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

6.5. NEW YSO CANDIDATES – A-29, A-31, AND BF-36 83relatively small absorption (N H = 2.5×10 22 cm −2 ; Table 5.2), hence may be located in front of thecore.The hard band (2.0–9.0 keV) ACIS image near ρ Oph B1 is shown in Figure 6.18 (right). Thereis a hard X-ray source (BF-36) about 30 ′′ south of the 1.3 mm peak B1-MM4. A class II sourceGY238 (BF-33) is also appeared near the center of SMM J16272−2430, which would be located inthe clumps, as supported by the large absorption of N H = 4×10 22 cm −2 (Table 5.2).6.5.2 Time variabilityFigure 6.19 shows the X-ray light curves (background is not subtracted) of A-29, A-31, and BF-36in the 2.0–9.0 keV band. We have detected solar-like flares from A-29 and BF-36 (§5.5). The lightcurve of A-31 also seems to be time variable; A-31 has only 12 photons and about 80 % of them(10/12) are concentrated in a limited time interval of ∼30 ks. We estimate the chance probabilityof such a photon clustering from a constant-flux source to be less than 1 % with the Kolmogorov-Smirnov test (Press et al., 1992). Thus the time variability of A-31 is also significant. The X-rayactive time intervals are shown with dotted vertical lines in Figure 6.19 and are referred as the flarephase hereafter. The hard band counts in the quiescent phase (time periods outside of the flarephase) for A-29, A-31, and BF-36 are 5, 2 and 81, while those of the expected background countsare 0.7, 0.5, and 1.8, respectively. This means that A-31 has no significant quiescent flux with ≥99.9 % of the confidence level (Poisson statistics; see §6.4.1).A-29FlareFlareA-31FlareBF-36Fig. 6.19.— Light curves of (left) A-29, (middle) A-31, and (right) BF-36 in the 2.0–9.0 keV band.The time bins of each data point are 2000 (A-29), 4000 (A-31), and 4000 (BF-36) s. The arrowsindicate the times of the flare phase (see text).