Highly Variable Quasar Candidates Selected from 4XMM-DR13 with Machine Learning

TL;DR

This paper identifies highly variable quasar candidates from the 4XMM-DR13 catalog using machine learning, revealing extremely faint quasars with significant X-ray flux changes over two decades.

Contribution

The study develops a random forest classifier to select faint, highly variable quasars from X-ray and multi-wavelength data, extending the known population to optically faint sources.

Findings

Identified 12 highly variable quasar candidates with flux changes over a factor of 10.

Extended the quasar variability study to optically faint sources around r~22.

Found that such extreme variability quasars are very rare, with none detected in ROSAT data.

Abstract

We present a sample of 12 quasar candidates with highly variable soft X-ray emission from the 4th XMM-newton Serendipitous Source Catalog (4XMM-DR13) using random forest. We obtained optical to mid-IR photometric data for the 4XMM-DR13 sources by correlating the sample with the SDSS DR18 photometric database and the AllWISE database. By cross-matching this sample with known spectral catalogs from the SDSS and LAMOST surveys, we obtained a training data set containing stars, galaxies, and quasars. The random forest algorithm was trained to classify the XMM-WISE-SDSS sample. We further filtered the classified quasar candidates with $\it{Gaia}$ proper motion to remove stellar contaminants. Finally, 53,992 quasar candidates have been classified, with 10,210 known quasars matched in SIMBAD. The quasar candidates have systematically lower X-ray fluxes than quasars in the training set, which indicates the classifier is helpful to single out fainter quasars. We constructed a sample of 12 sources from these quasars candidates which changed their soft X-ray fluxes by a factor of 10 over $\sim$ 20 years in the 4XMM-newton survey. Our selected highly variable quasar candidates extend the quasar sample, characterized by extreme soft X-ray variability, to the optically faint end with magnitudes around $r \sim 22$. None of the 12 sources were detected in ROSAT observations. Given the flux limit of ROSAT, the result suggests that quasars exhibiting variations of more than two orders of magnitudes are extremely rare.