216 research outputs found
Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities
Context: The gamma-ray binary LSI+61303 shows multiple periodicities. The
timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV
gamma-ray data both have found two close periodicities P1(GBI) = 26.49 \pm 0.07
d, P2(GBI)=26.92 \pm 0.07 d and P1(gamma)=26.48 \pm 0.08 d, P2(gamma) = 26.99
\pm 0.08 d. Aims: The system LSI+61303 is the object of several continuous
monitoring programs at low and high energies. The frequency difference between
f1 and f2 of only 0.0006 d(-1) requires long-term monitoring because the
frequency resolution in timing analysis is related to the inverse of the
overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope
has been monitoring the source at 15 GHz for five years and overlaps with
Fermi-LAT monitoring. The aim of this work is to establish whether the two
frequencies are also resolved in the OVRO monitoring. Methods: We analysed OVRO
data with the Lomb-Scargle method. We also updated the timing analysis of
Fermi-LAT observations. Results: The periodograms of OVRO data confirm the two
periodicities P1(OVRO) = 26.5 \pm 0.1 d and P2(OVRO) = 26.9 \pm 0.1 d.
Conclusions: The three indipendent measurements of P1 and P2 with GBI, OVRO,
and Fermi-LAT observations confirm that the periodicities are permanent
features of the system LSI+61303. The similar behaviours of the emission at
high (GeV) and low (radio) energy when the compact object in LSI+61303 is
toward apastron suggest that the emission is caused by the same periodically
(P1) ejected population of electrons in a precessing (P2) jet.Comment: 4 pages, 7 figures, A&A Letters in pres
Long-term OVRO monitoring of LS I +61º 303: confirmation of the two close periodicities
Context. The gamma-ray binary LS I +61° 303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P_(1,GBI) = 26.49 ± 0.07 d, P_(2,GBI) = 26.92 ± 0.07 d and P_(1,γ) = 26.48 ± 0.08 d, P_(2,γ) = 26.99 ± 0.08 d.
Aims. The system LS I +61°303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between ν_1 and ν_2 of only 0.0006 d^(-1) requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope has been monitoring the source at 15 GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring.
Methods. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations.
Results. The periodograms of OVRO data confirm the two periodicities and .
Conclusions. The three independent measurements of P_1 and P_2 with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LS I +61°303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LS I +61°303 is toward apastron suggest that the emission is caused by the same periodically (P_1) ejected population of electrons in a precessing (P_2) jet
Ab initio quantum Monte Carlo calculations of spin superexchange in cuprates: the benchmarking case of CaCuO
In view of the continuous theoretical efforts aimed at an accurate
microscopic description of the strongly correlated transition metal oxides and
related materials, we show that with continuum quantum Monte Carlo (QMC)
calculations it is possible to obtain the value of the spin superexchange
coupling constant of a copper oxide in a quantitatively excellent agreement
with experiment. The variational nature of the QMC total energy allows us to
identify the best trial wave function out of the available pool of wave
functions, which makes the approach essentially free from adjustable parameters
and thus truly ab initio. The present results on magnetic interactions suggest
that QMC is capable of accurately describing ground state properties of
strongly correlated materials.Comment: Published in Physical Review
Machine Learning Diffusion Monte Carlo Energies
We present two machine learning methodologies that are capable of predicting
diffusion Monte Carlo (DMC) energies with small datasets (~60 DMC calculations
in total). The first uses voxel deep neural networks (VDNNs) to predict DMC
energy densities using Kohn-Sham density functional theory (DFT) electron
densities as input. The second uses kernel ridge regression (KRR) to predict
atomic contributions to the DMC total energy using atomic environment vectors
as input (we used atom centred symmetry functions, atomic environment vectors
from the ANI models, and smooth overlap of atomic positions). We first compare
the methodologies on pristine graphene lattices, where we find the KRR
methodology performs best in comparison to gradient boosted decision trees,
random forest, gaussian process regression, and multilayer perceptrons. In
addition, KRR outperforms VDNNs by an order of magnitude. Afterwards, we study
the generalizability of KRR to predict the energy barrier associated with a
Stone-Wales defect. Lastly, we move from 2D to 3D materials and use KRR to
predict total energies of liquid water. In all cases, we find that the KRR
models are more accurate than Kohn-Sham DFT and all mean absolute errors are
less than chemical accuracy
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