Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                               
October 20, 1993
(Phone:  202/358-0883)

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-5565)


RELEASE:  93-192

MYSTERY OF "FASTEST FLICKERING" GALAXY SOLVED

   Since its discovery in the mid 1980s, Galaxy NGC 6814, located 
about 47 million light years from Earth, has held special interest 
with astronomers.  

   Of the billions of galaxies that make up the universe, a few 
galaxies "flicker," and this one "flickered" faster than any 
other.  Furthermore, it was the only one with a predictable 
flicker.  Every 3.4 hours it became brighter.  Then, 3.4 hours 
later, it became dimmer. 

   The reason for the flickering finally has become clear.  
However, the reason is not related to the galaxy.

   Astronomers using data from the orbiting ROSAT x-ray 
observatory have discovered two stars which orbit each other near 
the line of sight between Earth and the galaxy.  When the two 
stars -- called a binary system -- become aligned with each other 
in relation to Earth, the image appears much brighter.  When the 
binary stars move out of alignment, the becomes dimmer. 

   "Given the limited spatial resolution of the previous x-ray 
observatories, it simply was not possible to disentangle the two 
sources -- namely NGC 6814 and this binary star system," said Dr. 
Greg M. Madejski of the Universities Space Research Assocation at 
NASA's Goddard Space Flight Center (GSFC), Greenbelt, Md.  "It 
required the precise imaging capability of the ROSAT observatory, 
coupled with its large field of view.  In fact, these two stars 
have absolutely nothing whatever to do with NGC 6814."

   A report on the discovery has just been published in the 
science journal "Nature" by Madejski and other observer team 
members.

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   The galaxy is a member of a group known as the Seyfert 
galaxies.  They are distant celestial objects with very bright and 
highly active central regions or nuclei.  In visible light, the 
nuclei can outshine the rest of the galaxy and produce ample 
ultraviolet and x-ray radiation.

   According to Madejski and his colleagues, one of the binary 
stars probably is a small, dense star called a white dwarf which 
is pulling material from its companion.  In turn, the impact of 
this material on the white dwarf is producing the x-rays 
previously thought to be associated with NGC 6814.

   This galaxy was thought to be unique because the variations of 
its brightness not only were very fast for a Seyfert galaxy, but 
extremely regular.  The galaxy is about 100,000 times farther from 
Earth than the stars that caused the "optical illusion."  They are 
in the Milky Way.

   What appeared to be the unique characteristics of NGC 6814 were 
detected first by the European X-ray Satellite (EXOSAT) and 
confirmed by the Japanese orbiting observatory GINGA.

   Following these observations, a leading theory emerged that the 
periodicity was caused by a captured star orbiting a super-massive 
black hole at the galaxy's center.  A black hole is believed to be 
a collapsed star with a gravitational field so strong that not 
even light can escape it.

   Madejski said that despite ROSAT's findings about NGC 6814, 
other lines of evidence still favor a presence of massive black 
holes at the center of Seyfert galaxies.

   Collaborating with Madejski on the "Nature" paper were T. Jane 
Turner of the Universities Space Research Association at GSFC; 
Richard F. Mushotzky and Peter Serlemitsos of GSFC; Chris Done of 
GSFC and the Leicester University in Great Britain; Fabrizio Fiore 
of the Center for Astrophysics in Cambridge, Mass.; Marek Sikora 
of the Copernicus Astronomical Center in Warsaw, Poland and 
JILA/University of Colorado in Boulder; and Mitchell C. Begelman 
of JILA/University of Colorado.

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