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High-level script for fitting microlensing models with MulensModel

First things first:

This script aims at allowing easy access to fitting microlensing models with the MulensModel package and it's not the best place to learn the MulensModel itself.

Allowing easy access to many functions results in somehow complicated code, so if you want to learn MulensModel usage, then we suggest to start with other examples. Also please note that MulensModel has more capabilities than provided in this script.

Here all settings are passed via YAML files, which are human- and machine-readable. If the script syntax is unclear, then please search for information on YAML format files (or see this link).

Install required packages

I suggest to start with:

pip install -r requirements.txt

so that you have all required packages.

Basic usage

In this and following few sections I show how to fit model using EMCEE implementation of MCMC. The other possibility is to use MultiNest and it's presented at the end.

Example usage:

python ulens_model_fit.py ob08092-o4_minimal.yaml

should produce fitted model parameters in a few seconds. Please have a look at ob08092-o4_minimal.yaml - it has only basic settings. In many cases, one can fit a reasonable point-source point-lens model by just changing file name and mean value of t_0.

This minimal script does not plot the model. You can plot the model at the end of fitting, or run a separate script (ulens_model_plot.py) that only makes the plot. You have to take 3rd and 4th last line from output of the above script, add them in model secion of the YAML file and add the information where you want your plot to be saved:

python ulens_model_plot.py ob08092-o4_minimal_plot.yaml

This should produce file ob08092-o4_minimal_plot.png. Compare the two above YAML files to see the differences. You can remove three sections from plotting YAML file, because they're ignored anyway.

More complicated example that will also produce plots of the best model with residuals and the triangle plot:

python ulens_model_fit.py ob08092-o4.yaml

In ob08092-o4.yaml you can see how format of these YAML files mirrors MulensModel API - see the second line and compare it to MulensData API.

Binary lens fitting

You can specify the methods used for calculating magnification. For example, fit the first microlensing planet (calculations may take a few minutes):

python ulens_model_fit.py ob03235_1.yaml

Note that this YAML file will result in a warning message. The message is caused by the fact that in some cases, flux minus its uncertainty results in negative values which cannot be translated to magnitudes when plotting. The warning is related to only plotting, not fitting. You can ignore this warning.

There are many more features to be added - please let authors know what specific needs you have.

Please note that this code is a high-level example for MulensModel, but it uses fitting algorithms that are not part of MulensModel. The latter allows many microlensing calculations including chi^2 for given data and model parameters, but does not have built-in fitting capabilities.

Annual parallax fitting

Let's try to fit the parallax model for OB05086. First, fit model without parallax:

python ulens_model_fit.py ob05086_1.yaml

We see that some of the points are not well fit. Hence, we will try to fit the parallax model. First we update starting points based on results from the first fit (e.g., t_E = 100). We also add parallax parameters: pi_E_N and pi_E_E. We limit both of them to (-1, 1) range. There is one more piece of information definitely needed: event coordinates. These are: 18:04:45.71 -26:59:15.2. In YAML file it's under model and coords. Finally, we want to set parameter reference epoch: t_0_par. The fit will be much much slower without it. We choose an epoch close to t_0. This is not a fitting parameter, so it's placed in fixed_parameters. All these changes are in ob05086_2.yaml file. Please compare it previous file to see all changes. And run:

python ulens_model_fit.py ob05086_2.yaml

We see that chi^2 improved significantly - by more than 400. This clearly indicates the parallax model is better. We see it also on the lightcurve plot. However there is a problem. The blending flux is significantly negative, which is unphysical. Let's try degenerate solution, i.e., we change u_0 sign:

python ulens_model_fit.py ob05086_3.yaml

The chi^2 improved very slightly over positive u_0 parallax model, only by 2.3. In this model the blending flux is positive, so it's the best model. Congratulations!

Priors and constraints

It is possible to specify additional fit constraints in the input file. Currently, empirical t_E distribution and constraining negative blending flux are allowed. To see how it works, let's go back to ob08092-o4.yaml. If you carefully look at the output printed, you will see that the blending flux (flux_b_1 in output) is negative within 1-sigma. This is somehow unphysical. To prevent it, we can disfavor negative blending flux models - see ob08092-o4_prior_1.yaml:

python ulens_model_fit.py ob08092-o4_prior_1.yaml

You will see that the blending flux changed - the median increased and uncertainties decreased. Also, positive and negative uncertainties changed to asymmetric. You can have a look at triangle plots from two runs to see the difference.

We can also specify prior on t_E. In ob08092-o4_prior_2.yaml we use the results from Mroz et al. (2017):

python ulens_model_fit.py ob08092-o4_prior_2.yaml

In this case, the parameters don't change much because t_E is well constrained by the data.

For detailed description of different options, see fit_constraints in ulens_model_fit.py.

Fit using pyMultiNest

The pyMultiNest is one of the implementations of nested sampling - a method that has similar goals to frequently used MCMC approach. There are complicated posteriors in which EMCEE fails and pyMultiNest works without a problem. The latter method is also capable of automatically finding separate posterior modes and exploring each one separately.

The most basic usage of pyMultiNest is presented in ob08092-o4_minimal_MN.yaml. Note that instead of starting_parameters there are prior_limits (based on these settings it's decided which method will be used).

More advanced pyMultiNest input file is ob08092-o4_MN.yaml. It illustrates trivial degeneracy u0 vs. -u0. Note that multimodal option is turned on, so each mode is reported separately.

More options

There are many options and more are being added. The file ob03235_2_full.yaml presents all options currently available:

python ulens_model_fit.py ob03235_2_full.yaml

More information

  • Some more information on API can be found at the top of ulens_model_fit.py file - see docstrings for UlensModelFit class. Please keep in mind that all keywords are read from YAML type file.
  • Julian Dates are long numbers and which may cause problems (e.g., too many numbers to be displayed properly on axis label), hence, in this example we add 2450000 to all input data and subtract it from plots. Note that all calculations are carried out using full HJD, so e.g., Earth's positions are calculated for proper epochs.
  • If you want to plot to screen then do not provide the name of output file for plot, e.g., you can remove last line in ob08092-o4_minimal_plot.yaml.
  • In output, "Best model" is the one with the highest probability, which if different from the smallest chi2 model if informative priors are applied.
  • I have many plans to add more options and capabilities. Please let me know, what you need and I'll try to make it my priority.