Chandra Observations of a Young Embedded Magnetic B Star in the $\rho$ Ophiuchus Cloud

This paper reports on an analysis of two Chandra X-ray observations of theyoung magnetic B star rho Ophiuchus S1. X-ray emission from the star wasdetected in both observations. The average flux was almost the same in both,but during each observation the flux showed significant time variations by afactor of two on timescales of 20-40 ks. Each spectrum could be fit by eitheran absorbed power-law model with a photon index of ~3 or a thin-thermal plasmamodel with a temperature of ~2 keV and an extremely low metal abundance (<~0.1solar). The spectrum of the first observation has a weak-line feature at about6.8 keV, which might correspond to highly ionized iron Kalpha. In contrast, thespectrum of the second observation apparently shows a weak edge absorptioncomponent at E~4 keV. The continuum emission and log LX/Lbol ~ -6 are similarto those of young intermediate-mass stars (Herbig Ae/Be stars), although thepresence of a strong magnetic field (inferred from the detection of non-thermalradio emission) has drawn an analogy between rho Ophiuchus S1 and magneticchemically peculiar (MCP) stars. If the X-ray emission is thermal, the smallabundances that we derived might be related to the inverse first-ionizationpotential (FIP) effect, though there is no significant trend as a function ofFIP from our model fits. If the emission is non-thermal, it might be producedby high-energy electrons in the magnetosphere.Comment: 13 pages, 6 figures, To appear in the October issue of PASJ (vol.55, No. 5