TIME-DEPENDENT CALCULATIONS OF IONIZATION NEBULAE SURROUNDING SUPERSOFT X-RAY SOURCES

摘要

In this work we carry out a theoretical investigation of the time-dependent properties of ionization nebulae surrounding luminous supersoft X-ray sources. First discovered by the X-ray satellite Einstein and later found in substantial numbers with ROSAT, luminous supersoft X-ray sources have characteristic luminosities of ～10~(37)-10~(38) ergs s~(-1) and effective temperatures of ～4 x 10~5 K, and are generally highly variable in X-ray luminosity. Previously we predicted that such objects should be surrounded by substantial regions of ionized gas exhibiting properties distinct from those of other astrophysical nebulae. The discovery of a large ionization nebula surrounding the supersoft source CAL 83 in the Large Magel-lanic Cloud indicates that at least some supersoft X-ray sources are indeed associated with such nebulae. In contrast to our previous studies of such ionization nebulae that assume thermal and ionization equilibrium, in this work we relax these assumptions to study (1) the ionization evolution of the nebula after the sudden appearance of a supersoft X-ray source within the neutral interstellar medium (the "turn-on" evolution), (2) the recombination evolution of the decaying nebula after the central source ceases to radiate (the " turn-off " evolution), (3) the response of the nebula, both structurally and spectrally, to sources which vary periodically in luminosity, and (4) the ionization structure of the nebula, which develops around a source which is in motion relative to the ambient interstellar medium. Only the ionization evolutions of nebular hydrogen (H) and helium (He) are considered, and the gas is taken to be of uniform density at all times. We find a particularly interesting result for the case of a central source whose luminosity is both variable on timescales shorter than the recombination timescales of the gas and negligibly small (quiescent) for a large fraction q of the time. We show that in this case, the surface brightness at the center of the associated ionization nebula drops off only as S_0(1 - q)~(1/3), where S_0 is the central nebular surface brightness if the source radiated continuously. Our calculations of the nebular H~+ structure surrounding moving sources reveal a distinctive head-tail structure in the surface brightness distribution. As the source continually moves into the neutral gas ahead of it, the extent of the ionization in the forward direction is reduced, while an extended, low surface-brightness tail of recombining plasma is left in the wake of the source. We develop a practical method of quantifying the degree of azimuthal asymmetry in the nebula as a function of transverse source velocity, and use the method, in conjunction with measurements of the radial (Doppler) motion, to estimate an upper limit for the total velocity of the CAL 83 central source relative to the ambient gas.