Ingrid Stairs

Pulsars are compact objects which are remnants of once-massive stars. Most are lighthouses in radio frequencies, and studying them has led to significant discoveries of exoplanets, gravitational waves, and fundamental physics theories. An ongoing research area is the source of pulsar emission and, in turn, the strange way telescopes see some pulsars. These include intermittent pulsars and Rotating Radio Transients (RRATs) i.e. ones that turn on for some time but remain quiet otherwise. While the study of pulsars stretches back to 1967, a more contemporary discovery is that of Fast Radio Bursts (FRBs). The discovery in 2007 led to a burgeoning field in radio astronomy, which has enticed many current-generation telescopes. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is among the bank of telescopes that have been commissioned to find FRBs. This thesis addresses the development and deployment of a novel pipeline for finding new pulsars from galactic and extragalactic single-pulse events. We employ the vast astronomical database collected by the aptly named the CHIME/FRB collaboration over 29 months. The majority of the work was to sort through the CHIME/FRB database for RFI and known pulsars before implementing an unsupervised machine learning algorithm, DBSCAN. DBSCAN made clusters of transient astronomical events which are checked against known pulsars in extant catalogs before being followed up by a second CHIME backend instrument, CHIME/Pulsar. The process has led to 144 candidates, with 12 confirmed new pulsars. Many of the pulsars discovered are RRATs, objects that are atypical for a pulsar in that the pulsed emission is sporadic and intermittent. The continued study of RRATs have the potential to elucidate their intermittency and, by extension, the emission mechanism for pulsars. Thus adding to the current ~120 known RRATs will enable pulsar astronomers to make more substantial claims regarding the estimated ~10⁵ RRATs in the Milky Way Galaxy.

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The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a novel radio telescope that is predicted to detect up to several dozens of Fast Radio Bursts (FRBs) per day. However, CHIME’s FRB detection software pipeline is susceptible to a large number of false positive triggers from terrestrial sources of Radio Frequency Interference (RFI). This thesis details the description of intensityML, a software pipeline designed to generate waterfall plots and automatically classify radio bursts detected by CHIME without explicit RFI-masking and DM-refinement. The pipeline uses a convolutional neural network based classifier trained exclusively on the events detected by CHIME, and the classifier has an accuracy, precision and recall of over 99%. It has also successfully discovered several FRBs, both in real-time and from archival data. The ideas presented in this thesis may play a key role in designing future machine-learning models for FRB classification.

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Pulsars are rapidly spinning neutron stars emitting radiation about their magnetic field axes. Misalignment of the spin and magnetic axes causes a “lighthouse” effect where we observe radiation pulses, in time with the pulsar’s rotation. Millisecond pulsars are those which have accreted material from a companion star, spinning themselves up to rotate faster. Some millisecond pulsars occur in tight orbits with low mass companions; this combination can lead to the companion losing material, due to bombardment by energetic particles from the pulsar, presumably eventually destroying the companion. These pulsars are known as Black Widows (BWs). This thesis is an analysis of radio observations of PSR J2256-1024, a BW pulsar with a spin period of 2.294531816964939(10)ms.Observing pulsars, we can calculate the arrival times of individual pulses and compare these with those predicted from various models to find the best-fitting one. This process is known as pulsar timing. We present the timing solution for PSR J2256-1024. We find it has a 5.1091831284(9)hr binary orbit with a semimajor axis of 4.1(3)ltsec and a 0.0312(9)M⊙ companion. PSR J2256-1024 shows a radio eclipse over 7.8% of its orbit - approximately twice the size of the Roche lobe calculated for the companion. This confirms the picture of a Black Widow pulsar with material being stripped from the companion and forming a trailing cloud which blocks the pulsar signal. We also find evidence for variable clumps of material in the system.We present polarization profiles and mean flux densities at 350 MHz, 820MHz and 1500 MHz. We discuss polarization changes in the post-eclipse region, where the pulsar signal is transmitted through eclipsing material in the system, and find evidence of Faraday rotation. At one epoch, synchronous measurements of excess dispersion and rotation measure lead to a detection of a 3.9(0.6)mG line-of-sight magnetic field. This field occurs an estimated minimum 3.3(0.3) companion-Roche-lobe-radii from the companion. We believe this is the first successful detection of a magnetic field component in eclipsing material within a Black Widow system.

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Pulsars are neutron stars (NS) that produce beamed radio-frequency emission. Due to their rapid, steady rotation rate, this signal is detected as a series of pulses whose integrated profile is unusually stable over time. Pulsars in double neutron star (DNS) binary systems are a rare, but extremely useful, astronomical tool and have been used in tests of gravity theories in the strong-gravitational field limit. Rarer still are DNS systems in which both objects have been detected as pulsars; only one such system has been found thus far -- PSR J0737-3039A/B. Discovered over a decade ago, this system consists of one recycled pulsar, PSR J0737-3039A, and its companion, PSR J0737-3039B, which has since become undetectable.In any pulsar-related research, precise timing is necessary to produce meaningful results. The pulse time of arrivals (TOAs) are greatly affected by the medium through which the electromagnetic (EM) signal travels in both frequency-dependent and -independent ways. Even after accounting for such effects, many pulse profiles still exhibit frequency-dependent shape changes, which can greatly affect the precision of the timing results. Traditionally, corrections are applied to the TOAs after calculation in an ad hoc manner. In contrast to this, we explored the wideband timing algorithm developed by Timothy T. Pennucci and collaborators which accounts for frequency-dependent profile changes through a two-dimensional Gaussian pulse portrait model implemented in the TOA calculations. It was found that the portrait model is well-representative of the pulse profile shape over a wide frequency range. This method is also able to produce a robust set of wideband TOAs. The subsequent timing model, determined with TEMPO timing software, was found to be comparable to those produced from subbanded TOAs derived though more traditional methods. Some inconsistencies between the timing model astrometric and spin parameters of the wideband and subbanded data of this well-studied pulsar imply potential difficulties in achieving precise timing results not only for this pulsar, but for others, such as those used in pulsar timing arrays aiming to detect gravitational waves.

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With new, highly sensitive telescopes, increased computational power, and improved search algorithms, the present century has seen a great increase in the discovery of pulsars in globular clusters. These are typically fast-spinning 'millisecond' pulsars, and more often than not they are members of binary systems. Unlike in the Galactic field, millisecond pulsars in globular clusters are often found in eccentric systems because of disruptions and exchanges due to the high stellar density of the cluster environment.A long-standing problem is that of characterizing our sensitivity to pulsars in binary systems, particularly those with non-zero eccentricity. A pulsar's orbital motion modulates its observed pulse period, making its detection through standard Fourier analysis difficult or impossible. A common technique to mitigate this problem is the 'acceleration search', which corrects for uniform line-of-sight acceleration, but not higher-order variations. This is often a valid approximation, and many pulsars have been found this way. However, it is not clear where such a search breaks down.This is a problem with a many-dimensional phase space that includes all of the binary parameters, the pulsar parameters, and the various search inputs. Past studies have approached the problem analytically, and have made valuable insights; however, until recently they have been restricted to circular orbits, and have not accounted for pulsar brightness or signal digitization.Here I approach the problem empirically. I simulate 1.8 million pulsars in a variety of orbital configurations and explore the frequency of pulsar recovery across various dimensions of the phase space. I find in particular that, at very short orbital periods, high eccentricities make binary systems easier to detect.

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We present an updated analysis of pulse profiles and their arrival-times from PSR B1534+12, a 37.9-ms pulsar that is orbiting a neutron star. Such “double-neutron-star” systems are expected to undergo various relativistic effects, such as orbital decay and precession, due to the strong-field nature of the local gravitational field (Damour & Taylor, 1992). A high-precision timing model is derived that accounts for all astrophysical processes that systematically affect pulse arrival-times. In the process of generating this model, we constrain parameters that characterize the interstellar environment, relative motion of the pulsar, its spin properties, and binary parameters. We measure five “post-Keplerian” parameters that represent relativistic corrections to the standard Keplerian quantities that describe a binary orbit. These relativistic parameters are then used to test general relativity by comparing the measured values with those predicted by Einstein’s grav- itational theory. We conclude that general relativity is confirmed to within ∼ 0.35% of its predictions. The measurement of orbital decay contains a bias due to relative acceleration in the Galactic potential, and cannot be corrected for at this time due to an unreliable measure of distance; however, we can use this bias as a means to constrain the distance the pulsar should be from Earth in order for general relativity to be the correct theory of gravity. We find this distance to be d_GR = 1.037 ± 0.012 kpc. We also present evidence for pulse “jitter” in PSR B1534+12, which indicates short-term magnetospheric activity and has significant implications for the long-term improvement of timing precision. In a separate study, we present an analysis on pulse-profile evolution that has been previously linked to relativistic spin precession (Stairs et al., 2004). The current results of our precession analysis cannot confirm general relativity using this relativistic effect, but future studies and observations are needed to constrain the precession rate of PSR B1534+12.

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