Pulsars and neutron stars/History of applications of pulsars

Introduction
Within a year of the first pulsar discovery, Counselman & Shapiro (1968) were able to state that pulsars:

"... can be used to test general relativity, to study the solar corona, and to determine the earth's orbit and ephemeris time. The vector positions and transverse velocities of pulsars can be measured with radio interferometers; in combination with pulse-arrival-time data, the distance determination will yield the average interstellar electron density."

Even though this is remarkably complete, we show in this section that even more applications of pulsar astronomy are now known. Most of the pulsar applications are possible because the pulses are so regular that they can be used like the tick of a clock. The pulsar signal is originally emitted from the pulsar, but then the pulses travel through the interstellar medium where they get dispersed and scattered. The signal is affected by the Galactic magnetic fields. The pulses pass through the Solar system and there they are affected by the solar wind and the gravitational field of the Sun. The pulses are detected using space-based telescopes or travel through the Earth's atmosphere to be detected using ground-based detectors. By studying pulses over long time spans or with a wide range of different telescopes it is possible to disentangle many of the phenomena that have affected the shape and arrival times of the pulses. In turn, this allows pulsar astronomers to study the pulsar, the interstellar medium, the Galactic magnetic fields and the solar wind.

Relativity, theories of gravity and gravitational waves
The first mention of using a pulsar to test a general relativistic effect was published by Hoffmann (1968). Hoffmann’s work considered how the orbital motion of the Earth and the gravitational field of the Sun would affect pulsar observations. However, it was not until the discovery, in 1975, of the first binary system, PSR B1913+16, that it became clear of the huge importance that pulsar astronomy would play in studies of gravity theories.



The orbit for most of the binary pulsars can be well described using Keplerian orbital mechanics. However, there are a handful of systems, including the first pulsar binary system, for which the Keplerian formalism is not sufficient. For such pulsars, extra parameters are needed in order to describe the orbit. By measuring and studying these parameters it is possible not only to model the orbit, but also to test different theories of gravity. To date, using the original binary system and a more recent discovery (J0737-3039A/B ) no departure from the predictions of general relativity has been found.

Pulsar-white dwarfs systems are generally not as relativistic as the double neutron star systems. However, some theories of gravitation predict effects that depend upon the difference between the two masses of the orbiting objects (see e.g., Thibault & Esposito-Farese 1996). Freire et al. (2012) provided an example for testing theories using a long pulsar timing data set of a pulsar-white dwarf system (PSR J1738+0333).

We now know of many pulsars in orbit with another neutron star or white dwarf. One of the key goals for modern pulsar research is to find the first pulsar in orbit around a black hole. The closest, supermassive black hole is at the centre of our Galaxy, Sag A*. The prospects for probing the spacetime of Sgr A* with pulsars was described by Liu et al. (2012). However, it is also hoped that a pulsar will be discovered in orbit around a stellar mass black hole. Laguna & Wolsczan (1997) and Wex & Kopeikin (1999) described the analysis methods and type of research possible if such a system were found.

From its name, one would expect Newton's gravitational constant, G, to actually be constant. But that hasn't stopped astronomers attempting to place bounds on its possible variability using pulsar observations. Damour, Gibbons & Taylor (1988) showed how bounds could be made using binary pulsars. Thorsett (1996) showed how measurements of the masses of young and old neutron stars in pulsar binaries limit variations of the gravitational constant.

The potential use of pulsars to search for and study gravitational waves was described by Detweiler (1979) (see also Sazhin 1978). To date, no detection has been made, but many attempts have been made to search for the gravitational waves expected from supermassive binary black holes (e.g., Wyithe & Loeb 2003 or van Haasteren & Levin 2010), from cosmic strings (e.g., Caldwell, Battye & Shellard 1996) or from the inflationary era (e.g., Krauss 1985). The non-detections have led to a huge literature on upper bounds that can be placed on their existence and on the implications of such bounds. Jenet et al. (2004) carried out what is arguably the first gravitational wave astrophysics by using the non-detection of gravitational waves to rule out a postulated binary black hole system. Shannon et al. (2013) described how gravitational-wave limits from pulsar timing can constrain supermassive black hole evolution. Sampson, Cornish & McWilliams (2015) demonstrated how pulsar data can constrain the solution to the last parsec problem. Of course, the pulsar community is hoping for an actual gravitational wave detection. If a clear detection is made then it will be possible to test the predictions of general relativity and to probe supermassive binary black hole systems in our Universe.

Searches for dark matter
One of the key goals of modern-day astrophysics is to understand dark matter. The existence of any unmodelled matter along the line of sight between the pulsar and the Earth will affect the measured pulse arrival times. Various constraints on the existence of dark matter have therefore been placed using pulsar observations (e.g., Larachenkova & Doroshenko 1995, Siegel, Hertzberg & Fry 2007, Khmelnitsky, Rubakov 2014 and Clark, Lewis & Scott 2015).

Similarly Hosokawa, Ohnishi & Fukushima (1999) considered how the motion of intervening stars and MACHOs would affect pulsar timing residuals.

Pulsar-based time standards
Let's return back to Earth. Clocks and time standards are essential in many aspects of our lives - from satellite navigation to telecommunication systems. Around 70 time laboratories around the world currently provide input into our international time standard. We have noted above that the pulsar signal is, in some ways, analogous to the tick of a clock. Could pulsar observations be used to look for irregularities in terrestrial time standards or even combined with the Earth based clocks to provide an even more stable time standard? Guinot & Petit (1991) and Petit & Tavella (1996) discussed the challenges in making such a pulsar-based time standard. The most stable pulsars are needed in order to produce a pulsar-based time standard that is of comparable stability to atomic time standards. The first such pulsar-based time scale was presented by Hobbs et al. (2012). They reported on the possibility of some deviations between the pulsar and terrestrial time standards. Work is still progressing to confirm or deny such discrepancies.

The Galactic magnetic field
The pulsed signal is often highly polarized. Smith (1968) noted that the observed Faraday rotation of the plane-polarized radio waves from a pulsar can be explained as a combination of ionospheric and interstellar effects. The observed rotation depends upon the magnetic field strength and so observations of Faraday rotation provide a means to study magnetic fields in our Galaxy. Ekers et al. (1969) used the Vela pulsar to determine the longitudinal component of the Galactic magnetic field in the Orion arm. Han, Manchester & Qiao (1999) identified a likely magnetic field reversal in the Perseus arm of our Galaxy. Han et al. (2006) used a large number of pulsar observations to study the Galactic magnetic field in detail.

The interstellar medium
A large-scale study of interstellar medium electron density irregularities using pulsars was presented by Armstrong, Cordes & Rickett (1981). For most pulsars, a property, the dispersion measure, is known. The dispersion measure is a measurement of the integrated electron density along the line-of-sight from the pulsar to the Earth. If a pulsar’s distance is known, then these measurements can be used to measure the electron density in that line-of-sight. Such measurements can be used to develop a model of the electron density (see e.g., Cordes et al. 1991), which, in turn, can be used to estimate pulsar distances. The first major model was presented by Taylor & Cordes (1993). More recently, this was updated by Cordes & Lazio (2002) who developed a new model for the Galactic distribution of free electrons in the galaxy.

Pulsar dispersion measures are not constant. The dispersion measures for many pulsars slightly change over time. Phillips & Wolszczn (1991) described how observations of such variability could be used to study the properties of the interstellar medium.

Fiedler et al. (1987) provided the first observation of an extreme scattering event by observing an extra-Galactic source. The event is thought to occur because of a region of high density plasma passing the line-of-sight to the source being observed. Maitia, Lestrade & Cognard (2003) detected a 3-year long extreme scattering event in the direction of a millisecond pulsar. More events have been recently reported by Coles et al. (2015).

Direct evidence of discrete scattering structures in the interstellar medium was provided by Stinebring et al. (2001). The implications of such scattering structures have been described by

Walker et al. (2004).

Extra-solar planets
The first, confirmed extra-Solar planets were detected in 1992 using pulsar observations (Wolsczan & Frail 1992).

The Solar system
Pulsar observations have been used to probe many aspects of our solar system.

The Solar wind
Hollweg (1968) first described how pulsar observations can be used to study the solar corona. They noted that fluctuations in the pulse arrival times would be observed because of variations in the coronal electron density. The solar corona was studied in more detail by an occultation of NP 0532 by Counselman & Rankin (1972). More recent studies have considered both the electron density and the magnetic field of the solar corona (Ord, Johnston & Sarkissian 2007 and Smirnova, Chashei & Shishov 2009).

The Earth
An investigation into the Earth’s ionosphere using pulsar observations was suggested by Ulyanov et al. (2013).

Planetary masses and unknown objects
Ephemerides that provide the positions of solar system objects as a function of time have been developed. These ephemeris make use of the best available data on e.g., planetary motions and masses along with input from lunar laser ranging experiments, known times of eclipses and many more data sets. As described by Champion et al. (XX), pulsar observations can be used to check the predictions of the ephemerides. For known planets it is also possible to use pulsars to improve their mass determinations. Pulsars are currently being used to search for currently unknown objects in our solar system.

Navigation
Pulsar astronomers usually assume that the position of the observatory is known. However, by observing a sufficient number of pulsars it is possible to locate the telescope (here the pulsars are being used like a Galactic global-positioning-system, GPS). Bernhardt et al. (2011) are one of the many groups currently studying autonomous spacecraft navigation using pulsars.

Reference frames and calibration
Pulsar positions can be determined using the pulsar timing method (this is based on the ecliptic frame). Pulsar positions can also be determined using Very-long-baseline interferometry (VLBI) which uses the International Celestial Reference Frame. By comparing the positions determined using the two methods it is possible to determine the angles representing the differences between the two reference frames. Such work was presented by Bartel et al. (1996).