Number 175, April 25, 1994 by Phillip F. Schewe and Ben Stein|
EXPLORING MATTER ON A SCALE OF 10**-16 CM , 1000 times smaller than the
size of the proton, is possible at the HERA electron-proton collider at
Hamburg, Germany. In some of the HERA interactions ("diffraction scattering")
the electron merely grazes the proton, but in other interactions the electron,
which is immune to the strong nuclear force, burrows deep inside the proton
and scatters from individual constituent quarks or even from short-lived
virtual quarks popping into existence out of the vacuum. In these violent
"deep inelastic scattering" events the proton is usually shattered.
Speaking at last week's American Physical Society meeting in Virginia,
Allen Caldwell of Columbia University reported on a mysterious class of
events recorded at the ZEUS detector at HERA that seem to be both diffractive
and deep-inelastic in nature. In these events, a single high-energy quark
is ejected from the target proton, indicative of deep-inelastic scattering.
But the debris from what should have been a broken-up proton was nowhere
to be found. Caldwell had no solid explanation for this odd behavior, but
speculated that the electron might in this case be scattering from some
combination of quarks or gluons within the proton.
THE COSMIC X-RAY BACKGROUND (CXB) , the diffuse x-ray glow spread across
the sky, probably arises from discrete sources. The Japanese x-ray telescope
ASCA has now resolved about 30% of the CXB into individual sources so.
At the APS meeting, ASCA scientist Hajime Inoue went further and asserted
that there are reasons for believing that the x-ray output of active galaxies---the
most energetic category of galaxies---cannot account for the CXB and that
most ordinary galaxies, those not previously known for their long-term
x-ray emissions, probably contribute to the CXB. Indeed, an ASCA study
of one such galaxy, M33, did reveal the presence of a weak central x-ray
whose x-ray spectrum resembles that of active galaxies. One inference that
can be drawn from this, according to Inoue, is that every galaxy may have
a massive black hole at its center.
THE INVENTORY OF ELEMENTS IN SUPERNOVA REMNANTS , and the structure of
their progenitor stars, has been studied with ASCA by recording x-ray images
that correspond to emission lines from specific hot, multi-ionized shells
of matter surrounding the supernova blast. Textbook diagrams of heavy stars
on the eve of a supernova explosion show concentric layers of successively
heavier fusion products: carbon, oxygen, neon, silicon, sulphur, and iron
piled up at the heart of a star as it runs out of nuclear fuel. The force
of the subsequent supernova explosion flings the matter in these layers
out into space, where the layering of the elements is somewhat retained.
At the APS meeting, Robert Petre of NASA/Goddard presented separate Si,
S, Fe (etc.) ASCA pictures of remnants. These illustrate that fact that
some supernovas are symmetric, while others are asymmetric. For example,
pictures of the Cassiopeia-A remnant reveal the material to be more ringlike
than spherical, suggesting that the progenitor star had been rapidly rotating.
This is borne out by separate doppler maps of different parts of the Cas-A
remnant. The ASCA pictures of Cas A also show, for the first time, two
different x-ray processes at work in a single supernova: x rays from specific
elements (silicon and sulphur, say) in the north and east and, in the southwestern
corner of the remnant, "continuum" radiation coming from the
collisions between electrons and all sorts of ions.