Space Telescope Science Institute, Baltimore, MD

January 9, 1998


Teaming up space telescopes to make simultaneous ultraviolet and X-ray observations, astronomers may have solved a 20-year old mystery and possibly discovered a new class of X-ray star.

The unlikely suspect is a second magnitude star 600 light-years away in the middle point of the "W" in circumpolar constellation Cassiopeia. It turns out the mild-mannered looking star is ejecting 100 million degree flares into space -- 10 times hotter than typical flares ejected from our sun.

The results are being announced today to the 191st meeting of the American Astronomical Society in Washington, DC by Myron Smith of the Space Telescope Science Institute in Baltimore, MD. His co-investigators are Richard Robinson and Robin Corbet of the Goddard Space Flight Center in Greenbelt, MD.

The team pointed both the Hubble Space Telescope and the Rossi X-Ray Timing Explorer at the hot star gamma Cassiopeiae, simultaneously for one full day in March, 1996 in order to understand the origin of the bright, variable X-rays coming from this star. To their surprise they discovered the X-rays are probably produced by extraordinarily hot surface flares, which are completely unexpected and unpredicted for a star of this type.

A New Tack On A 20-Year Mystery

In the 1860's an Italian priest, Father Secchi, discovered that gamma Cassiopeiae shows bright emission in the hydrogen lines of its spectrum. His finding made this star the first member of the class of "B-emission" (Be) stars. These stars are hotter and several times more massive than the Sun and are known for their unexplained eruptions of mass from time to time.

Twenty years ago astronomers had discovered that surprisingly bright X-ray light came from this well-known star, there has been no consensus since then on the reason for these bright emissions.

Most "Be" stars generally show only faint X-ray emissions, similar to other hot stars. The X-ray emissions of gamma Cassiopeiae stand out because they originate from gas which is hot even by astronomical standards, 100 million degrees Kelvin. The emissions also show a zig-zag variability over only several seconds which is reminiscent of X-ray flares on the Sun and similar low-mass, "cool" stars.

Previous explanations of this X-ray behavior have suggested that the energy is created by the infall of gas onto a dense companion star, such as a neutron star. However, despite repeated searches no evidence has been found for a companion star.

The simultaneous observations of the X-ray and Hubble telescopes dramatically changed this view by showing that the slower X-ray variability in gamma Cassiopeiae is consistent with its ultraviolet variations. Comparison of these slow undulations with past X-ray observations suggest that they repeat with a period of 27 hours. The Hubble observations also indicate that gas lobes hang over the star, which is also consistent with a magnetic flare interpretation.

Confirming evidence comes from a repeated 27 hour period between the Hubble observations and ultraviolet variations of this star observed by the International Ultraviolet Explorer satellite two months earlier. Since a period of 27 hours is consistent with that expected with the star's rotational spin rate, Smith and his team believe the X-rays originate from the star's surface.

Though relatively cooler flares are common on the Sun, it is surprising that they occur on a Be-class star. Solar flares are caused by magnetic fields inside the Sun which then get tangled and finally cause violent explosions on the surface.

In contrast, the theory for massive stars predicts that they should not generate such fields. "Thus, at the present time, the precise mechanism that creates these X-rays is still unclear," Smith concludes.

Despite this uncertainty, Smith points out "there is no such thing as uniqueness in astronomy. Where there is one peculiar star, there are many more somewhere else. There is probably an entire class of these X-ray variables out there."

Robinson notes that "with the new generation of X-ray satellites recently or soon to be launched, it should be possible to discover a class of fainter B-emission stars that are variables just like gamma Cassiopeiae."

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

A figure caption may be obtained in postscript format via the World Wide Web at

FIGURE CAPTION: This plot shows the similarity of X-ray and ultraviolet light emissions from the B-emission star gamma Cassiopeiae on March 14--15, 1996. The UV emission from the HST has been inverted and rescaled to emphasize its correspondence with the slow component of the variations of the X-ray emission.

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