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Register a new userObservations of Mkn 421 using Pachmarhi Array of v{C}erenkov Telescopes
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Pachmarhi Array of v{C}erenkov Telescopes (PACT), based on wavefront sampling technique, has been used for detecting TeV gamma rays from galactic and extra-galactic $gamma $-ray sources. The Blazar, Mkn 421 was one such extra-galactic source observed during the winter nights of 2000 and 2001. We have carried out a preliminary analysis of the data taken during the nights of January, 2000 and 2001. Results show a significant gamma ray signal from this source during both these periods above a threshold energy of 900 GeV. The source was contemporaneously observed by CAT imaging telescope during the first episode of January 2000 while HEGRA CT1 was observing the source during the second episode. Both these observations have detected variable $gamma $-ray emission this source and they reported that it was flaring during both these periods. The light curve in the TeV gamma ray range derived from the first PACT observations during both these episodes is in agreement with that reported by other experiments. The analysis procedure and the preliminary results will be presented and discussed.
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Context:The HAGAR Telescope Array at Hanle, Ladakh has been regularly monitoring the nearby blazar Mkn 421 for the past 7yrs. Aims: Blazars show flux variability in all timescales across the electromagnetic spectrum. While there is abundant literature characterizing the short term flares from different blazars, comparatively little work has been done to study the long term variability. We aim to study the long term temporal and spectral variability in the radiation from Mkn 421 during 2009-2015. Methods: We quantify the variability and lognormality from the radio to the VHE bands, and compute the correlations between the various wavebands using the z-transformed discrete correlation function. We construct the Spectral Energy Distribution (SED) contemporaneous with HAGAR observation seasons and fit it with a one zone synchrotron self Compton model to study the spectral variability. Results: The flux is found to be highly variable across all time scales. The variability is energy dependant, and is maximum in the X-ray and Very High Energy (VHE) bands. A strong correlation is found between the Fermi-LAT (gamma) and radio bands, and between Fermi-LAT and optical, but none between Fermi-LAT and X-ray. Lognormality in the flux distribution is clearly detected. This is the third blazar, following BL~Lac and PKS~2155$+$304 to show this behavior. The SED can be well fit by a one zone SSC model, and variations in the flux states can be attributed mainly due to changes in the particle distribution. A strong correlation is seen between the break energy $gamma_b$ of the particle spectrum and the total bolometric luminosity.
The Pachmarhi Array of Cerenkov Telescopes consists of a distributed array of 25 telescopes that are used to sample the atmospheric Cerenkov Photon showers. Each telescope consists of 7 parabolic mirrors each viewed by a single photo-multiplier tube. Reconstruction of photon showers are carried out using fast timing information on the arrival of pulses at each PMT. The shower front is fitted to a plane and the direction of arrival of primary particle initiating the shower is obtained. The error in the determination of the arrival direction of the primary has been estimated using the {it split} array method. It is found to be $sim 2.4^prime ~$ for primaries of energy $ > 3 ~TeV$. The dependence of the angular resolution on the separation between the telescopes and the number of detectors are also obtained from the data.
Pachmarhi Array of v Cerenkov Telescopes (PACT) consists of a 5$times$5 array of v Cerenkov telescopes deployed over an area of 100 $m$ $times$ 80 $m$, in the form of a rectangular matrix. The experiment is based on atmospheric v Cerenkov technique using wavefront sampling technique. Each telescope consists of 7 parabolic mirrors mounted para-axially on an equatorial mount. At the focus of each mirror a fast phototube is mounted. In this experiment a large number of parameters have to be measured and recorded from each of the 175 phototubes in the shortest possible time. Further, the counting rates from each phototube as well as the analog sum of the 7 phototubes from each telescope (royal sum) need to be monitored at regular intervals during the run. In view of the complexity of the system, the entire array is divided into four smaller sectors each of which is handled by an independent field signal processing centre (FSPC) housed in a control room that collects, processes and records information from nearby six telescopes that belong to that sector. The distributed data acquisition system (DDAS) developed for the purpose consists of stand-alone sector data acquisition system (SDAS) in each of the four FSPCs and a master data acquisition system (MDAS). MDAS running in the master signal processing centre (MSPC) records data from each of the 25 telescopes. The data acquisition and monitoring PCs (SDAS and MDAS) are networked through LAN. The entire real time software for DDAS is developed in C under $linux$ environment. Most of the hardware in DDAS are designed and fabricated in-house. The design features and the performance of the entire system along with some other auxiliary systems to facilitate the entire observations will be presented.
Mkn 421 was repeatedly observed with BeppoSAX in 1997-1998. We present highlights of the results of the thorough temporal and spectral analysis discussed by Fossati et al. (1999) and Maraschi et al. (1999), focusing on the flare of April 1998, which was simultaneously observed also at TeV energies. The detailed study of the flare in different energy bands reveals a few very important new results: (a) hard photons lag the soft ones by 2-3 ks *a behavior opposite to what is normally found in High energy peak BL Lacs X-ray spectra*; (b) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. Moreover the spectral evolution during the flare has been followed by extracting X-ray spectra on few ks intervals, allowing to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase, and then receding. The spectral analysis confirms the delay in the flare at the higher energies, as above a few keV the spectrum changes only after the peak of the outburst has occurred. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process. A theoretical picture accounting for all the observational constraints is discussed, where electrons are injected at low energies and then progressively accelerated during the development of the flare.
A detection of Gamma-rays from Mkn 421 at energies above 1 TeV is reported, based on observations made in December 1994 - May 1995 with the first two HEGRA Cherenkov telescopes. >From the image analysis, 111 excess gamma candidates are obtained from the 26 h Telescope #1 (CT1) dataset (significance 4.0 sigma) and 218 from the 41 h Telescope #2 (CT2) dataset (significance 4.2 sigma) at zenith angles theta < 25 degrees. The combined significance is approx. 5.8 sigma. This is the second detection of Mkn 421 at TeV energies. The average excess rate is 4.3 +- 1.0 h^-1 for CT1 and 5.4 +- 1.3 h^-1 for CT2. Comparison with our contemporary observations of the Crab Nebula indicates that Mkn 421 has a steeper spectrum than the Crab Nebula above 1 TeV. Under the assumption that the spectrum of Mkn 421 follows a power law, we obtain a differential spectral index of 3.6 +- 1.0 and an integral flux above 1 TeV of 8 (+-2)_(Stat) (+6-3)_(Syst) x 10^-12 cm^-2 s^-1 from a comparison with Monte Carlo data. This flux is smaller than the Crab Nebula flux by a factor of 2.0 +- 0.8.
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