X-ray transientsBright novalike cosmic X-ray sources that develop rapidly over a few days and remain visible for several weeks or months. Ariel V observations, supported by simultaneous ground-based studies, led to the first optical identification of an X-ray transient source. This was A0620–00 (Nova Mon 1975), for a time in 1975 the brightest cosmic X-ray source ever seen, and found in the visible as a nova of about 11th magnitude. Both X-ray and optical stars then faded rapidly, to disappear by mid-1976. Subsequently, a star of 18th magnitude has been identified as the quiescent state of Nova Mon. Optical spectroscopy has shown the compact companion to be very probably a black hole of at least 3 solar masses.
The sky distribution of X-ray transients shows them to be galactic, typically at distances of 1–10 kiloparsecs. Several have been found to have recurrent outbursts, usually on a timescale of several months or years. It is believed that X-ray transients represent close binary systems in which the mass transfer is highly variable. The qualitative difference from most optical novae (e.g. Nova Cygni 1975, which had no detectable X-ray emission) may be due to the compact star being a neutron star or black hole, rather than a white dwarf as in the optical novae.