Bose, D

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29th International Cosmic Ray Conference Pune (2005) 4, 343346Observation of AGNs with PACTBose D. , Acharya B.S. , Chitnis V. R. , Singh B.B. and Vishwanath P. R.(a) Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India(b) Indian Institute of Astrophysics, Koramangala, Bangalore 560034, IndiaPresenter: Bose D. (debanjan@tifr.res.in), ind-bose-D-abs1-og23-oralWe report our observations on 4 AGNs viz., Mkn 421, Mkn501, IES1426+428 and ON231 belonging to a classof objects called blazars. The observations were carried out using the Pachmarhi Array of Cˇerenkov Telescopes(PACT) and span about 5 year period from 2000 to 2004. We discuss our methods of analysis adopted to extractthe gamma-ray signal from cosmic-ray background. We present our results on the emission of TeV gammarays from these objects.1. IntroductionAGNs have dominated extragalactic astronomy by virtue of their great luminosities. It is generally acceptedthat these AGNs are powered by black holes of mass 10M  . About 10% of all AGNs are more luminousat radio wavelengths than at optical ones and are, hence, called radio-loud. The radio emission is believedto originate in the associated jets which are aligned with the poles of the spinning black hole. If the jetsaim straight towards us, both the waves and the durations of outbursts are compressed by a large Dopplerfactor. This greatly enhances the power we receive. Such AGNs with jets directed towards us are also calledBlazars. Blazars are characterised by two parts in their Spectral Energy Distributions (SEDs). First part inSED rises smoothly from radio wavelengths upto a broad peak spanning the range from microwave to infraredwavelengths and is due to relativistic electrons radiating via synchrotron process. Second part is probably dueto inverse compton scattering of synchrotron photons by the same electrons and is characterised by a peakin SEDs in hard X-ray-  -ray band. On a power plot these SEDs show a two-humped shape [1]. One of thecharacteristic features of these blazars is their time variability on scales ranging from hours to years. Blazarsare further subdivided into two classes, BL Lac and optically violent variables (OVVs). BL Lacs are alsoclassified into two groups, Low-frequency BL Lacs (LBLs) and High-frequency BL Lacs (HBLs). Mkn 421,Mkn 501, 1ES1426+428, ON231 are examples of high frequency peaked BL Lac objects with  -ray emissionextending upto TeV energies. They are found to be in quiescent state most of the time but occasionally theyflare up. In the flaring state often they are the brightest objects in VHE  -rays. There are mainly two modelswhich are proposed to explain the production as well as variability of  -rays in these sources. One is called’lepton model’ and the other ’hadron model’ depending on the particle that is responsible for the production of -rays. The HE and VHE  -ray observations constrain both the models but do not rule either out. To furtherconstrain these models and also to understand the astrophysical processes in the environment of these objectsthey are being observed extensively by the ground based  -ray telescopes and also in other wavelengths. Wehave observed these sources with PACT on several occasions. In the following sections we will present ouranalysis procedure and results for the data collected on these sources from year 2000 to 2004.2. PACT ArrayPachmarhi array of Cˇerenkov telescopes (PACT) is located at Pachmarhi in central India (latitude 22 	 28  N,longitude 78 	 25  E, altitude 1075  ) [2]. PACT consists of 5  5 array of 25 telescopes spread over anarea of 80m  100m. Each telescope consists of 7 parabolic mirrors of diameter 0.9m with f/d  1 mountedpara-axially. Total reflector area is 4.45 m  per telescope. A fast phototube (EMI9807B) of 2 in.344 Bose D. et alTable 1. Observation log for Blazarsyear observation duration (mins.)Mkn 421 Mkn 501 1ES1426+428 ON 231z=0.030 z=0.034 z=0.129 z=0.1022000 3510. 710. – –2001 1960. – – –2002 1860. 510. 1520. –2003 1770. 840. 570. 510.2004 2270. 780. 870. 550.2005 930. – 960. –is mounted at the focus of each mirror. All the telescopes are equatorially mounted and are independentlysteerable in both E-W and N-S direction within  45 	 . The movement of the telescopes is remotely controlledby a low cost control system called Automated Computerized Telescope Orientation System (ACTOS). Thesystem can orient to the putative source from an arbitrary initial position with an accuracy of  (0.003  0.2) [3].The array is divided into 4 sectors with 6 telescopes in each. At the center of each sector there is a Field SignalProcessing Center (FSPC) where pulses from individual PMTs of telescopes in a sector are brought throughcoaxial cables of type RG213 and processed. High voltage to PMTs are controlled through a high voltagedivider unit (C.A.E.N. model SY170) which is controlled by CAMAC based controller module (C.A.E.N.model CY117B). A coincidence of 4 out of 6 telescopes generates the event trigger for a sector. Once an eventtrigger is generated CAMAC controller initiates the data recording process. A  based PC records TDC(timing) and QDC (photon density) information of 6 peripheral mirrors of telescopes along with other housekeeping informations for that event. In addition, information relevant to entire array is recorded in the centralMaster Signal Processing Center (MSPC) in the control room at the centre of the array. PACT has energythreshold of  800 GeV for  rays incident in the vertical direction and the corresponding collection area isﬀﬂﬁﬃ ! .3. ObservationsSince the year 2000 we have collected data on Mkn 421, Mkn 501, 1ES1426+428 and ON231 using PACT.Most of these runs were taken with all 4 sectors pointing to the source direction (ON-source). On the samenight background runs (OFF-source) were taken before or after the source runs with all telescopes pointing to adark region (a region with same declination as that of source but offset in RA) such that zenith angle coverageis same as source run. Some of the runs in year 2002 and in 2003 were taken with 2 sectors pointing at thesource and other 2 sectors at a background region. Data were also collected on fictitious sources, in thesecases the source region is an arbitrary one (a region devoid of any known " ray sources). Table 1 summarisesdurations for ON-source observation of these blazars. Similarly OFF-source observations were also carried outfor each of them with same durations.4. Data Analysis and ResultsA number of preliminary checks on the data were carried out before doing actual analysis. A large fraction ofdata collected (  50%) were rejected as they were found to be noisy. Telescopes having similar efficienciesfor source and background runs were retained for further analysis. The arrival direction of each shower isObservation of AGNs.... 345determined by reconstructing shower front using the relative arrival time of Cˇerenkov front at the telescopes.This Cˇerenkov photon front is fitted with a plane, the normal to this plane gives the direction of shower axis.By comparing the observed and expected arrival times of pulses at the telescopes events with bad fits arerejected. Thus the space angle between the direction of shower axis and the direction of source is obtained forall events with # 8 telescopes. The space angle distribution peaks around  1.5 	 and has a FWHM of  2.5 	 .The space angle distribution become narrower and the peak shifts to lower angle as the degrees of freedom(no. of telescopes with timing information) increases. Space angle distributions of all the source runs arecompared with respective distributions of background runs. For this comparison it is ensured that both sourceand background runs have the same zenith angle coverage as well as similar distribution of events as a functionof the no. of telescopes triggered. Further background space angle distribution is normalized with the sourcedistribution by comparing the shape of the distribution in 2.5 	 to 6.5 	 window. We chose this region becausethe angular resolution of the array in the estimation of arrival direction is such that we do not expect any  rayevents beyond 2.5 	 [4]. Beyond 6.5 	 , the statistics is poor, besides, this region is populated by events withpoor estimation of direction of arrival angles. Normalisation of distributions corresponding to the backgroundwith that of the source is necessary as these two data sets were taken at different times. The normalisationconstant is obtained as follows. We define$ %'&(*)ﬃ+,.-)ﬃ/10,32%54,16where S , , B , are the no. of source and background events in  th bin. The normalisation constant 2 % is chosensuch that $  % is minimum. After normalising the source and background distributions, the  ray signal isobtained as excess of source events over background in 0 	 to 2.5 	 region i.e.87:957<;=>@?5ACBED&)ﬃ+,.-F/G0,H2%I4,6This procedure is tested for fictitious source from which we do not expect to receive any  rays. We get a rateof  rays as 0.5  0.7 per minute, which sets the noise level for our  ray signal. The rate of " ray eventswere calculated for each run or part of a run using the respective duration of observation. Figure 1 gives thelight curves ( " ray rate vs MJD) for all the above mentioned sources from 2000 to 2004.5. Discussions and ConclusionsMkn 421 was found to be in flaring state in 2000, 2001 and in 2004 [6, 5], as confirmed by other groundbased " ray experiments. At present the ’Lepton model’ in which Self Synchrotron Compton (SSC) processis responsible for the production of TeV " rays is the widely accepted model for blazars. One of the featuresof this model is a strong correlation between X-ray and " ray emission. Mkn421 was indeed found to be quiteactive by RXTE in these periods most of the time. Over the last few years several multiwavelength campaignsare carried out to study correlations between different wavebands for Mkn421. At present correlation studybetween PACT data and data at other wavelengths that are simultaneous with PACT observations is in progress.Mkn501 was not found in flaring state over the last several years although in the past it was detected in flaringstate [7]. No out-burst is reported so far for 1ES1426+428 and ON231.All the data available so far, for Mkn421 and Mkn501, suggests strong correlation between X-ray and " ray.But recently Whipple observation has shown a flare where X-ray emission reached the peak a few days afterthe TeV emission [6]. This is very significant because if it is true then it will pose a serious challenge to the346 Bose D. et alFigure 1. Light Curves for Mkn421, Mkn501, 1ES1426 and ON231 from 2000 to 2004.standard SSC model as well the hadronic model. Therefore to understand the astrophysical processes in theseBlazar class of objects, it is necessary to study correlation between different wavebands through coordinatedmultiwavelength campaigns and also on more such objects.6. AcknowledgementsWe are grateful to all the members of PACT collaboration for their contributions towards making this projectsuccessful. We thank Prof. P.N. Bhat, S.S. Upadhya, K.S. Gothe, B.K. Nagesh, S.K. Rao, M.S. Pose, P.V.Sudershanan, S. Sharma, K.K. Rao, A.J. Stanislaus, P.N. Purohit, M.A. Rahman, A.I. D’Souza, J. Francis, andB.L.V. Murthy for their help and support.References[1] T.C.Weeks, Very High Energy Gamma-Ray Astronomy, IOP (2003).[2] Chitnis, V.R. et al., 27th ICRC, Hamburg, OG.2.05, 2793 (2001).[3] Gothe, K.S. et al., Indian Journal of Pure & Applied Physics, 38, 269 (2000).[4] Majumdar, P. et al., Astroparticle Physics, 18, 333 (2003).[5] Aharonian, F. et al., A&A 393, 89-99 (2002).[6] Blazejowski, M. et al., astro-ph/0505325 (2005).[7] Petry, D. et al., ApJ, 536:742-755 (2000).

Observation of AGNs with PACT

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