Multi-frequency study on markarian 421 during the first two years of operation of the MAGIC stereo telescopes
Beschreibung
vor 10 Jahren
Markarian 421 (Mrk~421) is one of the classical blazars at X-ray
and very high energies (VHE; $>$100 GeV). Its spectral energy
distribution (SED) can be accurately characterized by current
instruments because of its close proximity, which makes Mrk~421 one
of the best sources to study the nature of blazars. The goal of
this PhD thesis is to better understand the mechanisms responsible
for the broadband emission and the temporal evolution of Mrk~421.
The results might be applied to other blazars which cannot be
studied with this level of detail because their emissions are
weaker, or they are located further away. This thesis reports
results from $\sim$70 hours of observations with MAGIC in 2010 and
2011 (the first two years of the operation of the MAGIC stereo
telescopes), as well as the results from the multi-wavelength (MW)
observation campaigns in 2010 and 2011, where more than 20
instruments participated, covering energies from radio to VHE. The
MW data from the 2010 and 2011 campaigns show that, for both years,
the fractional variability $F_{\rm var}$ increases with the energy
for both the low-energy and the high-energy bumps in the SED of
Mrk~421. Furthermore, $F_{\rm var}(\text{optical})$ was similar to
$F_{\rm var}(\text{HE-$\gamma$-ray;$>$100 MeV})$, and $F_{\rm
var}(\text{X-ray})$ was similar to $F_{\rm
var}(\text{VHE-$\gamma$-ray})$. This observed characteristic is
expected from the strong correlation between the synchrotron
photons and the up-scattered photons by inverse-Compton effect
within the synchrotron self-Compton (SSC) emission model, thus
allowing for the first time of the consistency test on this widely
used theoretical model. During the MW campaign in 2010, we measured
the decay of a flaring activity during 13 days in March. We could
perform MW observations every day, which enables an unprecedented
characterization of the time-evolution of the radio to $\gamma$-ray
emission of Mrk~421. The broadband SEDs during this flaring
episode, resolved on timescales of one day, were characterized with
two leptonic scenarios: a one-zone SSC model, and a two-zone SSC
model where one zone is responsible for the quiescent emission
while the other (smaller) zone, which is spatially separated from
the former one, contributes to the daily-variable emission
occurring mostly at X-rays and VHE $\gamma$ rays. Both the one-zone
SSC and the two-zone SSC models can describe the daily SEDs.
However, the two-zone SSC model provides a better agreement to the
observed SED at the narrow peaks of the low- and high-energy bumps
during the highest activity. The proposed two-zone scenario would
naturally lead to the correlated variability in the X-ray and VHE
bands without variability in the optical/UV band, as well as to
shorter timescales for the variability in the X-ray and VHE bands
with respect to the variability in the other bands. This concept of
a second small emission region containing a narrow electron
spectrum in order to explain the short timescale flaring activity
in the X-ray and VHE bands could be generalized to other blazars.
The results from the 2010 March flaring activity of Mrk~421 are
reported in Sections~\ref{LightCurves} -- \ref{Discussion}, and
they are the main scientific achievement of this PhD thesis.
Preliminary results were reported (as an oral contribution) in the
33rd International Cosmic Ray Conference (Rio de Janeiro, July
2013), one of the most prestigious conferences in the field of the
VHE astronomy and astro-particle physics in general. The final
results (reviewed and approved within the \Fermic, MAGIC, and
VERITAS Collaborations) have been submitted for publication in the
Astronomy and Astrophysics journal in 2014 June. During the MW
campaign in 2011, Mrk~421 had an atypically high activity in the
optical band, together with a very low state in the X-ray/VHE band.
Typically, blazar emission models for Mrk~421 focus on the
explanation of the variability in the X-ray and $\gamma$-ray bands.
This data set is suitable for examining emission models and
estimate if they can describe the evolution of the whole broadband
SEDs including the variabilities in optical, X-ray, and
$\gamma$-ray bands. We found that the one-zone SSC model can
describe the relatively slow variation of the 2011 broadband SEDs.
The modeling of these SEDs shows that the main factor dominating
the spectral evolution could be the electron energy distribution
(EED), instead of the environmental parameters like the blob size
and the Doppler factor. To explain the featured high optical state
together with the low X-ray/VHE state, several changes were needed
in comparison to the typical state from 2009: a harder power-law
index in the first segment in the EED, a lower first break in the
EED, and a softer power-law index in the second segment in the EED.
Besides, these optical high states had synchrotron peak frequencies
10 times lower than the typical state, while their synchrotron peak
energy-fluxes were similar to those of the typical state. On the
contrary, the 2010~March flaring activity showed that a high peak
energy-flux was accompanied by a high peak frequency in comparison
to the typical state, which has also been observed on several other
blazars. This contrast showed that the broadband variability in the
emission of Mrk~421 during 2011 had a different \emph{flavor} with
respect to the typical blazar broadband flaring activity. This PhD
thesis shows that most variations in the SED of Mrk~421 can be
produced through changes in the EED, which could shed light into
how particles get accelerated in the vicinity of super-massive
black holes, or within the relativistic jets of the active galactic
nuclei. However, the results also show a large complexity in the
evolution of the broadband (radio to VHE $\gamma$-rays) SED. Thus
longer and deeper observations are needed to understand what
characteristics get repeated over time and hence typical, what
characteristics are atypical, and ultimately, whether the lessons
learned with Mrk~421 can be extended to high-synchrotron-peaked
blazars in general.
and very high energies (VHE; $>$100 GeV). Its spectral energy
distribution (SED) can be accurately characterized by current
instruments because of its close proximity, which makes Mrk~421 one
of the best sources to study the nature of blazars. The goal of
this PhD thesis is to better understand the mechanisms responsible
for the broadband emission and the temporal evolution of Mrk~421.
The results might be applied to other blazars which cannot be
studied with this level of detail because their emissions are
weaker, or they are located further away. This thesis reports
results from $\sim$70 hours of observations with MAGIC in 2010 and
2011 (the first two years of the operation of the MAGIC stereo
telescopes), as well as the results from the multi-wavelength (MW)
observation campaigns in 2010 and 2011, where more than 20
instruments participated, covering energies from radio to VHE. The
MW data from the 2010 and 2011 campaigns show that, for both years,
the fractional variability $F_{\rm var}$ increases with the energy
for both the low-energy and the high-energy bumps in the SED of
Mrk~421. Furthermore, $F_{\rm var}(\text{optical})$ was similar to
$F_{\rm var}(\text{HE-$\gamma$-ray;$>$100 MeV})$, and $F_{\rm
var}(\text{X-ray})$ was similar to $F_{\rm
var}(\text{VHE-$\gamma$-ray})$. This observed characteristic is
expected from the strong correlation between the synchrotron
photons and the up-scattered photons by inverse-Compton effect
within the synchrotron self-Compton (SSC) emission model, thus
allowing for the first time of the consistency test on this widely
used theoretical model. During the MW campaign in 2010, we measured
the decay of a flaring activity during 13 days in March. We could
perform MW observations every day, which enables an unprecedented
characterization of the time-evolution of the radio to $\gamma$-ray
emission of Mrk~421. The broadband SEDs during this flaring
episode, resolved on timescales of one day, were characterized with
two leptonic scenarios: a one-zone SSC model, and a two-zone SSC
model where one zone is responsible for the quiescent emission
while the other (smaller) zone, which is spatially separated from
the former one, contributes to the daily-variable emission
occurring mostly at X-rays and VHE $\gamma$ rays. Both the one-zone
SSC and the two-zone SSC models can describe the daily SEDs.
However, the two-zone SSC model provides a better agreement to the
observed SED at the narrow peaks of the low- and high-energy bumps
during the highest activity. The proposed two-zone scenario would
naturally lead to the correlated variability in the X-ray and VHE
bands without variability in the optical/UV band, as well as to
shorter timescales for the variability in the X-ray and VHE bands
with respect to the variability in the other bands. This concept of
a second small emission region containing a narrow electron
spectrum in order to explain the short timescale flaring activity
in the X-ray and VHE bands could be generalized to other blazars.
The results from the 2010 March flaring activity of Mrk~421 are
reported in Sections~\ref{LightCurves} -- \ref{Discussion}, and
they are the main scientific achievement of this PhD thesis.
Preliminary results were reported (as an oral contribution) in the
33rd International Cosmic Ray Conference (Rio de Janeiro, July
2013), one of the most prestigious conferences in the field of the
VHE astronomy and astro-particle physics in general. The final
results (reviewed and approved within the \Fermic, MAGIC, and
VERITAS Collaborations) have been submitted for publication in the
Astronomy and Astrophysics journal in 2014 June. During the MW
campaign in 2011, Mrk~421 had an atypically high activity in the
optical band, together with a very low state in the X-ray/VHE band.
Typically, blazar emission models for Mrk~421 focus on the
explanation of the variability in the X-ray and $\gamma$-ray bands.
This data set is suitable for examining emission models and
estimate if they can describe the evolution of the whole broadband
SEDs including the variabilities in optical, X-ray, and
$\gamma$-ray bands. We found that the one-zone SSC model can
describe the relatively slow variation of the 2011 broadband SEDs.
The modeling of these SEDs shows that the main factor dominating
the spectral evolution could be the electron energy distribution
(EED), instead of the environmental parameters like the blob size
and the Doppler factor. To explain the featured high optical state
together with the low X-ray/VHE state, several changes were needed
in comparison to the typical state from 2009: a harder power-law
index in the first segment in the EED, a lower first break in the
EED, and a softer power-law index in the second segment in the EED.
Besides, these optical high states had synchrotron peak frequencies
10 times lower than the typical state, while their synchrotron peak
energy-fluxes were similar to those of the typical state. On the
contrary, the 2010~March flaring activity showed that a high peak
energy-flux was accompanied by a high peak frequency in comparison
to the typical state, which has also been observed on several other
blazars. This contrast showed that the broadband variability in the
emission of Mrk~421 during 2011 had a different \emph{flavor} with
respect to the typical blazar broadband flaring activity. This PhD
thesis shows that most variations in the SED of Mrk~421 can be
produced through changes in the EED, which could shed light into
how particles get accelerated in the vicinity of super-massive
black holes, or within the relativistic jets of the active galactic
nuclei. However, the results also show a large complexity in the
evolution of the broadband (radio to VHE $\gamma$-rays) SED. Thus
longer and deeper observations are needed to understand what
characteristics get repeated over time and hence typical, what
characteristics are atypical, and ultimately, whether the lessons
learned with Mrk~421 can be extended to high-synchrotron-peaked
blazars in general.
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