An X-ray activity cycle on the young solar-like star ɛ Eridani

Abstract

Chromospheric Ca II activity cycles are frequently found in late-type stars, but there have been no systematic programs to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles goes from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. XMM-Newton has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ǫ Eridani, observed previously twice in 2003 and in early 2015 by XMM-Newton. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ǫ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity results to be 2 × 10 28 erg/s, with an amplitude that is only a factor 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ǫ Eridani in terms of solar magnetic structures: active regions, cores of active regions and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can describe the observed X-ray emission measure of ǫ Eridani only if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ǫ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ǫ Eridani. Our analysis revealed also that the X-ray cycle of ǫ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ǫ Eridani is thus explained by the high percentage of magnetic structures on its surface.

Detection of 2–4 GHz Continuum Emission from ϵ Eridani

“…While the exact physical origin of this flare is still uncertain, we use the apparent mean radio flaring rate, l = -0.05 min radio 1 , to predict a flare count for our net 10 hour exposure on ò Eri in 2019. We note that λ radio is several orders of magnitude higher than the observed X-ray flaring rate, λ X−ray ;4.6×10 −7 minute −1 , inferred from the detection of four X-ray flares by Coffaro et al (2020) over 6000days of observing.…”

“…is the emission measure distribution (EMD) of X-ray emitting coronal volumes. Setting EMD= 3.2×10 50 cm −3 at T≈3 MK (Coffaro et al 2020), and f c  40% below 4GHz based on magnetic flux freezing, we find that τ ff (ν)=1 occurs at ν ff 1 GHz. Hence, it is likely that gyroresonance absorption renders the stellar corona optically thick to radiation in the 2-6GHz frequency band.…”

“…Figure 4 shows that the stellar corona of ò Eri becomes optically thick below ∼6GHz. Using constraints from recent X-ray observations (Coffaro et al 2020) of òEri, we identify the radio frequency ν ff , below which κ ff (ν) dominates the net opacity. At any radio frequency, the free-free optical depth (τ ff ) is given by…”

“…Stellar flares are often linked to magnetic energy release episodes in the corona, with their event rates highly dependent on stellar activity cycles. Regular monitoring of ò Eri over the past 50 years has revealed the simultaneous operation of two dynamos (Metcalfe et al 2013;Coffaro et al 2020) with cycle periods of 2.95 years and 12.7 years. For comparison, the Sun also exhibits dual dynamos with a short ∼2yr cycle and a long 11yr cycle (Fletcher et al 2010).…”

Exploring the robustness of Keplerian signals to the removal of active and telluric features

“…Other peaks do not change significantly, which leads to a relative improvement of the planetary signal of 20, 23, and 39 per cent relative to the original data set for the three cases presented. We do not see the 3 year activity period in the data (Coffaro et al 2020), but it is possible that it is present as a change in the planet's period after the active lines are removed.…”