Why We Search

Despite the fact that we now know of more than 2000 pulsars, there are still many open questions in pulsar astronomy. The goal of all pulsar searches is to provide the means by which we can answer these questions. Below, I present a small selection of open²⁰ questions in pulsar astronomy.

19Survey pointings are usually tiled such that there is a maximum sensitivity drop of 50% for a pulsar that falls between two adjacent beams.

20Or, perhaps, ajar.

Introduction 25 The Galactic distribution of pulsars

With every new population study produced (see e.g. Johnston and Bailes, 1991; Lorimer et al., 2006; Manchester et al., 1978), those who determine the distributions must extrap-olate to contend with various selection biases present in pulsar observations. Increased frequency- and time-resolution backends, in combination with new or improved receivers, allow each successive generation of pulsar surveys to reduce the effects of many of these selection biases. As such, current pulsar surveys are probing the Galaxy to greater dis-tances and with greater sensitivity than has ever been achieved before (see e.g. Chapter 4). The discoveries from these surveys will be invaluable in producing accurate models of the Galactic pulsar distribution. Such models have a plethora of uses, from enabling predictions of the binary-neutron-star merger rate (an essential part of understanding the observable event rates for gravitational wave detectors, see e.g. Abbott et al., 2008;

Kalogera et al., 2004), to the examination of the spatial velocity of neutron stars (which has implications for the prevalence of supernovae types and their asymmetries, see e.g.

Hobbs et al., 2005).

The origin of magnetars

The boundary between the magnetars and the high B-field normal pulsars (see Figure 1.4) is not fully understood (see e.g. Lin and Zhang, 2004; Pivovaroff et al., 2000). The discovery of a magnetar showing pulsed radio emission in 2006 (Camilo et al., 2006), brought several interesting questions to the fore, namely: Are all magnetars radio emit-ters (and if not, what is the trigger mechanism for the production of radio emission) and do all high B-field pulsars exhibit magnetar-like behaviour? The small number of known magnetars makes these questions particularly difficult to answer. However, the ability of new pulsar surveys to detect objects such as the radio emitting magnetar J1622−4950 (Levin et al., 2010), will bring us closer to understanding whether magnetars are simply an evolutionary stage for radio pulsars or something else entirely. An expansion of the known magnetar population will also allow for detailed study of plasma under some of the most extreme conditions in the known universe (see e.g. Harding and Lai, 2006; Heyl and Hernquist, 2005).

The evolution of MSPs

Despite almost 15% of MSPs not belonging to binary systems, the formation mechanism for isolated MSPs is still not fully understood. The predominant theory, is that the particle wind from pulsars in tight binary orbits (P_b <24 hrs) with low mass companions (Mc < 0.05 M)²¹, the so-called ‘Black Widow’ systems (see e.g. Roberts, 2011), can evaporate their binary companions through ablation (see e.g. Bhattacharya and van den

21Such systems should constitute a significant fraction of pulsar binaries (Podsiadlowski et al., 2002).

Introduction 26 Heuvel, 1991). Prior to 2009, only three Black Widow binaries were known in the Galactic disk, with observations of these systems suggesting mass-loss rates much smaller than would be required for evaporation of the companion within a Hubble time (see e.g.

Stappers et al., 1996). While this result would seemingly rule out evaporation as a formation channel for isolated MSPs, the low-number statistics involved restricts any strong conclusions from being drawn from this result. The recent discovery of ten Black Widow systems (Ray et al., 2012b), of which PSR J1745+1017 (see Section 3.6) is one, may provide the means by which we can examine companion evaporation in more detail.

There are several alternatives to the evaporation theory presented above. For example, it has been suggested that isolated MSPs may be a possible final product of hierarchical triple systems (Freire et al., 2011; Portegies Zwart et al., 2011) or could even be formed through the core collapse of a white dwarf in a Type Ia supernova (Chen et al., 2010).

By discovering many isolated and binary MSPs, current and future pulsar surveys will provide the missing pieces in the isolated MSPs formation puzzle and will, undoubtedly, provide new insight in to all aspects of binary and stellar evolution.

Neutron star characteristics

The discovery of pulsars at the edges of the distribution, whether extremely massive, light, fast, slow, magnetic, young or energetic, is a key goal of current pulsar surveys.

These systems are important, as they allow us to rule out certain pulsar and neutron star models. An example of this can be seen in the many equation-of-state models ruled out through mass measurement of PSR J1614-2230 (see Figure 3 of Demorest et al., 2010). Similarly, the discovery of the fastest spinning pulsar, J1748-2446ad (Hessels et al., 2006), seemingly rules out neutron star radii of greater than 16 km, for a 2-M pulsar²². As the orbit of J1748-2446ad is almost perfectly circular and the pulsar’s emission is very faint, it is, unfortunately, not possible to constrain the mass of the pulsar through pulsar timing. The hypothetical discovery of a similar system to J1748-2446ad, in which both the pulsar’s mass and radius could be tightly constrained, would give the strongest yet indication as to the mass-radius relation for neutron stars and, thus, teach us much about the behaviour of supra-nuclear matter.

Pulsar-black hole binary systems

Known throughout the pulsar searching community as simply the ‘Holy Grail’, the discovery of a pulsar-black hole (PSR-BH) system (see e.g. Narayan et al., 1991) is, arguably, the most exciting and enticing prospect of current and future pulsar surveys.

These systems are thought to be formed through several channels (see e.g. Bethe and

22For a pulsar mass of 1.4M, this drops to 14.4 km.

Introduction 27 Brown, 1998; Pfahl et al., 2005; Voss and Tauris, 2003), with the largest number of PSR-BH systems expected to be composed of a normal pulsar in a tight binary with a low-mass black hole. The formation rate of such systems is still a matter of debate (see e.g. Bethe and Brown, 1998; Pfahl et al., 2005; Sipior et al., 2004), with the more optimistic rates suggesting one PSR-BH system for every 1,500-10,000 field pulsars (Sipior et al., 2004).

A different kind of PSR-BH system may be found by looking for a pulsar orbiting the black hole in the centre of our Galaxy, Sagittarius A* (Sgr A*). For example, Pfahl and Loeb (2004) suggest a population of∼1000 pulsars in<100 year orbits around Sgr A*, with 1-10 of these expected to be detectable with current radio telescopes. Although pulsar searches towards the Galactic centre are limited by scatter broadening and high background temperatures, several such searches have been performed, albeit with limited success (see e.g. Deneva et al., 2009a; Johnston et al., 2006).

The discovery of a PSR-BH system would be a landmark scientific moment, providing a peerless natural laboratory for the study of general relativity and black hole physics (Kramer et al., 2004).

Dececting gravitational waves

Perhaps the most impressive application for the MSP population, is as a timing array for directly detecting gravitational waves (Foster and Backer, 1990). Pulsar timing arrays offer a means by which correlated variations in the signal from pulsars at different posi-tions on the sky may be measured, variaposi-tions that would be indicative of the presence of a gravitational wave (Jenet et al., 2005)²³. Several collaborations of pulsar astronomers are currently working on being the first to directly detect gravitational waves (see e.g.

Demorest et al., 2012; Ferdman et al., 2010; Hobbs et al., 2010a; Yardley et al., 2011).

However, for pulsar timing arrays to work, they require a fairly even distribution of pre-cisely timed pulsar across the sky. Surveys with high time and frequency resolution are therefore vital for finding suitable pulsars for the timing array. Although the existence of gravitational waves has been inferred through measurements of orbital shrinkage of the Hulse-Taylor pulsar (Taylor and Weisberg, 1989), a direct detection of gravitational waves has so far eluded the scientific community. Pulsar timing arrays are expected to be sensitive to low-frequency gravitational waves (10⁻⁹to 10⁻⁷Hz), such as may be pro-duced by coalescing supermassive black holes, cosmic superstrings and relic gravitational waves from the big bang (see e.g. Jaffe and Backer, 2003; Maggiore, 2000).

23Pulsar timing arrays are also a vital tool for, among other things, providing independent constraints on the masses of the planets in the Solar System (Champion et al., 2010) and for the development of a pulsar-based time standard (Hobbs et al., 2012).