The high energy spectrum of Proxima Centauri simultaneously observed at X-ray and FUV wavelengths

International audienceThe M dwarf Proxima Centauri, the Sun’s closest stellar neighbour, is known to be magnetically active and it hosts a likely Earth-like planet in its habitable zone. High-energy radiation from the host star can significantly alter planetary atmospheres in close orbits. Frequent flaring may drive radiation-induced effects such as rapid atmospheric escape and photochemical changes. Therefore, understanding the characteristics of stellar radiation by understanding the properties of the emitting plasma is of paramount importance for a proper assessment of the conditions on Proxima Centauri b and exoplanets around M dwarfs in general. This work determines the temperature structure of the coronal and transition region plasma of Proxima Centauri from simultaneous X-ray and far-ultraviolet (FUV) observations. The differential emission measure distribution (DEM) was constructed for flaring and quiescent periods by analysing optically thin X-ray and FUV emission lines. Four X-ray observations of Proxima Centauri were conducted by the LETGS instrument on board of the Chandra X-ray Observatory and four FUV observations were carried out using the STIS spectrograph on board the Hubble Space Telescope. From the X-ray light curves, we determined a variation of the quiescent count rate by a factor of two within 20% of the stellar rotation period. To obtain the DEM, 18 optically thin emission lines were analysed (12 X-ray and six FUV). The flare fluxes differ from the quiescence fluxes by factors of 4–20 (FUV) and 1–30 (X-ray). The temperature structure of the stellar corona and transition region was determined for both the quiescence and flaring state by fitting the DEM(T) with Chebyshev polynomials for a temperature range log T = 4.25–8. Compared to quiescence, the emission measure increases during flares for temperatures below 0.3 MK (FUV dominated region) and beyond 3.6 MK (X-ray dominated region). The reconstructed DEM shape provides acceptable line flux predictions compared to the measured values. Using the DEM we provide synthetic spectra at 1–1700 Å, which may be considered as representative for the high-energy irradiation of Proxima Cen b during quiescent and flare periods. In future work these values can be used for planet atmosphere calculations which will ultimately provide information about how habitable Proxima Centauri b is.Key words: stars: individual: Proxima Centauri / stars: low-mass / stars: flare / stars: coronae / ultraviolet: stars / X-rays: star