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Strange New Star Appears in the Southern Sky

17 Gennaio 1991

Acting on information received from Danish scientists working with an X-ray telescope on a Soviet satellite, astronomers at the ESO La SiIla observatory in Chile have discovered a strange new star in the southern constellation of Musca (the Fly). Preliminary observations with the ESO 3.5-m New Technology Telescope indicate that the object is a nova, most probably a small, very dense star in a binary system whose brightness in a dramatic event has suddenly increased about 1000 times. The available optical and X-ray observations point to a quite unusual object.

The "international" story of Nova Muscae 1991

The new star [1], which is now referred to as Nova Muscae 1991, was first detected by the WATCH all-sky X-ray camera, an instrument built by the Danish Space Research Institute (Lyngby, Denmark) in collaboration with the Space Research Institute (Moscow) and installed on the Soviet GRANAT satellite which was launched in December 1989. Monitoring the stream of WATCH data on January 10, Søren Brandt, a Danish scientist at the GRANAT ground station in Crimea (USSR) noted the sudden emergence on the previous day of a new source of X-ray emission in the southern sky. This new X-ray source was unusually strong, in fact the second strongest in the sky at that moment and about twice as intense as the famous X-ray emitting Crab Nebula (the remnant of a star that exploded in the year 1054).

Brandt immediately transmitted the detailed data to Niels Lund at the Danish Space Research Institute in Denmark, Principal Investigator of the WATCH project. Noting that the new source was situated far down in the southern sky and therefore not observable from Europe, Niels Lund the same evening contacted scientists at the European Southern Observatory, suggesting that a search should be made for the visible image of the X-ray emitting object. A preliminary position of the X-ray source placed it in the direction of the constellation Musca within a circle with a one degree diameter. This position is within the bright, star-rich Milky Way band, about 20° from the south celestial pole. At the La Silla Observatory, a photographic plate was exposed during 10 min with the GPO telescope in the early morning hours of January 11. However, there was apparently no new object in the sky area of the WATCH X-ray source. The next night another plate with 45 min exposure and therefore showing much fainter objects was taken with the ESO Schmidt telescope by night assistant Guido Pizarro. Again, a first inspection revealed no new object in the field, brighter than magnitude 16 [2] Persisting, the astronomers then obtained a 90 min Schmidt plate in the morning of January 13. In the meantime, it had become known that also the Japanese Ginga satellite had observed X-ray emission from the Musca region and a somewhat more accurate position was now available. Still, thousands of stars were seen on the Schmidt plate within the X-ray "error box" (the area of uncertainty of the X-ray position).

The "new" star

The break-through and the discovery of the optical image of the X-ray emitting object was finally made in the afternoon of January 14 by ESO astronomers Massimo Della Valle and Brian Jarvis. By a careful comparison of the new Schmidt plates with earlier ones available in the La Silla archive, they detected a rather faint "new" star of magnitude 16 -17 near the border of the X-ray error box. That same night CCD images and spectra were obtained with the ESO New Technology Telescope and, to some surprise, it was seen that the star had brightened considerably during the preceding days; now the magnitude was almost 13. Moreover, the star was found to be rather blue, indicative of a hot, energy-rich object. These observations showed that the new X-ray source in Musca was most probably an exploding star (a "nova") whose (optical) light was still brightening.

One of the digital CCD images was transmitted in the morning of January 15 to the ESO Headquarters where ESO astronomer Richard West measured the celestial position of the "new" star to an accuracy of 0.2 arcseconds. It turned out that a very faint star (magnitude 21) of blue colour could be seen at exactly the same position on Schmidt plates obtained in 1976 and 1984; it is therefore most certainly the so-called progenitor, that is, the star that exploded. Official information about the identification of the Musca X-ray source was submitted by ESO to the International Astronomical Union (IAU) on January 15 and published the same day on IAU Circular No. 5165 with the suggestion that other ground-and space-based observatories join the observational campaign of this unusual object.

At this time, and according to astronomical custom, the star was given the designation "Nova Muscae 1991".

The nature of Nova Muscae 1991

About two "novae" are observed in the Milky Way Galaxy each year and some others are seen in other galaxies from time to time. A typical nova brightens in the course of a few days to a maximum and then its light slowly decreases over a period of several weeks or months. However, the present object is unusual as it began to emit X-rays, before it brightened optically. It now seems almost certain that it belongs to the small, select class of so-called "X-ray novae"; objects of this type have been detected in 1975, 1977, 1980 and 1989 [3] They are different from the "classical novae" in that they emit vastly more X-ray emission; in X-ray novae the ratio between the energy emitted at X-ray and optical wavelengths is about 1000, whereas for classical novae it is only 0.000l.

Nova events are believed to occur in binary stars and to be caused by transfer of matter from one component star to the other which is a very compact object, either a white dwarf star or a neutron star (or perhaps even a "black hole "). In all cases the matter collects in a disk around the compact star and from the inside of this disk it spirals closer and closer to the surface of the star.

In a "classical nova", the compact star is a white dwarf star that weighs about as much as the Sun, but is no larger than the Earth. At a certain moment, a thermonuclear explosion may take place in the hydrogen-rich material on the surface of the star which will blow much stellar material out into the surrounding space. The dramatic increase in the brightness during the nova outburst is caused by the light emitted from the hot, expanding envelope around the binary star. Contrary to a supernova explosion, the white dwarf star survives the violent event; some of these objects may actually experience repeated explosions and are known as "recurrent novae". Contrarily, in X-ray novae as Nova Muscae 1991, the compact object is thought to be an extremely dense neutron star, as heavy as the Sun, but with a diameter of only 10 -15 kilometres. The nova event is caused by an instability in the matter falling inwards from the disk towards the star and a sudden release of gravitational energy near the surface of the neutron star (there may perhaps also be a thermonuclear explosion involved), but in this case there is not enough matter to create an expanding envelope. In X-ray novae the increase in brightness is caused by the heating of the disk which then radiates strongly in X-ray as well as in optical light.

More observations to be made

Some neutron stars are detected as rapidly fluctuating pulsars in supernova remnants. No X-ray fluctuations have been observed from Nova Muscae 1991 so far, but satellite observations are continuing. It is expected that ultraviolet spectra will be obtained with the IUE satellite.

Optical spectra have been obtained on January 16 and 17 by German astronomer Manfred Pakull, working with the ESO 1.5-m spectrographic telescope at La Silla. They show a number of broad emission lines from hydrogen, helium and nitrogen atoms and ions in the disk, reflecting its present, highly energetic state and also that it rotates rapidly. A preliminary analysis indicates velocities of several hundred km/sec.

Nova Muscae 1991 is obviously a most interesting object which will now be studied with all available means. Thanks to an excellent international collaboration it was discovered when it was in a critical phase of transition.

Note

[1] This Press Release is accompanied by a B/W photo of Nova Muscae 1991.

[2] In the astronomical magnitude scale) smaller numbers signify brighter stars. A difference of 1 magnitude means an intensity difference of a factor 2.5. The brightest stars have magnitudes near O. The star Polans near the celestial North Pole has magnitude 2. The faintest objects which can be perceived with the unaided eye have magnitude 6. A star of magnitude 16 is 2.514 =400,000 limes fainter than Polaris.

[3] The objects are: VGIG MOll (Mon X-I) in 1975, V2107 Oph (HI705-25) in 1977, V822 Cyg (Cen X-4) in 1980 and V404 Cyg in 1989.

Contatti

Richard West
ESO EPR Dept
Garching, Germany
E-mail: information@eso.org

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