Engle, Scott G.Guinan, Edward F.Harper, Graham M.Cuntz, ManfredEvans, Nancy RemageNeilson, Hilding R.Fawzy, Diaa E.2023-06-162023-06-1620170004-637X1538-4357https://doi.org/10.3847/1538-4357/aa6159https://hdl.handle.net/20.500.14365/2574From 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 phasedependent FUV-UV emissions of the star that increase similar to 10-20 times with highest fluxes at similar to 0.90-0.95 phi, just prior to maximum brightness. Previously delta Cep. was found as potentially X-ray variable, using XMM-Newton observations. 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 were found to very closely match the previously observed X-ray behavior. From the combined data set, a similar to 4 increase in X-ray flux is measured, reaching a peak L-X = 1.7 x 10(29) erg s(-1) near 0.45 phi. The precise X-ray flux phasing with the star's pulsation indicates that the emissions arise from the Cepheid and not from a companion. However, it is puzzling that the maximum X-ray flux occurs similar to 0.5 phi (similar to 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 a new class of pulsation-induced X-ray variables.eninfo:eu-repo/semantics/openAccessstars: activitystars: atmospheresstars: individual (delta Cep)stars: variables: Cepheidsultraviolet: starsX-rays: starsI Lambda-10830 LineHubble-Space-TelescopeRadial-VelocitiesMagnetic-FieldsEmission-LinesYoung StarsMass-LossChromospheresSupergiantsPopulationArticle10.3847/1538-4357/aa61592-s2.0-85016786723