1,351 research outputs found
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
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
Hints of the existence of Axion-Like-Particles from the gamma-ray spectra of cosmological sources
Axion Like Particles (ALPs) are predicted to couple with photons in the
presence of magnetic fields. This effect may lead to a significant change in
the observed spectra of gamma-ray sources such as AGNs. Here we carry out a
detailed study that for the first time simultaneously considers in the same
framework both the photon/axion mixing that takes place in the gamma-ray source
and that one expected to occur in the intergalactic magnetic fields. An
efficient photon/axion mixing in the source always means an attenuation in the
photon flux, whereas the mixing in the intergalactic medium may result in a
decrement and/or enhancement of the photon flux, depending on the distance of
the source and the energy considered. Interestingly, we find that decreasing
the value of the intergalactic magnetic field strength, which decreases the
probability for photon/axion mixing, could result in an increase of the
expected photon flux at Earth if the source is far enough. We also find a 30%
attenuation in the intensity spectrum of distant sources, which occurs at an
energy that only depends on the properties of the ALPs and the intensity of the
intergalactic magnetic field, and thus independent of the AGN source being
observed. Moreover, we show that this mechanism can easily explain recent
puzzles in the spectra of distant gamma-ray sources... [ABRIDGED] The
consequences that come from this work are testable with the current generation
of gamma-ray instruments, namely Fermi (formerly known as GLAST) and imaging
atmospheric Cherenkov telescopes like CANGAROO, HESS, MAGIC and VERITAS.Comment: 16 pages, 7 figures. Replaced to match the published version in Phys.
Rev. D. Minor changes with respect to v
Statistical downscaling in Chile: selection of large-scale predictors and climate change projections
Experience gained at the Ural Turbine Works with retrofitting steam turbine units for thermal power stations
Examples of projects on retrofitting, modernizing, and renovating steam turbine units at thermal power stations implemented with participation of the Ural Turbine Works are given. Advanced construction and layout solutions were used in implementing these projects both on the territory of Russia and abroad. © 2013 Pleiades Publishing, Ltd
Directly tracing the vertical stratification of molecules in protoplanetary disks
We aim to directly trace the vertical location of the emitting surface of
multiple molecular tracers in protoplanetary disks. Our sample of disks
includes Elias 2-27, WaOph 6 and the sources targeted by the MAPS ALMA Large
Program. The set of molecules studied include CO isotopologues in various
transitions, HCN, CN, H2CO, HCO+, C2H and c-C3H2. The vertical emitting region
is determined directly from the channel maps, implementing accurate masking of
the channel emission to recover the vertical location of the emission surface
even at large radial distances from the star and for low-SNR lines. The
vertical location of the emitting layer is obtained for 4-10 lines in each
disk. IM Lup, HD163296 and MWC 480 12CO and 13CO show vertical modulations,
which are coincident with dust gaps and kinematical perturbations. We also
present estimates of the gas pressure scale height in the disks from the MAPS
sample. Compared to physical-chemical models we find good agreement with the
vertical location of CO isotopologues. In HD 163296 CN and HCN trace a similar
intermediate layer, for the other disks, the UV flux tracers and the vertical
profiles of HCN and C2H are lower than predicted in theoretical models. HCN and
H2CO show a highly structured vertical profile, possibly indicative of
different formation pathways. It is possible to trace the vertical locations of
multiple molecular species that trace a wide variety of physical and chemical
disk properties. The distribution of CO isotopologues are found at a wide range
of vertical heights 0.5-0.05. Other molecular lines are mostly found
at 0.15. The vertical layering of molecules is in agreement with
theory in some systems, but not in all, therefore dedicated chemical-physical
models are needed to further study and understand the emission surfaces.Comment: Accepted for publication in A&A. 29 pages, 28 figure
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
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