Astrophys. J., 838, 67-67 (2017/March-3)
The Secret Lives of Cepheids: δ Cep-the prototype of a new class of pulsating X-ray variable stars.
ENGLE S.G., GUINAN E.F., HARPER G.M., CUNTZ M., EVANS N.R., NEILSON H.R. and FAWZY D.E.
Abstract (from CDS):
From our Secret Lives of Cepheids program, the prototype Classical Cepheid, δ 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 ∼10-20 times with highest fluxes at ∼0.90–0.95φ, just prior to maximum brightness. Previously δ 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 δ 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 ∼4 increase in X-ray flux is measured, reaching a peak LX = 1.7 x 1029 erg s–1 near 0.45ph. 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 ∼0.5ph (∼3 days) later than the FUV-UV maximum. There are several other potential Cepheid X-ray detections with properties similar to δ Cep, and comparable X-ray variability is indicated for two other Cepheids: β Dor and V473 Lyr. X-ray generating mechanisms in δ Cep and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then δ Cep will be the prototype of a new class of pulsation-induced X-ray variables.
© 2017. The American Astronomical Society. All rights reserved.
stars: activity - stars: atmospheres - stars: individual: δ Cep - stars: variables: Cepheids - ultraviolet: stars - X-rays: stars - X-rays: stars
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