1,270 research outputs found
Study of the Flux and Spectral Variations in the VHE Emission from the Blazar Markarian 501, with the MAGIC Telescope
The blazar Markarian 501 (Mrk 501) was observed above 100 GeV with the MAGIC
Telescope during May, June and July 2005. The high sensitivity of the
instrument made possible the detection of the source with high significance in
each of the observing nights. During this observational campaign, the emitted
gamma-ray flux from Mkn 501 was found to vary by one order of magnitude, and
showed a high correlation with spectral changes. Intra-night flux variability
was also observed, with flux-doubling times of ~2 minutes. The data showed a
clear evidence of a spectral peak (in the nuFnu representation) during the
nights when the gamma-ray activity was highest. The location of this spectral
feature was found to be correlated with the emitted gamma-ray flux. In these
proceedings we discuss some of the results of this unprecedented spectral and
temporal analysis of Mrk 501 observations in the very high energy range.Comment: 4 pages, 3 figures, manuscript submitted on behalf of the MAGIC
collaboration to the First GLAST Symposium (oral presentation
A novel approach to identify blazar emission states using clustering algorithms
Even after decades of multi-wavelength (MWL) observations, blazars still
remain mysterious objects. Their extreme variability and variety of emission
characteristics observed during different time periods make it hard to
understand the fundamental processes behind their emission. Thus, a robust
identification and characterization of the different emission states among
blazars is vital to investigate the underlying processes causing the observed
emission. In this contribution, we present a novel technique to determine
emission states across MWL lightcurves (LCs) of blazars using a clustering
algorithm. Using the Extreme Deconvolution algorithm, we apply a Gaussian
Mixture model to the 12-year long-term LC of one of our archetypal blazars, Mrk
501. The two main advantages of the method are that, compared to more
conventional methods, such as the Bayesian block algorithm, it considers
multiple wavebands simultaneously and it is not dependent on the order in time
of the data points. This allows to assign data points to the same emission
state even though they are separated by other states in time. The well sampled
gamma-ray, X-ray and radio LCs used as input allow to identify six clusters.
The clustering is mainly driven by the X-ray flux, showing different levels of
quiescent, intermediate and high flux states. However, the radio flux reveals a
more complicated pattern, dividing some of the X-ray flux levels in low and
high-radio flux states. This suggests that multiple emission regions maybe
responsible for the radio to gamma-ray flux.Comment: 8 pages, 1 figure, Presented at the 38th International Cosmic Ray
Conference (ICRC 2023), 202
A method to measure the mirror reflectivity of a prime focus telescope
We have developed a method to measure the mirror reflectivity of telescopes. While it is relatively easy to measure the local reflectivity of the mirror material, it is not so straightforward to measure the amount of light that it focuses in a spot of a given diameter. Our method is based on the use of a CCD camera that is fixed on the mirror dish structure and observes simultaneously part of the telescope's focal plane and the sky region around its optical axis. A white diffuse reflecting disk of known reflectivity is fixed in the telescopes focal plane. During a typical reflectivity measurement the telescope is directed to a selected star. The CCD camera can see two images of the selected star, one directly and another one as a spot focused by the mirror on the white disk. The ratio of the reflected starlight integrated by the CCD from the white disk to the directly measured one provides a precise result of the product of (mirror area x mirror reflectivity)
Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380
We report the Fermi Large Area Telescope (LAT) detection of two
very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional
vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null
hypothesis that both the 134 and 122 GeV photons originate from unrelated
sources can be rejected at the 5.5 sigma confidence level. We therefore claim
that at least one of the two VHE photons is securely associated with the
blazar, making PKS 0426-380 the most distant VHE emitter known to date. The
results are in agreement with the most recent Fermi-LAT constraints on the
Extragalactic Background Light (EBL) intensity, which imply a
horizon for 100 GeV photons. The LAT detection of the two VHE
gamma-rays coincided roughly with flaring states of the source, although we did
not find an exact correspondence between the VHE photon arrival times and the
flux maxima at lower gamma-ray energies. Modeling the gamma-ray continuum of
PKS 0426-380 with daily bins revealed a significant spectral hardening around
the time of detection of the first VHE event (LAT photon index \Gamma\
1.4) but on the other hand no pronounced spectral changes near the detection
time of the second one. This combination implies a rather complex variability
pattern of the source in gamma rays during the flaring epochs. An additional
flat component is possibly present above several tens of GeV in the
EBL-corrected Fermi-LAT spectrum accumulated over the ~8-month high state.Comment: 5 pages, 1 table, 4 figures. Accepted by ApJ
A Novel Approach in Constraining Electron Spectra in Blazar Jets: The Case of Markarian 421
We report results from the observations of the well studied TeV blazar Mrk
421 with the Swift and the Suzaku satellites in December 2008. During the
observation, Mrk 421 was found in a relatively low activity state, with the
corresponding 2-10 keV flux of erg/s/cm^2. For the purpose
of robust constraining the UV-to-X-ray emission continuum we selected only the
data corresponding to truly simultaneous time intervals between Swift and
Suzaku, allowing us to obtain a good-quality, broad-band spectrum despite a
modest length (0.6 ksec) exposure. We analyzed the spectrum with the parametric
forward-fitting SYNCHROTRON model implemented in XSPEC assuming two different
representations of the underlying electron energy distribution, both well
motivated by the current particle acceleration models: a power-law distribution
above the minimum energy with an exponential cutoff at the
maximum energy , and a modified ultra-relativistic Maxwellian
with an equilibrium energy . We found that the latter implies
unlikely physical conditions within the blazar zone of Mrk 421. On the other
hand, the exponentially moderated power-law electron distribution gives two
possible sets of the model parameters: (i) flat spectrum with low minimum electron energy , and
(ii) steep spectrum with high minimum electron energy
. We discuss different interpretations of
both possibilities in the context of a diffusive acceleration of electrons at
relativistic, sub- or superluminal shocks. We also comment on how exactly the
gamma-ray data can be used to discriminate between the proposed different
scenarios.Comment: 18 pages, 2 figures; accepted for publication in the Astrophysical
Journa
Novel technique for monitoring the performance of the LAT instrument on board the GLAST satellite
The Gamma-ray Large Area Space Telescope (GLAST) is an observatory designed
to perform gamma-ray astronomy in the energy range 20 MeV to 300 GeV, with
supporting measurements for gamma-ray bursts from 10 keV to 25 MeV. GLAST will
be launched at the end of 2007, opening a new and important window on a wide
variety of high energy astrophysical phenomena . The main instrument of GLAST
is the Large Area Telescope (LAT), which provides break-through high-energy
measurements using techniques typically used in particle detectors for collider
experiments. The LAT consists of 16 identical towers in a four-by-four grid,
each one containing a pair conversion tracker and a hodoscopic crystal
calorimeter, all covered by a segmented plastic scintillator anti-coincidence
shield. The scientific return of the instrument depends very much on how
accurately we know its performance, and how well we can monitor it and correct
potential problems promptly. We report on a novel technique that we are
developing to help in the characterization and monitoring of LAT by using the
power of classification trees to pinpoint in a short time potential problems in
the recorded data. The same technique could also be used to evaluate the effect
on the overall LAT performance produced by potential instrumental problems.Comment: 2 pages, 1 figure, manuscript submitted on behalf of the GLAST/LAT
collaboration to First GLAST symposium proceeding
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