PUBLICATIONS OF THEASTRONOMICALSOCIETY OF THEPACIFIC—DISSERTATIONSUMMARY Preprint typeset using LATEX style emulateapj v. 10/09/06
MEASUREMENT OF VERY HIGH ENERGY GAMMA-RAY EMISSION FROM FOUR BLAZARS USING THE MAGIC TELESCOPE AND A COMPARATIVE BLAZAR STUDY
ROBERTMARCUSWAGNER
Current address: Max-Planck-Institut für Physik, München, Germany Electronic mail: robert.wagner@mpp.mpg.de
Thesis work conducted at Max-Planck-Institut für Physik (Werner-Heisenberg-Institut) Ph.D. thesis directed by Siegfried Bethke and Eckart Lorenz; Ph.D. degree awarded 2007 January 24
(Received 2007 August 9; Accepted 2007 August 13)
PASP Keywords:Astronomical Instrumentation — Astronomical Techniques — Quasars and Active Galactic Nuclei: individual (1ES 2344+514, BL Lacertae, Mrk 501, PG 1553+113) — Dissertation Summary
1. INTRODUCTION
All but one of the detected extragalactic very high energy (VHE; defined asE>100 GeV)γ-ray sources so far are ac- tive 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 ra- dio toγ-rays, and by continuum spectra dominated by non- thermal emission that consist of two distinct broad compo- nents. 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 rea- sonably well described by synchrotron-self-Compton mod- els (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 re- ferred to as high-frequency peaked (HBL; in the X-ray do- main) 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 theγ-ray production mechanisms, assumed to take place in the jets and to be linked to the mas- sive 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; e.g. Hauser & Dwek 2001) spectrum between about 0.3 and 30µm wavelength.
In the past, most of the VHEγ-ray–emitting blazars were discovered during phases of high activity, biasing our current observational record towards 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 tem- porarily active and are inactive between times of flaring, or whether there also exists a state of low but continuousγ-ray emission. In addition, the temporal and spectral properties of lowγ-ray emission states are mostly elusive as of to date. It is quite conceivable that, compared to a low state, the flare emis- sion 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 de- tection of VHE γ-rays from the four blazars Mrk 501,
1ES 2344+514, PG 1553+113, and BL Lacertae with the Ma- jor Atmospheric Gamma-Ray Imaging (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 VHEγ-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 four times the Crab Nebula flux were found. Energy spectra for the different flux lev- els are determined in the energy range between 100 GeV and 4 TeV. These show a clear spectral hardening with an increas- ing flux level, ranging from baseline spectra of
dF
dE = (0.56±0.04)×10−9 TeV cm2s
E
250 GeV
−2.39±0.06
to high flux-level spectra that can be described by dF
dE = (4.11±0.16)×10−9 TeV cm2s
E
250 GeV
−2.14±0.03
After accounting for EBL absorption effects, a hint for an inverse Compton peak is found with the peak energy mov- ing from 0.3±0.1 to 1.4±0.7 TeV 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 ofRδ−1≤1012cm. A significant spectral hardening of∆α= 0.4 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 1.5×1017GeV.
For the BL Lac object 1ES 2344+514 a differential energy spectrum during a state of lowγ-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
dF
dE =(1.18±0.13)×10−11 TeV cm2s
E
500 GeV
−2.95±0.12
1see also http://wwwmagic.mppmu.mpg.de/magic/factsheet/
2 Wagner 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
0.04≤FE>350 GeV≤0.11 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 ob- jects in this energy domain, PG 1553+113 and BL Lacertae.
PG 1553+113 was detected at a high significance level of 8.8σ during observations in 2005 April and May and from 2006 February to April. The source shows the softest VHEγ-ray spectrum observed so far. Between 95 and 500 GeV it can be described by a pure power law of the form
dF
dE =(0.18±0.03)·10−9 TeV cm2s ×
E
200 GeV
−4.21±0.25
Motivated by acceleration theories, an upper limit ofz<0.78 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, si- multaneously recorded optical data feature a substantial flare, which leaves room for interpretation of the non-existing cor- relations.
BL Lacertae, the prototype of the class of BL Lac objects, was detected at a significance level of 5.6σin data collected between 2005 August and October. It is the first LBL ob- ject seen in VHE γ-rays. A spectrum with a steep slope of α=−3.7±0.5 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 emit- ters is carried out. It compares and correlates the luminosi- ties, spectral slopes, and variability timescales of the observed emission among each other and with the individual estimated BH massesM•. Key findings are:
1. Only AGNs that host BHs with M• ≥108M show VHEγ-ray emission.
2. There are no obvious correlations of the VHE γ- ray emission properties with the BH masses. This also holds true for the flare duty cycles and the flare timescales. This result implies that the BH mass itself does not have a major impact on the VHEγ-ray emis- sion.
3. A hint at a marginal correlation between the intrinsic spectral hardness and the source distance is probably due to an EBL over-prediction by current EBL models.
4. The VHE luminosity and the corresponding X-ray lu- minosity show a trend towards a correlation, as ex- pected in leptonic acceleration models.
5. There is a 3.3σindication that theγ-ray luminosity is correlated with the spectral hardness in theE >100 GeV region. This correlation can be formulated as a decrease of∆Γ≈0.7 per decade of luminosity. This behavior is also found for those individual sources that were observed in different emission states.
6. Using the VHE luminosity distribution, the yet un- known distance of PG 1553+113 is constrained by as- suming that the properties of this blazar are not too different from the most extreme objects in the sam- ple. Conversely, a large distance of this blazar implies an unusually high luminosity or an unusually high jet Doppler factor. Depending on the strength of the as- sumptions made, the limit varies betweenz≤0.43 and
≤0.68.
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 s 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 starfield monitoring system has been commissioned. In con- trast 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 6.2◦×6.2◦ CCD image to those recorded in standard star catalogs.
Further, the implementation of the flux and light-curve cal- culation 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 VHEγastronomy. The observations were performed in 2004/2005 to understand and calibrate the tele- scope. We find a light curve that is compatible with a constant γ-ray emission ofFE>200 GeV= (2.16±0.07)×10−10cm−2s−1 and a differentialγ-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 experi- ments (with thresholdsE>300 GeV) and with model predic- tions.
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