X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

78 CHAPTER 6. INDIVIDUAL SOURCES6.4 Brown DwarfsImanishi et al. (2001b) did a systematic X-ray search from bona-fide and candidate brown dwarfsin ρ Oph. Here, we show details.6.4.1 X-ray detection from brown dwarfs in ρ OphIn the fields of view of obs-A and obs-BF, 18 late M dwarfs have been reported based on the watervapor absorption at λ = 2.4–2.5 µm (Wilking et al., 1999; Cushing et al., 2000). Among them,eight sources have the upper limit of the mass less than 0.08 M ⊙ , which we call “bona-fide browndwarfs” (here and after, BDs).Other seven sources have the lower limit of the mass less than0.08 M ⊙ , which is called “candidate brown dwarfs” (BD c s) 3 . Table 6.5 shows their names andspectral types.In the X-ray source catalogue (Table 5.2), we found X-ray counterparts for four BDs and BD c s(A-30 = GY37, A-38 = GY59, BF-86 = GY310, and BF-S2 = GY141). Also, a BD c GY5 maybe associated with a soft-band source A-S2, although Imanishi et al. (2001b) reported no X-rayemission from GY5 by using the NIR position of Barsony et al. (1997), which is ∼2 ′′ away fromthe 2MASS source. Since no apparent X-ray sources are found in the error radius of the other 11catalogued BDs and BD c s, we define a circle with a half radius of the PSF size around each IRposition, then manually count X-ray photons. We note that a rather small source radius is selectedso as to increase the signal-to-noise (S/N) ratio, particularly for faint X-ray sources. Nevertheless,as is demonstrated in the bright sources, the position error between IR and X-ray is always smallerthan the source radius, because both have generally similar dependence on a source off-axis angle;both have the smaller values for sources with smaller off-axis angle. Therefore, most of the X-rayphotons for the relevant BDs and BD c s, if any, may fall in the source circles. Table 6.5 shows theX-ray counts thus counted. The mean background counts are estimated from the same regions usedin §5.6. The soft band (0.5–2.0 keV) background counts (in units of 10 −2 counts arcsec −2 ) are 2.3and 2.2 for obs-BF and obs-A, respectively, while those in the hard (2.0–9.0 keV) band are aboutthree times larger, 6.8 and 6.5 for obs-BF and obs-A. The background counts in each source areaare given in Table 6.5. We then separately calculate the confidence level (CL) of X-ray detectionfor the soft, hard, and total (0.5–9.0 keV) bands. Based on the Poisson statistics, we define the CLasCL =N∑0 −1N ′ =0e −N N N ′bg bgN ′ ! , (6.2)3 Imanishi et al. (2001b) regarded GY31, GY163, and GY326 as BD cs. Now, however, the mass of these M dwarfsis regarded to be higher than the hydrogen burning limit (Wilking et al., 1999), hence we do not include them in theBD c sample.