# Sample of Cataclysmic Variables from 400d X-ray Survey

## TL;DR

This paper identifies and characterizes a sample of cataclysmic variables from the 400d X-ray survey, providing new insights into their luminosity function at low X-ray luminosities and predicting their detectability in future surveys.

## Contribution

It presents the first optical identification of CVs from the 400d X-ray survey and constrains their X-ray luminosity function at low luminosities, revealing a less steep slope than at higher luminosities.

## Key findings

- Eight CVs identified, four newly discovered.
- The CV X-ray luminosity function has a shallower slope at L_X ~ 10^29-10^30 erg s^-1.
- Predicted detection of thousands of CVs in future X-ray surveys.

## Abstract

We present a sample of cataclysmic variables (CVs) identified among the X-ray sources from the 400 square degree X-ray survey based on ROSAT pointing data (400d). The procedure of the CV selection among the X-ray sources using additional optical and infrared data from Sloan Digital Sky Survey and WISE survey is described. The results of the optical observations of the selected objects carried out mainly with the Russian-Turkish 1.5-m telescope (RTT-150) and the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (BTA) are presented. Some observations have also been performed with the Sayan Observatory 1.6-m AZT-33IK telescope. Currently we selected eight CVs, four of which were found for the first time in our work. Based on this sample, we have obtained preliminary constraints on the CV X-ray luminosity function in the solar neighborhood in the low luminosity range, L_X ~ 10^29-10^30 erg s^-1 (0.5-2 keV). We show that the logarithmic slope of the CV X-ray luminosity function in this luminosity range is less steep than at L_X > 10^31 erg s^-1. From our CV X-ray luminosity function estimates it follows that few thousand CVs will be detected in theSpectrum-Roentgen-Gamma (SRG) observatory all-sky X-ray survey at high Galactic latitudes, which will allow to obtain much more accurate measurements of CV X-ray luminosity function in the luminosity range L_X < 10^30-10^31 erg s^-1.