X-ray nova debuts in southern sky.

X-ray nova debuts in southern sky

Astronomers have detected a Milky Way star just as it began a rare and explosive flare-up, emitting an outburst of X-rays and optical light that dramatically boosted its brightness.

On Jan. 10, an X-ray camera aboard the Soviet satellite GRANAT first imaged this heavenly object, located in the southern constellation Musca ("The Fly"). The intense emission from the star, now dubed Nova Muscae 1991, made it the second strongest X-ray source in the southern sky. Four days later, astronomers Massimo Della Valle and Brian Jarvis of the European Southern Observatory (ESO) in La Silla, Chile, identified the optical counterpart of the bright X-ray source by comparing old and new photographic plates of the same sky region with X-ray images from GRANAT and the Japanese satellite Ginga.

High-resolution images taken with ESO's New Technology Telescope on the night of Jan. 14 showed that the star, invisible to the naked eye, had boosted its optical brightness a thousand-fold within the past few days. Moreover, its bluish spectrum indicated it was a hot, highly energetic star.

Della Valle and Jarvis say these observations suggest that the new identified object represents an unusual type of exploding star called an X-ray nova, which radiates more X-rays than optical light and has been detected only four times before. In contrast, the more common "classical nova" emits about 10,000 times as such optical light as X-ray radiation.

The astronomers note that both types of novas make their fiery debut when one member of a binary star system transfers mass to its more compact companion. The mass forms a disk around the compact star and eventually falls onto the star's surface. In a classical nova, the compact star is a white dwarf and the mass transfer triggers a thermonuclear explosion that blows out an expanding envelope of hot material surrounding the binary star system. This causes a dramatic increase in optical light.

But in Nova Muscae 1991 and other X-ray novas, the compact star is believed to be a neutron star, and the outburst -- prompted by the suddent release of gravitational energy as matter from the disk accretes onto the star's surface -- might not have a thermonuclear component. And unlike a classical nova, this type of explosion does not expel enough matter to create an expanding envelope. However, it contains sufficient energy to heat up the disk, causing it to radiate both X-ray and optical light. In contrast to a supernova, the compact-star component of both types of novas survives the explosion and may undergo repeated outbursts, astronomers note.

Researchers continue to analyze light from the flare-up, which remains strong. Preliminary observations with Ginga, made in early February, reveal fluctuations in the nova's X-ray emission, and researchers using ground-based telescopes are watching to see whether that change will alter the star's optical output. At ESO, German astronomer Manfred Pakull has detected several broad emission lines from hydrogen, helium and nitrogen atoms and ions in the disk -- an indication of its hot temperature and a sign that it rotates at a speed of several hundred kilometers per per second.