Credit: X-ray (NASA/CXC/NCSU/K.Borkowski et al.);
Optical (DSS)
Astronomers estimate that a star explodes as a supernova in our Galaxy, on average, about twice per century. In 2008, a team of scientists announced they discovered the remains of a supernova that is the most recent, in Earth's time frame, known to have occurred in the Milky Way.
The explosion would have been visible from Earth a little more than a
hundred years ago if it had not been heavily obscured by dust and gas.
Its likely location is about 28,000 light years from Earth near the
center of the Milky Way. A long observation equivalent to more than 11
days of observations of its debris field, now known as the supernova
remnant G1.9+0.3, with NASA's Chandra X-ray Observatory is providing new details about this important event.
The source of G1.9+0.3 was most likely a white dwarf star that
underwent a thermonuclear detonation and was destroyed after merging
with another white dwarf, or pulling material from an orbiting companion
star. This is a particular class of supernova explosions (known as Type Ia) that are used as distance indicators in cosmology because they are so consistent in brightness and incredibly luminous.
The explosion ejected stellar debris at high velocities, creating the
supernova remnant that is seen today by Chandra and other telescopes.
This new image is a composite from Chandra where low-energy X-rays are
red, intermediate energies are green and higher-energy ones are blue.
Also shown are optical data from the Digitized Sky Survey, with
appearing stars in white. The new Chandra data, obtained in 2011, reveal
that G1.9+0.3 has several remarkable properties.
The Chandra data show that most of the X-ray emission is "synchrotron radiation,"
produced by extremely energetic electrons accelerated in the rapidly
expanding blast wave of the supernova. This emission gives information
about the origin of cosmic rays - energetic particles that constantly
strike the Earth's atmosphere - but not much information about Type Ia
supernovas.
In addition, some of the X-ray emission comes from elements
produced in the supernova, providing clues to the nature of the
explosion. The long Chandra observation was required to dig out those
clues.
Most Type Ia supernova remnants are symmetrical
in shape, with debris evenly distributed in all directions. However,
G1.9+0.3 exhibits an extremely asymmetric pattern. The strongest X-ray
emission from elements like silicon, sulfur, and iron is found in the
northern part of the remnant, giving an extremely asymmetric pattern.
Another exceptional feature of this remnant is that iron, which is
expected to form deep in the doomed star's interior and move relatively
slowly, is found far from the center and is moving at extremely high
speeds of over 3.8 million miles per hour. The iron is mixed with
lighter elements expected to form further out in the star.
Because of the uneven distribution of the remnant's debris and their
extreme velocities, the researchers conclude that the original supernova
explosion also had very unusual properties. That is, the explosion
itself must have been highly non-uniform and unusually energetic.
By comparing the properties of the remnant with theoretical models,
the researchers found hints about the explosion mechanism. Their
favorite concept for what happened in G1.9+0.3 is a "delayed
detonation", where the explosion occurs in two different phases. First,
nuclear reactions occur in a slowly expanding wavefront, producing iron
and similar elements. The energy from these reactions causes the star to
expand, changing its density and allowing a much faster-moving
detonation front of nuclear reactions to occur.
If the explosion were highly asymmetric, then there should be large
variations in expansion rate in different parts of the remnant. These
should be measurable with future observations with X-rays using Chandra
and radio waves with the NSF's Karl G. Jansky Very Large Array.
Observations of G1.9+0.3 allow astronomers a special, close-up view
of a young supernova remnant and its rapidly changing debris. Many of
these changes are driven by the radioactive decay of elements ejected in
the explosion. For example, a large amount of antimatter
should have formed after the explosion by radioactive decay of cobalt.
Based on the estimated mass of iron, which is formed by radioactive
decay of nickel to cobalt to iron, over a hundred million trillion (ie
ten raised to the power of twenty) pounds of positrons, the antimatter
counterpart to electrons, should have formed. However, nearly all of
these positrons should have combined with electrons and been destroyed,
so no direct observational signature of this antimatter should remain.
A paper describing these results is available online
and will be published in the July 1, 2013 issue of The Astrophysical
Journal Letters. The first author is Kazimierz Borkowski of North
Carolina State University (NCSU), in Raleigh, NC and his co-authors are
Stephen Reynolds, also of NCSU; Una Hwang from NASA's Goddard Space
Flight Center (GSFC) in Greenbelt, MD; David Green from Cavendish
Laboratory in Cambridge, UK; Robert Petre, also from GSFC; Kalyani
Krishnamurthy from Duke University in Durham, NC and Rebecca Willett,
also from Duke University.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science and
flight operations from Cambridge, Mass.
Scale: Image is 8 arcmin across (About 60 light years)
Category: Supernovas & Supernova Remnants
Coordinates (J2000): RA 17h 48m 45s | Dec -27° 10' 00"
Constellation: Sagittarius
Observation Date: 15 pointings between Feb 2007 and Jul 2011
Observation Time: 362 hours (15 days 2 hours)
Obs. ID: 6708, 8521, 10111, 10112, 10928, 10930, 12689-95, 13407, 13509
Instrument: ACIS
References: Borkowski, K, et al, 2013, ApJ Letters (Submitted); arXiv:1305.7399
Color Code: X-ray (Red, Green, Blue); Optical (White/Cyan)
Distance Estimate: About 25,000 light years
