The Secret Lives of Cepheids: $\delta$ Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars

Abstract

From our Secret Lives of Cepheids program, the prototype Classical Cepheid, $\delta$ Cep, is found to be an X-ray source with periodic pulsation-modulated X-ray variations. This finding complements our earlier reported phase-dependent FUV--UV emissions of the star that increase $\sim$10--20 times with highest fluxes at $\sim 0.90-0.95\varphi$, just prior to maximum brightness. Previously, $\delta$ Cep was found as potentially X-ray variable, using XMM-Newton observations (Engle et al. 2014). Additional phase-constrained data were secured with Chandra near X-ray emission peak, to determine if the emission and variability were pulsation-phase-specific to $\delta$ Cep and not transient or due to a possible coronally-active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and very closely match the previously observed X-ray behavior. From the combined dataset, a $\sim$4$\times$ increase in X-ray flux is measured, reaching a peak $L_X$ = 1.7 $\times$ 10${}^{29}$ erg s${}^{-1}$ near 0.45$\varphi$. The precise X-ray flux phasing with the star's pulsation indicates that the emissions arise from the Cepheid and not a companion. However, it is puzzling that maximum X-ray flux occurs $\sim$0.5$\varphi$ ($\sim$3 days) later than the FUV--UV maximum. There are several other potential Cepheid X-ray detections with properties similar to $\delta$ Cep, and comparable X-ray variability is indicated for two other Cepheids: $\beta$ Dor and V473 Lyr. X-ray generating mechanisms in $\delta$ Cep and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then $\delta$ Cep will be the prototype of new class of pulsation-induced X-ray variables.

16 pages, 3 figures, 2 tables, accepted for publication in the Astrophysical Journal

References in corpus (11)

• X-Ray Spectroscopy of Stars
• Revealing δ Cephei's Secret Companion and Intriguing Past
• X-ray flares in Orion young stars. I. Flare characteristics
• Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
• Discovery of Cycle-to-cycle Modulated Spectral Line Variability and Velocity Gradients in Long-period Cepheids
• The Kepler Cepheid V1154 Cyg revisited: light curve modulation and detection of granulation
• Binary Properties from Cepheid Radial Velocities (CRaV)
• The Secret Lives of Cepheids: Evolutionary Changes and Pulsation-Induced Shock Heating in the Prototype Classical Cepheid δ Cep
• A new type of X-ray pulsar
• VEGA/CHARA interferometric observations of Cepheids. I. A resolved structure around the prototype classical Cepheid delta Cep in the visible spectral range
• On X-ray pulsations in beta Cephei-type variables

Cited by in corpus (11)

• The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required
• Stellar flares detected with the Next Generation Transit Survey
• A thin shell of ionized gas explaining the IR excess of classical Cepheids
• Probing Polaris' Puzzling Radial Velocity Signals - Pulsational (In-)Stability, Orbital Motion, and Bisector Variations
• Finding magnetic north: an extraordinary magnetic field detection in Polaris and first results of a magnetic survey of classical Cepheids
• Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-Term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk
• X-ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-Mass Companion
• X-rays in Cepheids: XMM-Newton Observations of $\eta$ Aql
• Period Variations for the Cepheid VZ Cyg
• First Detection of Radio Emission Associated with a Classical Cepheid
• Discovery of ionized circumstellar gas emission around the long-period Cepheid $\ell$ Carinae with ALMA