Changing states in quasar light curves

Quasars are known to show typical stochastic variability patterns in their light curves. The following figure shows the variabilty in the simulated quasar light curves.

If quasars show certain types of changes in their variability patterns, these changes might be signatures of accretion states in the quasars. The following shows the example ZTF light curves of the SDSS quasars:

We can simulate some types of changes in the quasar light curves. In the following figures, red data points represent the simulated changes with respect to the original data points (blue).

The quasar light curves are generally irregularly and inhomogeneously sampled because of observation coditions and constraints.

Machine learning methods

We develope machine learning methods that describe the distribution of normal quasar light curves, i.e., typical variability patterns in quasars, and then we find outliers in the estimated distributions as light curves showing changes, i.e. deviations from the typical quasar light curves. Our methods do not include any procedures of imputating data assuming uniform sampling of astronomical time-series data.

We detect changes in the time-series data in the space of dt, dm, and udm where dt is time difference between two observation epochs, dm is the corresponding flux difference, and udm is the corresponding uncertainty of the dm.

Mixture density network in Paper I

Public data

Training and test data in Paper I

The first set of the data is made of completely simulated light curves in the ZTF g-band.

• Normal quasar light curves: simulated_normal_lightcurves.tar.gz (about 43MB; 20,000 light curves)
• Mean-shift changes in the normal quasar light curves: simulated_meanshift_lightcurves.tar.gz (about 12MB; 5,000 light curves)
• Variance changes in the normal quasar light curves: simulated_variance_lightcurves.tar.gz (about 12MB; 5,000 light curves)
• Sum-of-exponentials changes in the normal quasar light curves: simulated_sumofexp_lightcurves.tar.gz (about 12MB; 5,000 light curves)

The above normal quasar light curves have three columns of time, magnitude, and uncertainty of magnitude. The other files have four columns of time, magnitude, uncertainty of magnitude, and tag. If the tag is 1, the data points represent the changes. The files of changes in the light curves also have parameter files with filename extension .param. See the Paper I for explanation of the parameters.

The second set of the data corresponds to the ZTF g-band light curves of the SDSS quasars and simulated changes in the light curves.

• SDSS quasar light curves: sdss_normal_lightcurves.tar.gz (about 1.3GB; 344,374 light curves)
• Mean-shift changes in the SDSS quasar light curves: sdss_meanshift_lightcurves.tar.gz (about 20MB; 5,000 light curves)
• Variance changes in the SDSS quasar light curves: sdss_variance_lightcurves.tar.gz (about 23MB; 5,000 light curves)
• Sum-of-exponentials changes in the SDSS quasar light curves: sdss_sumofexp_lightcurves.tar.gz (about 20MB; 5,000 light curves)
• Weibull changes in the SDSS quasar light curves: sdss_weibull_lightcurves.tar.gz (about 21MB; 5,000 light curves)

The formats of the above SDSS quasar light curves are same as those of the completely simulated light curves.