S. Milia

A Population of Fast Radio Bursts at Cosmological Distances

Mysterious Radio Bursts It has been uncertain whether single, short, and bright bursts of radio emission that have been observed are celestial or terrestrial in origin. Thornton et al. (p. 53; see the Perspective by Cordes) report the detection of four nonrepeating radio transient events with millisecond duration in data from the 64-meter Parkes radio telescope in Australia. The properties of these radio bursts indicate that they had their origin outside our galaxy, but it is not possible to tell what caused them. Because the intergalactic medium affects the characteristics of the bursts, it will be possible to use them to study its properties. Radio telescope data revealed four short, extragalactic, nonrepeating bursts of radio emission whose source is unknown. [Also see Perspective by Cordes] Searches for transient astrophysical sources often reveal unexpected classes of objects that are useful physical laboratories. In a recent survey for pulsars and fast transients, we have uncovered four millisecond-duration radio transients all more than 40° from the Galactic plane. The bursts’ properties indicate that they are of celestial rather than terrestrial origin. Host galaxy and intergalactic medium models suggest that they have cosmological redshifts of 0.5 to 1 and distances of up to 3 gigaparsecs. No temporally coincident x- or gamma-ray signature was identified in association with the bursts. Characterization of the source population and identification of host galaxies offers an opportunity to determine the baryonic content of the universe.

The High Time Resolution Universe Pulsar Survey - I. System configuration and initial discoveries

We have embarked on a survey for pulsars and fast transients using the 13-beam multibeam receiver on the Parkes Radio Telescope. Installation of a digital backend allows us to record 400 MHz of bandwidth for each beam, split into 1024 channels and sampled every 64 μs. Limits of the receiver package restrict us to a 340 MHz observing band centred at 1352 MHz. The factor of 8 improvement in frequency resolution over previous multibeam surveys allows us to probe deeper into the Galactic plane for short-duration signals such as the pulses from millisecond pulsars. We plan to survey the entire southern sky in 42 641 pointings, split into low, mid and high Galactic latitude regions, with integration times of 4200, 540 and 270 s, respectively. Simulations suggest that we will discover 400 pulsars, of which 75 will be millisecond pulsars. With ∼30 per cent of the mid-latitude survey complete, we have redetected 223 previously known pulsars and discovered 27 pulsars, five of which are millisecond pulsars. The newly discovered millisecond pulsars tend to have larger dispersion measures than those discovered in previous surveys, as expected from the improved time and frequency resolution of our instrument.

A Radio-loud Magnetar in X-ray Quiescence

As part of a survey for radio pulsars with the Parkes 64 m telescope, we have discovered PSR J1622-4950, a pulsar with a 4.3 s rotation period. Follow-up observations show that the pulsar has the highest inferred surface magnetic field of the known radio pulsars (B {approx}3 x 10{sup 14} G), and it exhibits significant timing noise and appears to have an inverted spectrum. Unlike the vast majority of the known pulsar population, PSR J1622-4950 appears to switch off for many hundreds of days and even in its on-state exhibits extreme variability in its flux density. Furthermore, the integrated pulse profile changes shape with epoch. All of these properties are remarkably similar to the only two magnetars previously known to emit radio pulsations. The position of PSR J1622-4950 is coincident with an X-ray source that, unlike the other radio pulsating magnetars, was found to be in quiescence. We conclude that our newly discovered pulsar is a magnetar-the first to be discovered via its radio emission.

Transformation of a Star into a Planet in a Millisecond Pulsar Binary

Timing observations of a millisecond pulsar reveal a planet that is far denser than any known planet. Millisecond pulsars are thought to be neutron stars that have been spun-up by accretion of matter from a binary companion. Although most are in binary systems, some 30% are solitary, and their origin is therefore mysterious. PSR J1719−1438, a 5.7-millisecond pulsar, was detected in a recent survey with the Parkes 64-meter radio telescope. We show that this pulsar is in a binary system with an orbital period of 2.2 hours. The mass of its companion is near that of Jupiter, but its minimum density of 23 grams per cubic centimeter suggests that it may be an ultralow-mass carbon white dwarf. This system may thus have once been an ultracompact low-mass x-ray binary, where the companion narrowly avoided complete destruction.

The High Time Resolution Universe Pulsar Survey – III. Single-pulse searches and preliminary analysis

We present the search methods and initial results for transient radiosignals in the High Time Resolution Universe (HTRU) survey. The HTRUsurveys single-pulse search, the software designed to perform thesearch and a determination of the HTRU surveys sensitivity to singlepulses are described. Initial processing of a small fraction of thesurvey has produced 11 discoveries, all of which are sparsely emittingneutron stars, as well as provided confirmation of two previouslyunconfirmed neutron stars. Most of the newly discovered objects lie inregions surveyed previously, indicating both the improved sensitivity ofthe HTRU survey observing system and the dynamic nature of the radiosky. The cycles of active and null states in nulling pulsars, rotatingradio transients (RRATs) and long-term intermittent pulsars are exploredin the context of determining the relationship between these populationsand of the sensitivity of a search to the various radio-intermittentneutron star populations. This analysis supports the case that manyRRATs are in fact high-null-fraction pulsars (i.e. with a null fractionof {\gsim}0.95) and indicates that intermittent pulsars appear distinctfrom nulling pulsars in their activity cycle time-scales. We find thatin the measured population, there is a deficit of pulsars with typicalemission time-scales greater than {\tilde}300 s that is not readilyexplained by selection effects. The HTRU low-latitude survey will becapable of addressing whether this deficit is physical. We predict thatthe HTRU survey will explore pulsars with a broad range of nullingfractions (up to and beyond 0.999), and at its completion is likely toincrease the currently known RRATs by a factor of more than 2.

The High Time Resolution Universe Pulsar Survey – IV. Discovery and polarimetry of millisecond pulsars

We present the discovery of six millisecond pulsars (MSPs) in the High Time Resolution Universe (HTRU) survey for pulsars and fast transients carried out with the Parkes radio telescope. All six are in binary systems with approximately circular orbits and are likely to have white dwarf companions. PSR J1017-7156 has a high flux density and a narrow pulse width, making it ideal for precision timing experiments. PSR J1446-4701 and PSR J1125-5825 are coincident with gamma-ray sources, and folding the high-energy photons with the radio timing ephemeris shows evidence of pulsed gamma-ray emission. PSR J1502-6752 has a spin period of 26.7 ms, and its low period derivative implies that it is a recycled pulsar. The orbital parameters indicate it has a very low mass function, and therefore a companion mass much lower than usually expected for such a mildly recycled pulsar. In addition we present polarization profiles for all 12 MSPs discovered in the HTRU survey to date. Similar to previous observations of MSPs, we find that many have large widths and a wide range of linear and circular polarization fractions. Their polarization profiles can be highly complex, and although the observed position angles often do not obey the rotating vector model, we present several examples of those that do. We speculate that the emission heights of MSPs are a substantial fraction of the light cylinder radius in order to explain broad emission profiles, which then naturally leads to a large number of cases where emission from both poles is observed.

Radio emission evolution, polarimetry and multifrequency single pulse analysis of the radio magnetar PSR J1622-4950

Here we report on observations of the radio magnetar PSR J1622-4950 at frequencies from 1.4 to 17 GHz. We show that although its flux density is varying up to a factor of ~10 within a few days, it has on average decreased by a factor of 2 over the last 700 days. At the same time, timing analysis indicates a trend of decreasing spin-down rate over our entire data set, again of about a factor of 2 over 700 days, but also an erratic variability in the spin-down rate within this time span. Integrated pulse profiles are often close to 100 per cent linearly polarized, but large variations in both the profile shape and fractional polarization are regularly observed. Furthermore, the behaviour of the position angle of the linear polarization is very complex - offsets in both the absolute position angle and the phase of the position angle sweep are often seen and the occasional presence of orthogonal mode jumps further complicates the picture. However, model fitting indicates that the magnetic and rotation axes are close to aligned. Finally, a single pulse analysis has been carried out at four observing frequencies, demonstrating that the wide pulse profile is built up of narrow spikes of emission, with widths that scale inversely with observing frequency. All three of the known radio magnetars seem to have similar characteristics, with highly polarized emission, time-variable flux density and pulse profiles, and with spectral indices close to zero.

The High Time Resolution Universe Pulsar Survey – V. Single-pulse energetics and modulation properties of 315 pulsars

We report on the pulse-to-pulse energy distributions and phase-resolved modulation properties for catalogued pulsars in the southern High Time Resolution Universe intermediate-latitude survey. We selected the 315 pulsars detected in a single-pulse search of this survey, allowing a large sample unbiased regarding any rotational parameters of neutron stars. We found that the energy distribution of many pulsars is well described by a log-normal distribution, with few deviating from a small range in log-normal scale and location parameters. Some pulsars exhibited multiple energy states corresponding to mode changes, and implying that some observed ‘nulling’ may actually be a mode-change effect. PSR J1900−2600 was found to emit weakly in its previously identified ‘null’ state. We found evidence for another state-change effect in two pulsars, which show bimodality in their nulling time-scales; that is, they switch between a continuous-emission state and a single-pulse-emitting state. Large modulation occurs in many pulsars across the full integrated profile, with increased sporadic bursts at leading and trailing sub-beam edges. Some of these high-energy outbursts may indicate the presence of ‘giant pulse’ phenomena. We found no correlation with modulation and pulsar period, age or other parameters. Finally, the deviation of integrated pulse energy from its average value was generally quite small, despite the significant phase-resolved modulation in some pulsars; we interpret this as tenuous evidence of energy regulation between distinct pulsar sub-beams.

The High Time Resolution Universe Pulsar Survey –VIII. The Galactic millisecond pulsar population

We have used millisecond pulsars (MSPs) from the southern High Time Resolution Universe (HTRU) intermediate latitude survey area to simulate the distribution and total population of MSPs in the Galaxy. Our model makes use of the scale factor method, which estimates the ratio of the total number of MSPs in the Galaxy to the known sample. Using our best fit value for the z-height, z=500 pc, we find an underlying population of MSPs of 8.3(\pm 4.2)*10^4 sources down to a limiting luminosity of L_min=0.1 mJy kpc^2 and a luminosity distribution with a steep slope of d\log N/d\log L = -1.45(\pm 0.14). However, at the low end of the luminosity distribution, the uncertainties introduced by small number statistics are large. By omitting very low luminosity pulsars, we find a Galactic population above L_min=0.2 mJy kpc^2 of only 3.0(\pm 0.7)*10^4 MSPs. We have also simulated pulsars with periods shorter than any known MSP, and estimate the maximum number of sub-MSPs in the Galaxy to be 7.8(\pm 5.0)*10^4 pulsars at L=0.1 mJy kpc^2. In addition, we estimate that the high and low latitude parts of the southern HTRU survey will detect 68 and 42 MSPs respectively, including 78 new discoveries. Pulsar luminosity, and hence flux density, is an important input parameter in the model. Some of the published flux densities for the pulsars in our sample do not agree with the observed flux densities from our data set, and we have instead calculated average luminosities from archival data from the Parkes Telescope. We found many luminosities to be very different than their catalogue values, leading to very different population estimates. Large variations in flux density highlight the importance of including scintillation effects in MSP population studies.

The High Time Resolution Universe Pulsar Survey - VI. An artificial neural network and timing of 75 pulsars

We present 75 pulsars discovered in the mid-latitude portion of the High Time Resolution Universe survey, 54 of which have full timing solutions. All the pulsars have spin periods greater than 100 ms, and none of those with timing solutions are in binaries. Two display particularly interesting behaviour; PSR J1054{5944 is found to be an intermittent pulsar, and PSR J1809{0119 has glitched twice since its discovery. In the second half of the paper we discuss the development and application of an articial neural network in the data-processing pipeline for the survey. We discuss the tests that were used to generate scores and nd that our neural network was able to reject over 99% of the candidates produced in the data processing, and able to blindly detect 85% of pulsars. We suggest that improvements to the accuracy should be possible if further care is taken when training an articial