Pulsars and Their Types
This article was written by Shreya Umesh Prabhu, fourth year BSMS Physics undergraduate at IISER Thiruvananthapuram.
Neutron stars are one of the three endpoints of stellar evolution, the other 2 being white dwarfs and black holes. Some neutron stars emit radiation along their magnetic axis. When the magnetic axis and the rotation axis of such neutron stars are misaligned, the observer can detect the radio emissions from the star as the light cone sweeps past across the earth (see animation). Such stars are called pulsars.
Link for animation: https://drive.google.com/file/d/15Uflpykqj2tk11qXW91_n271frghyEe5/view?usp=sharing (This is an animation of a 1.3-second pulsar, including an accompanying pulse profile.)
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The time between two pulses detected is called the pulsar period, which is stable and can be accurately determined. The pulsars, therefore, act as celestial clocks. The pulsars are powered by the rotational kinetic energy, and as the star loses its energy for relativistic particles and radiation, pulsars eventually slow down. Depending on the period and the spin-down rate, pulsars can be categorized into various types. The graph of periods vs. period derivatives (Fig), known as the P-Pdot diagram, shows the distribution of pulsars in different areas of the plot. The central part of the diagram contains normal pulsars with a time period greater than 30 milliseconds. The bottom left of this diagram shows the pulsars with a time period of 1ms to 30ms and a very slow spin-down rate; these are called the millisecond pulsars (MSPs). The top right corner of the diagram contains magnetars (Anomalous X-ray pulsars or soft gamma repeater) with magnetic fields up to 1013 to 1015G and a time period of 2–12 sec.
Millisecond pulsars (MSPs) are of particular interest to astronomers due to their very stable time periods. These are known to have very different properties and evolutionary histories than the normal pulsars. MSPs are usually accompanied by a white dwarf or a neutron star or, sometimes, another pulsar in a binary system. It is proposed that MSPs are formed when a neutron star accreting the matter from its binary companion is spun up. Therefore, these are also known as recycled pulsars. Some of the MSPs are also found isolated. Theories propose that the isolated MSPs result when the binary companion is completely ablated from the system. Spider MSPs (redbacks and black widows) are the MSPs with very low mass companions, and these are thought to be a missing link for isolated MSPs.
References:
https://www.atnf.csiro.au/outreach/education/everyone/pulsars/index.html
