X-ray properties of RR Lyrae and Cepheid variables from eROSITA

TL;DR

Using the first eROSITA all-sky survey, the paper searches for X-ray emission from RR Lyrae and Cepheid pulsators by cross-matching optical catalogs with X-ray sources and deriving luminosities from Gaia distances. The analysis yields seven RR Lyrae and eight Cepheid X-ray counterparts, with OGLE-BLG-RRLYR-00252 remaining the sole plausible RR Lyrae detection; Cepheids generally show higher X-ray luminosities and an apparent Lx–period trend for classical Cepheids, while Type II Cepheids are more luminous at similar periods. Many RR Lyrae associations are likely spurious, due in part to crowding, misclassifications, or unrelated background sources, and several X-ray fluxes may be affected by optical loading given the short eROSITA exposures. The results support a shock-driven origin of X-ray emission in pulsators and emphasize the importance of phase-resolved follow-up with XMM-Newton or Chandra to confirm detections and quantify the underlying physics.

Abstract

We present a search for X-ray counterparts to RR Lyrae and Cepheid variables using data from the first eROSITA all-sky survey. We identify seven RR Lyrae and eight Cepheid variables with positional matches to X-ray sources. While most Cepheid associations appear reliable, the RR Lyrae matches are predominantly spurious. Only one source, OGLE-BLG-RRLYR-00252, appears to be a plausible RR Lyrae detection, potentially representing the first observational evidence of X-ray emission from a star of this type. Its inferred luminosity suggests that RR Lyrae stars are intrinsically at least two orders of magnitude fainter in X-rays than the brightest Cepheids. We also observe a tentative increase in X-ray luminosity with pulsation period among classical Cepheids, and higher luminosities in type II Cepheids at comparable periods. These trends may reflect intrinsic differences in atmospheric structure and shock efficiency, offering new insight into the mechanisms driving high-energy emission in pulsating stars.

Figures (5)

• Figure 1: Sorted X-ray luminosities of RR Lyrae stars (blue) and Cepheids (orange) detected with eROSITA. Horizontal error bars represent uncertainties arising from flux measurement errors and distance uncertainties.
• Figure 2: Distance distributions of RR Lyrae stars (left) and Cepheids (right) in our eRASS1 cross-matched sample, using distances from Gaia2021AJ....161..147B. Blue histograms show the full pulsator samples, while vertical dashed red lines indicate those with candidate X-ray counterparts.
• Figure 3: X-ray luminosity as a function of pulsation period for classically pulsating stars with X-ray counterparts detected in eRASS1. Tentative X-ray associations for RR Lyrae stars have been excluded for clarity.
• Figure 4: eROSITA light curves in the 0.2--10 keV band for bet Dor (left) and l Car (right). Pulsation phase 0.0 corresponds to the maximum of the optical brightness.
• Figure 5: Comparison of optical and X-ray light curves phased with the pulsation period. The optical data are from the AAVSO database (bet Dor, l Car, and zet Gem), the TESS satellite (AX Cir and MY Pup), and the OGLE Survey (remaining objects).