Galactic X-ray Transients in the First eROSITA All Sky Survey

TL;DR

We address the problem of Galactic X-ray transients with timescales $<$30 years by performing a blind cross-match of the eROSITA All Sky Survey (eRASS1), ROSAT 2RXS, and Gaia catalogs. We combine X-ray spectral data with optical Gaia information to classify 738 transients into YSO, SS/IB, IB, IB/ACO, CV, XRB, LPV, PSR, and WR, including ~40 compact-object systems (8 new WD CVs, 3 XRBs, and a pulsar). The eROSITA spectra reveal two common types: a thermal-type peak around $\sim$1 keV and a soft, $<0.2$ keV peak requiring a power-law component, with giants among transients showing higher $L_{\mathrm{X}}$. Giants among transients show higher $L_{\mathrm{X}}$ than main-sequence or YSO systems, suggesting different emission mechanisms and binary interactions. These results inform emission mechanisms, transient fractions, and Galactic transient rates for blind X-ray sky surveys, highlighting the power of eROSITA–Gaia cross-matched catalogs for population studies.

Abstract

Although a multitude of studies have focused on targeted observations of Galactic X-ray transients, blind surveys and population studies have been limited. We have used the ROSAT, eROSITA and Gaia source catalogs to find Galactic X-ray transients having timescales $<$30 years. We report the properties of 738 transients found in our search, majority of which are active stars or interacting binaries. We have also found $\sim$40 compact object systems among which are at least 8 newly-identified white-dwarf systems, 3 known X-ray binaries, and one known pulsar. We use eROSITA (soft X-ray) spectra of the Galactic transients to show that two distinct types of flaring systems are prevalent: one having peak around 1 keV, well fit by thermal models, and another having peak below 0.2 keV and requiring a power-law component. Our study also reveals that single star or interacting binary systems (X-ray transients) involving giant stars exhibit significantly higher X-ray luminosities than systems involving only main-sequence stars or young stellar objects. Finally, we discuss the properties of the transients in the context of their putative emission mechanisms, the fraction of transients with respect to the total population, and the rates of Galactic transients expected in blind searches of the X-ray sky.

Figures (17)

• Figure 1: Histograms of the flux values (absorbed, assuming photon index 2) of 2RXS and eRASS1 catalogs. The approximate 2RXS flux threshold limit is marked with the vertical line at $4\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$
• Figure 2: Plot showing the matching radius versus the fraction of matched sources (total cross-matched sources/20655) for the cross-match of eRASS1 with Gaia DR2 and 2RXS catalogs. We chose 40" and 2" (black and green vertical line respectively) as the matching radius for eRASS1/2RXS cross-match and eRASS1/Gaia DR2 cross-match respectively. Magenta and light blue data points correspond to the above mentioned cross-matches but with eRASS1 sources shifted by 1'.
• Figure 3: Histogram of the 2RXS-to-eRASS1 flux ratio for the 10671 sources resulting from the cross-match of eRASS1/2RXS using a 40" matching radius. The error bars on each bin are described in the text. The red curve represents a Gaussian fit to the histogram and indicates a mean value (for the flux ratio) of $1.15\pm 0.01$ and a corresponding standard deviation of $1.86\pm0.01$.
• Figure 4: Flowchart showing the procedure for selecting Galactic X-ray transients in the eRASS1 catalog.
• Figure 5: Locations of Galactic transients in Galactic co-ordinates, highlighting the open clusters and molecular clouds to which YSOs belong. Various classes of Galactic transients mentioned in section 3.1 are color coded and the varying marker correspond to the different X-ray luminosities (in erg s$^{-1}$) of the transients, as shown in the legend.