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Astron. Astrophys. 342, 69-86 (1999)

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1. Introduction

Mrk 501 was discovered as a source of TeV-[FORMULA]-radiation in 1995 by the Whipple group (Quinn et al. 1996). The observation was confirmed later by the HEGRA collaboration (Bradbury et al. 1997). Together with two other extragalactic TeV [FORMULA]-ray sources detected so far, Mrk 421 (Punch et al. 1992; Petry et al. 1996) and 1ES 2344+514 (Catanese et al. 1998), Mrk 501 belongs to a sub-population of Active Galactic Nuclei (AGNs), the so-called BL Lac objects. Flux variability on various time scales, ranging from dramatic flares of Mrk 421 in May 1996 with durations of about 1 h (Gaidos et al. 1996) to a state of high flaring activity of Mrk 501 which lasted several months (e.g. Protheroe et al. 1997), is a characteristic feature of the TeV emission observed from BL Lac objects. This agrees well with the general properties of BL Lac objects - highly variable AGNs without significant optical line emission, but showing a strong nonthermal (synchrotron) component of radiation from radio to X-ray wavelengths (e.g. Urry & Padovani 1995).

The correlated flares of BL Lac objects in the keV energy band and in the TeV energy band, discovered for the first time during simultaneous observations of Mrk 421 by the Whipple and ASCA detectors (Takahashi et al. 1996; Buckley et al. 1996), strongly support the commonly accepted view that both components originate in a relativistic jet, with Doppler factors [FORMULA], due to synchrotron and inverse Compton (IC) radiation of the same population of ultrarelativistic electrons (for a review see e.g. Ulrich et al. 1997). Since in the Thomson regime the IC cooling time [FORMULA] is proportional to [FORMULA], and since the Compton scattering boosts ambient photons with energies [FORMULA] up to [FORMULA], the characteristic time of [FORMULA]-ray emission decreases with energy as [FORMULA]. This explains in a natural way the less dramatic variations of the MeV/GeV [FORMULA]-ray fluxes during the keV/TeV flares; the relatively low energy electrons, responsible for the GeV IC photons as well as for the optical/UV synchrotron radiation do not respond as rapidly to changes of the physical conditions in the jets as the high energy electrons do. In addition, the expected hard spectra of IC radiation below 100 GeV explain the low fluxes of GeV [FORMULA]-rays from Mrk 421, and their non-detection by EGRET in the case of Mkn 501 and 1ES 2344+514. This implies that the VHE [FORMULA]-ray region, combined with X-ray observations, is likely to be the most important window of the electromagnetic spectrum to infer the highly non-stationary processes of particle acceleration and their radiation in BL Lac objects. Imaging Atmospheric Cherenkov Telescope (IACT) detectors, characterized by large effective detection areas of [FORMULA] m2 and an effective suppression of the background of cosmic rays, are well suited to access this very informative "TeV" channel. This was convincingly demonstrated by the Mkn 501 observations with several Cherenkov telescopes located in the Northern Hemisphere during the extreme activity of the source in 1997 (Protheroe et al. 1997).

During the first two years after its discovery as a TeV [FORMULA]-ray source, Mkn 501 showed rather low fluxes at a level significantly below the Crab flux (Quinn et al. 1996; Bradbury et al. 1997). However, in March 1997 the source went into a state of highly variable and strong emission with maximum fluxes roughly 10 times that of the Crab. According to the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) (Remillard & Levine 1997), the high X-ray activity of the source started in March 1997 and continued until October 1997. Apparently the period of high activity coincided with the period of the visibility of the source by ground-based optical instruments. Thus it was possible to continuously monitor the source during this extremely bright emission period with several IACTs, i.e. with CAT (Barrau et al. 1997), HEGRA (Aharonian et al. 1997a), TACTIC (Bhat et al. 1997), Whipple (Catanese et al. 1997), and the Telescope Array (Hayashida et al. 1998).

The HEGRA experiment is located on the Roque de los Muchachos on the Canary Island of La Palma, (lat. [FORMULA] N, long. [FORMULA] W, 2200 m a.s.l.). The HEGRA collaboration operates 6 Cherenkov telescopes. A system of at present four telescopes (telescopes CT3, CT4, CT5, and CT6) is used as a single detector for stereoscopic air shower observations (Daum et al. 1997). The two telescopes, CT1 (Mirzoyan et al. 1994; Rauterberg et al. 1995) and CT2 (Konopelko et al. 1996), are currently operated each as independent detectors. The IACT system is characterized by a high sensitivity and excellent spectroscopic capabilities. The stand alone telescopes CT1 and CT2 have been used to considerably extend the Mkn 501 time coverage, in particular during moonshine periods, when the stereoscopic system was not operated.

In this paper (Part I) the results obtained from the IACT system data are presented. The companion paper (Part II) describes in detail the results from CT1 and CT2 data.

The basic concept of the IACT array is the stereoscopic approach based on simultaneous detection of air showers by [FORMULA] telescopes under widely differing viewing angles. With the stereoscopic technique an angular resolution of [FORMULA] per photon, an energy resolution of [FORMULA] per photon, and a suppression of the isotropic cosmic ray background on the trigger level and by image analysis by a factor of the order of 100 is achieved. Thus [FORMULA]-ray observations with unprecedented signal to noise ratio and excellent spectroscopic capabilities are possible. Furthermore, since a hardware trigger requiring the coincident detection of air showers by at least two telescopes strongly suppresses triggers caused by the night sky background light or by local muons, the energy threshold of a stereoscopic telescopes system is mainly limited by Cherenkov photon statistics. As a consequence, the IACT system achieves an energy threshold as low as 500 GeV despite the relatively small size of the telescope mirrors of [FORMULA] (the energy threshold is defined as the energy at which the [FORMULA]-ray detection rate peaks for Crab type spectra with differential photon indices of [FORMULA]). The flux sensitivity of the IACT system for episodic TeV [FORMULA]-ray phenomena with durations of the order of 1 h is about 1/4 Crab 1 (for S/[FORMULA]=5[FORMULA]), which corresponds to a [FORMULA]-flux at 1 TeV of [FORMULA]. This energy flux sensitivity combines nicely with the comparable energy flux sensitivities of the current X-ray instruments like ASCA, BeppoSAX and RXTE for the study of the high energy emission of BL Lacs, especially of Mkn 501 and of Mkn 421. These two sources proved to release a comparable amount of nonthermal energy in X-rays and in TeV [FORMULA]-rays, with an average energy flux in both channels exceeding [FORMULA]. During strong flares of these sources with fluxes up to 10 Crab, a 2 minute exposure is sufficient for the IACT system to detect a statistically significant [FORMULA]-ray signal, and a 1 h exposure suffices for a measurement of the differential energy spectrum.

This paper is organized as follows: Hardware issues are briefly summarized in Sect. 2. Analysis methods are presented in Sect. 3. Subsequently, the results concerning the TeV-[FORMULA]-ray emission from Mkn 501 in 1997 are discussed. In Sect. 4 the 1997 light curve of Mkn 501 is presented and possible correlations between the flux amplitude and the spectral slope are explored. The most rapid time scales of flux variability are discussed in Sect. 5. The correlation of the TeV-fluxes with the keV-fluxes as measured with the RXTE All Sky Monitor are studied in Sect. 6. Implications on models of the non-thermal [FORMULA]radiation from BL Lac objects are discussed in Sect. 7.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998