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Since their discovery 50 years ago, neutron stars have continually astonished. From the first-discovered radio pulsars to the powerful “magnetars” that emit sudden bursts of X-rays and γ-rays, from the so-called Isolated Neutron Stars to Central Compact Objects, observational manifestations of neutron stars are surprisingly varied, with most properties totally unpredicted. The challenge is to cement an overarching physical theory of neutron stars and their birth properties that can explain this great diversity. Here I briefly survey the disparate neutron star classes, describe their properties, highlight recent results, and describe efforts at “grand unification” of this wealth of observational phenomena.
The LOFAR Tied Array All-Sky Survey (LOTAAS) is an ongoing all northern sky survey for pulsars and transients. It is one of the first large scale pulsar surveys conducted at an observing frequency below 200 MHz. The unique set-up of the survey is the simultaneous formation of 222 beams for each survey pointing by coherently adding signals from the central 6 LOFAR stations. This represents the first SKA-like pulsar survey. As of 12 September 2017, the survey has completed 1456 pointings, more than two-thirds of the total. The survey has discovered 61 new pulsars via Fourier-based periodicity searches and a further 5 via single pulse searches. I present the survey approach and distinctive features including a discussion of an improved machine learning classifier used to identify the best candidates produced by the pipeline for further investigation. I present a summary of the discoveries so far including the first binary pulsar and the pulsar with the longest spin period of 23.5 s.
The ongoing Green Bank North Celestial Cap pulsar survey is using the Green Bank Telescope to search for pulsars and transients over 85% of the celestial sphere. The survey has resulted in over 150 new pulsars, among which are high-precision millisecond pulsars, several binary pulsars, including at least one relativistic double neutron star system, nulling pulsars, and several nearby millisecond pulsars. We find no fast radio bursts in the survey to date. We present these results and discuss the future prospects for the survey.
This paper details on the discovery of 21 pulsars using the Giant Metrewave Radio Telescope (GMRT) from targeted (Fermi directed search) and blind surveys (GMRT High Resolution Southern Sky - GHRSS) and results from the follow up studies. We discovered seven millisecond pulsars (MSPs) in the Fermi directed searches, which are the first Galactic MSPs discovered with the GMRT. We have discovered 13 pulsars (including a MSP and two mildly recycled pulsars) with the GHRSS survey, which is an off-Galactic-plane survey at 322 MHz with complementary target sky (declination range −40 deg to −54 deg) to other ongoing low-frequency surveys by GBT and LOFAR. The simultaneous time-domain and imaging study for localising pulsars and transients and efficient candidate investigation with machine learning are some of the features of the GHRSS survey, which are also finding application in the SKA design methodology.
Since the launch of the Fermi Gamma-ray Space Telescope in 2008, the onboard Large Area Telescope (LAT) has detected gamma-ray pulsations from more than 200 pulsars. A large fraction of these remain undetected in radio observations, and could only be found by directly searching the LAT data for pulsations. However, the sensitivity of such “blind” searches is limited by the sparse photon data and vast computational requirements. In this contribution we present the latest large-scale blind-search survey for gamma-ray pulsars, which ran on the distributed volunteer computing system, Einstein@Home, and discovered 19 new gamma-ray pulsars. We explain how recent improvements to search techniques and LAT data reconstruction have boosted the sensitivity of blind searches, and present highlights from the survey’s discoveries. These include: two glitching pulsars; the youngest known radio-quiet gamma-ray pulsar; and two isolated millisecond pulsars (MSPs), one of which is the only known radio-quiet rotationally powered MSP.
For fifty years astronomers have been searching for pulsar signals in observational data. Throughout this time the process of choosing detections worthy of investigation, so called ‘candidate selection’, has been effective, yielding thousands of pulsar discoveries. Yet in recent years technological advances have permitted the proliferation of pulsar-like candidates, straining our candidate selection capabilities, and ultimately reducing selection accuracy. To overcome such problems, we now apply ‘intelligent’ machine learning tools. Whilst these have achieved success, candidate volumes continue to increase, and our methods have to evolve to keep pace with the change. This talk considers how to meet this challenge as a community.
The high time resolution afforded by coherent dedispersion has enabled precision pulsar timing, detailed studies of pulsar morphology, and has led to conclusions about the radio emission mechanism. The advance of technology in the last 50 years has enhanced the capability of coherent dedispersion, now used for most pulsar observing, by nearly six orders of magnitude. Although coherent dedispersion is now done mostly in software, in “earlier days” several novel hardware devices for real-time processing were developed.
We have used LOFAR to perform targeted millisecond pulsar surveys of Fermi γ-ray sources. Operating at a center frequency of 135 MHz, the surveys use a novel semi-coherent dedispersion approach where coherently dedispersed trials at coarsely separated dispersion measures are incoherently dedispersed at finer steps. Three millisecond pulsars have been discovered as part of these surveys. We describe the LOFAR surveys and the properties of the newly discovered pulsars.
The first few binary pulsars revealed the richness of evolution possible in binary systems containing neutron stars. Products of different evolutionary routes, in high and low mass binaries, as well as examples of evolution affected by the pulsar wind were among the first ten objects discovered. This article presents a historical review of the impact of binary pulsars on the early development of ideas regarding the evolution of neutron stars in binary systems.
Black widows and redbacks are binary systems consisting of a millisecond pulsar in a close binary with a companion having matter driven off of its surface by the pulsar wind. X-rays due to an intrabinary shock have been observed from many of these systems, as well as orbital variations in the optical emission from the companion due to heating and tidal distortion. We have been systematically studying these systems in radio, optical and X-rays. Here we will present an overview of X-ray and optical studies of these systems, including new XMM-Newton and NuStar data obtained from several of them, along with new optical photometry.
Transitional millisecond pulsars (tMSPs), which are systems that harbor a pulsar in the throes of the recycling process, have emerged as a new source class since the discovery of the first such system a decade ago. These systems switch between accretion-powered low-mass X-ray binary (LMXB) and rotation-powered radio millisecond pulsar (RMSP) states, and provide exciting avenues to understand the physical processes that spin-up neutron stars to millisecond periods. During the last decade, three tMSPs, as well as a candidate source, have been extensively probed using systematic, multi-wavelength campaigns. Here we review the observational highlights from these campaigns and our general understanding of tMSPs.
Over the last fifty years since the discovery of pulsars, our understanding of where and how pulsars emit the radiation we observe has undergone significant revision. The location and mechanisms of high-energy radiation are intimately tied to the sites of particle acceleration. The evolution of emission models has paralleled the development of increasingly more sensitive telescopes, especially at high energies. I will review the history of pulsar emission modeling, from the early days of gaps at the polar caps, to outer gaps and slot gaps in the outer magnetosphere, to the present era of global magnetosphere simulations that locate most acceleration and high-energy emission in the current sheets.
The recent revelation that there are correlated period derivative and pulse shape changes in pulsars has dramatically changed our understanding of timing noise as well as the relationship between the radio emission and the properties of the magnetosphere as a whole. Using Gaussian processes we are able to model timing and emission variability using a regression technique that imposes no functional form on the data. We revisit the pulsars first studied by Lyne et al. (2010). We not only confirm the emission and rotational transitions revealed therein, but reveal further transitions and periodicities in 8 years of extended monitoring. We also show that in many of these objects the pulse profile transitions between two well-defined shapes, coincident with changes to the period derivative. With a view to the SKA and other telescopes capable of higher cadence we also study the detection limitations of period derivative changes.
New simultaneous X-ray and radio observations of the archetypal mode-switching pulsar PSR B0943+10 have been carried out with XMM-Newton and the LOFAR, LWA and Arecibo radio telescopes in November 2014. They allowed us to better constrain the X-ray spectral and variability properties of this pulsar and to detect, for the first time, the X-ray pulsations also during the X-ray-fainter mode. The combined timing and spectral analysis indicates that unpulsed non-thermal emission, likely of magnetospheric origin, and pulsed thermal emission from a small polar cap are present during both radio modes and vary in a correlated way.
PSR B0943+10 is an old non-recycled pulsar which for decades has been mostly known for its rapid and spontaneous radio mode switching. Recently, Hermsen et al. (2013) discovered correlated changes in the thermal X-ray emission from the polar cap, thus demonstrating that radio modes are not just a product of the local changes in the radio emission region, but a sign of some global magnetospheric transformation. At about the same time, owing to the commissioning of the new generation of low-frequency radio arrays, the broadband observations at the lowest edge of ionospheric transparency window became available. At these radio frequencies profile morphology and the single-pulse properties of PSR B0943+10’s emission become very dynamic, providing details not only about the emission itself, but also about the conditions in the polar gap. Here, I will present the recent results of the LOFAR observations of PSR B0943+10 and discuss their contribution to the multiwavelength picture.
Pulsars were discovered on the basis of their individual pulses, first by Jocelyn Bell and then by many others. This was chart-recorder science as computers were not yet in routine use. Single pulses carry direct information about the emission process as revealed in the detailed properties of their polarization characteristics. Early analyses of single pulses proved so dizzyingly complex that attention shifted to study of average profiles. This is turn led to models of pulsar emission beams—in particular the core/double-cone model—which now provides a foundation for understanding single-pulse sequences. We mention some of the 21stC single-pulse surveys and conclude with a brief discussion of our own recent analyses leading to the identification of the pulsar radio-emission mechanism of both slow and millsecond pulsars.
The aim of this work is to explore the connection between variability in single pulse intensity and periodic switching of the position angle (PA) of the linear polarisation and how this relates to the radio emission mechanism. There are five pulsars reported in the literature for which the PA is seen to periodically change in tandem with the variability in their pulse shapes. This behaviour is seemingly incompatible with two well established models of the radio emission mechanism. The purpose of this study is to investigate in a systematic way whether this phenomenon is common or if only happens in special cases, using a high-quality sample of pulsar data observed with the Parkes telescope. We show that the connection between polarisation variability and intensity variability is more common than previously expected.
The Crab pulsar has a striking radio profile, dominated by two pulse components (the main pulse and interpulse) which are comprised of giant pulses. These pulses are randomly occurring, they extend to extremely high flux densities, and are closely aligned with emission across the entire electromagnetic spectrum. The Crab, like many pulsars, exhibits scintillation – a pattern in frequency and time arising from interfering scattered images. The pattern varies with location, with the physical scale over which it changes by order unity corresponding to the spatial resolution of the scattering surface. For the Crab, the scattering is in the nebula and the estimated spatial resolution is of order the light cylinder radius. Comparing scintillation spectra of the two components, we infer a difference in physical location of the same order.
Pulsar polarization has been a fruitful area of study since the first discovery of pulsars 50 years ago. Polarization gives information on the geometry of the star, the location of the radio emission in the magnetosphere, the physics behind the radio emission mechanism and a plethora of phenomenology. Here, I will restrict myself to a brief outline of recent work in pulsar polarization using observations taken with the Parkes radio telescope over the past decade.
A radio polarization study of gamma-ray-detected pulsars reveals a surprising tendency for the magnetic and rotation axes to be relatively aligned. This provides tension with gamma-ray models, which disfavour such alignment. The lack of correlation between these findings and those derived from the gamma-ray light curves suggests problems in the models. To make the data consistent with a random orientation of the magnetic field the emission regions could be assumed to extend outside what is traditionally thought to be the open-field-line region in a magnetic inclination angle dependent way. Both acceptance and rejection of this hypothesis has important consequences. Finally, a unification scheme is proposed to explain the observational differences between gamma-ray loud and gamma-ray quiet radio pulsars. This unification scheme takes the orientation of the line of sight and the magnetic inclination angle to be key parameters affecting both the radio and gamma-ray light-curve morphology.