Resolved Companions of Cepheids: Testing the Candidates With X-Ray Observations*

Abstract

We have made XMM-Newton observations of 14 Galactic Cepheids that have candidate resolved (≥5 ′′ ) companion stars based on our earlier HST WFC3 imaging survey. Main-sequence stars that are young enough to be physical companions of Cepheids are expected to be strong X-ray producers in contrast to field stars.XMM-Newton exposures were set to detect essentially all companions hotter than spectral type M0 (corresponding to 0.5 M ⊙ .) The large majority of our candidate companions were not detected in X-rays, and hence are not confirmed as young companions. One resolved candidate (S Nor #4) was unambiguously detected, but the Cepheid is a member of a populous cluster. For this reason, it is likely that S Nor #4 is a cluster member rather than a gravitationally bound companion. Two further Cepheids (S Mus and R Cru) have X-ray emission that might be produced by either the Cepheid or the candidate resolved companion.A subsequent Chandra observation of S Mus shows that the X-rays are at the location of the Cepheid/spectroscopic binary. R Cru and also V659 Cen (also X-ray bright) have possible companions closer than 5 ′′ (the limit for this study) which are the likely source of X-rays. One final X-ray detection (V473 Lyr) has no known optical companion, so the prime suspect is the Cepheid itself. It is a unique Cepheid with a variable amplitude.The 14 stars that we observed with XMM constitute 36% of the 39 Cepheids found to have candidate companions in our HST/WFC3 optical survey. No young probable binary companions were found with separations of ≥ 5 ′′ or 4000 AU.

X-Rays in Cepheids: Identifying Low-mass Companions of Intermediate-mass Stars*

“…Observations of V473 Lyr and η Aql are discussed in Evans et al 2020bEvans et al , 2021, respectively. Finally, a survey was made with XMM-Newton of possible resolved companions (Evans et al 2016b). In this section we discuss these observations in three subsections: upper limits in cases where the Cepheid was not detected (Table 2), cases where the Cepheid was detected (Table 3), and cases where a low-mass companion was detected (Table 4).…”

“…A series of observations were conducted with XMM-Newton of Cepheids with possible resolved low-mass companions (Evans et al 2016b). The resolved companion candidates are separated from the Cepheids typically by 10″; hence, these observations also provide an X-ray observation of the Cepheid and any possible close companion as well.…”

“…For most observations, the exposure time only covered ±0.03 in phase. The upper limits are based on a 3σ detection since the positions of the sources are known, as discussed in Evans et al (2016b). Uncertainties for the upper limits were estimated from the standard deviation of the local background.…”

Radio Stars: From kHz to THz

“…However, recent work by N. Evans (CfA) and collaborators provides a new motivation for re-examining these stars in the radio. Using Chandra and X-Ray Multi-Mirror Mission (XMM-Newton) observations of Cepheids, they have discovered surprising bursts of X-rays at a pulsation phase just after maximum radius (Engle et al 2014(Engle et al , 2017Evans et al 2016). One possible explanation is that these bursts are caused by shocks related to the pulsation process; another is that they are a flare-like phenomenon triggered by the collapse of the atmosphere.…”

Hubble Space Telescope Snapshot Survey for Resolved Companions of Galactic Cepheids*

“…The current paper (Paper III) follows with a discussion of possible companions closer to the Cepheid. These studies are supplemented with a discussion of XMM-Newton X-ray observations of systems with possible companions, in order to determine whether the companion is young and active enough to be a physical companion of the Cepheid (Paper IV; Evans et al 2016b). The study of resolved companions is complemented by a discussion of radial velocities for northern Cepheids (Evans et al 2015).…”