Mind the Gap - A High Mass Neutron Star or Low Mass Black Hole?
This month’s Astrophysical Journal published a discovery of
a binary black hole merger/collision of two highly dissimilar objects, as
identified by the LIGO and VIRGO gravitational telescopes (GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass
Black Hole with a 2.6 Solar Mass Compact Object, The Astrophysical Journal
Letters, 896:L44 (20pp), 2020 June 20).
This is actually quite a big deal, both in terms of the mass of the objects
detected (one object was a about 24 times the mass of the sun and the other was about 2.6 solar masses) and its possible impact on
astrophysical theory and even nuclear physics.
That’s because the smaller object is in the gap between the highest mass
neutron stars so far confidently discovered and the lowest mass black holes
discovered.
To quote the Astrophysical Journal paper:
“The highly unequal mass ratio of
-0.112 +0.009 or -0.008 and unusual secondary mass of -2.59 +0.09 or -0.08 Solar mass make the source of GW190814
unlike any other compact binary coalescence observed so far. The average mass
ratio for BBH (binary black hole) coalescences detected by the LVC during O1
and O2 is 0.9 (Roulet & Zaldarriaga 2019),
and an inference of the underlying population predicted that 99% of detectable
BBHs have mass ratios q>=0.5 (Fishbach
& Holz 2020). However, the paucity of events
from O1 and O2 means that this picture is limited. Indeed, the discovery of
GW190412 has already changed the picture substantially (Abbott
et al. 2020d).”
When stars reach the end of their main sequence life (when
most of their nuclear fuel has been exhausted) they will undergo a collapse,
due to the intrinsic high gravitational field created by their mass. Nuclear burning (note that people often say
burning, though it is actually nuclear fusion) prevents this from occurring
during their time on the main sequence as the outward pressure from the energy
caused by the fusion reaction counterbalances the inward pressure of
gravity. But when that stops, something
has to give.
What the end point of that collapse is, depends on a number
of parameters, with the most important one being the mass of the original star.
·
It is thought that the smallest stars, red
dwarfs, can last for extremely long periods (on the order of a trillion years)
of time, as they burn their nuclear fuel relatively slowly. They eventually probably collapse into white
dwarf stars.
·
Smaller stars, like the sun, eventually go nova
(after billions of years), expanding outward, but not explosively, losing some
mass. In time the core of the star
contracts to a “white dwarf”, about the size of the Earth, a highly compressed
object. The contraction is stopped by
something called electron degeneracy, which puts a brake on the gravitational
collapse.
·
Larger stars also burn through their nuclear fuel,
though at a much faster rate (tens of millions of years). After a series of stages of nuclear fusion,
which create the various different lighter elements (iron and lighter), they
run out of fusional fuel and a supernova results. This is a massive explosion, truly astronomical. These last moments also create the elements
that are heavier than iron, and it is thought that all of these elements that
we see in the universe ultimately came from supernovae.
·
It is these large stars which leave behind
either a neutron star or a black hole as supernova remnants. Depending on the mass of the originating
large star, the result may be a neutron star (the smaller massive stars) or a
black hole (really big massive stars).
o
If a star is not too large, the neutron star is
formed, as the collapse is stayed by neutron degeneracy. An over-simplified way to think of this is as
one mass of huge nuclear material, where the nuclear matter is so close
together (“touching” so to speak) that only nuclear repulsion (quantum neutron
degeneracy) stays the collapse. Up until
this discovery, the largest neutron stars that had been discovered were thought
to be about 2 solar masses or a bit over.
o
However, for really massive stars even nuclear
repulsive forces won’t stop the collapse.
It is thought that the collapse continues until a singularity is
reached, a strange object of “infinite density” (again over-simplifying), where
the laws of physics as we understand them don’t apply. Up until this discovery, the smallest black
holes that had been discovered were thought to be about 5 solar masses or a bit
under.
It is possible that this recently discovered object could be
a “heavy” neutron star or a “light” black hole.
The authors of the paper are inclined to the latter explanation, though
the mechanism through which such a relatively small black hole could form is
not well established.
On the other hand, it could be a “heavy” neutron star. If so, it might have significant implications
for our understanding of the atom itself (i.e. the “standard model”). Most currently plausible theories of the neutron
stars don’t allow for a neutron star of this size (i.e. the currently accepted
neutron star equations of state don’t predict this). Since the equation of state relies on
evidence and extrapolations from evidence of investigations of nuclear
reactions here on Earth, it throws in some doubt our understanding of those
fundamental atomic interactions. Thus,
the standard model itself might require some significant alterations.
The finding also upsets some astrophysical
expectations. Basically, it was thought
that a merger/collision of very high mass stars, where one star was so much
larger than the other, would be a rather rare occurrence. The fact that this one showed up relatively
early in the observing history of the LIGO-type gravitational telescopes upsets
that assumption. To use Bayesian-speak,
this will definitely result in updating some priors.
From the Astrophysical Journal paper:
“GW190814ʼs secondary mass lies
in the hypothesized lower mass gap of 2.5–5Me (Bailyn et al. 1998; Özel et al. 2010;
Farr et al. 2011; Özel et al. 2012) between known NSs and BHs. It is heavier
than the most massive pulsar in the Galaxy (Cromartie et al. 2019), and almost
certainly exceeds the mass of the 1.61–2.52 solar mass primary component of
GW190425, which is itself an outlier relative to the Galactic population of
BNSs (Abbott et al. 2020a). On the other hand, it is comparable in mass to two
BH candidates: the 2.7 solar mass merger remnant of GW170817(Abbott et al. 2019b)
and the 2.6–6.1solar mass compact object (95% confidence interval) discovered
by Thompson et al. (2019). It is also
comparable to the millisecond pulsar PSR J1748−2021B (Freire et al. 2008), whose mass is claimed as 2.74+/-
0.21 solar mass at 68% confidence. However, this estimate, obtained via
measurement of the periastron advance, could be inaccurate if the system
inclination is low or the pulsar’s companion is rapidly rotating (Freire et al.
2008). In sum, it is not clear if GW190814ʼs secondary is a BH or an NS.”
It is also worth
noting that what I called a “LIGO-type telescope” isn’t a telescope in the
usual sense of the word. It actually
detects the effect of gravitational waves on space-time, via careful
measurements of distances, which indicate a perturbation in local space-time as
the gravitational waves pass by. But,
since it’s purpose is to detect a form of radiation (gravitational radiation in
this case), the term telescope seems fair.
It is also worth noting that this merger/collision is thought to have occurred in a galaxy about 240 million parsecs from Earth (or about a billion light years).
Note: The BBC website had a good article on this, which is
where the images in this blog originated.
So, that was interesting, but you should think about reading
a nice SF book that features a neutron star:
The Witches’ Stones Book 1: Rescue from the Planet of the Amartos
So,
go on a romantic science fiction adventure with an agent of Earth’s Terra
Federation’s counter-intelligence group “The Agency”, to prevent the mystical
and powerful Witches’ Stones from falling into the hands of the shadowy
power-mad developing galactic empire, known to Earthlings only as The
Organization. Key to this mission is the
rescue of an apparently unassuming Earth girl, who holds the key to the
galactic balance of power, via her ability (unknown to her) to psychically
unlock the energies of the Witches’ Stones.
Amazon U.K.: http://www.amazon.co.uk/gp/product/B008PNIRP4
Amazon Canada: https://www.amazon.ca/dp/B008PNIRP4
Amazon Germany: https://www.amazon.de/dp/B008PNIRP4
Amazon Australia: https://www.amazon.com.au/dp/B008PNIRP4
Amazon Japan: https://www.amazon.co.jp/dp/B008PNIRP4
Amazon India: https://www.amazon.in/dp/B008PNIRP4
Amazon Spain: https://www.amazon.es/dp/B008PNIRP4
Amazon France: https://www.amazon.fr/dp/B008PNIRP4
Witches’ Stones Book One – Rescue from the Planet of the Amartos
Sarah
Mackenzie had trained as a space ship mechanic at the Space Port of her home
city on Earth. She left Earth to explore the galaxy, and, some months later,
landed a dream assignment, to become the ship mechanic of an Explorer ship, the
Beth 117.
The Beth
was on its way to a planet at the edge of the galaxy, where its crewmembers
were to search for the Witches’ Stones, or amartos, the mysterious crystals,
which the Witches of the world, Kordea, use to channel and augment their
psychic energies.
Sarah has
no idea that she, herself, happens to be Stone-sensitive, just like the Witches
are. Under perilous circumstances, she comes across the cache of the Stones
which the Explorers are looking for, and, unwittingly, “keys” them, igniting a
psychic blaze that attracts the attention of The Organization, the implacable
foe of the Terra Confederation, the centuries-old star-spanning government of
most of the human race, and its non-human allies. To make use of amarto-energy,
The Organization needs, not just the Stones, but also amarto-sensitive
individuals whom they enslave to the devices which they have developed in their
pursuit of galactic domination. Thus, they want not just the cache of Stones;
they also want Sarah.
To forestall
galactic war, rescuers, from a counter-intelligence group, known as The Agency,
are sent to the Planet of the Amartos. A fast scoutship, manned by an Agent and
a Pilot, must try to fetch Sarah and the amartos, bringing them to a safe haven
among the Kordean Witches.
Sarah,
herself, has to deal with serious conflicts. In the psychic realm she must
choose between The Organization and the Kordean Witches, while retaining
mastery over her own mind. In the physical reality, she has become the centre
of an armed battle between the Terran scoutship and a military task force sent
by The Organization to capture her and the Stones. Her determination to keep
control of her own self sends her into unexplored mental realities, while
exciting but dangerous physical events swirl around her and the crew of the
scoutship, Camin.
To further
complicate things, she senses within herself, the beginnings of an attraction
to the handsome Agent sent to rescue her. However, she’s merely a naive young
woman from Earth; surely, her hopes are beyond realization....
The novel
is about 100,000 words, or 250 pages. It is the first book in the Witches'
Stones series, which explores the struggle for power among the Terra
Confederation, the Kordean Witches and The Organization, as well as the
personal and romantic entanglements of the characters. Book 2 and Book 3 complete the series.
No comments:
Post a Comment