[Music throughout]This is J0030, a type
of dead star called a pulsar, located about 1,100 light-years
away in the constellation Pisces. Observing
J0030 in X-rays, astronomers have now made the most precise
and reliable measurements of any pulsar’s size. And they’ve discovered
that J0030's appearance differs dramatically from textbook depictions.
NASA’s Neutron star Interior
Composition Explorer, or NICER, is a telescope on the International
Space Station. NICER makes extremely detailed X-ray measurements
of neutron stars, and its data provided this unprecedented glimpse
of J0030. A neutron star is the crushed
core of a massive star that exploded in a supernova. Pulsars,
like J0030, are rapidly spinning neutron stars
that sweep beams of energy across our line of sight, much like a lighthouse.
A pulsar is so dense that its gravity
bends the fabric of space-time around it. NICER’s precise
X-ray measurements allow scientists to take advantage of this effect
to see light from the far side of the pulsar. This is
a pulsar-sized object about 16 miles across but with much less
mass. We only see light from the side of the object nearest
to Earth. But as its mass increases, the object warps
space-time and acts like a lens to show us light from the far side.
This has the strange effect of making a pulsar look bigger than it really is.
The more mass an object of a given size contains, the more
it distorts space-time and the more we see of its far side.
Textbooks show pulsars with two hot spots on the surface,
directly opposite each other at the magnetic poles.
As the pulsar spins, the spots come in and out of view, creating regular
changes in its X-ray brightness. If the pulsar’s mass is
low, the spots disappear when they rotate to the far side.
But if the mass is high enough, the hot spots may never completely disappear.
Using NICER data, two teams
of scientists examined different models for the shapes, and even the number,
of hot spots on J0030. Both
arrived at the same conclusion — the pulsar is around 16 miles across
and about 1.4 times the Sun’s mass.
This represents the most precise measurement yet of a pulsar’s size,
with an uncertainty of less than 10%. The spots themselves
don’t match the textbook image, though. From Earth, we look down onto
J0030’s northern hemisphere. Both teams say there are no
spots there at all. Contrary to the simple magnetic dipole model,
all the spots appear in the southern hemisphere and are not necessarily
in shapes we might expect. One team, led by researchers
at the University of Amsterdam, suggests J0030
has one small circular spot and another long, crescent-shaped one.
The other team, led by researchers at the Universities of
Maryland and Illinois, finds two oval hot spots.
Their sizes, shapes and locations closely match those derived from
the other model. However, the Maryland-led team also finds
a third, cooler spot located slightly askew
of the pulsar’s south rotational pole, just at the edge of our view of the pulsar.
Scientists are still trying to determine why
J0030’s spots take on these shapes and arrangements, but for now it’s
clear that pulsar magnetic fields are more complex than originally
assumed. NICER’s measurements of
J0030 have opened a new chapter in our understanding of neutron
stars. As it continues to study other pulsars,
we’ll learn even more about the common characteristics — and
individual quirks — of these incredible objects.
[Pulsar grid animations by Sharon Morsink, University of Alberta. Pulsar magnetosphere animation by Alice Harding, Constantinos Kalapotharakos, and Zorawar Wadiasingh, NASA’s Goddard Space Flight Center]
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