* Astronomy

Maximum of the variable star Delta Cephei is at 01:10 UT, 18th March 2017.

Title: The Secret Lives of Cepheids: delta Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars
Author: Scott G. Engle, Edward F. Guinan, Dr. Graham M. Harper, Prof. Manfred ****z, Dr. Nancy R. Evans, Dr. Hilding Neilson, Dr. Diaa E. Fawzy

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 ~10--20 times with highest fluxes at ~0.90-0.95 phi, 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 behaviour. From the combined dataset, a ~4x increase in X-ray flux is measured, reaching a peak LX = 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 a companion. However, it is puzzling that maximum X-ray flux occurs ~0.5 phi (~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.

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Delta Cephei is at minimum at 20:50 UT, 17th February 2017.

Maximum of Delta Cephei variable star is at 02:40 UT, 3rd February 2017