scholarly journals Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks

2020 ◽  
Vol 636 ◽  
pp. A52 ◽  
Author(s):  
J. H.-H. Chan ◽  
M. Millon ◽  
V. Bonvin ◽  
F. Courbin
Keyword(s):  
Accretion Disks ◽  
Measurement Errors ◽  
Transfer Functions ◽  
Light Curves ◽  
Time Shift ◽  
Reverberation Mapping ◽  
Central Engine ◽  
Photometric Monitoring ◽  
The Impact ◽  
Multi Band

With the advent of high-cadence and multi-band photometric monitoring facilities, continuum reverberation mapping is becoming of increasing importance for the measurement of the physical size of quasar accretion disks. The method is based on measuring the time it takes for a signal to propagate from the center to the outer parts of the central engine, assuming the continuum light curve at a given wavelength has a time shift of the order of a few days with respect to light curves obtained at shorter wavelengths. We show that with high-quality light curves, this assumption is no longer valid and that light curves at different wavelengths are not only shifted in time, but also distorted: in the context of the lamp-post model and thin-disk geometry, the multi-band light curves are, in fact, convolved by a transfer function whose size increases with wavelength. We illustrate the effect with simulated light curves in the Large Synoptic Survey Telescope (LSST) ugrizy bands and examine the impact on the delay measurements when using three different methods, namely JAVELIN, CREAM, and PyCS. We find that current accretion disk sizes estimated from JAVELIN and PyCS are underestimated by ∼30% and that unbiased measurements are only obtained with methods that properly take the skewed transfer functions into account, as the CREAM code does. With the LSST-like light curves, we expect to achieve measurement errors below 5% with a typical two-day photometric cadence.

GRAVITATIONAL RADIATIONS FROM THE PRECESSION CENTRAL ENGINE IN GAMMA-RAY BURSTS

2013 ◽  
Vol 23 ◽  
pp. 281-283
Author(s):  
MOU-YUAN SUN ◽  
TONG LIU ◽  
WEI-MIN GU ◽  
JU-FU LU
Keyword(s):  
Black Hole ◽  
Accretion Disks ◽  
Gamma Ray ◽  
Local Group ◽  
Complex Structure ◽  
Outer Part ◽  
Gamma Ray Bursts ◽  
Light Curves ◽  
Central Engine ◽  
Black Hole Masses

The ultra-relativistic precessing jet in gamma-ray bursts (GRBs) may be responsible for the complex structure in GRBs' light curves. In this work, we study the gravitational radiations of jet precession induced by neutrino-dominated accretion disks around black holes. In our model, the jet and the inner part of the disk may precess along with the black hole, which is driven by the outer part of the disk. Gravitational radiations are therefore expected to be significant from this precession system. Based our numerical results, we find that it is possible for DECIGO and BBO to detect such gravitational radiations regardless of GRBs' black hole masses, particularly for GRBs in the Local Group.

scholarly journals Assessment of Spatiotemporal Peak Shift of Intra-Urban Transportation Taking a Case in Bangkok, Thailand

2021 ◽  
Vol 13 (12) ◽  
pp. 6777
Author(s):  
Masanobu Kii ◽  
Yuki Goda ◽  
Varameth Vichiensan ◽  
Hiroyuki Miyazaki ◽  
Rolf Moeckel
Keyword(s):  
Developing Countries ◽  
Travel Time ◽  
Demand Management ◽  
Likelihood Estimation ◽  
Road Transport ◽  
Peak Shift ◽  
Time Shift ◽  
Peak Time ◽  
Management Scenarios ◽  
The Impact

Reducing congestion has been one of the critical targets of transportation policies, particularly in cities in developing countries suffering severe and chronic traffic congestions. Several traditional measures have been in place but seem not very successful. This paper applies the agent-based transportation model MATSim for a transportation analysis in Bangkok to assess the impact of spatiotemporal transportation demand management measures. We collect required data for the simulation from various data sources and apply maximum likelihood estimation with the limited data available. We investigate two demand management scenarios, peak time shift, and decentralization. As a result, we found that these spatiotemporal peak shift measures are effective for road transport to alleviate congestion and reduce travel time. However, the effect of those measures on public transport is not uniform but depends on the users’ circumstances. On average, the simulated results indicate that those measures increase the average travel time and distance. These results suggest that demand management policies require considerations of more detailed conditions to improve usability. The study also confirms that microsimulation can be a tool for transport demand management assessment in developing countries.

scholarly journals Classification of multiwavelength transients with Machine Learning

2020 ◽  
Author(s):  
K Sooknunan ◽  
M Lochner ◽  
Bruce A Bassett ◽  
H V Peiris ◽  
R Fender ◽  
...  
Keyword(s):  
Machine Learning ◽  
Small Sample ◽  
Light Curves ◽  
Machine Learning Techniques ◽  
Optical Data ◽  
Test Time ◽  
Test Accuracy ◽  
Training Set ◽  
The Impact

Abstract With the advent of powerful telescopes such as the Square Kilometer Array and the Vera C. Rubin Observatory, we are entering an era of multiwavelength transient astronomy that will lead to a dramatic increase in data volume. Machine learning techniques are well suited to address this data challenge and rapidly classify newly detected transients. We present a multiwavelength classification algorithm consisting of three steps: (1) interpolation and augmentation of the data using Gaussian processes; (2) feature extraction using wavelets; (3) classification with random forests. Augmentation provides improved performance at test time by balancing the classes and adding diversity into the training set. In the first application of machine learning to the classification of real radio transient data, we apply our technique to the Green Bank Interferometer and other radio light curves. We find we are able to accurately classify most of the eleven classes of radio variables and transients after just eight hours of observations, achieving an overall test accuracy of 78%. We fully investigate the impact of the small sample size of 82 publicly available light curves and use data augmentation techniques to mitigate the effect. We also show that on a significantly larger simulated representative training set that the algorithm achieves an overall accuracy of 97%, illustrating that the method is likely to provide excellent performance on future surveys. Finally, we demonstrate the effectiveness of simultaneous multiwavelength observations by showing how incorporating just one optical data point into the analysis improves the accuracy of the worst performing class by 19%.

Blind Identification of Impact Forces From Multiple Remote Vibratory Measurements

2000 ◽  
Author(s):  
J. Antunes ◽  
P. Izquierdo ◽  
M. Paulino
Keyword(s):  
Explicit Knowledge ◽  
Transfer Functions ◽  
Dynamical Behavior ◽  
Practical Interest ◽  
Operating Conditions ◽  
Blind Identification ◽  
Minimum Entropy ◽  
Response Measurement ◽  
Identification Techniques ◽  
The Impact

Abstract Structures and mechanical components are often subjected to impulsive forces. There is a need for identification techniques which enable monitoring of such loads under operating conditions. For safety reasons and convenience, force identification must often be based on response motions sensed at accessible locations, remote from the impact points. In our previous work we presented techniques for the experimental identification of both isolated impacts and complex rattling forces on a beam, generated at a single and also at several impacting supports. The system dynamical behavior was modeled using traveling flexural beam waves. Although successful, these techniques obviously assume a good understanding of the system dynamic parameters. This is not always the case, a fact that highlights the practical interest of blind identification techniques. This relatively recent field, connected with higher-order statistics, avoids any explicit knowledge of the system transfer functions or impulse responses. Our previous work, based on a single response measurement, is extended in this paper to include several simultaneous responses. We develop a multi-trace version of Wiggins minimum-entropy blind deconvolution algorithm. From numerical simulations and experiments, it is shown that the robustness to noise contamination is increased by using multiple response data. These results suggest that blind identification techniques will prove very useful in practical situations.

scholarly journals Impact of measurement errors on the inferred stellar asteroseismic ages

2018 ◽  
Vol 620 ◽  
pp. A168 ◽  
Author(s):  
G. Valle ◽  
M. Dell’Omodarme ◽  
P. G. Prada Moroni ◽  
S. Degl’Innocenti
Keyword(s):  
Effective Temperature ◽  
Measurement Errors ◽  
Regression Models ◽  
Linear Models ◽  
Projection Pursuit ◽  
Observational Constraints ◽  
Error Source ◽  
Projection Pursuit Regression ◽  
The Impact ◽  
Age Estimates

Aims. We aim to perform a theoretical investigation on the direct impact of measurement errors in the observational constraints on the recovered age for stars in main sequence (MS) and red giant branch (RGB) phases. We assumed that a mix of classical (effective temperature Teff and metallicity [Fe/H]) and asteroseismic (Δν and νmax) constraints were available for the objects. Methods. Artificial stars were sampled from a reference isochrone and subjected to random Gaussian perturbation in their observational constraints to simulate observational errors. The ages of these synthetic objects were then recovered by means of a Monte Carlo Markov chains approach over a grid of pre-computed stellar models. To account for observational uncertainties the grid covers different values of initial helium abundance and mixing-length parameter, that act as nuisance parameters in the age estimation. Results. The obtained differences between the recovered and true ages were modelled against the errors in the observables. This procedure was performed by means of linear models and projection pursuit regression models. The first class of statistical models provides an easily generalizable result, whose robustness is checked with the second method. From linear models we find that no age error source dominates in all the evolutionary phases. Assuming typical observational uncertainties, for MS the most important error source in the reconstructed age is the effective temperature of the star. An offset of 75 K accounts for an underestimation of the stellar age from 0.4 to 0.6 Gyr for initial and terminal MS. An error of 2.5% in νmax resulted the second most important source of uncertainty accounting for about −0.3 Gyr. The 0.1 dex error in [Fe/H] resulted particularly important only at the end of the MS, producing an age error of −0.4 Gyr. For the RGB phase the dominant source of uncertainty is νmax, causing an underestimation of about 0.6 Gyr; the offset in the effective temperature and Δν caused respectively an underestimation and overestimation of 0.3 Gyr. We find that the inference from the linear model is a good proxy for that from projection pursuit regression models. Therefore, inference from linear models can be safely used thanks to its broader generalizability. Finally, we explored the impact on age estimates of adding the luminosity to the previously discussed observational constraints. To this purpose, we assumed – for computational reasons – a 2.5% error in luminosity, much lower than the average error in the Gaia DR2 catalogue. However, even in this optimistic case, the addition of the luminosity does not increase precision of age estimates. Moreover, the luminosity resulted as a major contributor to the variability in the estimated ages, accounting for an error of about −0.3 Gyr in the explored evolutionary phases.

scholarly journals Small impacts on the giant planet Jupiter

2018 ◽  
Vol 617 ◽  
pp. A68 ◽  
Author(s):  
R. Hueso ◽  
M. Delcroix ◽  
A. Sánchez-Lavega ◽  
S. Pedranghelu ◽  
G. Kernbauer ◽  
...  
Keyword(s):  
Giant Planet ◽  
Interplanetary Dust ◽  
Light Curves ◽  
Dust Particles ◽  
Video Observation ◽  
Interplanetary Dust Particles ◽  
The Past ◽  
Giant Impacts ◽  
The Masses ◽  
The Impact

Context. Video observations of Jupiter obtained by amateur astronomers over the past 8 years have shown five flashes of light with durations of 1–2 s, each observed by at least two observers that were geographically separated. The first three of these events occurred on 3 June 2010, 20 August 2010, and 10 September 2012. Previous analyses of their light curves showed that they were caused by the impact of objects of 5–20 m in diameter, depending on their density, with a released energy comparable to superbolides on Earth of the class of the Chelyabinsk airburst. The most recent two flashes on Jupiter were detected on 17 March 2016 and 26 May 2017 and are analyzed here. Aims. We characterize the energy involved together with the masses and sizes of the objects that produced these flashes. The rate of similar impacts on Jupiter provides improved constraints on the total flux of impacts on the planet, which can be compared to the amount of exogenic species detected in the upper atmosphere of Jupiter. Methods. We extracted light curves of the flashes and calculated the masses and sizes of the impacting objects after calibrating each video observation. We also present results from a systematic search of impacts on >72 000 video amateur observations with a customized software that is based on differential photometry of the images. An examination of the number of amateur observations of Jupiter as a function of time over the past years allows us to interpret the statistics of these impact detections. Results. The cumulative flux of small objects (5–20 m or larger) that impact Jupiter is predicted to be low (10–65 impacts per year), and only a fraction of them are potentially observable from Earth (4–25 observable impacts per year in a perfect survey). These numbers imply that many observers are required to efficiently discover Jupiter impacts. Conclusions. We predict that more impacts will be found in the next years, with Jupiter opposition displaced toward summer in the northern hemisphere where most amateur astronomers observe. Objects of this size contribute negligibly to the abundance of exogenous species and dust in the stratosphere of Jupiter when compared with the continuous flux from interplanetary dust particles punctuated by giant impacts. Flashes of a high enough brightness (comparable at their peak to a +3.3 magnitude star) could produce an observable debris field on the planet. We estimate that a continuous search for these impacts might find these events once every 0.4–2.6 yr.

scholarly journals Long-term Photometric Monitoring of FUor and FUor-like Objects

2018 ◽  
pp. 240-248
Author(s):  
E. Semkov ◽  
S. Ibryamov ◽  
S. Peneva ◽  
A. Mutafov
Keyword(s):  
Light Curve ◽  
Significant Role ◽  
Stellar Evolution ◽  
Photographic Plate ◽  
Light Curves ◽  
Curve Shape ◽  
Rate Of Increase ◽  
Fu Ori ◽  
Photometric Monitoring

A phenomenon with a significant role in stellar evolution is the FU Orionis (FUor) type of outburst. The first three (classical) FUors (FU Ori, V1515 Cyg and V1057 Cyg) are well-studied and their light curves are published in the literature. But recently, over a dozen new objects of this type were discovered, whose photometric history we do not know well. Using recent data from photometric monitoring and data from the photographic plate archives we aim to study, the long-term photometric behavior of FUor and FUor-like objects. The construction of the historical light curves of FUors could be very important for determining the beginning of the outburst, the time to reach the maximum light, the rate of increase and decrease in brightness, the pre-outburst variability of the star. So far we have published our results for the light curves of V2493 Cyg, V582 Aur, Parsamian 21 and V1647 Ori. In this paper we present new data that describe more accurate the photometric behavior of these objects. In comparing our results with light curves of the well-studied FUors (FU Ori, V1515 Cyg and V1057 Cyg), we conclude that every object shows different photometric behavior. Each known FUor has a different rate of increase and decrease in brightness and a different light curve shape.

scholarly journals Magnetic field transport in compact binaries

2020 ◽  
Vol 641 ◽  
pp. A133
Author(s):  
N. Scepi ◽  
G. Lesur ◽  
G. Dubus ◽  
J. Jacquemin-Ide
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Magnetic Flux ◽  
Large Scale ◽  
Compact Object ◽  
Light Curves ◽  
Momentum Transport ◽  
Local Magnetization ◽  
Angular Momentum Transport ◽  
The Impact

Context. Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints on angular momentum transport mechanisms. Aims. We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large-scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations. Methods. We developed a version of the disk instability model that evolves the density, the temperature, and the large-scale vertical magnetic flux simultaneously. We took into account the accretion driven by turbulence or by a magnetized outflow with prescriptions taken, respectively, from shearing box simulations or self-similar solutions of magnetized outflows. To evolve the magnetic flux, we used a toy model with physically motivated prescriptions that depend mainly on the local magnetization β, where β is the ratio of thermal pressure to magnetic pressure. Results. We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe light curves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as Ṁ−2/3. Conclusions. Models of DNe and LMXB light curves typically require the outer, viscous disk to be truncated in order to match the observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, with the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.

scholarly journals Impact of climate change on hydropower production in the Upper Danube

2021 ◽  
Author(s):  
Ignacio Martin Santos ◽  
Mathew Herrnegger ◽  
Hubert Holzmann
Keyword(s):  
Climate Change ◽  
Spatial Resolution ◽  
Transfer Functions ◽  
Danube River ◽  
Emission Scenarios ◽  
Hydropower Generation ◽  
Climate Change Projections ◽  
Impact Of Climate Change ◽  
The Future ◽  
The Impact

<p>In the last two decades, different climate downscaling initiatives provided climate scenarios for Europe. The most recent initiative, CORDEX, provides Regional Climate Model (RCM) data for Europe with a spatial resolution of 12.5 km, while the previous initiative, ENSEMBLES, had a spatial resolution of 25 km. They are based on different emission scenarios, Representative Concentration Pathways (RCPs) and Special Report on Emission Scenarios (SRES) respectively.</p><p>A study carried out by Stanzel et al. (2018) explored the hydrological impact and discharge projections for the Danube basin upstream of Vienna when using either CORDEX and ENSEMBLES data. This basin covers an area of 101.810<sup></sup>km<sup>2</sup> with a mean annual discharge of 1923 m<sup>3</sup>/s at the basin outlet. The basin is dominated by the Alps, large gradients and is characterized by high annual precipitations sums which provides valuable water resources available along the basin. Hydropower therefore plays an important role and accounts for more than half of the installed power generating capacity for this area. The estimation of hydropower generation under climate change is an important task for planning the future electricity supply, also considering the on-going EU efforts and the “Green Deal” initiative.</p><p>Taking as input the results from Stanzel et al. (2018), we use transfer functions derived from historical discharge and hydropower generation data, to estimate potential changes for the future. The impact of climate change projections of ENSEMBLE and CORDEX in respect to hydropower generation for each basin within the study area is determined. In addition, an assessment of the impact on basins dominated by runoff river plants versus basins dominated by storage plants is considered.</p><p>The good correlation between discharge and hydropower generation found in the historical data suggests that discharge projection characteristics directly affect the future expected hydropower generation. Large uncertainties exist and stem from the ensembles of climate runs, but also from the potential operation modes of the (storage) hydropower plants in the future.</p><p> </p><p> </p><p>References:</p><p>Stanzel, P., Kling, H., 2018. From ENSEMBLES to CORDEX: Evolving climate change projections for Upper Danube River flow. J. Hydrol. 563, 987–999. https://doi.org/10.1016/j.jhydrol.2018.06.057</p><p> </p>

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