Phase-modulated X-ray Emission from Cepheids due to Pulsation-Driven Shocks

TL;DR

This study uses hydrodynamic simulations to investigate how pulsation-driven shocks in Cepheid stars produce phase-dependent X-ray emissions, revealing a two-component emission mechanism linked to shocks and quiescent processes.

Contribution

The paper demonstrates, through simulations, that shocks can account for observed X-ray variability in Cepheids, introducing a two-component emission model based on phase-dependent behavior.

Findings

Shocks can reproduce observed X-ray fluxes during maximum radius phases.

Cepheids exhibit two-component X-ray emission: shock-induced and quiescent.

Simulations cover a wide range of mass-loss rates and X-ray luminosities.

Abstract

Cepheids are pulsating variable stars with a periodic chromospheric response at UV wavelengths close to their minimum radius phase. Recently, an X-ray variable signature was captured in observations during the \emph{maximum} radius phase. This X-ray emission came as a surprise and is not understood. In this work, we use the modern astrophysical code, PLUTO, to investigate the effects of pulsations on Cepheid X-ray emission. We run a number of hydrodynamic numerical simulations with a variety of initial and boundary conditions in order to explore the capability of shocks to produce the observed phase-dependent X-ray behavior. Finally we use the Simulated Observations of X-ray Sources (SOXS) package to create synthetic spectra for each simulation case and link our simulations to observables. We show that, for certain conditions, we can reproduce observed X-ray fluxes at phases 0.4--0.8 when the Cepheid is at maximum radius. Our results span a wide range of mass-loss rates, $2\times10^{-13}$--$3\times10^{-8}$ $M_\odot$ yr$^{-1}$, and peak X-ray luminosities, $5\times10^{-17}$--$1.4\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$. We conclude that Cepheids exhibit \textit{two component emission} with (a) shock waves being responsible for the phase dependent variable emission (phases 0.2 - 0.6), and (b) a separate quiescent mechanism being the dominant emission mechanism for the remaining phases.