GSoC’17 — Report 4
Big Update of this phase is that CrossCorrelation has been merged into the Stingray 🎉 . It feels great when your code gets submitted into a project that has the potential to be a go-to library for astronomical data analysis.

Since last time, I have opened up two Pull Requests. One is one Bispectrum and other is on AutoCorrelation. In particular, work on Bispectrum was very challenging, since I could not find enough help regarding algorithm implementation online.
A few words on Bispectrum. Bispectrum is the Higher Order Spectral analysis method. In simple words, it is Higher order version of PowerSpectrum. It is mostly used to study non-linear interactions which are not possible in the lower order spectra like Power Spectrum. There are two ways in literature to calculate Bispectrum, a direct way and an indirect way. Direct way uses application of convolution theorem, to calculate Bispectra.

Another way to calculate Bispectrum is by using a means of Correlation. Fourier Transform of AutoCorrelation function gives us Powerspectrum of time series. Similarly, Bispectrum is calculate as a Fourier transform of Triple AutoCorrelation function also called as third cumulant. My implementation of Bispectrum involves, indirect method. A third order cumulant of time series is calculated first and its fourier transform is taken to calculate Bispectra. Bispectrum tutorial notebook is also opened as PR in the notebooks repository. Below is the screenshot of Bispectrum plots from notebook repository.

Matteo has not used Bispectrum in his work alot, so he says that he needs to verify results somehow before Bispectrum can be merged into Stingray repository.
The method to tackle was AutoCorrelation. Since, CrossCorrelation been implemented keeping in mind that AutoCorrelation will subclass from it. So, implementation was as easy as calling constructor of CrossCorrelation with same lightcurve as its input arguments. Some tests were included to verify the implementation and as I write this blog, PR on AutoCorrelation has also been merged into master branch 🎉 .
Second Evaluations are coming up next 😨. Hopefully, I pass that as well. That is it for this blog. More stuff after the second evaluations.