4. What are pulsars, neutron stars, and quark stars?

Pulsars were first discovered in 1967 by Anthony Hewish and Jocelyn Bell [18,19,20,21] using the radio telescope shown here:

**Figure 4.1:** The Cambridge interplanetary scintillation telescope ("the 4-acre array"). The telescope was designed and constructed by Hewish and Bell to measure the angular sizes of distant radio sources, based on how much they twinkle as they shine through the solar wind. Pulsars were discovered serendipitously during this work. (Photo courtesy Graham Woan)

Pulsars are very small, very dense, rapidly-spinning stars that form when the pressure in the core of a large massive star can no longer resist gravity and it collapses. In the process, the outer layers of the star are ripped apart in a supernova explosion. The remains of a supernova that occurred in 1054 AD, called the Crab Nebula, are shown in this photo. After the initial discovery of pulsars, a pulsar was also found in the Crab Nebula and was named after it.

**Figure 4.2:** Crab Nebula and Pulsar: N.A.Sharp/NOAO/AURA/NSF [22]. The inset sequence of images show the pulsar's brightness varying millisecond by millisecond as it completes a full rotation every 33.367 milliseconds.

Pulsars have masses comparable to our Sun, but radii of only around 10km (the size of a city). This corresponds to a matter density of a billion tons per teaspoon.

Such ultra-dense stars are called neutron stars because at such high density (comparable to that of an atomic nucleus) matter consists mostly of neutrons. The cores of neutron stars are even denser than an atomic nucleus, and may also contain hyperons, which are like neutrons and protons but heavier. At even higher density matter dissolves into a mixture of "up", "down", and "strange" quarks [23], which are the constituents of neutrons, protons, and hyperons. If the density gets high enough near the center, some of these stars might actually have quark cores (and properly be called hybrid stars). There might also be stars made entirely of strange quarks, though we don't know enough about the properties of quarks to be sure of this. Most of the time we will just use the term "neutron star" to be concise, but keep in mind that these more exotic objects are also possible.

The reason some neutron stars are observed as pulsars is that these stars have intense magnetic fields and emit beams of electromagnetic radiation from their magnetic poles. The beams sweep through space as the star rotates, forming a cosmic lighthouse. Imagine the beam from a lighthouse sweeping over you with every rotation of the lighthouse search-light. You would see one pulse of light with each rotation. Similarly, if the beam of electromagnetic radiation from a neutron star sweeps over the Earth, we observe a radio pulse each time the star rotates. Thus, objects observed in this way are called pulsars. Note that at times the radio pulses are scattered and dispersed by interstellar gas and dust, so they can appear and disappear unpredictably. See section 3.2.1 of [24] for details.

Since the initial discovery by Hewish and Bell, several thousand pulsars have been detected electromagnetically using ground-based radio telescopes. These known pulsars have spin rates varying from once every few seconds to hundreds of times per second. The fastest observed spin rate is 642 times per second! It is believed that our Galaxy contains a hundred thousand or more rapidly-spinning neutron stars, most of which are (as yet) undetected. This is because either the pulsar's beam misses the Earth, the radio pulses are smeared out by cosmic clouds or dust, or the spinning neutron star does not have an intense enough magnetic field to produce a beam.

Einstein@Home is one of the most powerful techniques available to search for gravitational waves from previously undetected dense spinning stars. Because gravitational waves are not beamed (they are emitted in all directions, though non-uniformly) and would not arrive in pulses, and because they might not be associated with any detectable radio pulses, one could invent new names for such sources. For example one could call them "gravitational-wave pulsars", "GWENs" (for Gravitational-Wave Emitting Neutron Stars), or "gravitars". But to keep things as simple as possible, in this document we will use the term "pulsars" for such sources.

The detection of gravitational waves from pulsars would provide a new means to discover and locate neutron stars, and might eventually provide unique insights into the nature of matter at high densities.

Please see references [11,12,24,13] for more information about pulsars, neutron stars, and quark stars.

Einstein@Home S3 Analysis Summary	Last Revised: 2005.09.11 16:22:17 UTC
Copyright © 2005 Bruce Allen for the LIGO Scientific Collaboration	Document version: 1.97