Context :

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 \sim 1 Myr to \sim 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 _ { x } / L _ { 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 \alpha emission in ultracool dwarfs that the low ionization fraction in the cool brown dwarf atmospheres may suppress magnetic activity .