Searching Harder, Localizing Better, Classifying Faster: Optimizing Fast Radio Burst Detection And Analysis

Fast Radio Bursts (or FRBs) are millisecond-duration transients of extragalactic origin. They exhibit dispersion caused by propagation through an ionized medium, and quantified by Dispersion Measure (DM). Around 800 FRBs (24 repeaters) have been discovered; so far, 24 FRBs have been confidently associated with a host galaxy. In this thesis, we discuss multiple new FRB search and analysis techniques and the corresponding tools that enable us to search for FRBs harder, localize them better, and classify candidates faster. We discuss five open-source software suites that can be used in FRB analysis. These suites are used to distinguish between FRBs and radio frequency interference (RFI), model FRB properties, search for periodic activity, calculate the probability of an association between an FRB and the host galaxy, and unify data processing across multiple data formats. We then present a robust comparative analysis of clustering algorithms to group candidates from REALFAST transient search system at the Karl G. Jansky Very Large Array. We design a performance metric that optimizes for a few pure clusters, i.e., clusters with either astrophysical or noise candidates. We show that using sky location along with DM/time improves clustering performance, and propose a strategy that can be used to decide which clustering algorithm is most fit for a particular application. We present a dense sample of bursts from the repeating FRB\,121102, discovered using our software. Using the Arecibo Telescope, we detected 133 bursts in 3~hours of data observed at 1.4\,GHz. We determine the properties of the bursts using robust spectro-temporal modeling. We find that the bursts are band-limited, with a lack of emission below 1.3\,GHz. We find the wait time distribution to be log-normal in form with a peak at 75\,s. Poissonian and Weibull distributions do not describe the burst rate distribution well. The cumulative energy distribution can be described using a broken power-law model, with the break at $(2.3\pm0.2)\times 10^{37}$~ergs and a high-energy slope of $-1.8\pm0.2$. Motivated by the banded nature of FRB\,121102 bursts, we perform a simulation study to show that commonly used analyses of band-limited FRBs lead to observational biases. We show that all the observed shapes in the energy distributions of repeaters can be explained using these biases. We then recommend techniques to correct these biases: modeling burst spectra to robustly estimate the intrinsic properties, and using bursts that are within the observing band for energy distribution analyses. Finally, we discuss the REALFAST search and analysis pipeline, compare it to the search pipelines on single-dish telescopes, and highlight the advantages of using an interferometer. Primarily, every detection with REALFAST comes with a precise localization that can be used to associate the FRB to a host galaxy. We then discuss five repeating FRBs that were localized using REALFAST

Irreducible Atlanto-Axial Dislocation in Neglected Odontoid Fracture Treated with Single Stage Anterior Release and Posterior Instrumented Fusion

It is a well-know fact that type 2 odontoid fractures frequently go into nonunion. If left untreated, patients may develop irreducible atlantoaxial dislocation (AAD). We describe the surgical management of two patients with neglected odontoid fractures and irreducible AAD treated with single stage anterior release followed by posterior instrumented fusion. Both patients presented with history of neglected trauma and progressive myelopathy. Traction under anesthesia failed to achieve reduction of AAD. Anterior release was done by trans-oral approach in one patient and retrophayngeal approach in the other. Posterior fixation was performed with transarticular screws in both the patients. Both patients had full neurological recovery and demonstrated fusion at follow-up. Anterior release followed by posterior instrumented correction may be an effective alternative to the traditional means of treating irreducible dislocations associated with neglected odontoid fractures

Probabilistic Association of Transients to their Hosts (PATH)

We introduce a new method to estimate the probability that an extragalactic transient source is associated with a candidate host galaxy. This approach relies solely on simple observables: sky coordinates and their uncertainties, galaxy fluxes and angular sizes. The formalism invokes Bayes' rule to calculate the posterior probability P(O_i|x) from the galaxy prior P(O), observables x, and an assumed model for the true distribution of transients in/around their host galaxies. Using simulated transients placed in the well-studied COSMOS field, we consider several agnostic and physically motivated priors and offset distributions to explore the method sensitivity. We then apply the methodology to the set of 13~fast radio bursts (FRBs) localized with an uncertainty of several arcseconds. Our methodology finds nine of these are securely associated to a single host galaxy, P(O_i|x)>0.95. We examine the observed and intrinsic properties of these secure FRB hosts, recovering similar distributions as previous works. Furthermore, we find a strong correlation between the apparent magnitude of the securely identified host galaxies and the estimated cosmic dispersion measures of the corresponding FRBs, which results from the Macquart relation. Future work with FRBs will leverage this relation and other measures from the secure hosts as priors for future associations. The methodology is generic to transient type, localization error, and image quality. We encourage its application to other transients where host galaxy associations are critical to the science, e.g. gravitational wave events, gamma-ray bursts, and supernovae. We have encoded the technique in Python on GitHub: https://github.com/FRBs/astropath.Comment: In press, ApJ; comments still welcome; Visit https://github.com/FRBs/astropath to use and build PAT

VLA/Realfast Detection of a Burst from FRB180916.J0158+65 and Tests for Periodic Activity

We report on the detection of a burst from FRB180916 by realfast/VLA and present software for interpreting fast radio bursts (FRB) periodicity. We demonstrate a range of periodicity analyses with bursts from FRB180916, FRB121102 and FRB180814. Our results for FRB180916 and FRB121102 are consistent with published results. For FRB180814, we did not detect any significant periodic episodes. The realfast-detected and other high-frequency bursts for FRB180916 tend to lie at the beginning of the activity window, indicating a possible phase-frequency relation. The python package $\texttt{frbpa}$ can be used to reproduce and expand on this analysis to test models for repeating FRBs.Comment: Published in Research Notes of the AA

VLA/Realfast Detection of a Burst from FRB 180916.J0158+65 and Tests for Periodic Activity

We report on the detection of a burst from FRB 180916 by realfast/Very Large Array and present software for interpreting fast radio bursts (FRB) periodicity. We demonstrate a range of periodicity analyses with bursts from FRB 180916, FRB 121102 and FRB 180814. Our results for FRB 180916 and FRB 121102 are consistent with published results. For FRB 180814, we did not detect any significant periodic episodes. The realfast-detected and other high-frequency bursts for FRB 180916 tend to lie at the beginning of the activity window, indicating a possible phase-frequency relation. The python package frbpa can be used to reproduce and expand on this analysis to test models for repeating FRBs

An unidentified Fermi source emitting radio bursts in the Galactic bulge

We report on the detection of radio bursts from the Galactic bulge using the real-time transient detection and localization system, realfast. The pulses were detected commensally on the Karl G. Jansky Very Large Array during a survey of unidentified Fermi $\gamma$-ray sources. The bursts were localized to subarcsecond precision using realfast fast-sampled imaging. Follow-up observations with the Green Bank Telescope detected additional bursts from the same source. The bursts do not exhibit periodicity in a search up to periods of 480s, assuming a duty cycle of < 20%. The pulses are nearly 100% linearly polarized, show circular polarization up to 12%, have a steep radio spectral index of -2.7, and exhibit variable scattering on timescales of months. The arcsecond-level realfast localization links the source confidently with the Fermi $\gamma$-ray source and places it nearby (though not coincident with) an XMM-Newton X-ray source. Based on the source's overall properties, we discuss various options for the nature of this object and propose that it could be a young pulsar, magnetar, or a binary pulsar system.Comment: Submitted to The Astrophysical Journa