88 research outputs found
Relativistic Positron-Electron-Ion Shear Flows and Application to Gamma-Ray Bursts
We present Particle-in-Cell simulation results of relativistic shear flows
for hybrid positron-electron-ion plasmas and compare to those for pure e+e- and
pure e-ion plasmas. Among the three types of relativistic shear flows, we find
that only hybrid shear flow is able to energize the electrons to form a
high-energy spectral peak plus a hard power-law tail. Such electron spectra are
needed to model the observational properties of gamma-ray bursts.Comment: 18 pages, 5 figures. Paper accepted by APJ
GeV breaks in blazars as a result of gamma-ray absorption within the broad-line region
Spectra of the brightest blazars detected by the Fermi Gamma-ray Space
Telescope Large Area Telescope cannot be described by a simple power law model.
A much better description is obtained with a broken power law, with the break
energies of a few GeV. We show here that the sharpness and the position of the
breaks can be well reproduced by absorption of gamma-rays via photon--photon
pair production on HeII Lyman recombination continuum and lines. This implies
that the blazar zone lies inside the region of the highest ionization of the
broad-line region (BLR) within a light-year from a super-massive black hole.
The observations of gamma-ray spectral breaks open a way of studying the BLR
photon field in the extreme-UV/soft X-rays, which are otherwise hidden from our
view.Comment: 4 pages, 3 figures, 2 tables; published in ApJ Letter
Estimating oil and gas recovery factor via machine learning
Master of ScienceDepartment of GeologyBehzad Ghanbarian AlavijehMohammad B ShadmandWith recent advances in artificial intelligence, machine learning (ML) approaches have become an attractive tool in petroleum engineering, particularly for reservoir characterizations. A key reservoir property is hydrocarbon recovery factor (RF) whose accurate estimation would provide decisive insights to drilling and production strategies. Therefore, this study aims to estimate the hydrocarbon RF from various reservoir characteristics, such as porosity, permeability, pressure, and water saturation via the ML. We applied three regression-based models including the extreme gradient boosting (XGBoost), support vector machine (SVM), and stepwise multiple linear regression (MLR) and various combinations of three databases to construct ML models and estimate the oil and/or gas RF. Using two databases and the cross-validation method, we evaluated the performance of the ML models. In each iteration 90 and 10% of the data were respectively used to train and test the models. The third independent database was then used to further assess the constructed models. For both oil and gas RFs, we found that the XGBoost model estimated the RF for the train and test datasets more accurately than the SVM and MLR models. However, the performance of all the models were unsatisfactory for the independent databases. Results demonstrated that the ML algorithms were highly dependent and sensitive to the databases based on which they were trained. Results of statistical tests revealed that such unsatisfactory performances were because the distributions of input and output features in the train datasets were significantly different from those in the independent databases (p-value < 0.05)
Scaling of Relativistic Shear Flows with Bulk Lorentz Factor
We compare Particle-in-Cell simulation results of relativistic electron-ion
shear flows with different bulk Lorentz factors, and discuss their implications
for spine-sheath models of blazar versus gamma-ray burst (GRB) jets.
Specifically, we find that most properties of the shear boundary layer scale
with the bulk Lorentz factor: the lower the Lorentz factor, the thinner the
boundary layer, and the weaker the self-generated fields. Similarly, the
energized electron spectrum peaks at an energy near the ion drift energy, which
increases with bulk Lorentz factor, and the beaming of the accelerated
electrons gets narrower with increasing Lorentz factor. This predicts a strong
correlation between emitted photon energy, angular beaming and temporal
variability with the bulk Lorentz factor. Observationally, we expect systematic
differences between the high-energy emissions of blazars and GRB jets.Comment: 27 pages, 8 figures, to be submitte
VHE Gamma-Ray Induced Pair Cascades in Blazars and Radio Galaxies: Application to NGC 1275
Recent blazar detections by HESS, MAGIC, and VERITAS suggest that
very-high-energy (VHE, E > 100 GeV) gamma-rays may be produced in most, if not
all, types of blazars, including those that possess intense circumnuclear
radiation fields. In this paper, we investigate the interaction of nuclear VHE
gamma-rays with the circumnuclear radiation fields through gamma-gamma
absorption and pair production, and the subsequent Compton-supported pair
cascades. We have developed a Monte-Carlo code to follow the spatial
development of the cascade in full 3-dimensional geometry, and calculate the
radiative output due to the cascade as a function of viewing angle with respect
to the primary VHE gamma-ray beam (presumably the jet axis of the blazar). We
show that even for relatively weak magnetic fields, the cascades can be
efficiently isotropized, leading to substantial off-axis cascade emission
peaking in the Fermi energy range at detectable levels for nearby radio
galaxies. We demonstrate that this scenario can explain the Fermi flux and
spectrum of the radio galaxy NGC 1275.Comment: Accepted for publication in The Astrophysical Journa
Optical Spectral Variability of the Very-High-Energy Gamma-Ray Blazar 1ES 1011+496
We present results of five years of optical (UBVRI) observations of the
very-high-energy gamma-ray blazar 1ES 1011+496 at the MDM Observatory. We
calibrated UBVRI magnitudes of five comparison stars in the field of the
object. Most of our observations were done during moderately faint states of
1ES 1011+496 with R > 15.0. The light curves exhibit moderate, closely
correlated variability in all optical wavebands on time scales of a few days. A
cross-correlation analysis between optical bands does not show significant
evidence for time lags. We find a positive correlation (Pearson's r = 0.57;
probability of non-correlation P(>r) ~ 4e-8) between the R-band magnitude and
the B - R color index, indicating a bluer-when-brighter trend. Snap-shot
optical spectral energy distributions (SEDs) exhibit a peak within the optical
regime, typically between the V and B bands. We find a strong (r = 0.78;
probability of non-correlation P (>r) ~ 1e-15) positive correlation between the
peak flux and the peak frequency, best fit by a relation with k = 2.05 +/- 0.17. Such a correlation is
consistent with the optical (synchrotron) variability of 1ES 1011+496 being
primarily driven by changes in the magnetic field.Comment: Accepted for publication in ApJ. 16 pages, including 7 figure
Time dependent gamma-ray production in the anisotropic IC pair cascade initiated by electrons in active galaxies
New high energy emission features have been recently discovered by the
Cherenkov telescopes from active galaxies e.g., a few minutes variability time
scale of TeV emission from Mrk 501 and PKS 2155-304, sub-TeV -ray
emission from GeV peaked blazar 3C 279, and TeV emission from two nearby active
galaxies, M87 and Cen A, which jets are inclined at a relatively large angle to
the line of sight. These results have put a new light on the high energy
processes occurring in central parts of active galaxies stimulating more
detailed studies of -ray emission models. Here we report the results of
a detailed analysis concerning the most general version of the model for the
-ray production by leptons injected in the jet which interact with the
thermal radiation from an accretion disk (the so called {\it external inverse
Compton model}). We investigate the -ray spectra produced in an
anisotropic Inverse Compton (IC) pair cascade in the whole volume above
the accretion disk. The cascade -ray spectra are obtained for different
locations of the observer in respect to the direction of the jet. We also study
the time evolution of this -ray emission caused by the propagation of
the relativistic leptons along the jet and the delays resulting from different
places of the origin of -rays above the accretion disk. We discuss the
main features of such a cascade model assuming constant injection rate of
electrons along the jet. We are investigating two models for their different
maximum energies: constant value independent on the distance along the jet or
limited by the synchrotron energy losses considered locally in the jet. The
model is discussed in the context of blazars observed at small and large
inclination angles taking as an example the parameters of the two famous
sources Cen A and 3C 279.Comment: 16 pages, 18 figures, accepted for publication in MNRA
- …