552 research outputs found
Gradient boosting decision trees classification of blazars of uncertain type in the fourth Fermi-LAT catalog
The deepest all-sky survey available in the -ray band - the last
release of the Fermi-LAT catalogue (4FGL-DR3) based on the data accumulated in
12 years, contains more than 6600 sources. The largest population among the
sources is blazar subclass - 3743, of which are classified as BL
Lacertae objects (BL Lacs) or Flat Spectrum Radio Quasars (FSRQs), while the
rest are listed as blazar candidates of uncertain type (BCU) as their firm
optical classification is lacking. The goal of this study is to classify BCUs
using different machine learning algorithms which are trained on the spectral
and temporal properties of already classified BL Lacs and FSRQs. Artificial
Neural Networks, \textit{XGBoost} and LightGBM algorithms are employed to
construct predictive models for BCU classification. Using 18 input parameters
of 2219 BL Lacs and FSRQs, we train (80\% of the sample) and test (20\%) these
algorithms and find that LightGBM model, state-of-the-art classification
algorithm based on gradient boosting decision trees, provides the highest
performance. Based on our best model, we classify 825 BCUs as BL Lac candidates
and 405 as FSRQ candidates, however, 190 remain without a clear prediction but
the percentage of BCUs in 4FGL is reduced to 5.1\%. The -ray photon
index, synchrotron peak frequency, and high energy peak frequency of a large
sample are used to investigate the relationship between FSRQs and BL Lacs
(LBLs, IBLs, and HBLs).Comment: Accepted for publication in MNRA
Electromagnetic emission of white dwarf binary mergers
It has been recently proposed that the ejected matter from white dwarf (WD)
binary mergers can produce transient, optical and infrared emission similar to
the "kilonovae" of neutron star (NS) binary mergers. To confirm this we
calculate the electromagnetic emission from WD-WD mergers and compare with
kilonova observations. We simulate WD-WD mergers leading to a massive, fast
rotating, highly magnetized WD with an adapted version of the
smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial
conditions for the ejecta such as escape velocity, mass and initial position
and distribution. The subsequent thermal and dynamical evolution of the ejecta
is obtained by integrating the energy-conservation equation accounting for
expansion cooling and a heating source given by the fallback accretion onto the
newly-formed WD and its magneto-dipole radiation. We show that magnetospheric
processes in the merger can lead to a prompt, short gamma-ray emission of up to
erg in a timescale of - s. The bulk of the ejecta
initially expands non-relativistically with velocity and then it
accelerates to due to the injection of fallback accretion energy. The
ejecta become transparent at optical wavelengths around days
post-merger with a luminosity - erg s. The X-ray
emission from the fallback accretion becomes visible around -
day post-merger with a luminosity of erg s. We also predict
the post-merger time at which the central WD should appear as a pulsar
depending on the value of the magnetic field and rotation period.Comment: 12 pages, Accepted for publication in JCA
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