Radio Transients Surveys

Abstract

Introduction - Key Science Goals, Context, and The Fast/Slow Transient Split

The paper will present an overview of the science, techniques, and state-of-the-art of surveys for radio transients. The paper is aimed to non-experts such as early graduate students who are interested in working in the field, but will also provide detailed information for researchers currently working in the field. Progress in the field is rapid so there will be a heavy emphasis on long-term science goals and technqiues. I will discuss important developing topics such as fast radio bursts, where there is still considerable uncertainty about the significance of current discoveries.

The science and techniques of radio transient surveys are divided into domains: fast and slow. The dividing line is a characteristic time scale of 1 second. On short time scales, coherent emission processes such as the pulsar mechanism dominate. On longer time scales, incoherent synchrotron processes dominate. Technically, slow transient searches are in the domain of interferometric imaging. Fast transient searches have been traditionally the domain of high bandwidth single dish timing. Interferometric techniques, however, are becoming more important for fast transient searches. The next generation of large telescopes are primarily large-N interferometers. As these new telescopes are commissioned, interferometric techniques must be expoited in order for fast transient searches to advance scientifically.

The science of radio transients has primarily been driven by follow-up of optical and high energy events such as gamma-ray bursts and supernovae. Surveys have the opportunity to discover new classes of transient that are not known or predicted by multi-wavelength information. Serendipitous discovery has demonstrated that radio transient surveys can be critical for discovery of dust-obscured objects and coherent emitters that would not have been otherwise discovered. There is the potential for new radio transient surveys to flip the dominant paradigm in which radio discovery leads to optical and high energy follow-up.

The paper is timely because of the launch of major surveys with the Very Large Array and LOFAR and the planned launch of surveys with ASKAP and MeerKAT. Pre-cursor surveys with the VLA, the Allen Telescope Array, Parkes, Arecibo, and the GBT provide an important foundation for planning. The coming of Advanced LIGO GW interferometer results at the end of the decade will heighten interest in disovery of radio transient counterparts to GW events.

Fast Transient Science

Pulsars

Pulsars are the most common and most intensively studied of all radio transient sources. Since discovery of pulsars in 1967 (Hewish et al., 1969) and millisecond pulsars in 1982 (Backer et al., 1982), the study of pulsars has provided deep insights into stellar evolution, the structure of nuclear material, general relativity, and the interstellar medium. The extensive achievements of pulsar science have effectively created a separate and distinct sub-field of radio transient science. The study of pulsars has led to two Nobel Prizes in Physics (Hewish, for the discovery of pulsars; Hulse and Taylor, 1993, for the discovery of gravitational waves from a binary pulsar system). The power of pulsars to probe such a wide range of physics is the ultimate promise of radio transient discovery.

Pulsar phenomena and observational techniques have been reviewed extensively and recently (e.g., Lorimer et al., 2004; Kramer, 2008; Lorimer et al., 2010; Manchester, 2013). Briefly, pulsars are spinning neutron stars with strong magnetic fields that are misaligned to the spin axis. Radio emission is created through particle acceleration at the magnetic poles. This emission appears as time variable emission as the radio beam sweeps past the observer. Pulsars are primarily detected through this periodic emission with some notable exceptions discussed below.

More than 2400 pulsars are known, primarily as the result of large area surveys conducted with large single dish radio telescopes Approximately, 10% of pulsars have periods \({\stackrel{\scriptstyle <}{\scriptstyle \sim}}10\) milliseconds and are known as millisecond pulsars (MSPs). Pulsar periods span from 1.4 milliseconds to 8.5 seconds (Hessels et al., 2006; Young et al., 1999). Pulsar spectra are typically steep with indices \(\alpha\approx -1.7\) , with some notable exceptions of flat spectra . The large majority of known pulsars are in the galactic plane with notable populations in globular clusters and the Galactic Center (Johnston et al., 2006; Deneva et al., 2009; Shannon et al., 2013; Eatough et al., 2013). Extragalactic pulsars have only been detected in the Magellanic Clouds (e.g., Ridley et al., 2013). More discussion of properties? luminosity range, DM?, polarization properties, spectra, lifetimes

Intermittency of the pulsed radio emission is an important property for radio transient science. Many pulsars exhibit time variable emission properties, including cases where the periodic emis