Radio Transients

The bursting and transient universe is one of the major areas of unexplored phase space. Traditional observations have greatly expanded our knowledge of the steady-state properties of many astrophysical phenomena, but the physics of the dynamic universe remains elusive due to the difficulty of observing short lived and impulsive events. Across all areas of astronomy, transient science has become a major focus of new and proposed observatories including ROTSE, SNAP, Swift, GLAST, LIGO, and Ice Cube. Since radio can observe the magnetic fields and non-thermal processes which drive many dynamic astronomical events, transient radio observations could provide key observations for understanding the bursting and transient universe.

A number of historical searches for transient radio emission have been made. Most of these searches focused on looking for prompt GRB emission, including the Cambridge Low Frequency Synthesis Telescope search by Dessenne et al. (1996), the solar spectrometer search by Benz & Paesold (1998), and the FLIRT observations by Balsano (1999). The transient surveys by Baird et al. (1976) with small arrays and Katz et al. (2003) with the distributed wide field elements of STARE attempted to perform blind surveys for radio transients. All of these attempts have suffered from the constraints inherent in analog receivers and simple transient identification systems. The All Sky Monitor team has designed the hardware and software tools needed to perform a modern search for radio transients, as detailed in the whitepaper Morales et al. (2004). These advances are the basis for several proposed radio transient surveys, including the GASE search for prompt GRB emission near 30 MHz, MWA observations from 80–300 MHz as part of this proposal, and potential observations with the LWA from 20–90 MHz. Taken together, these observations will create a comprehensive survey of radio transients at low radio frequencies which is ~6 orders of magnitude deeper than any previous survey (as measured by the intrinsic luminosity volume rate).

Potential sources of radio transient emission fall into four broad categories:

Explosive Events - Gamma Ray Bursts and radio supernovae may both produce short and long duration transient radio signals. The “afterglow” emission from GRBs and supernovae light curves will provide signals at the upper frequencies of the MWA which are delayed from the initial explosion and slowly rise in intensity over a few weeks to months. GRBs and supernovae may also produce prompt pulses of coherent emission during the initial explosion. The theorized prompt signals are produced by coherent emission near the external shock, and mechanisms range from current oscillations at the shock (Usov & Katz 2000) to synchrotron maser activity just behind the shock (Sagiv & Waxman 2002).

Stellar and Planetary Emission - The Sun and Jupiter are well known sources of transient radio emission, and the MWA should extend these studies to nearby stars and planets. While the MWA does not quite have the sensitivity to observe solar type activity from nearby G class stars, it will observe the stellar transient activity of more active stellar systems. The radio bursts from hot-Jupiter type planets may also be detected, offering insight into the magnetic fields of these unique objects and the solar winds of their parent stars.

Compact Objects - The launching of relativistic jets and knots from black hole accretion systems produces bright x-ray flares, and is a poorly understood phenomenon central to understanding the physics behind Active Galactic Nuclei (AGN), micro-quasars and gamma-ray bursts. Simultaneous transient radio and x-ray observations could provide crucial insight into the physics behind the launching of relativistic jets.

Serendipity - The MWA transient survey opens a new area of phase space, and may detect unexpected transient sources. Some of the more exotic possibilities include coincident observations with LIGO and the neutrino detectors, or SETI observations.

The MWA spans the upper frequencies associated with coherent radio sources and the lower frequencies of non-thermal MHD processes. This makes transient observations with the MWA particularly useful for understanding the non-equilibrium processes which drive dynamic astrophysical systems. The All Sky Monitor will be more than six orders of magnitude more sensitive than previous transient surveys in the band and will cover a much broader range of frequencies, transient durations, and dispersion measures.

Radio Transient Collaboration

The Radio Transient Collaboration is based around the All Sky Monitor (ASM) software analysis system, which is designed to use the computational resources of modern parallel supercomputers and forms the software foundation for the transient search with the MWA and future transient surveys on other proposed observatories.

The ASM analysis on the MWA uses the visibilities and ionospheric calibration to create calibrated, residual images of the full field-of-view, which are searched for transient emission ranging from 1/2 second to many months duration. The ASM is inherently a search for extra-statistical emission in a five dimensional space. The five parameters of the search are position in the sky (two dimensions), start time, duration of the emission, and dispersion measure.

The sensitivity of any transient search is determined by the statistical distribution of the background, and thus the statistics of the residual image must be very well characterized and controlled. Since we are searching a very large number of independent locations, start times, durations and dispersions, there is a large number of trial searches which must be accounted for. For example, a five sigma excursion due simply to the background noise is expected every few seconds (4 sigma corresponds to 1 in 3.2e4, and there are more than this many pixels in a single MWA field-of-view, 5 sigma is reached in a few snapshots). Accounting for this trials penalty, the threshold sensitivity for the ASM is approximately ten sigma for a five sigma deviation beyond what is expected from background fluctuations in one year of searching (sigmas do not add linearly), or equivalently a sensitivity about two times less than if the location and properties of the transient were known in advance. The ASM is designed to accurately calculate calibrated residual images and their pixel-by-pixel statistics to enable a sensitive and robust search for transient radio sources.

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