Time Domain Astrophysics
The MWA is a great instrument for studying pulsars at frequencies below 300 MHz. Observing at low frequencies allows us to take advantage of the steep spectrum of pulsars (the median spectral index is -1.6). The array will open exciting opportunities in several areas of pulsar science.
- Probes of the interstellar medium through observations of scattering, scintillation and Faraday rotation: all these effects become more prominent at lower radio frequencies (e.g. dispersion smearing, goes as wavelength cubed; Faraday rotation, goes a wavelength squared; pulse broadening, goes as wavelength to the 4th power). Measurements of dispersion and scattering are important ingredients for Galactic electron density models (Cordes & Lazio 2002). In addition, pulsar scintillation studies lead to valuable insights into the nature of interstellar turbulence, as well as into theories of interstellar scattering.
- Pulsar phenomenology: The MWA will enable high sensitivity single-pulse studies at lower frequencies than hitherto. Phenomena such as pulse nulling, drifting of sub-pulses and mode switching are more pronounced at low frequencies, and are keys to a better understanding of still-mysterious emission mechanisms.
- Pulsar surveys: At 200 MHz the MWA can reach a 5-sigma detection limit of ~0.2 mJy in 30 minutes of integration. Deep surveys toward specific targets at low dispersion measures (e.g. globular clusters, the Magellanic clouds) show good promise, especially for millisecond pulsars which characteristically have steeper spectra than normal pulsars.
- Giant pulses: These are very strong and very steep spectrum, but have been detected from only four pulsars (Johnston & Romani 2003), due to limitations of conventional pulsar observing and data processing strategies. Many more prospective candidates can be monitored with the MWA for long periods. The potential also exists for detection of giant pulses from nearby external galaxies.
A bright giant pulse from the Crab Nebula pulsar PSR B0531+21 from Arecibo observations at 430 MHz; the dispersed and dedispersed pulses are also shown. This giant pulse is ~2x10^4 times stronger than typical normal pulses. Right: Palomar image of the Crab nebula that hosts the pulsar. Such bright, very steep spectrum giant pulses can potentially be detected with the MWA demonstrator out to ~1 Mpc. Credit: (R. Bhat (MIT Haystack), P. Scowen (Palomar/ASU)
Bursts and Transient Events
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 program, 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.