X-ray Detection from Bona-fide and Candidate Brown Dwarfs in the ρ Ophiuchi Cloud with Chandra

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 7 sources above 99.9% confidence level. Therefore ∼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 AV vs. 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 Xray-detected sources is in the range of 0.3–90×10 ergs s, while the luminosity ratio of X-ray to bolometric (LX/Lbol) is 10 –10, 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. Subject headings: stars: low-mass, brown dwarfs — X-rays: stars — ISM: individual (ρ Ophiuchi cloud)