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

60 CHAPTER 6. INDIVIDUAL SOURCESrespectively (Inoue, 1985). If we regard that most of the observed column density (5.2×10 22 cm −2 )originates from the circumstellar matter and use the iron abundance of 0.3 solar, we predict themaximum equivalent width to be ∼15 eV, which is significantly lower than the observed value of∼200 eV. We therefore require non-spherical geometry; a larger amount of matter should be presentout of the line-of-sight. A possible scenario is that YLW16A has a face-on geometry (Sekimoto etal., 1997) and the disk is responsible for the fluorescent 6.4 keV line. Since we see no time delay(reflection timescale) with 10 4 s between the flare on-set and 6.4 keV iron line appearance, thedistance of the star and the reflecting region should be less than 20 AU, which is typical of thedistance between the stellar surface and the inner edge of the disk (Feigelson & Montmerle, 1999).Fig. 6.2.— (left): (top) Light curve of BF-64 = YLW16A and (middle and bottom) time profile ofthe best-fit temperature and the emission measure. (right): Spectrum of YLW16A in phases 7–9.The upper panel shows data points (crosses) and the best-fit model (solid line), while the lowerpanel shows residuals from the best-fit model. Line structure exists at ∼6.4 keV and ∼6.7 keV.