Astronomers Find First Direct Evidence Linking
Black Holes and Supernovae
Astronomers believe that black holes are the evolutionary endpoints of
stars at least 10 to 15 times as massive as the Sun. If such a star
undergoes a violent
supernova explosion, the core
of the star it leaves behind may gravitationally collapse in on
itself, creating a
singularity - an entity with
zero volume and infinite
density - a black hole.
The first direct evidence that black holes are created by supernova
explosions comes from a source known as GRO J1655-40, which lies
10,000 light years away in the direction of the constellation
Scorpius. It was discovered in 1994 by BATSE as an X-ray nova. Because there was also an
optical nova at the same time, it is also known as Nova Sco 1994. The
source had several short X-ray outburst events until the
summer of 1995, when it went quiet. Results from this initial series
of outbursts showed it to be a black hole having a mass of 7 solar
masses (and one of the best measured black hole masses at that !). It
is in a binary star system with a normal star, and has an orbital
period of about 2 days.
X-ray Timing Explorer's (RXTE) All-Sky Monitor (ASM) instrument first
detected a subsequent outburst in spring of 1996. This activity
lasted well into 1997 and was well observed by RXTE. Data for this
source from RXTE was also used to show evidence of framedragging (http://rxte.gsfc.nasa.gov/docs/xte/learning_center/discover_1197.html) near a black hole. This source also exhibits
superluminal jets, and is one of
two microquasars in our
Radio observations of GRO J1655-40 in August and September 1994
showed jets of gas|
shooting out at up to 92 percent of the speed of light.
In the latest news, focus has turned to the companion star of GRO J1655-40.
Researchers from the Institute of Astrophysics of the Canary Islands
and the University of California, Berkeley have detected large
abundances of heavy elements in this companion star - elements
created in such
abundances only in supernova explosions. They think that the supernova
that created GRO J1655-40 may be responsible for flinging this material
into its companion star.
Rafael Rebolo of the Institute of Astrophysics of the Canary Islands
told ABC News, "What we thought was that this star could have retained
a signature, the fingerprint, of the formation process of the black
hole." In other words, an exploding star blows off a large quantity
of material and if there is a companion star anywhere nearby, it
should contain material evidence of that explosion.
This is exactly what the researchers found. By looking at the
spectrum of GRO J1655-40's companion, they found that its
atmosphere contained overabundances of oxygen, magnesium, silicon, and
sulphur. These heavy elements should not exist in such large
quantities in a normal star - they were likely expelled during the
explosion of GRO J1655-40. "The only way you can produce an excess of
these elements is through several billions of degrees," Rebolo said
to the Associated Press (AP), "The only way to reach these temperatures is
when a star goes to a supernova situation."
John Cowan, professor at the University of Oklahoma in Norman,
commented to the AP, "The explanation offered by the researchers is
the most straightforward and logical one. There could be some other
explanation, but it would have to be more convoluted." According to
ABC News, Rebolo said that the elements could not have come from the
original gas cloud that formed the stars. "We don't know of any other
way to enrich the companion star so drastically with [these]
elements." He added to Reuters News Service, "This is the first time
these elements have been observed in a star linked to a black hole.
The content is quite anomalous. We argue that these anomalies are
related to the fate of the precursor of the black hole. There is no
alternative way to explain this."
The supernova explosion that created GRO J1655-40 likely happened a million
years ago. Long enough, Rebolo reported to ABC, that the clouds of
expanding gas shot outward have since dissipated. Supernova remnants, the
expelled gas from a supernova explosion, can remain visible for 100,000
years before being dispersed into space.
Rebolo and his collaborators in Spain and at UC Berkeley used the Keck I
telescope in Hawaii - their findings are published in the Sept. 9, 1999
issue of Nature along with Professor Cowan's commentary.