# bayesian.multiband: Estimating the time delay between doubly-lensed multi-band... In timedelay: Time Delay Estimation for Stochastic Time Series of Gravitationally Lensed Quasars

## Description

bayesian.multiband produces posterior samples of all the model parameters including the time delay between doubly-lensed multi-band light curves.

## Usage

  1 2 3 4 5 6 7 8 9 10 bayesian.multiband(data.band1, data.band2, n.bands = 2, theta.ini = c(0.01, 0.01, 100, 100, 0.5), delta.ini, delta.uniform.range = c(-500, 500), delta.proposal.scale = 1, tau.proposal.scale = 1, tau.prior.shape = 1, tau.prior.scale = 1, sigma2.proposal.scale = 0.5, sigma2.prior.shape = 1, sigma2.prior.scale = 1e-7, rho.proposal.scale = 0.1, beta.prior.diag = 10 * c(0.1, 0.01, 1e-3, 1e-5)^2, micro = 3, timesc = 100, adaptive.frequency = 100, adaptive.delta.factor = 0.1, adaptive.tau.factor = 0.1, adaptive.sigma2.factor = 0.1, adaptive.rho.factor = 0.1, sample.size, warmingup.size) 

## Arguments

 data.band1 A (n_1 by 5) matrix composed of the data for doubly-lensed images oberved in band 1; the first column contains n_1 observation times, the second column contains n_1 magnitudes for image A, the third column has n_1 measurement error standard deviations for image A, the fourth column include n_1 magnitudes for image B, and the fifth column contains n_1 measurement error standard deviations for image B. data.band2 A (n_2 by 5) matrix composed of the data for doubly-lensed images oberved in band 2; the first column contains n_2 observation times, the second column contains n_2 magnitudes for image A, the third column has n_2 measurement error standard deviations for image A, the fourth column include n_2 magnitudes for image B, and the fifth column contains n_2 measurement error standard deviations for image B. n.bands The number of bands used to obtain the data. For now, only two bands are allowed. The authors plan to modify the code to allow more bands in the near future. theta.ini A vector for initial values of the OU processes for two bands, i.e., (σ_1, σ_2, τ_1, τ_2, ρ), where σ_1 is the short-term variability (standard deviation) for band 1, σ_2 is the short-term variability (standard deviation) for band 2, τ_1 is the timescale for band 1, and τ_2 is the timescale for band 2, and ρ is the cross-correlation parameter between two bands. Defaults are (0.01, 0.01, 100, 100, 0.5). delta.ini Initial values of the time delay. delta.uniform.range The range of the Uniform prior distribution for the time delay. Default range is set to (-500, 500). delta.proposal.scale The proposal scale of the Metropolis step for the time delay. Default is 1. tau.proposal.scale The proposal scale of the Metropolis-Hastings step for τ_j (j=1, 2) Default is 1. tau.prior.shape The shape parameter of the Inverse-Gamma hyper-prior distribution for τ_j. Default is 1. tau.prior.scale The scale parameter of the Inverse-Gamma hyper-prior distribution for τ_j. Default is 1. sigma2.proposal.scale The proposal scale of the Metropolis-Hastings step for σ^2_j (j=1, 2). Default is 0.5. sigma2.prior.shape The shape parameter of the Inverse-Gamma hyper-prior distribution for σ^2_j. Default is 1. sigma2.prior.scale The scale parameter of the Inverse-Gamma hyper-prior distribution for σ^2_j. If no prior information is available, we recommend using 1e-7 (default). rho.proposal.scale The proposal scale of the Metropolis-Hastings step for ρ. Default is 0.1. beta.prior.diag The diagonal elements of the covariance matrix in the multivariate Gaussian prior for β (polynomial regression coefficients for microlensing adjustment). If such information is not available, these are set to 10 * c(0.1)^2 if micro = 0, 10 * c(0.1, 0.01)^2 if micro = 1, 10 * c(0.1, 0.01, 1e-3)^2 if micro = 2, and 10 * c(0.1, 0.01, 1e-3, 1e-5)^2 if micro = 3. micro A non-negative integer less than or equal to 3. It determines the order of a polynomial regression model that accounts for the long-term trend of microlensing effect. Default is 3. timesc It scales the observation time for fitting polynomial of microlensing, i.e., time / timesc, so that the coefficients are not too small. Default is 100. adaptive.frequency The adaptive MCMC is applied for every specified frequency. If it is specified as 500, the adaptive MCMC is applied to every 500th iterstion. Default is 100. adaptive.delta.factor The factor, exp(\pmadaptive.delta.factor), multiplied by the proposal scale of the time delay for adaptive MCMC. Default is 0.1. adaptive.tau.factor The factor, exp(\pmadaptive.tau.factor), multiplied by the proposal scale of τ_j for adaptive MCMC. Default is 0.1. adaptive.sigma2.factor The factor, exp(\pmadaptive.tau.factor), multiplied by the proposal scale of σ^2_j for adaptive MCMC. Default is 0.1. adaptive.rho.factor The factor, exp(\pmadaptive.tau.factor), multiplied by the proposal scale of ρ for adaptive MCMC. Default is 0.1. sample.size The number of the posterior samples of each model parameter. warmingup.size The number of burn-in samples for MCMC.

## Details

The function bayesian.multiband produces posterior samples of the model parameters, where the time delay (delta) is of primary interest. For now, this function only supports doubly-lensed data observed in two bands. The authors plan to generalize this code to account for more than two bands and more than two lens.

Please note that when astronomical time series data are loaded on R by read.table, read.csv, etc., some decimal places of the the observation times are automatically rounded because R's default is to load seven effective digits. For example, R will load the observation time 51075.412789 as 51075.41. This default will produce many ties in observation times even though there is actually no tie in observation times. To prevent this, please type "options(digits = 11)" before loading the data if the observation times are in seven effective digits.

## Value

The outcomes of bayesian.multiband are composed of:

 delta A vector for m posterior samples of the time delay. beta An m by k + 1 matrix containing posterior samples of the polynomial regression coefficients, where m is the size of the posterior sample, and k is the polynomial order for microlensing. rho A vector for m posterior samples of the cross-correlation parameter. sigma An m by 2 matrix containing posterior samples of the short-term variability (standard deviation) of the O-U process. The first column is composed ot the m posterior samples of σ_1 in band 1, and the second column contains the m posterior samples of σ_2 in band 2. tau An m by 2 matrix containing posterior samples of the timescale of the O-U process. The first column is composed ot the m posterior samples of τ_1 in band 1, and the second column contains the m posterior samples of τ_2 in band 2. tau.accept.rate The acceptance rate of the MCMC for τ_1 and τ_2. sigma.accept.rate The acceptance rate of the MCMC for σ_1 and σ_2. delta.accept.rate The acceptance rate of the MCMC for the time delay rho.accept.rate The acceptance rate of the MCMC for ρ.

## Author(s)

Zhirui Hu and Hyungsuk Tak

## References

Hyungsuk Tak, Kaisey Mandel, David A. van Dyk, Vinay L. Kashyap, Xiao-Li Meng, and Aneta Siemiginowska (2017). "Bayesian Estimates of Astronomical Time Delays between Gravitationally Lensed Stochastic Light Curves," The Annals of Applied Statistics, 11 (3), 1309-1348. Hyungsuk Tak, Xiao-Li Meng, and David A. van Dyk (2018), "A Repelling-Attracting Metropolis Algorithm for Multimodality", Journal of Computational and Graphical Statistics, 27 (3), 479-490. Zhirui Hu and Hyungsuk Tak (2020+), "Modeling Stochastic Variability in Multi-Band Time Series Data," arXiv:2005.08049.

## Examples

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39  # Loading datasets data(simple.band1) data(simple.band2) # Doubly-lensed quasar data set observed in two bands # Each data set contains doubly-lensed light curves observed in one band. head(simple.band1) head(simple.band2) # The length of each data set (i.e., number of observation times) # do not need to be the same. dim(simple.band1) dim(simple.band2) ############################################### # Time delay estimation via Bayesian approach # ############################################### # Cubic microlensing model (m = 3) output <- bayesian.multiband(data.band1 = simple.band1, data.band2 = simple.band2, n.bands = 2, theta.ini = c(0.01, 0.01, 100, 100, 0.5), delta.ini = 100, delta.uniform.range = c(-500, 500), tau.proposal.scale = 1, tau.prior.shape = 1, tau.prior.scale = 1, sigma2.proposal.scale = 0.5, sigma2.prior.shape = 1, sigma2.prior.scale = 1e-7, rho.proposal.scale = 0.1, beta.prior.diag = 10 * c(0.1, 0.01, 1e-3, 1e-5)^2, micro = 3, timesc = 100, adaptive.frequency = 100, adaptive.delta.factor = 0.1, adaptive.tau.factor = 0.1, adaptive.sigma2.factor = 0.1, adaptive.rho.factor = 0.1, sample.size = 100, warmingup.size = 100) names(output) # hist(output$delta, 20) # plot(output$delta, type = "l") # acf(output$delta) # output$delta.accept.rate 

timedelay documentation built on July 2, 2020, 2:41 a.m.