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

Chapter 8Systematic Study of YSO FlaresWe detected 71 X-ray flares from the ρ Oph X-ray sources. Using this enormous sample, we makea systematic study on flare activity of low-mass YSOs in this chapter.8.1 Flare RateTable 8.1 shows the number of sources and detected flares sorted by several physical parameterssuch as class, age, mass, and binarity. We should note that our flare detection method (§5.5) largelydepends on the quiescent flux level; the higher quiescent count rate results in a higher sensitivityto smaller amplitude flares due to small statistical errors. We hence show the mean quiescent fluxfor each class in the last column of Table 8.1. Following Stelzer et al. (2000), the flare rate (F ) isdefined asF = ( ¯τ r + ¯τ d )N FNT obs±√σ 2 τ r+ σ 2 τ d√NFNT obs, (8.1)where ( ¯τ r , ¯τ d ) and (σ τr , σ τd ) are the mean value and uncertainty of (τ r , τ d ), N and N F are thenumber of sources and detected flares, and T obs is the duration of the observations (≈100 ks). Allderived values of F are much higher than that derived with ROSAT (F ∼ 1 %: Stelzer et al. 2000),which may be primarily due to the extended sensitivity in the hard X-ray band of the Chandraobservations, because the flare activity (flux increase) is clearer in the harder X-ray band.We see larger flare rate of class III+III c than that of class II. However, this is simply due tothe effect of the flares with unusually long timescales (A-2, A-63, and BF-96). In fact, if we regardthose unusual flares are affected by other unknown components (unresolved flares, for example) andexclude them, F of class III+III c becomes ∼5.0 % (parentheses in Table 8.1), which is significantlysmaller than those of class I–II sources. Such a tendency is consistent with the ROSAT results (F107