Highly Variable Quasar Candidates Selected from 4XMM-DR13 with Machine Learning
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.
Authors 9
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Shenzhen Technology University
Affiliation as printed
Shenzhen Key Laboratory of Ultraintense Laser and Advanced Material Technology , Center for Intense Laser Application Technology , and College of Engineering Physics , Shenzhen Technology University , Shenzhen 518118 , People's Republic of China;
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Shenzhen Technology University
Affiliation as printed
Shenzhen Key Laboratory of Ultraintense Laser and Advanced Material Technology , Center for Intense Laser Application Technology , and College of Engineering Physics , Shenzhen Technology University , Shenzhen 518118 , People's Republic of China;
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National Astronomical Observatories
Affiliation as printed
CAS Key Laboratory of Optical Astronomy , National Astronomical Observatories , Beijing 100101 , People's Republic of China
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Leiden University · University of Groningen · Leiden Observatory
Affiliation as printed
Kapteyn Astronomical Institute , University of Groningen , PO Box 800 , 9700 AV Groningen , The Netherlands
Leiden Observatory , Leiden University , Einsteinweg 55 , 2333 CC Leiden , The Netherlands
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Shenzhen Technology University
Affiliation as printed
Shenzhen Key Laboratory of Ultraintense Laser and Advanced Material Technology , Center for Intense Laser Application Technology , and College of Engineering Physics , Shenzhen Technology University , Shenzhen 518118 , People's Republic of China;
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Affiliation as printed
Department of Astronomy , School of Physics and Material Sciences , Guangzhou University , Guangzhou 510006 , People's Republic of China
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Affiliation as printed
Department of Astronomy , School of Physics , Peking University , Beijing 100871 , People's Republic of China
Kavli Institute for Astronomy and Astrophysics , Peking University , Beijing 100871 , People's Republic of China
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Affiliation as printed
School of Computer Science , South China Normal University , 510631 Guangzhou , People's Republic of China
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National Astronomical Observatories
Affiliation as printed
CAS Key Laboratory of Optical Astronomy , National Astronomical Observatories , Beijing 100101 , People's Republic of China
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