Discovered: The Youngest And Brightest Fast-Spinning Pulsar Ever Known; Contradicts Known Theory

A new batch of unusual stellar remnants (neutron stars) has been discovered and one of these challenges the currently held view of astronomy about pulsars. NASA’s Fermi Gamma Ray Telescope has discovered nine pulsars at one go, taking its discovery count past 100. Eight are simply the garden variety pulsar, but dimmer than the ones discovered so far, while one is an interesting millisecond pulsar a superfast spinning pulsar. The fact that this is the brightest and youngest millisecond pulsar ever discovered adds to the excitement.

The discovered pulsars. The millisecond one is green. (Courtesy: NASA/Fermi/Lat Collaboration)

What is a Pulsar?

“Normal” Pulsars

A pulsar is a neutron star, which emits radiation from its polar regions. A neutron star is a very compact object, resulting from the collapse of a very massive star (whose core mass is about 1.4 to 2.5 solar masses). These are extremely dense objects made up of only neutrons. Imagine shrinking the entire sun to a the radius of a few blocks in a city (say 8-10 km) and you’ll get the picture of how extreme these objects really are.

How a pulsar radiates

When these dense objects convert a part of their rotational energy into radiation, they become pulsars. The pulsation happens through a small angle opening at the two poles of the spherical object. When one of these point towards Earth, we see a pulse. Since the rotation axis and the radiation axis (which is the magnetic axis) are not aligned perfectly with each other, the pulsar acts like a lighthouse. We thus don’t get a continuous pulse of radiation, but periodic flashes. Pulsars are extremely periodic in the radiation and, thus, they make perfect timekeepers (often being more accurate than atomic clocks).

The pulsar at the Crab Nebula. The Crab Pulsar is the most famous pulsar known, attached to the 1054 supernova event. Note the jets of radiation given out! The radiation is in X-Ray Band.(Courtesy: NASA/Chandra X-Ray Telescope)

Millisecond Pulsars

When a neutron star accretes matter (generally from a companion star), it also gains a lot of angular momentum, making it spin faster. Thus, if the neutron star accretes matter, they tend to spin up. The period of rotation might reduce considerably over millions of years. Neutron stars have variable rates of rotation. The typical ones go from as slow as 5 revolutions per minute to as high as 4000 revolutions per minute.

How a millisecond pulsar forms by accreting matter from a companion star

Sometimes, the spin up of a pulsar can be so high that it can rotate more than 42,000 times a minute (or 700 times a second!!) These have a pulse period of about 1.4 milliseconds (1.4 thousandths of a second!) and are called millisecond pulsars for obvious reasons. It should be quite clear that the pulsar spin up takes a long time over several hundreds of millions of years.

What’s wrong with this new discovery?

Here’s where the problem lies! The newest millisecond pulsar that Fermi has discovered is just 25 million years old, a baby in cosmic terms. If our picture of the pulsar formation is correct, we cannot have such a young pulsar. Further, the pulsar is the brightest one ever observed. Named PSRJ1823-3021A, as per catalogue, the pulsar rotates 183.8 times a second and is 27,000 light years away from Earth. It has incredible luminosity in the gamma ray spectrum! At this moment, no one knows what to do with this one.

Along with this wonderful find, Fermi has also discovered eight other dim pulsars in the vicinity. The study was presented at the Max Planck Institute yesterday i.e. on the 3rd of November ’11.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.