pulsar was first discovered in late 1967 by Cambridge University graduate
student Jocelyn Bell. She noticed a
peculiar blip in the strip-chart data from the new Interplanetary Scintillation
Array, the radio telescope that she had recently participated in building.
She had the nagging feeling that she has seen something similar a few
days earlier at the same sky position. Searching
through the many meters of strip-chart recorder paper that the radio telescope
generated, she found similar blips. There
were repeating radio pulses coming from the same fixed position in the sky.
She had discovered the first pulsar.
It was a Nobel-Prize-winning discovery (but not for her).
Bell and her thesis supervisor, Prof. Anthony Hewish, seriously considered the
possibility that they might be detecting the repeating signals from a high
technology alien civilization. The
Hewish group even used "LGM-1" (for little green men) to name signal
they had discovered. However, now
that they understood what to look for, the group soon found similar repeating
radio pulses coming from several other positions in the sky.
It was ultimately concluded that the pulses were not signals from
little green men, but rather periodic radio bursts from a rapidly rotating
neutron star that was beaming out radio waves along its magnetic poles.
The radio beam swept the galaxy as the neutron star spun on its
rotational axis, like the light beam of a lighthouse.
The observed radio blips picked up by the Cambridge antenna were made
when the pulsar's radio beam swept past the antennaís spot on the Earth every
few seconds. Bell and her colleagues
were observing the radio emissions of a rapidly rotating neutron star.
When a normal
star burns up all the fuel consumed in the nuclear fusion processes, it goes
into gravitational collapse and produces a supernova explosion.
If the remnant left behind by that titanic explosion has a mass of 5
solar masses or more, the gravitational force is so strong that the collapsing
star forms a black hole. If
the stellar corpse has a mass of 2.5 solar masses or less, the repulsion from
the nuclear force between neutrons comes into equilibrium with the attraction of
gravity, and the collapsing object becomes a neutron star. (Don't
ask what happens if the collapsing star's final mass is between 2.5 and 5 solar
masses; nobody knows!)
The neutron star that forms in a supernova has a radius of around 10 kilometers (the size of a small town), a density of about 1017 kg/m3 (100 trillion times the density of water), and a spin rate that can be as high as 716 rotations per second at first, but gradually slows down to one rotation per second or less as the neutron star ages and sheds it initial angular momentum over a time of a few hundred thousand years or so. The pulsar that Bell discovered, now given the name PSR B1919+21, is located in our galaxy at a distance of 978.5 light years from Earth, rotates once every 1.3373 seconds, and has a mass of 1.4 times that of our Sun.
1967 pulsar discovery, radio astronomers have been very busy.
They have found over 3,300 more pulsars, and that number is still
growing. Typically, these pulsars
have super-strong magnetic fields of 1010 tesla or more, the result
of concentrating the magnetic flux trapped in the conducting stellar interior as
the parent star collapses. The
angular momentum of the parent star is also preserved during the collapse.
This leads to tiny rotational periods as fast as a few milliseconds for "new"
pulsars. As we said above, older
pulsars are known to "spin down" as they age and eventually reach rotational
periods measured in seconds. Some of
the pulsars are isolated single stars, while others share a binary orbit with
another star. Such binary pulsars
have even been used to observe the rotational slowing down of the binary system
as it radiates gravitational waves and loses energy and angular momentum.
Another of the
peculiarities of pulsars is their very high average linear velocity.
While most Population-I stars (like our Sun) are observed to have
velocities around 30 km/s, the average velocity of pulsars is more than an order
of magnitude greater, around 450 km/s or 0.15% the speed of light.
Five known pulsars even have velocities exceeding 1,000 km/s (0.33% c).
In the cases of the relatively recently formed Crab pulsar (950 years
old) and Vela pulsar (about 10,000 years old), the velocity direction is known
to be roughly aligned with the spin-axis of the pulsar.
anomalously large value of the pulsar velocity raises many questions:
Why is it so large? Is it the
result of a brief "kick" from the supernova as it was forming the pulsar?
Or did some unknown process produce continuous acceleration?
Did the acceleration come and go in an initial burst, or is it still
theoretical attempts to explain such large pulsar velocities have focused on a
brief "kick". They assume that the
supernova that formed the pulsar was asymmetrical and that, on a time scale of
0.1 to 10 seconds, it boosted the pulsar to its observed high velocity.
These models assume asymmetrical matter ejection or asymmetrical neutrino
emission due to fluid instabilities in the rotation of the collapsing parent
star. Another approach suggests that
the acceleration is related to the super-strong magnetic field of the neutron
star (1011 to 1012 tesla is needed), implementing a
mechanism that creates the velocity boost either through neutrino oscillations
or electromagnetic radiation.
All such attempted explanations up to now have had problems. A large supernova explosion asymmetry, which must occur for every pulsar and result in a "kick" large enough to account for the observed final velocity, seems unlikely. For the other theories magnetic fields of at least 1011 tesla are required, while most pulsars with velocities exceeding 300 km/s have magnetic fields of less than 5 x 108 tesla. Thus, the high pulsar velocity remains an astrophysical mystery.
In a June-2022
paper published in The Astrophysical Journal a Chinese team of
astrophysicists, Zheng Li, et al, suggest a new pulsar acceleration
mechanism: the neutrino rocket. Their
model ties pulsar acceleration to its spin-down rate and supplies a mechanism by
which a pulsar can be continuously accelerated along its spin axis.
The work seems to supply the missing puzzle-piece for understanding why
pulsars have such high velocities.
face value, obtaining rocket action from emission of neutrinos seems
impossible, even ridiculous. Neutrinos,
very low-mass leptons with no electric charge, are typically produced only in
very low-probability nuclear reactions involving the weak force or in slow
weak-interaction decay processes like beta decay.
As a rule of thumb, neutrinos are very hard to make and very hard to
detect. However, in the bizarre
neutron star environment, where several solar masses of neutrons are jammed in
together at nuclear density so that they form a superfluid medium, unlikely
scenarios can become rather likely.
a charged particle like an electron undergoes rapid circular motion, it emits a
form of electromagnetic radiation called cyclotron radiation (see my
column #176 in the March-2015 issue of Analog).
Invoking electro-weak unification, the Li paper argues that when neutrons
undergo the high acceleration of very rapid circular motion, as would be the
case in superfluid vortexes in the interior of a neutron star, they should
analogously produce weak-interaction cyclotron radiation, radiating
neutrino plus anti-neutrino pairs that carry off energy and angular momentum.
Because of the parity violation in weak interactions, the emitted
neutrino and anti-neutrino are emitted in the same direction.
That direction is backwards along the spin axis of the neutron
star, so that the pulsar should be accelerated forward along that
axis. This weak-interaction
cyclotron radiation is made possible by the large population of neutron
superfluid vortexes that should be present within the neutron star.
The neutrino-radiation process removes energy and angular momentum from
the neutron star, contributing to the spin-down of pulsars, a well-observed but
poorly explained phenomenon.
neutron stars are formed, the angular momentum of the collapsing star system
must remain unchanged, so that the leisurely rotation of the parent star becomes
the rapid millisecond-level rotation period of the newly collapsed
nuclear-density object. Shrinking
the radius increases the spin rate of the neutron star, like an ice dancer
pulling in her arms to spin faster.
new pulsars like the Crab and Vela have rotation period measured in
milliseconds. As the pulsar ages,
its rotation period grows due to angular momentum lost to electromagnetic
radiation (including the radio beam of the pulsar) and to cyclotron neutrino
emission. At first, electromagnetic
radiation drives the spin down, but as the rotation slows, the neutrino emission
comes to dominate the spin- down process.
the Li paper, the authors show data of the correlation between rotation period
and spin-down rate, that rate rising as the rotation period grows.
They also show a rough growth in velocity as the pulsar rotation period
becomes longer, i.e., as continued acceleration boosts the velocity.
They can fit this dependence and other correlations with their neutrino
rocket model. We note that the
predicted acceleration of the pulsar is not very large, but it has a large
effect on the velocity because it always pushes in the same direction and acts
for many hundred-thousands of years.
This, then, is currently the best explanation of how and why the observed pulsars have such high velocities. They are all converting their enormous stored angular momentum to linear propulsion and accelerating in the process.
we have a grandiose new way of building rockets, using neutron stars as the
driving engines, accelerating as they shoot neutrinos out their backside along
their spin axis. Could an advanced
civilization exploit this mechanism for their own interstellar purposes?
Benford and Larry Niven have recently given us Bowl
of Heaven (2012), Shipstar (2014), and Glorious
(2020), a trilogy of hard SF novels describing a star-scale interstellar
spaceship powered by a moving star, created by advanced aliens and being
colonized by space-faring humans.
similar could be done with a neutrino-rocket-propelled neutron star.
The enormous bowl structure of the Benford-Niven shipstar could be
replaced by a planet in orbit around the neutron star, brought along for the
ride as the neutrino rocket accelerates the entire system.
The acceleration isnít large, so any interstellar neutrino-rocket trips
would have to be generational, on the scale of many thousand years.
Steering, control, and perhaps boosting neutrino acceleration might be
problems, but not beyond the reach of the imagination of our best hard SF
writers. Any takers out there?
G. Cramer's 2016 nonfiction book describing his transactional interpretation of
quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions,
(Springer, January-2016) is available online as a hardcover or eBook at:
editions of John's hard SF novels Twistor
Einstein's Bridge are available online at:
John's new hard-SF novel Fermi's Question, sequel to Einstein's Bridge, will be available soon from Baen Books.
View Columns Online: Electronic reprints of 222 or more of "The
A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott, and R. A. Collins, "Observation of a Rapidly Pulsating Radio Source", Nature 217, 5130 (1968), doi:10.1038/217709a0.
The Neutrino Rocket Hypothesis:
Zheng Li, Qiu-He Peng, Miao Kang6, Xiang Liu, Ming Zhang, Yong-Feng Huang, and Chih-Kang Chou, "Neutrino Rocket Jet Model: An Explanation of High-velocity Pulsars and Their Spin-down Evolution", The Astrophysical Journal 931, 123 (2022), doi: 10.3847/1538-4357/ac6cdd.