Since the discovery of pulsars in 1967 and the discovery of the first millisecond pulsar (a pulsar that rotates hundredths of times in a second) in 1982, pulsars have been used to study a wide variety of physical and astrophysical problems. The applications range from gravitational physics and cosmology to neutron star seismology. In many areas, pulsars provide the only tool accessible to us to study physical environments under extreme conditions.

Pulsars can be used not only to study neutron stars properties but also as tools for many physical and astrophysical studies.

• Probing the interstellar medium: it is well known that, because of the presence in the interstellar medium of free-electrons, a wide band radio signal is dispersed: low frequencies will arrive at the observer after higer ones.
  

  The time delay depends on the electronic column density between the radio source and the observer. For a pulsed signal such delay is measurable and gives estimates on the free electronic density for those pulsars wich have indipendently determined distances.

• Searching for extrasolar planets: if a pulsar is in orbit with another body the distance between the radio source and the Earth changes along the orbit. Diffrent pulses hence cover diffrent distances and arrive at the observer at different times. In particular an observer measures a sinusiodal variation in the times of arrival (TOA) of the pulses of a pulsar belonging to a binary sistem. Given the high stability of the pulsar signals (in particular of millisecond pulsars) and the accuracy of the TOAs measurements, even the presence of a planetary sized objects orbiting a radio pulsar is detectable.

  

  In 1992 Professor Alex Wolszczan discovered the first extrasolar planet during a pulsar search with the Arecibo radiotelescope. For this discovery professor Wolszczan has also been honored by the country of Poland in having his likeness featured on a stamp celebrating the past millennium!

• Detecting gravitational waves: the aforementioned stability of millisecond pulsars enable us in principle to detect any kind of distortion introduced in the highly predictable times of arrival of these objects. The passage of a gravitational wave (produced e.g. by the coalescence of two neutron stars or two black holes) would distort the space-time and produce a change in the path followed by the radio waves from a pulsar and hence a change in its TOAs.

  

  To catch such a large scale signal (and to distinguish it's signature from timing noise or other sources of uncertainties) it is necessary to monitor the TOAs of many millisecond pulsars in different points of the sky, creating a sort of timing array.

• Testing general relativity: a pulsar included in a binary system containing two compact bodies (e.g. two neutron stars) is a perfect laboratory to test the predictions of Einstein's general relativity. The presence of a strong gravitational field, indeed, affects the shape of the space-time near the binary system and hence, as mentioned before, the lengh of the path followed by the radio waves and by consequence the times of arrival of the pulses. From the analysis of the TOAs is possible to measure not just the spin (period and period derivative) and keplerian orbital parameters (binary period, projected semimajor axis, eccentricity, time of passage at the periastron and longitude of periastron) but also up to 5 post-keplerian parameters connected with relativistic effects: the periastron advance, the orbital decay due to loss of energy in the form of gravitational waves, the parameter gamma, that measures the gravitational redshift and the time dilatation and the range and shape of the Shapiro delay produced by the deformations of the space-time in the pulsar's surroundings. These parameters are connected with the orbital parameters of the binary system and with the masses of the two stars. Measuring two post-keplerian (pk) parameters hence allows to determine univoquely and separately the masses of the two compact objects. Measuring others pk parameters opens the possibility of testing the prediction of general relativity.

  

  Such a test has been made for the first time by professor Russel Hulse and professor Joseph Taylor of the Princeton University (awarded with the Nobel prize for Physics in 1993) by measuring the periastron advance of the double neutron star system (DNS) containing the pulsar PSR B1913+16.

  The discovery of radio pulsars in DNSs has allowed to give an estimate of the coalescence rate of such systems and hence of the possibility for gravitational wave detectors, as the italo-french VIRGO, to detect gravity wave bursts produced in this kind of event.

• Probing the gas and potential well of globular clusters: a Globular Cluster (GC) is a great ball of stars, densely packed, which contains hundred of thousands of individual stars. In a GC nucleus the stars are so close that in a cubic parsec can be more than 1000 stars (for comparison, in Sun's vicinity there is no stars in a cubic parsec). Because of the high density, close encounters between stars aren't rare and lead to the formation of binary system, where millisecond (or 'recycled') pulsars are believed to form. Indeed roughly 40% of recycled pulsars are found in GCs. Because of the highly stable temporal behaviour of these objects, the acceleration along the line of sight produced by the gravitational potential well of Globular Clusters on these objects can be measured.

  

  It is then possible to dynamically infer the ratio between the light emitted by globular cluster's stars and the GC's total mass (given by visible stars and non luminous matter - central black holes, for instance) that causes the pulsar's motion thowards the center of the Cluster. The study of the more than 20 millisecond pulsars found in the Globular Cluster 47 Tucanae have also allowed the first detection of gas in a GC.

Last update: 13-Jan-2006