A study of very high energy {γ-ray [gamma-ray] emission from AGNs and constraints on the extragalactic background light [Elektronische Ressource] / Daniel Mazin

Pgoehringer

Sujets

• Astronomie

Informations

• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2007
• Nombre de lectures: 16
• Langue: English
• Poids de l'ouvrage: 33 Mo

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PHYSIK-DEPARTMENT
A study of very high energy –ray emission
from AGNs and constraints on the
extragalactic background light
Dissertation
von
Daniel Mazin
TECHNISCHE UNIVERSITÄT
MÜNCHEN
Technische Universit¨at Munc¨ hen
Max-Planck-Institut fur¨ Physik
(Werner-Heisenberg-Institut)
A study of very high energy
γ-ray emission from AGNs and
constraints on the extragalactic
background light
Daniel Mazin
Vollst¨ andiger Abdruck der von der Fakult¨ at fur¨ Physik der Technischen Univer-
sit¨at Munc¨ hen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. J. Buras
Prufer¨ der Dissertation:
1. Hon.-Prof. Dr. S. Bethke
2. Univ.-Prof. Dr. F. von Feilitzsch
Die Dissertation wurde am 27.11.2007 bei der Technischen Universit¨ at Munc¨ hen
eingereicht und durch die Fakult¨ at fur¨ Physik am 27.11.2007 angenommen.4Abstract
This thesis studies very high energy (VHE) γ-radiation from active galactic nu-
clei (AGNs) using the novel Major Atmospheric Gamma-ray Imaging Cherenkov
(MAGIC) telescope on La Palma. A significant part of the thesis is devoted to
the development of the analysis tools and the analysis of MAGIC data taken
on AGNs. New precision results on the well known AGN Markarian 421 as
well as the discovery of VHE γ-ray emission from the AGNs Markarian 180 and
1ES 1011+496 are presented. These data are used to investigate the violent pro-
cesses within AGNs, the most prominent extragalactic objects of non-thermal
radiation. Furthermore, all available VHE γ-ray spectra of AGNs are used to
derive constraints on the density of the extragalactic background light (EBL),
which represents the integrated light emitted during the entire star and galaxy
evolution of the Universe.
The well known AGN Markarian 421 (redshift z =0.030) was measured with
MAGIC from December 2004 to April 2005. For the first time an energy spectrum
of the source has been derived down to 100GeV. The spectral domain between
100GeV and 500GeV was not accessible prior to these MAGIC observations. The
fluxofMarkarian421isfoundtovarybetween0.5–2Crabunits(integrated
above 200 GeV), a rather low state when compared to previously published data.
Although the flux varies on a day-by-day basis, no short-term variability is ob-
served. A clear correlation is observed between γ-rays and X-rays fluxes, whereas
no significant correlation between γ-rays and optical data is seen. The spectral
energy distribution between 100 GeV and 3 TeV shows a clear deviation from a
power law. The deviation persists after correcting for the effect of attenuation by
the extragalactic background light, and most likely is source–inherent. There is a
rather clear indication of an inverse Compton peak around 100 GeV. The spectral
energy distribution of Markarian 421 can be fitted by a one-zone synchrotron self-
Compton model suggesting once again a leptonic origin of the very high energy
γ-ray emission from this blazar. Within this model physics parameters of the jet
of Markarian 421 are constrained using reasonable assumptions on the shape of
the X-ray spectrum.
The AGN Markarian 180 (redshift z =0.045) was observed to have an optical
outburst in 2006 March, triggering a Target of Opportunity observation with the
MAGIC telescope. The source was observed for a total of 12.4 hours, and VHE
12
γ-ray emission was detected with a significance of 5.5 σ. An integral flux above
−11 −2 −1200 GeV of (2.3±0.7)×10 cm s is measured, corresponding to 11% of the
Crab Nebula flux. A rather soft spectrum with a photon index of−3.3± 0.7is
determined. No significant flux variation is found. It is the first time that VHE
γ-rays are detected from the direction of Markarian 180.
Also the discovery of VHE γ-ray emission from the AGN 1ES 1011+496 was
triggered by an optical outburst in March 2007. 1ES 1011+496 was observed
with the MAGIC telescope from March to May 2007 for a total of 18.7 hours.
The analysis of the data results in an excess of 6.2σ with an integrated flux above
−11 −2 −1200GeV of (1.58±0.32)· 10 photons cm s . Previous MAGIC observations
in March–April 2006 during a lower optical state resulted in upper limits only,
which suggests that the VHE emission state is related to the optical emission
state. The redshift of z =0 .212 makes 1ES 1011+496 the most distant source
observed to emit VHE γ-rays up to date.
In the second part of the thesis, VHE γ-ray spectra of AGNs are used to derive
limits on the EBL density. The EBL consists of the light produced by galaxies
during the entire history of the Universe. This radiation, redshifted according to
its emission epoch, extends from the ultraviolet to the far-infrared range. VHE
γ-rays are attenuated by the photons of the EBL via pair production, which
leaves an imprint in the measured spectra from distant sources. A new method
to derive constraints on the EBL is presented. The method does not rely on a pre-
defined EBL model but uses rather a large number of generic shapes constructed
from a grid in EBL density vs. wavelength. Spectral data from all known TeV
blazars are utilized, making this the most complete study so far. Limits on
the EBL are derived for three TeV blazar spectra (Markarian 501, H 1426+428,
1ES 1101-232) individually and for all spectra combined. The latter leads to
significantly stronger constraints over a wide wavelength range from the optical
(∼ 1 micron) to the far-infrared (∼ 80 microns). The limits are only a factor of
2 to 3 above the absolute lower limits derived from source counts. In the mid-
infrared our limits are the strongest constraints derived from TeV blazar spectra
so far. A high density of the EBL around 1 micron, reported by direct detection
experiments, can be excluded. The results can be interpreted in two ways: (i)
The sources resolved by galaxy counts account for the large fraction of the EBL,
leaving only little room for additional components by e.g. the first stars or (ii)
the assumptions about the underlying physics are not valid, which would require
substantial changes in the standard emission models of TeV blazars.
Using the knowledge of the minimum EBL level one can deduce an upper
limit for the distance of AGNs observed at VHE γ-rays. The method uses the
argument that the intrinsic photon index after deconvoluting the EBL absorption
cannot be harder than Γ = 1.5. PG 1553+113 is a BL Lac object of unknown
redshift, newly detected in the GeV-TeV energy range by H.E.S.S. and MAGIC.
Here we apply the above method to the combined H.E.S.S. and MAGIC spectrum
and derive an upper limit on the redshift of z<0.69. Moreover, we find that a3
redshift above z =0.42 implies a possible break of the intrinsic spectrum at about
200GeV. Assuming that such a break is absent, we derive a much stronger upper
limit of z<0.42. Alternatively, this break might be attributed to an additional
emission component in the jet of PG 1553+113. This would be the first evidence
for a second component is detected in the VHE spectrum of a blazar.4Zusammenfassung
Diese Doktorarbeit untersucht sehr hochenergetische (VHE) Gammastrahlung
von aktiven galaktischen Kernen (AGN) unter Verwendung des neuartigen Major
Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Teleskops auf der In-
sel La Palma. Ein Teil der Arbeit ist der Entwicklung von Analysemethoden
und der Analyse von AGN Beobachtungen mit MAGIC gewidmet. Es werden
neue Pr¨ azisionsmessungen des etablierten AGNs Markarian 421 sowie die Ent-
deckung von VHE Gammastrahlung aus der Richtung der AGNs Markarian 180
und 1ES 1011+496 vorgestellt. Diese Daten werden verwendet, um die physikalis-
chen Prozesse innerhalb der Jets von AGNs zu studieren. Des weiteren werden
alle vorhandenen VHE Gamma-Spektren von AGNs verwendet, um die Dichte des
extragalaktischen Hintergrundlichtes (EHL) einzuschr¨ anken, welches das ub¨ er alle
Epochen ausgestrahlte Licht der gesamten Sterne und Galaxien des Universums
darstellt.
Der bekannte AGN Markarian 421 (Rotverschiebung z =0 .030) wurde mit
MAGIC von Dezember 2004 bis April 2005 beobachtet. Hiermit konnte das Quel-
lenspektrum zum ersten mal runter bis 100 GeV gemessen werden. Der Spektral-
bereich zwischen 100 GeV and 500 GeV war vor den MAGIC Beobachtungen nicht
zuganglic¨ h. Der mit MAGIC gemessene Fluss von Markarian 421 variiert zwischen
0,5 – 2 Krebsnebel Einheiten (integriert ub¨ er 200 GeV), was einem eher niedrigen
Flusszustand entspricht, verglichen mit vorherig publizierten Messungen. ver-
gleicht. Obwohl der Fluss von Tag zu Tag variiert, konnte keine signifikante
¨Anderung des Flusses innerhalb einzelner N¨achte beobachtet werden. Eine klare
Korrelation wurde zwischen den Flussen¨ im VHE γ–Bereich und im Ron¨ tgen–
Bereich festgestellt, allerdings wurde keine Korrelation zwischen den Flussen¨ im
VHE γ–Bereich und dem optischen Bereich gefunden. Das Energiespektrum er-
streckt sich von 100 GeV bis 3 TeV und zeigt eine klare Abweichung von einem
Potenzgesetz. Die Abweichung verbleibt auch nach der Korrektur des Spektrums
f¨ur den Effekt der extragalaktischen Absorption und hat wahrscheinlich einen
intrinsischen Ursprung. Es gibt ein klares Anzeichnen