We can begin by knowing what pulsars are.
Pulsars are rotating neutron stars observed to have radiation pulses at very regular intervals
that typically range from milliseconds to seconds.
A binary pulsar has an additional feature of a binary companion, which is often a white
dwarf or a neutron star.
So in simple terms, a binary pulsar is a pulsar orbiting another star.
Binary pulsars are one of the very few celestial objects that allow scientists to test general
relativity because of the intense gravitational fields in their vicinity.
Although it is normally difficult or impossible to observe the binary companion directly,
its presence can be inferred from the timing of the pulsar’s pulses, which can be measured
with extreme precision by radio telescopes.
Binary pulsars form when a neutron star forms in a binary system.
If the system remains bound after the supernova explosion, a normal pulsar in an eccentric
orbit around a main-sequence star (the secondary) exists for a short period.
If the secondary expands at the end of its main-sequence lifetime, it can transfer mass
to the neutron star companion, causing the system to remain as an X-ray binary for some
time.
One of two things can happen when mass transfer stops.
The secondary will either remain a white dwarf indefinitely, resulting in a recycled binary
pulsar with a white dwarf companion, or it will explode, resulting in a neutron binary
star.
In special cases, the recycled pulsar might ‘attack’ its companion, either by tidal
forces or radiation, leaving behind a black widow binary pulsar.
These systems normally consist of a millisecond pulsar and a very low-mass companion.
Joseph Taylor and Russell Hulse found the first binary pulsar at Arecibo Observatory
in 1974.
The binary pulsar is known as PSR 1913 + 16.
They demonstrated that it was losing energy through gravitational radiation at the rate
predicted by Albert Einstein’s theory of general relativity.
Their discovery earned them the Nobel Prize in Physics in 1993.
Apart from the Arecibo dish, the pulsar is so faint that it is difficult to make meaningful
observations from any radio telescope on the planet.
PSR 1913 + 16 was significant in that it provided the first method of detecting gravitational
waves.
The immense interacting gravitational fields of the two stars were altering the regularity
of the radio pulses, and by monitoring and analyzing their fluctuations, Taylor and Hulse
discovered that the stars were rotating progressively faster around each other in an increasingly
tight orbit.
According to Einstein’s theory of general relativity, two neutron stars orbiting a shared
center of mass emit gravitational waves, which would sweep away orbital energy and force
the two stars to move closer together and decrease their orbital period.
This orbital decay is thought to occur because the system is losing energy in the form of
gravitational waves, resulting in a reduction of orbital distance between the two stars.
This discovery provided the first experimental evidence for Albert Einstein’s prediction
of gravitational waves in his general theory of relativity.
The PSR B1913+16 binary pulsar study also led to the first accurate measurement of neutron
star masses using relativistic time effects.
When two bodies are close together, the gravitational field becomes stronger, time slows down, and
the time between pulses or ticks is lengthened.
The pulsar clock then regains time as it travels more slowly through the weakest region of
the field.
Time dilation, a specific relativistic phenomenon, similarly works around the orbit.
This relativistic time delay is the difference between what one would expect to see if the
pulsar traveled at a constant distance and speed around its companion in a circular orbit
and what is actually observed.
Another interesting process seen in the binary pulsar is a slow wobble of its spin axis.
The wobble, known as “geodetic spin precession,” is caused by the curvature of spacetime induced
by the companion.
Because of the wobble, astronomers can see different parts of the pulsar’s “lighthouse
beam” then they would normally see.
These data will yield a two-dimensional map of the beam until it precesses away from Earth,
which will most likely happen in a few decades.
Binary pulsar astronomy remains an extremely active field in modern astrophysics, and the
next decade will surely bring new discoveries as well as surprises from currently known
objects.
If you liked this video, please consider subscribing and click the notification bell so you do
not miss any upcoming videos.