Letter to the Editor

Quiet, but not silent

The X-ray activity of the Maunder minimum star HD 166620

¹ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen Sand 1 72076 Tübingen, Germany
² 501 Campbell Hall, University of California at Berkeley Berkeley CA 94720, USA
³ Max-Planck-Institut für extraterrestrische Physik Giessenbachstrasse 1 85748 Garching, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 November 2025
Accepted: 26 December 2025

Abstract

As the only known unambiguous star in a Maunder minimum-like chromospheric activity state, the properties of HD 166620 can provide valuable insight into the behaviour of the Sun during the historic extended low-states of its activity cycle. The coronal X-ray activity of HD 166620 has so far only been probed with a ROSAT/HRI observation in 1996, near the chromospheric activity maximum before the star entered its grand minimum around 2004. We conducted a deep XMM-Newton observation of HD 166620 during its chromospheric Ca II H&K activity grand minimum to achieve a better understanding of its magnetic activity. We detected HD 166620 with an X-ray luminosity of log L_X(erg s⁻¹) = 26.56^+0.10_−0.12, corresponding to log(L_X/L_bol = −6.58^+0.10_−0.12 and an X-ray surface flux of log F_X(erg s⁻² s⁻¹ = 3.97^+0.10_−0.12. With respect to the earlier ROSAT observation, the X-ray brightness of HD 166620 has decreased by a factor of 2.5 during its Maunder minimum-like state. To place its X-ray properties into context, we constructed an X-ray sample of late-type stars within 10 pc of the Sun. The activity of HD 166620 is below the levels of all other K dwarfs in the 10 pc sample. The corona of HD 166620 during its grand minimum emits at the level of the solar background corona, which implies that it has no large active magnetic structures. Along with long-term Ca II H&K monitoring of HD 166620, this result provides evidence that the solar activity during the Maunder minimum was not reduced significantly below the levels seen during its present-day cycle minima. The similar X-ray surface flux of HD 166620 and the modern quiet Sun, and also their Rossby number near the critical value of spin-down models, suggest a connection between the regime of weakened magnetic braking and the occurrence of Maunder minimum states.