Abstract
We present results of an X-ray search from bona fide and candidate brown dwarfs in the ρ Ophiuchi cloud cores with the Chandra X-ray Observatory. The selected areas are two fields near the cloud center and are observed with the ACIS-I array of a 17' × 17' size and a ~100 ks exposure. Among 18 bona fide and candidate brown dwarfs listed by the infrared spectroscopy, we find X-ray emission from seven sources above 99.9% confidence level. Therefore, ~40% of the infrared-selected brown dwarfs in this cloud emit X-rays. For the brightest four sources, the X-ray spectra are made and are fitted with a thin-thermal plasma model of a temperature 1-2.5 keV. The X-rays are also time variable with rapid flares from two of the brown dwarfs. Assuming 2 keV temperature and using the empirical relation of AV versus NH, we estimate the X-ray luminosity or its upper limit of the other faint or non-X-ray sources. The X-ray luminosity (LX) of the X-ray-detected sources is in the range of 0.3-90 × 1028 ergs s-1, while the luminosity ratio of X-ray to bolometric (LX/Lbol) is 10-5-10-3, similar to those of low-mass pre-main-sequence and dMe stars. All these results suggest that the X-ray origin of brown dwarfs is the same as low-mass stars; strong magnetic activity at the stellar surface.
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