JSTOR

1. INTRODUCTION

All but one of the detected extragalactic very high energy (VHE; defined as ) γ‐ray sources so far are active galactic nuclei (AGNs) of the blazar type. Blazars are believed to have highly relativistic plasma outflows (“jets”) closely aligned to our line of sight and are characterized by a highly variable electromagnetic emission ranging from radio to γ‐rays and by continuum spectra dominated by nonthermal emission that consist of two distinct broad components. While the low‐energy bump arises from synchrotron emission of electrons, the origin of the high‐energy peak is still debated. It is commonly explained by inverse Compton radiation of ultrarelativistic electrons, accelerated by shocks moving along the jets at relativistic bulk speed, and can be reasonably well described by synchrotron self‐Compton models (SSC; e.g., Coppi 1992; Costamante & Ghisellini 2002). Hadronic models (Mannheim et al. 1996; Mücke et al. 2003), however, can also explain the observed features. Depending on the location of the low‐energy peak, blazars are often referred to as high‐frequency peaked (HBL; in the X‐ray domain) or low‐frequency peaked (LBL; in the near‐IR/optical) BL Lac objects (Fossati et al. 1998).

The prime scientific interest in VHE blazar studies is twofold: (1) to understand the γ‐ray production mechanisms, assumed to take place in the jets and to be linked to the massive black hole (BH) in the center of the AGN, and (2) to use the VHE γ‐rays as a probe of the extragalactic background light (EBL; Hauser & Dwek 2001) spectrum between about 0.3 and 30 μm wavelength.

In the past, most of the VHE γ‐ray–emitting blazars were discovered during phases of high activity, biasing our current observational record toward high emission states. Although variability in the X‐ray, optical, and radio domain was found, the VHE variability is often found to be the most intense and violent one. Before this work, VHE flux doubling times as short as 15 minutes had been observed (Gaidos et al. 1996). It still remains an open question whether blazars are only temporarily active and are inactive between times of flaring or whether there also exists a state of low but continuous γ‐ray emission. In addition, the temporal and spectral properties of low γ‐ray emission states are mostly elusive as of to date. It is quite conceivable that, compared to a low state, the flare emission state either is due to a different population of accelerated particles or originates from a different region in the AGN, or both.

This dissertation (Wagner 2006, 2007) reports the detection of VHE γ‐rays from the four blazars Mrk 501, 1ES 2344+514, PG 1553+113, and BL Lacertae with the Major Atmospheric Gamma‐Ray Imaging Cerenkov (MAGIC) telescope, currently the largest imaging Cerenkov telescope in operation (Baixeras et al. 2004; Cortina et al. 2005). It has a mirror area of 236 m² and a low trigger energy threshold of 50–60 GeV at low zenith angles.1 The measurements are interpreted in connection with optical and X‐ray data and in the framework of SSC models. The results are used in a synoptic comparison of all currently known VHE γ‐ray–emitting HBLs.

2. OBSERVATIONS AND INTERPRETATION

Mrk 501 was observed in 2005 during MJD 53,518–53,567. A baseline of about 25% of the Crab Nebula flux, as well as high flux states of up to 4 times the Crab Nebula flux, were found. Energy spectra for the different flux levels are determined in the energy range between 100 GeV and 4 TeV. These show a clear spectral hardening with an increasing flux level, ranging from baseline spectra of to high flux level spectra that can be described by After accounting for EBL absorption effects, a hint for an inverse Compton peak is found with the peak energy moving from to with increasing flux level. This evidence is strengthened by the observation of this peak also in the measured spectra, which are free of uncertainties of the EBL model used.

Two observation nights show an unprecedentedly rapid flux variability with doubling times of ≤5 minutes. These fast flares are used to infer significant limits on the size of the acceleration region of . A significant spectral hardening of within less than 20 minutes is found. The well‐defined fast flares are further used to infer a lower limit on the quantum gravity energy scale of .

For the BL Lac object 1ES 2344+514 a differential energy spectrum during a state of low γ‐ray activity is inferred for the first time, which between 140 GeV and 5 TeV can be described by a power law of the form The study of the 24 day VHE γ‐ray light curve of 1ES 2344+514 (MJD 53,585–53,736) yields a flux level of times the Crab Nebula flux. This level is compatible with the lowest fluxes measured so far (1995–2005) from 1ES 2344+514. A homogeneous one‐zone SSC model describes both a flare previously observed on 1995 December 20 and the low emission state measured in this work.

Predictions for the detectability of blazars in VHE γ‐rays with MAGIC led to the discovery of two new objects in this energy domain, PG 1553+113 and BL Lacertae. PG 1553+113 was detected at a high significance level of during observations in 2005 April and May and from 2006 February to April. The source shows the softest VHE γ‐ray spectrum observed so far. Between 95 and 500 GeV it can be described by a pure power law of the form Motivated by acceleration theories, an upper limit of on the undetermined redshift of PG 1553+113 is inferred by assuming a maximum hardness of the intrinsic spectrum. While the light curve does not show day‐scale variability, simultaneously recorded optical data feature a substantial flare, which leaves room for interpretation of the nonexisting correlations.

BL Lacertae, the prototype of the class of BL Lac objects, was detected at a significance level of in data collected between 2005 August and October. It is the first LBL object seen in VHE γ‐rays. A spectrum with a steep slope of between 100 GeV and 1 TeV is found for BL Lacertae.

3. A COMPARATIVE BLAZAR STUDY

For the first time a synoptic study of all 11 high‐frequency–peaked BL Lac–type objects detected as VHE γ‐ray emitters is carried out. It compares and correlates the luminosities, spectral slopes, and variability timescales of the observed emission among each other and with the individual estimated BH masses . Key findings are:

4. INSTRUMENTATIONAL ADVANCES

One of the main design goals of MAGIC was a fast response to γ‐ray burst (GRB) alerts from satellite monitors such as Swift. Typical burst durations of long bursts range from 10 to 100 s. Thus, for successful GRB observations, MAGIC must be able to point to GRBs, which occur randomly distributed on the sky, in a time as short as possible. The challenge is a great one, as the telescope has a weight of ≈60 tons and a mechanical diameter of about 20 m. The technical part of the dissertation deals with the implementation of a drive system that permits fast slews with angular velocities of 150° per 30 s about the azimuth axis and >90° per 20 s about the elevation axis. For determining the tracking accuracy of the telescope, a star field monitoring system has been commissioned. In contrast to optical astronomy, γ‐ray sources are typically too dim in the optical to be used for centering and monitoring the source position. Therefore, we measure the pointing position by comparing stellar positions determined from a CCD image to those recorded in standard star catalogs.

Further, the implementation of the flux and light‐curve calculation within the framework of the MAGIC Analysis and Reconstruction Software (Wagner & Bretz 2003) is discussed. The application of the analysis chain is exemplified on the first large data sample taken with MAGIC on the Crab Nebula, the standard candle in VHE γ‐astronomy. The observations were performed in 2004/2005 to understand and calibrate the telescope. We find a light curve that is compatible with a constant γ‐ray emission of and a differential γ‐ray spectrum that can be described by a curved power law in the energy range of 95 GeV to 6.5 TeV. The results are quite compatible with those of other experiments (with thresholds ) and with model predictions.