Credit: X-ray (NASA/CXC/ESO/F.Vogt et al);
Optical (ESO/VLT/MUSE & NASA/STScI)
Astronomers have discovered a special kind of neutron star
for the first time outside of the Milky Way galaxy, using data from
NASA's Chandra X-ray Observatory and the European Southern Observatory's
Very Large Telescope (VLT) in Chile.
Neutron stars are the ultra dense cores of massive stars that collapse and undergo a supernova explosion. This newly identified neutron star is a rare variety that has both a low magnetic field and no stellar companion.
The neutron star is located within the remains of a supernova — known as 1E 0102.2-7219 (E0102 for short) — in the Small Magellanic Cloud, located 200,000 light years from Earth.
This new composite image
of E0102 allows astronomers to learn new details about this object that
was discovered more than three decades ago. In this image, X-rays
from Chandra are blue and purple, and visible light data from VLT's
Multi Unit Spectroscopic Explorer (MUSE) instrument are bright red.
Additional data from the Hubble Space Telescope are dark red and green.
Oxygen-rich
supernova remnants like E0102 are important for understanding how
massive stars fuse lighter elements into heavier ones before they
explode. Seen up to a few thousand years after the original explosion,
oxygen-rich remnants contain the debris ejected from the dead star's
interior. This debris (visible as a green filamentary structure in the
combined image) is observed today hurtling through space after being
expelled at millions of miles per hour.
Chandra observations of E0102 show that the supernova remnant is
dominated by a large ring-shaped structure in X-rays, associated with
the blast wave of the supernova. The new MUSE data revealed a smaller
ring of gas (in bright red) that is expanding more slowly than the blast
wave. At the center of this ring is a blue point-like source of X-rays.
Together, the small ring and point source act like a celestial bull's
eye.
The combined Chandra and MUSE data suggest that this source is an
isolated neutron star, created in the supernova explosion about two
millennia ago. The X-ray energy signature, or "spectrum," of this source
is very similar to that of the neutron stars located at the center of
two other famous oxygen-rich supernova remnants: Cassiopeia A (Cas A) and Puppis A. These two neutron stars also do not have companion stars.
The lack of evidence for extended radio emission or pulsed X-ray
radiation, typically associated with rapidly rotating highly-magnetized
neutron stars, indicates that the astronomers have detected the
X-radiation from the hot surface of an isolated neutron star with low
magnetic fields. About ten such objects have been detected in the Milky
Way galaxy, but this is the first one detected outside our galaxy.
But how did this neutron star end up in its current position,
seemingly offset from the center of the circular shell of X-ray emission
produced by the blast wave of the supernova? One possibility is that
the supernova explosion did occur near the middle of the remnant, but
the neutron star was kicked away from the site in an asymmetric
explosion, at a high speed of about two million miles per hour. However,
in this scenario, it is difficult to explain why the neutron star is,
today, so neatly encircled by the recently discovered ring of gas seen
at optical wavelengths.
Another possible explanation is that the neutron star is moving
slowly and its current position is roughly where the supernova explosion
happened. In this case, the material in the optical ring may have been
ejected either during the supernova explosion, or by the doomed
progenitor star up to a few thousand years before.
A challenge for this second scenario is that the explosion site would
be located well away from the center of the remnant as determined by
the extended X-ray emission. This would imply a special set of
circumstances for the surroundings of E0102: for example, a cavity
carved by winds from the progenitor star before the supernova explosion,
and variations in the density of the interstellar gas and dust
surrounding the remnant.
Future observations of E0102 at X-ray, optical, and radio wavelengths
should help astronomers solve this exciting new puzzle posed by the
lonely neutron star.
A paper describing these results was published in the April issue of Nature Astronomy, and is available online.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for E0102-72.3:
Scale: Image is about 2.85 arcmin (165 light years) across
Category: Supernovas & Supernova Remnants
Coordinates (J2000): RA 01h 04m 02.40s | Dec -72° 01´ 55.30"
Constellation: Tucana
Observation Date: 28 pointings between 2/01/2003 - 03/19/2017
Observation Time: 113 hours 21 seconds (4 days 17 hours 21 seconds)
Obs. ID: 3519-3520, 3544-3545, 5123-5124, 5130-5131, 6042-6043, 6074-6075, 6758-6759, 6765-6766, 8365,9694, 10654-10656, 11957, 13093, 14258, 15467, 16589, 18418, 19850
Instrument: ACISAlso Known As: SN010102-72
References: Vogt, F. et al, 2018, Nature Astronomy, arXiv:1803.01006
Color Code: X-ray (blue, purple); Optical (red, green)
Distance Estimate: About 200,000 light years
Source: NASA’s Chandra X-ray Observatory
