The Transient Radio Sky: Pulsars and Fast Radio Bursts

Abstract

This thesis focuses on targeted searches of two radio transients associated with neutron stars (NS): Pulsars and Fast Radio Bursts (FRBs), and the study of their properties over different timescales. Pulsars emit beams of electromagnetic radiation along their magnetic axis, which is detected mainly as pulses at radio frequencies. They are means to study stellar evolution, to place limits on the equation of state for ultra-dense matter, to map the free electron distribution of our Galaxy and are superb natural laboratories in which to test theories of gravity in the strong-field regime. FRBs are an observational phenomena consisting of bright flashes of millisecond duration, detected so-far exclusively at radio frequencies. Although their astrophysical origin remains a mystery, it is proposed that their narrow and coherent pulses probe the large distance they travelled through; thus, FRBs could become powerful cosmological tools to probe the epoch of re-ionization, to test the homogeneity and isotropy of the Universe, and to constrain the weak equivalence principle, to name a few. <br /> Chapter 5 presents the ongoing drift-scan pulsar survey using the world largest single-dish radio telescope, the Five-hundred-meter Aperture Spherical radio Telescope (FAST). It reports the follow-up campaign of 10 pulsars discovered in the early commissioning phase of FAST with the use of the 100-m Effelsberg radio telescope. Highlights are PSR J1951+4724, a young and energetic pulsar with nearly 100% of linearly polarized flux and visible up to an observing frequency of 8 GHz, and PSR J2338+4824, a pulsar in a 95.2-day binary with a Carbon-Oxygen White dwarf (WD). Given the orbital parameters, the companion is estimated to have a minimum mass of 1.029 M, placing it as the widest known binary system with a massive WD. Additionally, PSR J2338+4824 seems to be a long-term nulling pulsar given the high non-detection rate, which is not consistent with the diffractive scintillation timescale. With the full set of pulsars in addition to the 11 FAST pulsars followed up by the Parkes radio telescope, a population analysis is performed. It is shown that FAST seems to be discovering an old population of pulsars. <br /> Chapter 6 studies the long-term evolution of the magnetic field of millisecond pulsars (MSPs) to understand their lower values when compared to the normal pulsar population. According to the standard scenario, they are formed from pulsars in binary systems, where the millisecond rotation is caused by the accretion of matter and angular momentum from their companion. However, how the magnetic field decays through accretion is not well understood. An alternative hypothesis is explored, in which the decay is due to ambipolar diffusion before the accretion process. The observed binary systems are used to constrain the time available for the decay based on the current pulsar companion: a Helium WD, a Carbon-Oxygen WD, or another NS. With a simplified model without baryon pairing, it is shown that the process agrees with the general distribution of observed magnetic field strengths in binary systems.

Chapter 7 presents an extensive multi-wavelength campaign on the first discovered repeating FRB, FRB 121102. Three radio telescopes: Effelsberg, Green Bank, and the Arecibo Observatory, were used to shadow higher energy experiments with the Gran Telescope Canaria (optical), NuSTAR (X-ray) and INTEGRAL (gamma-ray). From the 36 bursts detected with Effelsberg, one has a pulse width of 39 ms, which is the widest burst ever detected from FRB 121102. With one burst detected during simultaneous NuSTAR observations, a 5σ upper limit of 5 x 10^47 erg on the 3-79 keV energy of an X-ray burst counterpart is placed. With the roughly four years of data with Effelsberg, it is found a periodicity of 161+/-5 days, confirming the potential periodicity reported recently in the literature. Comparing the wait times between consecutive bursts within a single observation to Weibull and Poisson distributions, it is shown that if the few events with millisecond separation are excluded, the arrival times are Poisson distributed. It is proposed that such closely spaced bursts may be the components of one broad burst. Finally, it is found that the bursts’ cumulative energy distribution with energies from ~10^38-10^39 erg is well described by a power-law with a slope of gamma = -1.1 +/- 0.2. It is proposed that a single power-law might be a poor descriptor of the data over many orders of magnitude.