Astronomers have solved the riddle of the millisecond pulsar, which alternates brightness modes regularly.
Illustration of the J1023 millisecond pulsar changing modes. Credit:ESO/M. Grain fairs
The answer lies in the sudden ejection of material from the pulsar in a short amount of time. Events that resemble the ejection of a cosmic cannonball.
Pulsar
This one object is a product or the end result of a dead massive star. So when a massive star explodes and leaves an incredibly dense center of the star, something ends up being bintang neutron and something happened black hole.
Among these neutron stars, there are those that rotate or spin very fast every second, even the rotation speed can reach milliseconds with radio emission that changes rhythmically or periodically.
Astronomers call it, pulsar or pulsating radio source or a pulsed radio source. When it was first discovered in 1967, Jocelyn Bell found radio signals repeating every 1.3 seconds. And astronomers had suspected that this object was a pulsating star.
However, no star can beat that fast. That’s why astronomers then concluded that this object is a neutron star that rotates very fast.
Pulsars also have very strong magnetic fields that flow particles along the magnetic poles accelerating the particles until they reach relativistic speeds. This is what causes the formation of beams of light emanating from both poles. As the pulsar rotates, the beam of light sweeps across the cosmos much like the light from a lighthouse sweeps across its surroundings.
When a beam of light from a pulsar sweeps across Earth or is within the observer’s field of view, astronomers can detect the presence of this object. But, as the light recedes and dissipates as the pulsar rotates, to observers on Earth, the pulsar’s light appears to be pulsing.
Millisecond Pulsars
In the 1980s, astronomers discovered a pulsar that spins extraordinarily fast, with a spin period of up to milliseconds, or one pulsar’s rotation takes only a few revolutions in a thousandth of a second. We know it as a millisecond pulsar.
So fast, there is a pulsar that rotates 716 times per second!
When the millisecond pulsar was first observed, it certainly puzzled astronomers. The pulsar’s mechanism converts rotational kinetic energy into radiation. And along with the radiation of this energy over time, the rotation should slow down. When a new pulsar is formed, this object rotates very fast, but as it gets older, aka over time, the pulsar’s rotation will slow down by a few seconds. However, in the 1980s, astronomers discovered a pulsar that was old and supposed to be slowing down was still spinning very fast and spinning in milliseconds!
The mystery of this unusually fast spin is that the pulsar is part of a multiple star system. When it becomes part of a multiple star system, the pulsar will accrete or pull material from its partner star. As a result, the old pulsar undergoes a rejuvenation process that accelerates its rotation by a short period of only a few milliseconds!
Transition Millisecond Pulsars
Illustration of the millisecond pulsar J1023. Credit: ESA
In 2013, astronomers discovered for the first time traces of millisecond pulsars with strange behavior in the globular cluster M28. The pulsar alternates from emitting X-rays to radio emitting.
In 2019, astronomers discovered a pulsar of the same type in the constellation Sextans. PSR J1023+0038 or J1023.
Pulsar J1023 is 4500 light years away and is part of a multiple star system. For decades, pulsar J1023 has been actively pulling material from its companion star and creating an accretion disk around the pulsar. Over time, the material in the accretion disk also experiences a pull which results in the material in the disk being attracted to the pulsar.
Since the process of matter accumulation began, the pulsar beam of light that swept through the cosmos disappeared. The pulsar thus has two alternating modes. There are times when the pulsar is in high or bright mode when it emits bright X-rays, and in low or dim mode when the pulsar dims to X-ray frequencies and emits radio light.
The Pulsar J1023 will be in each mode for a few seconds or minutes and then within seconds it is switching modes. This mode shift is a puzzle for astronomers.
This shift appears to be due to interactions between the pulsar’s wind, the flow of high-energy particles away from the pulsar, and matter flowing toward the pulsar.
When the pulsar is in low mode, material flowing towards the pulsar is ejected in thin bursts perpendicular to the accretion disk.
Meanwhile, gradually, the accumulated material approaching the pulsar collides with the pulsar’s wind. As a result, there is heating of the material. At this time, the system is in bright mode and glows in X-ray, ultraviolet, and visible light. However, this lump of hot gas can also be lost in the end by jet bursts. When the hot material on the disk is reduced, the system dims and goes into low mode.
Observation
The international team of astronomers managed to unravel the riddle of this millisecond pulsar mode change from observations with 12 telescopes, both telescopes on Earth and space telescopes. Among them are the VLT (Very Large Telescope), NTT (New Technology Telescope), and the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope. Observations over two nights in June 2021 showed that the pulsar J1023 switched modes 280 times from bright to dim.
In the future, the presence of the ELT (Extremely Large Telescope) could become an important instrument for uncovering information regarding the influence of the pulsar’s changing mode behavior on the abundance of matter, dynamics, and the energetics of the material flowing around the pulsar.
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2023-08-30 12:00:00
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