We present results of an X-ray search from bona-fide and candidate brown dwarfs in the \rho 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 ^ { \prime } \times 17 ^ { \prime } size and a \sim 100 ks exposure .
Among 18 bona-fide and candidate brown dwarfs listed by the infrared spectroscopy , we find X-ray emission from 7 sources above 99.9 % confidence level .
Therefore \sim 40 % of the infrared-selected brown dwarfs in this cloud emit X-rays .
For the brightest 4 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 2 of the brown dwarfs .
Assuming 2 keV temperature and using the empirical relation of A _ { V } vs . N _ { H } , we estimate the X-ray luminosity or its upper limit of the other faint or non-X-ray sources .
The X-ray luminosity ( L _ { X } ) of the X-ray-detected sources is in the range of 0.3–90 \times 10 ^ { 28 } ergs s ^ { -1 } , while the luminosity ratio of X-ray to bolometric ( L _ { X } / L _ { bol } ) is 10 ^ { -3 } –10 ^ { -5 } , 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 .