There are many kinds of celestial objects in the Universe but we are far from knowing them all. XMM-Newton may have discovered a new one: a very luminous soft X-ray source that is pulsating extremely rapidly in the central region of the Andromeda galaxy. This unusual object could be a new kind of accreting white dwarf.

These X-ray images of the central region of the Andromeda Galaxy were taken in June (above) and December 2000 (below) with XMM-Newton's EPIC-MOS camera. The newly discovered X-ray Nova XMMU J004234.1+411808 and a transient supersoft X-ray pulsating source are marked by red and yellow circles respectively (courtesy of S. Trudolyubov). The differences between the two images can be clearly seen by blinking between the two images. Photo: ESA

The spiral Andromeda galaxy (M31), only 2.5 million light years away, is in many respects similar to our own. Because it is inclined to our line-of-sight there is less intervening material in the way and thus the many hundreds of X-ray sources that it contains can easily be observed.

Previous X-ray missions, such as Einstein and ROSAT, and NASA's Chandra observatory, which is currently operating, had together detected several hundred sources throughout the Andromeda galaxy. In two recent observations, XMM-Newton's EPIC-pn and EPIC-MOS cameras detected most of the previously known sources in Andromeda's central region, but also found 10 objects that have significantly brightened or dimmed between June and December 2000.

Most of these sources are likely to be X-ray binary systems, where a compact object such as a black hole or neutron star is tearing matter from a companion star. The X-ray emission from some such sources can be highly variable. One of the bright new sources (XMMU J004234.1+411808) - probably an X-ray nova - was discovered in June, but had disappeared six months later when XMM-Newton peered again at this region of the sky.

The outburst of an X-ray nova over a few days is caused when huge amounts of matter in a swirling accretion disk fall very suddenly on to the compact object. While not many of these objects have been observed - only two dozen in the whole Universe in the past 30 years - the observed behaviour of the new source is characteristic of an X-ray nova.

A cool 'supersoft' source
Of more interest scientifically is XMM-Newton's discovery of an unusual 'supersoft' source. These are much cooler accreting binary systems having a temperature less than a million degrees Kelvin - ten to a hundred times less than other sources in M31. Astronomers believe that the X-ray emission from such 'cool' sources is created by steady thermonuclear burning of great amounts of matter pulled from the companion star.

View of the central region of the Andromeda galaxy from XMM-Newton's EPIC-pn instrument. The positions of ten variable sources are indicated with circles. The newly discovered supersoft source is identified by the circle labelled 9. Photo: ESA

"One of the bright soft-spectrum sources in our field, bright in June but undetectable in December, showed significant oscillations in its X-ray flux," explains Dr. Julian Osborne of Leicester University, United Kingdom, first author of a paper to be published in the journal Astronomy and Astrophysics.

"Our surprise came from the fact that this object is pulsating with a 14-minute period, much shorter than all known supersoft systems which have periods ranging from a few days to a few hours. Often in these systems we can see a periodic modulation of the flux due to the outer edges of the disk or the secondary star regularly obscuring the central object. But here with a 14-minute interval, the orbit would have to be very small indeed. Clearly something exceptional is happening."

This new source (referenced as XMMU J004319.4+411758) could be a white dwarf, a remarkably dense and compact star, with an extremely short orbital period sucking matter from a secondary star. However in this case the object would have to be very close by because a very short orbital period means a very small mass-losing star and such binary systems cannot be very bright. It could only appear bright if it were nearby, and this is unlikely because there are very few stars between us and M31.

A more plausible explanation is that the pulsation indicates that the white dwarf possesses a magnetic field large enough to modulate the X-ray emission, yet not large enough to lock the star's spin and rotation periods.

A strong magnetic spinner
"Until now we had never seen the spin period of a white dwarf in a supersoft source," says Julian Osborne. "Because the pulse period is not orbital, the only thing it can stem from is the spin of the white dwarf. One can imagine that the white dwarf is pulling material off an egg-shaped companion at such a high rate that the material can continuously burn hydrogen into helium. But in this object the luminous material is being confined to a small region on the white dwarf, presumably by its strong magnetic field."

This interpretation would correspond to a type of so-called 'intermediate polar', in which a strongly magnetic white dwarf accretes gas from a close companion star. Intermediate polars are typically much fainter than the new supersoft source discovered by XMM, because they normally accrete at a lower rate which does not cause nuclear burning on the white dwarf. Intermediate polars are thus not usually supersoft sources.

A schematic diagram of an 'intermediate polar'. Photo: ESA

The new object is currently in this intermediate polar configuration. It is likely to evolve into a polar after the high accretion rate phase (which makes it a supersoft source) ends. A polar has no accretion disk and its rotation is rigidly locked to the secondary star.

Clarification of what precisely is occurring in XMMU J004319.4+411758 will require further study, as will the identification of the other X-ray sources observed by XMM-Newton in the M31 central region with the various types of compact objects. This will involve analysis of the data that was also obtained by XMM-Newton's Optical Monitor. Several more XMM-Newton observations are scheduled for the coming months.