The Secret Lives of Cepheids: δ Cep—The Prototype of a New Class of Pulsating X-Ray Variable Stars{sup *}

Journal Article · Mon Mar 20 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA6159· OSTI ID:22869177

Engle, Scott G.; Guinan, Edward F. ^[1]; Harper, Graham M. ^[2]; Cuntz, Manfred ^[3]; Evans, Nancy Remage ^[4]; Neilson, Hilding R. ^[5]

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1. Department of Astrophysics and Planetary Science, Villanova University, Villanova, Pennsylvania 19085 (United States)
2. Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, Colorado 80309 (United States)
3. Department of Physics, University of Texas at Arlington, Science Hall, Arlington, Texas 76019 (United States)
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 (United States)
5. Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4 (Canada)

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 L{sub X} = 1.7 × 10{sup 29} erg s{sup −1} near 0.45ϕ. 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 ∼0.5ϕ (∼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.

• OSTI ID: 22869177
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 838; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BRIGHTNESS
CEPHEIDS
COMPARATIVE EVALUATIONS
EMISSION
PERIODICITY
PULSATIONS
STELLAR ATMOSPHERES
ULTRAVIOLET RADIATION
X RADIATION
X-RAY DETECTION
X-RAY SOURCES