Cataclysmic Variables and AM CVn Binaries in SRG/eROSITA + Gaia: Volume Limited Samples, X-Ray Luminosity Functions, and Space Densities
Antonio C. Rodriguez, Kareem El-Badry, В. Ф. Сулейманов, A. F. Pala, S. R. Kulkarni, Boris Gaensicke, Kaya Mori, R. Michael Rich, Arnab Sarkar, Tong Bao, R. Lopes de Oliveira, Gavin Ramsay, Paula Szkody, M. J. Graham, Thomas A. Prince, Ilaria Caiazzo, Zachary P. Vanderbosch, Jan van Roestel, Kaustav K. Das, Yu-Jing Qin, M. M. Kasliwal, Avery Wold, Steven L. Groom, Daniel Reiley, Reed Riddle
Abstract
Abstract We present volume-limited samples of cataclysmic variables (CVs) and AM CVn binaries jointly selected from SRG/eROSITA eRASS1 and Gaia DR3 using an X-ray + optical color–color diagram (the “X-ray Main Sequence”). This tool identifies all CV subtypes, including magnetic and low-accretion rate systems, in contrast to most previous surveys. We find 23 CVs, 3 of which are AM CVns, out to 150 pc in the Western Galactic Hemisphere. Our 150 pc sample is spectroscopically verified and complete down to L X = 1.3 × 10 29 erg s −1 in the 0.2–2.3 keV band, and we also present CV candidates out to 300 pc and 1000 pc. We discovered two previously unknown systems in our 150 pc sample: the third nearest AM CVn and a magnetic period bouncer. We find the mean L X of CVs to be 〈 L X 〉 ≈ 4.6 × 10 30 erg s −1 , in contrast to previous surveys which yielded 〈 L X 〉 ∼ 10 31 −10 32 erg s −1 . We construct X-ray luminosity functions that, for the first time, flatten out at L X ∼ 10 30 erg s −1 . We infer average number, mass, and luminosity densities of ρ N,CV = (3.7 ± 0.7) × 10 −6 pc −3 , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>5.0</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1.0</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>ρ</mml:mi> <mml:msub> <mml:mi>L</mml:mi> <mml:mi mathvariant="normal">X</mml:mi> </mml:msub> </mml:msub> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>2.3</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.4</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>26</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi>erg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi mathvariant="normal">s</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:msubsup> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , respectively, in the solar neighborhood. Our uniform selection method also allows us to place meaningful estimates on the space density of AM CVns, ρ N,AM CVn = (5.5 ± 3.7) × 10 −7 pc −3 . Magnetic CVs and period bouncers make up 35% and 25% of our sample, respectively. This work, through a novel discovery technique, shows that the observed number densities of CVs and AM CVns, as well as the fraction of period bouncers, are still in tension with population synthesis estimates.