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arXiv · 2007.11545

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

Sofia-Paraskevi Moschou·Nektarios Vlahakis·Jeremy J. Drake·Nancy Remage Evans·Hilding R. Neilson·Joyce Ann Guzik·John ZuHone
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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.

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Sofia-Paraskevi Moschou, Nektarios Vlahakis, Jeremy J. Drake, Nancy Remage Evans, Hilding R. Neilson, Joyce Ann Guzik, John ZuHone. 2020-07-22. Phase-modulated X-ray Emission from Cepheids due to Pulsation-Driven Shocks. https://arxiv.org/abs/2007.11545

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