X-ray emission of brown dwarfs: towards constraining the dependence on age, luminosity, and temperature

Aims. We observed brown dwarfs in different evolutionary stages with the Chandra X-ray Observatory with the aim to disentangle the influence of different stellar parameters on the X-ray emission of substellar objects. The ages of our three targets (HR 7329 B, Gl 569 Bab, and HD 130948 BC) are constrained by them being companions to main-sequence stars of known age. With both known age and effective temperature or bolometric luminosity, the mass can be derived from evolutionary models. Methods. Combining the new observations with previous studies presented in the literature yields a brown dwarf sample that covers the age range from ~1 Myr to ~1 Gyr. Since the atmospheric temperature of brown dwarfs is approximately constant at young ages, a sample with a large age spread is essential for investigating the possible influence of effective temperature on X-ray activity. Results. Two out of three brown dwarfs are detected with Chandra , with variable lightcurves and comparatively soft spectra. Combining our results with published data allows us to consider a subsample of high-mass brown dwarfs (with 0.05-0.07 $M_\odot$), thus eliminating mass from the list of free parameters. We find evidence that X-ray luminosity declines with decreasing bolometric luminosity steeper than expected from the canonical relation for late-type stars ($L_{\rm x}/L_{\rm bol} = 10^{-3...-5}$). Effective temperature is identified as a likely parameter responsible for the additional decline of X-ray activity in the more evolved (and therefore cooler) brown dwarfs of the “high-mass” sample. In another subsample of brown dwarfs characterized by similar effective temperature, the X-ray luminosity scales with the bolometric luminosity without indications for a deviation from the canonical range of 10 -3...-5 observed for late-type stars. Conclusions. Our findings support the idea that effective temperature plays a critical role for the X-ray activity in brown dwarfs. This underlines an earlier suggestion based on observations of chromospheric H α emission in ultracool dwarfs that the low ionization fraction in the cool brown dwarf atmospheres may suppress magnetic activity.