A Peculiar X-Ray Transient Source, AX J1842.8–0423, Discovered with ASCA

A new transient X-ray source, AX J1842.8-0423, was discovered with ASCA in 1996 October in the Scutum arm region. The source exhibited an absorption-corrected 2–10 keV flux of 5.2 × 10⁻¹² erg s⁻¹ cm⁻², with insignificant intensity variability. The continuous spectrum is approximated by a power-law of photon index 2.9, absorbed by a hydrogen column of NH ∼ 5 × 10²² cm⁻². The spectrum also exhibits the Fe-K emission line with an extremely large equivalent width of ∼ 4000 eV, of which the centroid energy is ∼ 6.8 keV. The overall spectrum can be reproduced by a thin-thermal plasma-emission model having a temperature of ∼ 5.1 keV, on the condition that the heavy-element abundance is allowed to increase to $3.0^{+4.3}_{-0.9}$ solar abundance. The source was undetectable in a previous observation in 1993 October, as well as in a subsequent observation in 1996 April. The overall source behavior is thus quite peculiar, but may be explained in terms of a close binary involving a magnetized white dwarf viewed from pole-on inclination, where the unusually strong Fe-K line may be the result of resonant scattering of line photons.