  This paper reports on an analysis of two Chandra X-ray observations of the
young magnetic B star rho Ophiuchus S1. X-ray emission from the star was
detected in both observations. The average flux was almost the same in both,
but during each observation the flux showed significant time variations by a
factor of two on timescales of 20-40 ks. Each spectrum could be fit by either
an absorbed power-law model with a photon index of ~3 or a thin-thermal plasma
model with a temperature of ~2 keV and an extremely low metal abundance (<~0.1
solar). The spectrum of the first observation has a weak-line feature at about
6.8 keV, which might correspond to highly ionized iron Kalpha. In contrast, the
spectrum of the second observation apparently shows a weak edge absorption
component at E~4 keV. The continuum emission and log LX/Lbol ~ -6 are similar
to those of young intermediate-mass stars (Herbig Ae/Be stars), although the
presence of a strong magnetic field (inferred from the detection of non-thermal
radio emission) has drawn an analogy between rho Ophiuchus S1 and magnetic
chemically peculiar (MCP) stars. If the X-ray emission is thermal, the small
abundances that we derived might be related to the inverse first-ionization
potential (FIP) effect, though there is no significant trend as a function of
FIP from our model fits. If the emission is non-thermal, it might be produced
by high-energy electrons in the magnetosphere.
