blazar B2 0806+35 using data from Swift, Fermi, and TACTIC observations. We analysed Swift/UVOT, Swift/XRT, Fermi/LAT, and TACTIC data to construct the sepctral energy distribution for different epochs over last 7 yr. We modelled the spectral energy distributions using one zone leptonic blazar emission model and emission from optically thick geometrically thin accretion disc around the black hole. It is shown that the photon spectrum in the GeV-TeV energy range is steep and the source is not a good candidate to study intergalactic magnetic field. It is also shown that the accretion disc is systematically receding from the central black hole and the accretion flow is possibly advection dominated.

Strong X-ray and gamma-ray flares have been detected in February 2010 from the high synchrotron peaked blazar Mrk 421 (z = 0.031). With the motivation of understanding the physics involved in this flaring activity, we study the variability of the source in X-ray and gamma-ray energy bands during the period February 10-23, 2010 (MJD 55237-55250). We use near simultaneous X-ray data collected by MAXI, Swift-XRT and gamma-ray data collected by Fermi-LAT and TACTIC along with the optical V-band observations by SPOL at Steward Observatory. We observe that the variation in the one day averaged flux from the source during the flare is characterized by fast rise and slow decay. Besides, the TeV gamma-ray flux shows a strong correlation with the X-ray flux, suggesting the former to be an outcome of synchrotron self Compton emission process. To model the observed X-ray and gamma-ray light curves, we numerically solve the kinetic equation describing the evolution of particle distribution in the emission region. The injection of particle distribution into the emission region, from the putative acceleration region, is assumed to be a time dependent power law. The synchrotron and synchrotron self Compton emission from the evolving particle distribution in the emission region are used to reproduce the X-ray and gamma-ray flares successfully. Our study suggests that the flaring activity of Mrk 421 can be an outcome of an efficient acceleration process associated with the increase in underlying non-thermal particle distribution. (C) 2016 Elsevier B.V. All rights reserved.

We have observed Markarian 501 in Very High Energy (VHE) gamma-ray wavelength band for 70.6 hours from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of similar to 66.3 hours of clean data revealed the presence of a TeV gamma-ray signal (686 +/- 77 gamma-ray events) from the source direction with a statistical significance of 8.89? above 850 GeV. Further, a total of 375 +/- 47 gamma-ray like events were detected in 25.2 hours of observation from 22 - 27 May, 2012 with a statistical significance of 8.05 sigma 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=f(0)E(-Gamma) with f(0)=(2.27 +/- 0.38)x10(-11) photons cm(-2) s(-1) TeV-1 and Gamma=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. (C) 2017 Elsevier B.V. All rights reserved.

The nearby (z = 0.031) TeV gamma-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 gamma-rays with 6.9 sigma 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. (C) 2016 Elsevier B.V. All rights reserved.

In this paper, we present results of TeV gamma-ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z = 0.182) 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 39.62 h and no evidence of TeV gamma-ray activity is found from the source. The corresponding 99% confidence level upper limit on the integral flux above a threshold energy of 1.1 TeV is estimated to be 3.41 x10(-12) photons cm(-2) s(-1) (i.e. <23% Crab Nebula flux) assuming a power law differential energy spectrum with photon index 3.0, as previously observed by the MAGIC and VERITAS telescopes. For the study of multi-wavelength emission from the source, we use nearly simultaneous optical, UV and X-ray data collected by the UVOT and XRT instruments on board the Swift satellite and high energy gamma-ray data collected by the Large Area Telescope on board the Fermi satellite. We also use radio data at 15 GHz from OVRO 40 m telescope in the same period. No significant increase of activity is detected from radio to TeV gamma-rays from 1ES1218+304 during the period from March 1, 2013 to April 15, 2013. (C) 2014 Elsevier B.V. All rights reserved.

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.

The TeV atmospheric Cherenkov telescope with imaging camera (TACTIC) γ -ray telescope has been in operation at Mt. Abu, India since 2001 to study TeV γ -ray emission from celestial sources. During the last 10 years, apart from consistently detecting a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0 σ in ∼25 h, the telescope has also detected flaring activity from Mrk 421 and Mrk 501 on several occasions. Although we used Crab Nebula data partially, in some of the reported results, primarily for testing the validity of the full data analysis chain, the main aim of this work is to study the long term performance of the TACTIC telescope by using consolidated data collected between 2003 and 2010. The total on-source data, comprising ∼402 h, yields an excess of ∼(3742±192) γ -ray events with a statistical significance of ∼19.9 σ . The off-source data, comprising ∼107 h of observation, is found to be consistent with a no-emission hypothesis, as expected. The resulting γ -ray rate for the on-source data is determined to be ∼(9.31±0.48) h −1 . A power law fit (d Φ /d E = f 0 E − Γ ) with f 0 ∼ (2.66±0.29) × 10 −11 cm −2 s −1 TeV −1 and Γ ∼ 2.56±0.10 is found to provide reasonable fit to the inferred differential spectrum within statistical uncertainties. The spectrum matches reasonably well with that obtained by other groups. A brief summary of the improvements in the various subsystems of the telescope carried out recently, which has resulted in a substantial improvement in its detection sensitivity (viz., ∼5 σ in an observation period of ∼13 h as compared to ∼25 h earlier) are also presented in this paper. Encouraged by the detection of strong γ -ray signals from Mrk 501 and Mrk 421 on several occasions, 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.

In this paper, we present results of TeV γ–ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The obse rvations are primarily motivated by the unusually hard GeV-TeV spectrum of t he source despite its relatively large redshift. The source is observed in the TeV energy range with theTACTIC from March 1, 2013 to April 15, 2013 (MJD 56352–56397) for a total observation time of 39.62 h 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 1.1 TeV is estimated to be 3.41×10 photons cm s (i.e< 23% Crab Nebula flux) assuming a power law differential Email address: kksastro@barc.gov.in (K K Singh) Preprint submitted to New Astronomy January 26, 2022 energy spectrum with photon index 3.0, as previously observ ed by theMAGIC andV ERITAS telescopes. For the study of multi-wavelength emission fro m the source, we use nearly simultaneous optical, UV and and X– ray data collected by the UVOT and XRT instruments on board the Swift satellite and high energy γ–ray data collected by the Large Area Telescope on board the Fermi satellite. We also use radio data at 15 GHz from OVRO 40 m telescope in the s ame period. No significant increase of activity is detected from ra dio to TeVγ–rays from 1ES1218+304 during the period from March 1, 2013 to April 15, 2013.