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Near the end of its life, a star more massive than our Sun finds
itself no longer able to support its own weight from the crush of
gravity and so it collapses, producing an expanding shock wave that
sweeps through the surrounding gas, creating what is called a
supernova remnant. All that remains of the original star is a
dense, compact object known as a
neutron star. Magnetars are the latest addition to the "zoo" of
neutron stars and they are truly exotic beasts with magnetic fields
hundreds of millions of times stronger than have ever been seen on
Earth.
The story which led to the prediction
of magnetars and then to their
discovery is given elsewhere. Here we will focus on the other part
of the story, the supernova remnants born at the same time as
magnetars and the diffuse emission produced by the energetic
outpourings of the magnetars.
All four of the soft gamma-ray repeaters that we currently know are
located in or near a supernova remnant. It was this discovery that led
astronomers to determine that soft gamma-ray repeaters were in our
Galaxy and the nearby galaxy known as the Large Magellanic
Cloud. Through the study of these supernova remnants, astronomers were
able to infer that soft gamma-ray repeaters were solitary young
neutron stars speeding away from their birthplace at 3 million miles
per hour.
Theories predict that the same process which can produce the fantastic
bursts of hard X-ray emission that give soft gamma-ray repeaters their
name, can also accelerate particles (electrons, protons, etc) to
speeds approaching the speed of light. As the saying goes, "where
there's smoke there's fire" and this case is no exception. Most of the
energy released by the burst event is carried away by these high
energy particles and not the gamma-ray burst itself. As the particles
spiral in the surrounding magnetic field, they too emit radiation,
creating extended nebulae called "plerions". Provided there is some
way to confine the outflow, these plerions act as "wind bags"
containing the accumulated history of this bursting activity. The
study of these plerionic supernova remnants (detailed below) gives us
a new and different perspective on the magnetar phenomena. |
Figure Caption: Two views toward SGR 1806-20. These radio images,
taken with the Very Large Array, were made 1.4 years apart on 1994
March (left) and 1995 August (right). A jet-like feature is seen to
rotate on the sky during this time. This behavior is thought to result
from the sudden injection of high energy particles from the magnetar.
SGR 1900+14: New
Evidence of Particle Injection by a Magnetar Until
recently you could have been excused for thinking that SGR 1900+14 was
the least interesting of the four SGRs. It was somewhat inactive as a
gamma-ray source, bursting a few times in 1979 and again in 1993. An
approximate position for SGR 1900+14 has been determined via
triangulation of burst arrival times from the satellites in the
Interplanetary Network (IPN). In the next figure we
superimpose this long needle-like region on a radio image of the sky
taken in this direction. There is an X-ray source interior to this
which is thought to be the quiescent X-ray emission from SGR
1900+14. The X-ray source, which goes by the name of RX
J190717+0919.3, lies just outside the galactic supernova remnant
G42.8+0.6. This is a fairly ordinary-looking supernova remnant located
some 15,000 light years from the Sun, and is less than 10,000 years
old. It is likely (but still unproven) that SGR 1900+14 and G42.8+0.6
were born in the same explosive event but SGR 1900+14 caught up with
and punched through the expanding supernova shock.
In late May and late August of this year SGR 1900+14 awoke from its
slumber and began bursting like never before. Numerous bright
bursts occurred during these two active periods. On August 27 an
intense burst occurred that was reminiscent of the famous March 5 1979
burst from SGR 0526-66, causing the detectors of several satellites to
saturate. Click
here
to see the light
curve of the August 27 burst as detected by the Ulysses satellite.
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| Shortly after the August 27 burst we observed the
region around the quiescent X-ray emission from SGR 1900+14 using the
Very Large Array in Socorro, New Mexico. While prior observations of
this region have shown only empty sky, our observations in early
September 1998 detected a new radio source coincident (within the
position uncertainties) with the X-ray source. As an added bonus the
discovery of this nebula gives us a position for SGR 1900+14 with
sufficient accuracy to search for an optical counterpart. Perhaps even
more interestingly than the detection of the radio source was the fact
that it was short-lived, fading below detection in only one week. At
the top of this page we show a movie of this fast fading source. In a
forthcoming scientific paper we argue that we have detected a radio
nebula shining in the light of particles accelerated to nearly the
speed of light by the same event that gave rise to the bright
gamma-ray bursts. |
Figure Caption: Two views toward SGR 1900+14. These radio images,
taken with the Very Large Array, were made on 1998 June 25 (left) and
September 3 (right). The position of the quiescent X-ray source RX
J190717+0919.3, thought to indicate the approximate location of SGR
1900+14, is indicated by the circle. The radio source turned on
shortly after the bright gamma-ray burst of August 27th and then
rapidly faded from view a week later.
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SGR 1627-41: The Newest
SGR SGR 1627-41 is the newest member of the SGR
family. See the
NASA press release announcing this exciting discovery. While its
position is not well-determined yet, there is only one known supernova
remnant in the region: G337.0-0.1. It
is a compact supernova remnant 35,000 light years from the Sun which
lies in the CTB 33 complex (see below), a young region with plenty of
active star formation going on. |
Figure Caption: A radio image of the CTB33 complex taken with the
Australia Telescope Compact Array (ATCA). The supernova remnant
G337.0-0.1 is the bright, compact shell of emission just left of the
center of the image. Most of the remaining diffuse and filamentary
emission is from hot, thermal gas ionized by young stars in the CTB 33
complex.
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