Detailed study of the binary system LS I +613̊03 in VHE gamma-rays with the MAGIC telescope [Elektronische Ressource] / Tobias Jogler

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• Astronomie

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2009
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 42 Mo

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Technische Universitat Munchen
Max-Planck-Institut fur Physik
(Werner-Heisenberg-Institut)
Detailed study of the binary system
LS I +61 303 in VHE gamma-rays
with the MAGIC telescope
Tobias Jogler
Vollst andiger Abdruck der von der Fakult at fur Physik der Technischen Universitat 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 Disseration wurde am 09.09.2009 bei der Technischen Universiatt Munc hen eingereicht
und durch die Fakult at fur Physik am 19.11.2009 angenommen.Abstract
In this thesis, the LS I +61 303 X-ray binary system is studied in great detail in Very High
Energy (VHE,E > 50 GeV) gamma-rays with the Major Atmospheric Gamma-ray Imaging
Cherenkov (MAGIC) telescope. X-ray binaries were long suspected to be potential gamma-
ray sources. In the 1970’s, the COS B satellite reported gamma-rays from the vicinity of
LS I +61 303. Even after the more accurate measurements of the EGRET detector in the late
1990’s, the association of LS I +61 303 as the source of the gamma-rays remained ambiguous
until MAGIC (with its higher angular resolution measurements) identi ed LS I +61 303 as
the VHE gamma-ray source.
The total observation time of 225 hours on LS I +61 303 places the binary system among
the best studied VHE gamma-ray sources by MAGIC. The data were taken between 2005
and 2008 in three distinct observation campaigns (OC) each with more than 50 hours of
observation time.
The huge amount of data taken on LS I +61 303 and the dense sampling of the system’s
orbital period allowed the detection of a periodicity in the VHE gamma-ray emission. This
is the rst time that a periodic modulation in a light curve could be proven in observations
performed by MAGIC and the second time in the eld of VHE gamma-ray astronomy. The
1obtained period ofP = 26:60 0:45 d is compatible with the best measured orbital period
of LS I +61 303 obtained by radio measurements.
Further investigations of the system were carried out in orbital phase bins of = 0:1,
where the phases () reach from 0{1.0 within one orbit. LS I +61 303 is signi cantly detected
in all OC’s only in the 0.6{0.7 phase range close to the apastron of the compact object. In
this phase range, the light curve shows an outburst of 2{4 days’ duration while measurements
- for example around the periastron passage of the compact object - yield only upper limits.
The spectrum of the gamma-ray excess derived from the outbursts is well described by a
simple power law:
(2:60:2 0:2 )stat systdF E12 1 2 1= (2:6 0:3 0:8 ) 10 TeV cm s (1)stat syst
dE 1:0
In the most recent measurements conducted in 2007 and 2008, additional signi cant uxes
were measured in the 0.8{1.0 phase range, not seen in previous observations. The derived
spectrum in this phase range is poorly described by a simple power law. The spectrum is
tted better by a power law with an energy-dependent spectral index of the following form:
( 3:940:33 )+((1:310:65 ) log(E))stat statdF E12 1 2 1= (39:96:7 12:0 )10 TeV cm sstat syst
dE 0:3
(2)
iThis hint for spectral variability might be caused by a di erent production mechanism
of the VHE gamma-rays dominating the emission at these phase ranges compared to the
periodic outburst.
The diurnal light curves were investigated for ux variability on short timescales (less than
a few hours). Several nights present hints for short-time variability. The most promising of
those nights indicates a ux increase by a factor of three on a timescale as short as 30 minutes.
Similar behavior is observed in the X-rays by several measurements performed in the last
few years.
In addition to the VHE gamma-ray measurements, multiwavelength studies have been
carried out at radio frequencies and X-ray energies in several observation campaigns. The
LS I +61 303 X-ray spectra obtained in this thesis are all compatible with absorbed power
laws. The spectral index of these power laws lies between 1:34{1:87 in agreement with
previously published results by other observations. No prominent accretion disc feature such
as iron-line emission or black-body radiation - as expected in the case of a microquasar - is
detected in X-rays.
Evidence for a correlation between the ux levels in simultaneously taken X-ray and VHE
gamma-ray data is found on a level of 3:4 signi cance. A correlation study on the phase bin
averaged light curve in X-ray and VHE gamma-rays was performed in a distinct observation
campaign. A hint for correlation on the 2 signi cance level is obtained by this study.
The correlated uxes in the two energy bands can be explained by assuming that X-ray
and VHE gamma-ray emission originate from the same particle population. In this case,
the higher ux at X-ray energies favors a leptonic origin of the emission. In this scenario,
the X-rays are attributed to synchrotron radiation of relativistic electrons. These electrons
produce the VHE gamma-ray emission due to inverse Compton upscattering of UV photons
provided by the companion star.
Theoretical models try to explain the emission of LS I +61 303 either in terms of a
microquasar scenario or by proposing that a compact pulsar wind is responsible for the
VHE gamma-ray emission. In terms of the observational results presented here, several
microquasar models can be excluded by the measured VHE gamma-ray light curve and
spectra. Nevertheless, two other leptonic microquasar models are compatible with the VHE
gamma-ray emission.
In the case of the pulsar wind models, the scenario - attributing the emission to a mono-
energetic free pulsar wind - can be rejected. On the other hand, the free pulsar wind,
consisting of a power law distribution of electrons, can at least explain the VHE gamma-ray
part of the presented results very well.
Two models attribute the VHE gamma-ray emission to the shocked (by the stellar wind)
pulsar wind. One of the models suggests a hadronic origin of the VHE gamma-rays, which is
improbable due to the X-ray/VHE gamma-ray ux correlation described above but cannot
be fully excluded.
The spectral energy density derived in this work provides important information for the
further development of the models.
The technical part of this thesis describes the development and implementation of theMAGIC II camera control software.
In addition, the stability of the telescope’s performance was studied between 2005{2008
in terms of investigating the e ects on the Crab Nebula light curve and on its spectrum
which are generated by the various hardware setups, the observation modes, the background
light conditions and the zenith angle ranges used over a time span of four years. The sta-
bility study exhibits only a slight systematic increase both of the integral ux (by 12%) and
the spectral index of = 0 :2, after the change to the 2 GHz read-out system of MAGIC.
This systematic uncertainty is small compared to statistical uncertainties of measurements
of weak sources like LS I +61 303. Generally, the MAGIC telescope displayed a very stable
performance between 2005 and 2008.Zusammenfassung
Gegenstand dieser Dissertation ist die detaillierte Studie des R ontgenstrahlen emittieren-
den Bin arsystems LS I +61 303 in sehr hochenergetischen Gammastrahlenbereichen (E >
50 GeV) mithilfe des Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Teleskops.
Schon fruh wurde die These aufgestellt, R ontgenstrahlen emittierende Bin arsysteme seien
potentielle Quellen von Gammastrahlung: In den 1970er Jahren konnte erstmals Gam-
mastrahlung in der unmittelbaren N ahe von LS I +61 303 durch den Satelliten COS B
gemessen werden. Nachdem selbst pr azisere Messungen des EGRET detectors in den 1990er
Jahren LS I +61 303 nicht eindeutig als Gammastrahlenquelle identi zieren konnten, gelang
es schlie lich mithilfe der hohen Winkelau osung des MAGIC Teleskops, LS I +61 303 in
sehr hochenergetischen Gammastrahlenbereichen zu beobachten.
Aufgrund der immensen Beobachtungsdauer von insgesamt 225 Stunden geh ort das Bin ar-
system LS I +61 303 zu den den am besten untersuchten sehr hochenergetischen Gamma-
strahlenquellen des MAGIC-Projekts. Die Daten wurden in drei verschiedenen Observations-
zyklen aufgenommen, die jeweils ub er 50 Stunden Beobachtungszeit umfassen und zwischen
2005 und 2008 stattfanden.
Die gro e Menge an Daten, die an LS I +61 303 genommen wurden und die dicht
aufeinander folgenden, stichprobenartigen Messungen der Umlaufbahnperiode des Systems
erm oglichten die Entdeckung einer Periodizit at innerhalb der sehr hochenergetischen Gam-
mastrahlenemission. Erstmals im Rahmen des MAGIC-Projekts und zum zweiten Mal in-
nerhalb der sehr hochenergetischen Gammastrahlenstronomie konnte auf diese Weise die
periodische Modulation einer Lichtkurve bewiesen werden. Deren gemessene Periode von
1
P = 26:60 0:45 d stimmt mit der am pr azisesten gemessenen Umlaufbahnperiode von
LS I +61 303, abgeleitet von Messungen im Radiobereich, ub erein.
Weitergehende Untersuchungen des Systems fanden in den Intervallen der orbitalen Phasen
mit einer Spanne von = 0:1 statt, dabei reichen die orbitalen Phasen () von 0{1.0 in-
nerhalb einer Umlaufbahn. In allen drei Observationszyklen kann LS I +61 303 nur im
Phasenbereich 0.6{0.7, n