Pulsars, magnetars and RRATs

Pulsars are fascinating objects which impact on many areas of astronomy and physics. Observed as celestial sources where the emission is in the form of a highly periodic train of pulses, they are generally believed to be rotating and highly magnetised neutron stars. This combination of strong magnetic fields and rapid rotation generates huge voltages across the star and in its magnetosphere, leading to the formation of ultra-relativistic beams of electrons and positrons and ultimately to beamed radiation at wavelengths that range from low radio frequencies to high-energy gamma rays. These beams sweep across the sky as the star rotates so an observer who happens to lie in the path of a beam sees one pulse per revolution of the star. Over 1,600 pulsars are now known and their pulse periods cover the range between 1.4 milliseconds and 12 seconds (see the ATNF Pulsar Catalogue at www.atnf.csiro.au/research/pulsar/psrcat).

Pulsars come in several different flavours, distinguished by different period ranges and/or pulse emission properties. Radio pulsars are divided into two main groups, millisecond pulsars and `normal' pulsars. Most millisecond pulsars have periods in the range 1 - 30 ms as their name suggests, but a principal distinguishing characteristic is a very slow spin-down rate, indicating a weak external magnetic field. It is believed that both the rapid spin and the low magnetic field for these pulsars result from accretion of mass and angular momentum from a binary companion star in an earlier accretion phase. The system may be visible as an X-ray pulsar during this phase. The vast majority of pulsars are normal (again as the name suggests!) with periods mostly in the range 30 ms to 8 s and large period derivatives, indicating ages of a few million years or less and relatively strong magnetic fields, typically of order 10¹² Gauss. A separate class of pulsars known as Anomalous X-ray Pulsars (AXPs) or Soft Gamma-ray Repeaters (SGRs) are generally detected at X-ray or low gamma-ray energies and have long periods in the range 5 - 12 s. These pulsars have a very rapid spin-down rate, indicating surface magnetic fields as large as 10¹⁵ Gauss. Despite the rapid spin-down of these "magnetars", the loss of rotational kinetic energy is insufficient to power the observed X-ray and gamma-ray emission. It is believed that decay of the super-strong magnetic fields is the main energy source powering the emission in these sources.

These different groups of pulsars are easily distinguished on a plot of pulsar period derivative (or spin-down rate) versus pulsar period (Figure 1). Millisecond pulsars are grouped in the lower left of the diagram, whereas AXPs are at the top right. About 75% of all millisecond pulsars are members of a binary system, that is, in orbit around another star, whereas only a few per cent of normal pulsars are binary. Young pulsars, similar to those associated with the Crab and Vela supernova remnants, live in the upper left of the diagram and evolve to the right, possibly along lines of constant magnetic field, joining the vast bulk of pulsars in the "pulsar island" centred around period 0.6 s and period derivative 10^-15.

Figure 1: Pulsar period derivative versus pulsar period for all known Galactic disk pulsars. Pulsars discovered at Parkes in major surveys using the 20-cm multibeam receiver, RRATs and AXPs are indicated. Binary pulsars are indicated by a circle around the symbol. Lines of constant characteristic age, t_c = P/(2), surface dipole magnetic field, B_s ~ (P)^1/2, and the spin-up line, which represents the minimum period that a pulsar can attain through accretion of mass from a companion star, are shown (click on image for larger version).

The Parkes multibeam pulsar surveys have been remarkably successful, finding over 750 (nearly half) of the known pulsars. These discoveries have not only provided a superb database for various studies including pulsar and binary evolution, the Galactic distribution of pulsars and interstellar magnetic fields, they have also revealed many individual objects which are of special interest. Standing out among these is the first-known double pulsar, PSR J0737-3039A/B, a remarkable system which is providing the most stringent tests yet of relativistic theories of gravitation and giving new insights into pulsar magnetospheric physics (e.g., McLaughlin et al. 2005, Kramer et al. 2006). But there have been many others, for example, PSR J1119-6127, just 1700 years old and lying in the centre of the supernova remnant G292.2-0.5 (Crawford et al. 2001), and PSR J1740-3052, a binary pulsar in an eccentric 230-day orbit with a companion star which has a mass of at least 11 times that of the Sun (Stairs et al. 2001) and which just might be a black hole.

The Parkes multibeam survey has been particularly successful at finding relatively young but long-period pulsars. As Figure 1 shows, ten or so pulsars like this, lying across the top of the distribution of normal pulsars, have been found where none were previously known. The large period derivatives of these pulsars imply that they have very strong magnetic fields. In fact, some lie in the part of the P diagram occupied by AXPs. Until recently, there was no direct connection between these high-B radio pulsars and the AXPs other than their similar location on the P diagram. Searches for X-ray emission from the long-period radio pulsars have revealed X-ray emission from some pulsars (e.g., Kaspi & McLaughlin 2005), but the emission is weak and has a thermal spectrum, quite unlike that from magnetars and probably originating directly from the hot neutron-star surface. Conversely, most searches for radio emission from magnetars have been unsuccessful. Pulsed emission at low frequencies, ~100 MHz, from several magnetars has been claimed by Russian astronomers (e.g., Malofeev et al. 2005), but other observers have failed to reproduce these results (e.g., Lorimer & Xilouris 2000).

This situation has changed dramatically with the recent discovery of strong pulsed radio emission from the magnetar PSR J1809-1943 (XTE 1810-197) by Camilo et al. (2006). Pulses were first detected using the 20-cm multibeam receiver on the Parkes radio telescope, but were subsequently detected at much higher frequencies using both Parkes and the Green Bank Telescope (GBT). Individual pulses can be seen at frequencies as high as 42 GHz, making this pulsar one of the strongest known at high radio frequencies and showing that it has an essentially flat radio spectrum. Interestingly, this pulsar was not detected in the Parkes multibeam survey although it was in the survey area, placing a limit on the flux density in 1997 - 1998 of less than 10% of the present value. This clearly shows that the pulsed emission is highly variable. The radio brightening of PSR J1809-1943 may have occurred in 2003 when there was a large increase in the X-ray emission. It is possible that similar variability may account for the non-confirm-ation of the Russian detections of other magnetars.

The Parkes multibeam pulsar survey also turned up another class of transient source, the so-called RRATs, an acronym standing for Rotating RAdio Transients. Reanalysis of the survey data with a detection algor-ithm sensitive to single dispersed pulses has revealed a total of eleven such sources (McLaughlin et al. 2006). RRATs are characterised by the emission of isolated pulses at intervals ranging from a few minutes to a few hours. Radio searches are plagued by various types of interference including impulsive bursts from electrical equipment. However, these interference bursts are undispersed and so can be separated from distant astrophysical sources by the dispersion that the signal suffers in propagating through the interstellar medium (Figure 2). Repeated detection of pulses from the same direction in the sky with the same dispersion confirms a real astrophysical source.

Figure 2: Isolated pulses from two RRATs, PSR J1443-60 and PSR J1819-1458. The lower part of each plot shows the dispersion of the pulse as a function of frequency and the upper plot shows the de-dispersed pulse profile (click on image for larger version). Image credit: M McLaughlin

An analysis of the arrival times of the pulses for each source showed that pairs of pulses were separated by intervals which were always a multiple of a given shorter interval. The smallest of these common factors ranged between 0.42 and 6.79 s for the different sources and is identified as the rotation period of an underlying neutron star. This identification was confirmed using standard pulsar-timing techniques which showed that, for at least three of the sources, the periods increase slowly with time in exactly the same way as normal pulsar periods. In one of these sources, which has a period of 4.26 s, the implied surface dipole magnetic field is 5 × 10¹³ G, placing it right up with the high-B pulsars and not too far from the AXPs on the P diagram (Figure 1). RRATs have much longer periods on average than normal pulsars, again suggesting a connection with the high-B pulsars. However, as with the AXPs, the emission properties are very different. In no case have two consecutive pulses been observed from a RRAT, showing that the emission is strong for a time less than the rotation period of the neutron star. The emission must be beamed as for a normal pulsar, otherwise the strict periodicity would not be observed, but why it is so short-lived is a mystery. Isolated and very strong individual pulses have been observed from the nearby pulsar B0656+14 (Weltevrede et al. 2006) which, had the pulsar been more distant, would have been identified as a RRAT. It is possible that RRATs are a terminal stage in the life of a pulsar or that they are a distinct class, possibly different from birth. Their highly intermittent nature makes them very hard to find and consequently there is likely to be a large undetected population of them in the Galaxy, maybe comparable to the population of ordinary pulsars.

These recent discoveries highlight the fascination of pulsars. Even after nearly 40 years, these incredibly precise clocks continue to surprise us with their diversity and unexpected properties. It is an interesting contrast that these most predictable of objects continue to be so unpredictable!

References

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R N Manchester
(Dick.Manchester@csiro.au)