New distance measures for classifying X-ray astronomy data into stellar classes

TL;DR

This paper introduces normalized Wasserstein and Zolotarev distances to classify X-ray sources in astronomy, effectively distinguishing between different source types and correcting previous misclassifications in a large dataset.

Contribution

It proposes novel distance-based metrics for classifying X-ray sources using photon interarrival times, with detailed statistical analysis and application to real astronomical data.

Findings

Metrics effectively differentiate source classes

Identification of misclassified sources in the dataset

Some sources previously labeled as extragalactic are likely young stars

Abstract

The classification of the X-ray sources into classes (such as extragalactic sources, background stars, ...) is an essential task in astronomy. Typically, one of the classes corresponds to extragalactic radiation, whose photon emission behaviour is well characterized by a homogeneous Poisson process. We propose to use normalized versions of the Wasserstein and Zolotarev distances to quantify the deviation of the distribution of photon interarrival times from the exponential class. Our main motivation is the analysis of a massive dataset from X-ray astronomy obtained by the Chandra Orion Ultradeep Project (COUP). This project yielded a large catalog of 1616 X-ray cosmic sources in the Orion Nebula region, with their series of photon arrival times and associated energies. We consider the plug-in estimators of these metrics, determine their asymptotic distributions, and illustrate their finite-sample performance with a Monte Carlo study. We estimate these metrics for each COUP source from three different classes. We conclude that our proposal provides a striking amount of information on the nature of the photon emitting sources. Further, these variables have the ability to identify X-ray sources wrongly catalogued before. As an appealing conclusion, we show that some sources, previously classified as extragalactic emissions, have a much higher probability of being young stars in Orion Nebula.