First eROSITA study of nearby M dwarfs and the rotation-activity relation in combination with TESS

Abstract

We present the first results with the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission, and we combine the new X-ray data with observations with the Transiting Exoplanet Survey Satellite (TESS). We used the SUPERBLINK proper motion catalog of nearby M dwarfs as input sample to search for eROSITA and TESS data. We extracted Gaia DR2 data for the full M dwarf catalog, which comprises ~9000 stars, and we calculated the stellar parameters from empirical relations with optical/IR colors. Then we cross-matched this catalog with the eROSITA Final Equatorial Depth Survey (eFEDS) and the first eROSITA all-sky survey (eRASS1). After a meticulous source identification in which we associated the closest Gaia source with the eROSITA X-ray detections, our sample of M dwarfs is defined by 687 stars with SpT = K5..M7 (673 from eRASS1 and 14 from eFEDS). While for eRASSl we used the data from the source catalog provided by the eROSITA_DE consortium, for the much smaller eFEDS sample, we performed the data extraction, and we analyzed the X-ray spectra and light curves. This unprecedented data base for X-ray emitting M dwarfs allowed us to place a quantitative constraint on the mass dependence of the X-ray luminosity, and to determine the change in the activity level with respect to pre-main-sequence stars. TESS observations are available for 489 of 687 X-ray detected M dwarfs. By applying standard period search methods, we were able to determine the rotation period for 180 X-ray detected M dwarfs. This is about one-forth of the X-ray sample. With the joint eROSITA and TESS sample, and combining it with our compilation of historical X-ray and rotation data for M dwarfs, we examined the mass dependence of the saturated regime of the rotation-activity relation. A first comparison of eROSITA hardness ratios and spectra shows that 65% of the X-ray detected M dwarfs have coronal temperatures of ~0.5 keV. We performed a statistical investigation of the long-term X-ray variability of M dwarfs by comparing the eROSITA measurements to those obtained ~30 yr earlier during the ROSAT all-sky survey (RASS). Evidence for X-ray flares is found in various parts of our analysis: directly from an inspection of the eFEDS light curves, in the relation between RASS and eRASSl X-ray luminosities, and in a subset of stars that displays hotter X-ray emission than the bulk of the sample according to the hardness ratios. Finally, we point out the need to obtain X-ray spectroscopy for more M dwarfs to study the coronal temperature-luminosity relation, which is not well constrained by our eFEDS results.

A first eROSITA view of ultracool dwarfs

“…The EM-weighted mean temperatures are kT = 0.73 ± 0.11 keV for WDS J04469-1117AB and kT = 0.76±0.10 for TWA 22AB. These mean temperatures are higher than the values derived for early-M dwarfs from eROSITA spectra (Magaudda et al 2021). While this might be surprising at first sight, this is likely to be attributed to a combination of the young age, which is ∼ 10 Myr for TWA 22AB (Teixeira et al 2009) and 60 − 300 Myr for WDS J04469-1117AB (Shkolnik et al 2009), combined with their evident flaring activity.…”

“…However, we have shown that even a clearly wrong spectral model leads to a difference in the CF on the 30 % level only. We may caution that many of the UCDs detected in eRASS1 might have been in a high-activity flaring state during the eROSITA measurement, and therefore the plasma temperatures could be higher than those of the M dwarfs studied in Magaudda et al (2021). In fact, the two UCDs with the highest number of collected counts for which we analyzed the eRASS1 light curves both show evidence for flares (see Sect.…”

“…The literature data for UCDs (black symbols with SpT M6 and later in Fig. 6 2020) and Magaudda et al (2021). While these mid-M dwarfs show X-ray to bolometric luminosity ratios at or below the canonical saturation limit of log (L x /L bol ) ≈ −3, for most of the UCDs the X-ray to bolometric luminosity ratio is much higher, and about half of them have log (L x /L bol ) values above the upper envelope observed in mid-M dwarfs.…”

“…3.1), also known as WDS J04469-1117A and B and listed with SpT M5 + M6 in GagnéM (based on measurements presented by Shkolnik et al 2009). The other one is Gaia DR2 5355751581627180288, alias TWA 22 AB, which is -contrary to its historical assignment to the TW Hya association -a member of the β Pic moving group (Malo et al 2013) and an astrometric binary (Bonnefoy et al 2009) with two components of SpT M6. Both objects for which we are performing eRASS1 timing and spectral analysis are, thus, not genuine UCDs.…”

“…ML_RATE_1 and ML_RATE_2 and applying the rate-to-flux conversion factor CF M,12 = (7.37 ± 0.55) • 10 −13 erg/cts/cm 2 . This value was derived from fits of a thermal model (APEC) to eFEDS spectra of early-M dwarfs studied by Magaudda et al (2021). Note that in Magaudda et al (2021) a slightly higher value is reported which refers to a broader eROSITA energy band (0.2 − 5.0 keV), but the same approach and the same sample of M dwarf eROSITA spectra was used to derive the value for the 0.2 − 2.3 keV band (that is the sum of eRASS1 energy bands '1' and '2') cited above specifically for our work.…”