Sky & Telescope - December 2024 - 9

EQUATION OF STATE PLOT: ANNA WATTS (UNIVERSITY OF AMSTERDAM); VISUALIZATION OF NEUTRON STAR AND BLACK HOLE MERGER: SCIENTIFIC VISUALIZATION: T. DIETRICH (POTSDAM
UNIVERSITY AND MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS), N. FISCHER, S. OSSOKINE, H. PFEIFFER (MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS), T. VU. NUMERICALRELATIVITY
SIMULATION: S.V. CHAURASIA (STOCKHOLM UNIVERSITY), T. DIETRICH (POTSDAM UNIVERSITY AND MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS)
Mass (solar masses)
STARS
Neutron Stars Might Be Squishy Inside
IN PINNING DOWN the properties of
the closest and brightest neutron star
observed so far, Devarshi Choudhury
(University of Amsterdam) and colleagues
have ruled out both the plainest
and the strangest ideas describing the
dense matter in neutron star cores.
When stars of a certain mass collapse
into neutron stars, most of their
atoms break down into neutrons. But
conditions are weirder in the center,
where the pressure can grow to the
limits of unbearable. The key to understanding
how weird lies in the equation
of state, which describes how density
changes with pressure.
If the densest matter is " stiff, " then
increasing a neutron star's mass, and
thus its internal pressure, wouldn't
cause many (or any) interior changes
- the neutron star would simply grow
in size. But if the equation of state calls
for a " squishy " interior, then increasing
mass would alter the core in some way,
perhaps squeezing quarks out of neutrons.
Depending on how squishy the
matter is, a neutron star may stay the
GRAVITATIONAL WAVES
Black Hole Ate Neutron
Star (Probably)
GRAVITATIONAL-WAVE astronomers
have identified ripples in spacetime
from the collision and coalescence of a
neutron star with what's likely one of
the smallest black holes ever found.
When massive stars die in supernovae,
their imploded cores can become
neutron stars or - if they're big enough
- black holes. Neutron stars top out at
around 2½ solar masses; black holes file
in beyond that.
But starting in the late 1990s,
observers noticed a gap between the
heaviest neutron stars and the lightest
black holes, the latter of which tended
to weigh in with at least 5 Suns' worth.
Subsequent theoretical work suggested
it's difficult for stars to make black
holes of only a few solar masses.
same size as mass is added, or it might
even shrink.
To unravel the equation of state,
astronomers are measuring masses and
radii of spinning neutron stars known
as pulsars using the Neutron Star
Interior Composition Explorer (NICER)
on the International Space Station. To
measure size and heft, NICER clocks the
arrival of X-rays from hot regions near
pulsars' magnetic poles as they whirl
around like frenetic lighthouses.
NICER scientists have now obtained
data on the closest pulsar from their
list, PSR J0437-4715, 512 light-years
away. It's fast, pulsing 174 times per
second - quicker than the whirling
knives of a kitchen blender - and it's
bright, which makes for precise measurements.
The astronomers find a
radius between 10.7 and 12.3 kilometers
(6.6-7.6 miles), and with the aid of
previously gathered radio data, around
1.4 times the Sun's mass.
These measurements have placed
some of the best limits yet on what
kinds of material might exist inside
2.75
3.00
2.50
1.25
1.50
1.75
2.00
2.25
1.00
8
9 10 11 12
13
14
Radius (kilometers)
pNew mass and radius measurements from
J0437 (red) and two other pulsars rule out the
stiffest and squishiest states of matter. Also
shown are some representative equations of
state (cobra-shaped lines).
neutron stars, ruling out scenarios on
both extremes. " Our new result points
us towards slightly softer (squishier!)
equations of state, " says team member
Anna Watts (also at University of
Amsterdam). " But the very softest are
still unlikely, as are now the stiffest. "
That means that neutron star cores
are probably not just neutrons, though
exactly what exotic particles might
reside there is still up in the air.
¢MONICA YOUNG
pThis simulation shows a black hole tearing
a neutron star apart before merging with it, an
event called tidal disruption.
Yet astronomers have found a handful
of objects in this putative gap, and
one of the hopes for gravitational-wave
detectors is that they'll help us determine
just how empty this gap really is.
Now, scientists with the LIGO-VirgoKAGRA
(LVK) collaboration have found
another object in the breach.
On May 29, 2023, the LIGO Livingston
detector in Louisiana wiggled as
gravitational waves passed through it in
an event dubbed GW230529. (The other
detectors were offline at the time or not
sensitive enough to pick up the event.)
Based on careful analysis of the signal,
the collaboration concluded that the
gravitational waves marked the merger
of two objects: a smaller one with a
mass between 1.2 and 2 Suns, and a
larger one between 2.5 and 4.5 Suns.
The mass of the smaller one indicates
it's almost certainly a neutron star. The
larger one is most likely a black hole,
the scientists conclude in the August 1st
Astrophysical Journal Letters; however,
they cannot completely rule out that it's
a massive neutron star.
GW230529 is only one of more than
81 events detected during the first half
of LVK's fourth observing run, which
will continue until June 2025. These
will join the roughly 100 already found
in previous observing runs, either by
the LVK collaboration or by independent
teams trawling the data.
¢ CAMILLE M. CARLISLE
sk yand tele scope .o r g * DECEMBER 2024
9
15 16
J0740
Nucleonic
Quark
Hybrid
Hyperon
J0437
J0030

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Sky & Telescope - December 2024

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