2.1.1 Gamma-Ray Bursts

The afterglow emission from GRBs is generated via particle acceleration in the decelerating blast wave. At late times, the peak of the afterglow spectrum is at radio energies, and thus radio studies provide the most interesting probes for GRBs science. Based on previous studies (Chandra & Frail Reference Chandra and Frail2012), we expect that the GRBs seen by ASKAP will be long-lasting but intrinsically fainter on average than shorter wavelength afterglows. Given the sensitivity of ASKAP, this means that those GRBs studied by VAST will, in general, be the rare but important nearby (∼1 500 Mpc) events. In the relativistic expansion phase (up to 100 d after the initial explosion), ASKAP monitoring in conjunction with optical and X-ray facilities will allow us to measure physical parameters such as the total kinetic energy of the explosion, the density of the circumburst medium, and the geometry of the outflow.

At late times, when the blast wave has decelerated to sub-relativistic velocities, the jetted outflows expand and become more isotropic. Under these circumstances, the total (i.e. calorimetric) energy of GRB blast waves may be estimated—a key but difficult parameter to measure (Frail et al. Reference Frail2005). Instead of relying on rare, nearby events, VAST may also obtain a robust beaming-independent estimates of GRB energies from a large sample of single-epoch flux density measurements (and limits) of afterglows at late times (Shivvers & Berger Reference Shivvers and Berger2011).

As the relativistic beaming is totally suppressed at late time, radio monitoring is the best solution to identify ‘orphan afterglows’, i.e., those GRBs whose jet is beamed away from us (Rhoads Reference Rhoads1997). While they would lack a gamma-ray trigger by definition, such systems could be distinguished from other classes of transients through the evolution and luminosity of their light curves, their location towards galaxies that resemble other GRB hosts, and their probable association with SNe (Soderberg et al. Reference Soderberg2010). The discovery of a population of orphan afterglows would allow us to constrain the beaming fraction of GRBs—a key quantity for understanding the true GRB event rate. It would also open up an order of magnitude more objects which can be scrutinised in detail. This may also give insights on the jet structure of GRBs and clarify the puzzling lack of jet breaks from the Swift satellite (Racusin et al. Reference Racusin2009).

2.1.2 Supernovae

While thermonuclear (Type Ia) SNe have proved to be extremely useful in measuring the acceleration of the Universe, no Type Ia SN has ever been detected at radio wavelengths (Panagia et al. Reference Panagia, Van Dyk, Weiler, Sramek, Stockdale and Murata2006; Hancock, Gaensler, & Murphy Reference Hancock, Gaensler and Murphy2011; Chomiuk et al. Reference Chomiuk2012), indicating a very low density of circumstellar material. In contrast, many core-collapse SNe (those of Type Ib, Ic, and the various Type II sub-classes) have been detected and monitored at radio wavelengths (Weiler et al. Reference Weiler, Panagia, Montes and Sramek2002; Ryder et al. Reference Ryder2004; Soderberg Reference Soderberg, Immler, Weiler and McCray2007). Radio observations replay the late phases of stellar evolution (in reverse), and reveal clues about the late-time evolution of the progenitor system.

With VAST we will conduct an unbiased census of core-collapse SNe (a still undetermined fraction of which emit at radio wavelengths), allowing us to match radio detections against optically discovered SNe. We will also detect new radio SNe that may have gone undetected in the optical or infrared (IR) due to significant amounts of dust (Kankare et al. Reference Kankare2008). The prevalence of such heavily obscured SNe is virtually unconstrained, with hints that they may constitute a nontrivial subpopulation. Recently, SN 2008cs (Kankare et al. Reference Kankare2008) was detected in the IR and confirmed in the radio bands, and SN 2008iz (Brunthaler et al. Reference Brunthaler, Menten, Reid, Henkel, Bower and Falcke2009) was serendipitously discovered in radio observations of M 82. SN 2008iz has remained undetected in the optical, IR, and X-ray bands despite sensitive searches (Brunthaler et al. Reference Brunthaler2010). Discovery of new SNe is valuable for tying down the current rate of massive star formation in the Universe and for understanding how and when SNe explode in star-bursting galaxies. Radio emission from core-collapse SNe is detectable within days, and they reach their peak luminosity at 1.4 GHz as much as 1 year after the explosion, making them ideal targets to detect and study with the sensitivity and cadence of VAST (see Section 4).

Beyond the large population of standard core-collapse SNe, there exist sub-classes of objects that are of particular interest. For instance, radio observations of a recent type Ib/c supernova, SN2009bb, showed that it was expanding at mildly relativistic velocities (β≈0.8) powered by the central engine (Soderberg et al. Reference Soderberg2010). Such SNe are usually associated with GRBs and are discovered by their intense but short-lived gamma-ray energy emission. Yet, SN2009bb had no detected gamma-ray counterpart. It has been suggested that the blast waves produced in such engine-driven SNe could be responsible for the acceleration of ultra-high-energy cosmic rays (Chakraborti et al. Reference Chakraborti, Ray, Soderberg, Loeb and Chandra2011). However, the present limits on the neutrino flux expected as a by-product of cosmic-ray production in GRBs are at least a factor of 3.7 below predictions (Abbasi et al. Reference Abbasi2012). Radio surveys with the capability of completing the sample of these explosions will improve our understanding of particle acceleration processes and the diversity of core-collapse events (see Lien et al. Reference Lien, Chakraborty, Fields and Kemball2011 for predictions on the detection rates of core-collapse radio SNe for a range of SKA pathfinder instruments).

2.1.3 Gravitational Wave Counterparts

Gravitational wave (GW) events are expected to be compact and involve significant energies—both GRBs and SNe are considered to be among likely sources of both electromagnetic and GW emission. In an analogous manner, other classes of GW events may also generate significant transient radio emission, especially if they occur in dense environments. We discuss the particular case of mergers of neutron star binaries in Section 2.3.4. For all classes of GW events, however, localisation will be a significant challenge, with search areas expected to be of the order of 10–100 deg² for planned GW detectors (Fairhurst Reference Fairhurst2011). The large instantaneous sky coverage of ASKAP makes it particularly valuable in the search for electromagnetic counterparts to GW events, although there are significant uncertainties in the requisite search sensitivities and timescales. The search area estimates mentioned above are computed for the Advanced LIGO (Harry et al. Reference Harry2010) and Advanced VIRGO (Accadia et al. Reference Accadia2011) detectors. These systems are planned to come online in the 2014–2015 timeframe, meaning that ASKAP will be well positioned to make discoveries in this parameter space.

2.2.1 Pulse Broadening from Scattering

The well known temporal broadening of pulsar pulses caused by scattering from electron-density variations will affect any source of short-duration bursts, including those that will be targeted by VAST. Pulse broadening from sources in the Galactic centre can be broader than 1 000 s at 1 GHz. Bursts from extragalactic sources at cosmological distances can similarly be broadened, especially if the line of sight intersects another galaxy but also from the intergalactic medium (IGM). Scattering in the Milky Way, in other galaxies, and in the IGM therefore define pulse-broadening horizons beyond which pulses with specific widths will not be seen (see e.g. Cordes Reference Cordes2012). Pulse broadening from scattering can be distinguished from source-intrinsic effects by identifying across the ASKAP band the strong frequency dependence ∼∝ν⁻⁴ of scatter broadening.

2.2.2 Extreme Scattering Events

ESEs are flux density variations marked by decreases in flux density bracketed by increases (Figure 1). There is widespread agreement that ESEs are caused by refractive lens-like density perturbations along the line of sight to a background source (e.g. Fiedler et al. Reference Fiedler1987b; Romani, Blandford, & Cordes Reference Romani, Blandford and Cordes1987). The refracting lenses which cause ESEs must be Galactic in origin and probably lie within a few kpc, but in the 24 years since their discovery, no satisfactory physical model has emerged for the ESE phenomenon. An analysis of existing data suggests that ESEs are unlike any other identified component of the ISM and that they are present in vast numbers in the Galaxy (with a density ∼10³ pc⁻³). If the refracting properties are taken at face value, they imply regions with pressures ∼1 000 times higher than the diffuse ionised ISM. One suggestion is that they are associated with neutral, self-gravitating gas clouds (Walker Reference Walker, Haverkorn and Goss2007), in which case they would make a substantial contribution to the total baryonic mass content of the Galaxy. Alternatively, they may represent small-scale density enhancements created by converging flows in a turbulent medium (Jenkins & Tripp Reference Jenkins and Tripp2001), but it is unclear whether such structures could persist for the observed ESE durations.

Figure 1.

An extreme scattering event in Q0954+658 at 2.7 GHz (lower) and 8.1 GHz (upper) adapted from Fiedler et al. (Reference Fiedler1987b). For clarity, an offset of 1 Jy has been added to the top trace. The strong frequency dependence of ESEs and the necessity of regular sampling of the light curves over a long period is evident. At ASKAP frequencies (∼1 GHz), the amplitude of the flux density decrease would likely have been even larger, and the flux density would likely have increased to an even higher value during the start of the event.

A wide-area survey with daily cadence would allow us to address the physical composition and origin of the objects responsible for ESEs in a comprehensive manner.

Population statistics: One of the more comprehensive ESE surveys (Fiedler et al. Reference Fiedler1994b) provided a total coverage of approximately 600 source-years; VAST has the potential to achieve a coverage that is larger by a factor of roughly 300, searching for ESEs towards sources with much lower flux densities. In turn, an increased coverage will constrain the spatial density and distribution of refracting lenses in the Galaxy, including whether they are associated with particular Galactic structures (Fiedler et al. Reference Fiedler, Pauls, Johnston and Dennison1994a). A larger number of ESEs will also allow investigation into any potential diversity in light-curve shapes, and whether any such diversity can be understood in terms of simple models (Clegg et al. Reference Clegg, Fey and Lazio1998) or the extent to which more complex models of the density structures, background sources, or both, are required.

Real-time characterisation of ESEs: A straightforward approach to elucidating the nature of the structures responsible for ESEs would be to conduct additional observations of a source while it is undergoing an ESE. Events like those seen towards quasars B0954+658 and B1749+096 (Fiedler et al. Reference Fiedler1994b) or B1741−038 (Fiedler et al. Reference Fiedler, Johnston, Waltman and Ghigo1992; Clegg, Fey, & Fiedler, Reference Clegg, Fey and Fiedler1996) should be seen at the rate of roughly 60 per year in an all-sky monitoring survey with ∼1 mJy RMS sensitivity, with ∼10 in progress at any given moment. Thus, in contrast to the GBI monitoring program, for which only one ESE was identified while it was ongoing, VAST will enable real-time follow-up.

Some key follow-up measurements could be made at higher frequencies, particularly in the optical to X-ray bands, which should directly reveal the dense neutral gas clouds responsible for ESEs via scattering, absorption and refraction (Draine Reference Draine1998). Additionally, H i observations would reveal any associated neutral structures, Faraday rotation measurements may constrain the magnetic field strength within the structures, and high-resolution observations could confirm predicted changes in the apparent structure of the source. While there have been previous efforts in conducting such observations (Clegg et al. Reference Clegg, Fey and Fiedler1996; Lazio et al. Reference Lazio2000, Reference Lazio, Gaume, Claussen, Fey, Fiedler and Johnston2001a), only one source has been observed in such a manner (B1741−038), and, for some of the measurements, only a single observation during the actual ESE could be obtained. An all-sky survey could detect multiple ESEs during the course of a year, allowing ample opportunity to trigger follow-up observations with other telescopes and at other wavelengths.

2.2.3 Interstellar Scintillation of AGNs

Observations of decimetre variability in extragalactic sources (e.g. Hunstead Reference Hunstead1972) led quickly to the recognition that the sources needed to have extremely compact components, be affected by interstellar scintillation, or both (Armstrong, Spangler, & Hardee Reference Armstrong, Spangler and Hardee1977; Spangler et al. Reference Spangler, Fanti, Gregorini and Padrielli1989). The need for extremely compact components, apparently violating the so-called inverse Compton catastrophe, became even more acute with the discovery of ‘flickering’ (Heeschen Reference Heeschen1984), and later intra-day variability.

The MASIV 5-GHz Very Large Array (VLA) survey (Lovell et al. Reference Lovell2003, Reference Lovell2008) and other observations (Kedziora-Chudczer et al. Reference Kedziora-Chudczer1997; Dennett-Thorpe & de Bruyn Reference Dennett-Thorpe and de Bruyn2002; Bignall et al. Reference Bignall2003, Reference Bignall, Macquart, Jauncey, Lovell, Tzioumis and Kedziora-Chudczer2006) have established that the fast—intra-day and inter-day—flux density variations at centimetre wavelengths exhibited by a subset of compact quasars are predominantly caused by ISS rather than intrinsic variability. Intra-day variability refers to sources whose characteristic timescale is less than 24 h, while inter-day variability refers to sources whose timescale exceeds this.

Scintillating AGNs are of astrophysical interest because the small angular sizes that they must possess in order to exhibit rapid ISS require brightness temperatures near, or possibly in excess of, the 10¹² K inverse Compton limit for incoherent synchrotron radiation. In several cases over the past decade, it appeared that the Doppler boosting factors required in order to reconcile the apparent brightness temperature with the inverse Compton limit were well in excess of the typical values determined from Very Long Baseline interferometric observations of quasars. Estimates of the Doppler boosting factors are subject to considerable uncertainty related to the distance of the scattering material from the Earth, but many of the most rapid IDVs appear to require Doppler factors of a few tens (Macquart et al. Reference Macquart, Kedziora-Chudczer, Rayner and Jauncey2000; Rickett, Kedziora-Chudczer, & Jauncey Reference Rickett, Kedziora-Chudczer and Jauncey2002; Bignall et al. Reference Bignall, Macquart, Jauncey, Lovell, Tzioumis and Kedziora-Chudczer2006; Macquart & de Bruyn Reference Macquart and de Bruyn2007).

More recently, it appears that the fastest manifestations of IDV are associated with sources whose lines of sight intersect more local, albeit much more inhomogeneous, patches of turbulence than previously supposed. Thus, there has been a transfer of our ignorance away from the physics of the AGN and onto the physics of interstellar turbulence. However, this issue is not yet conclusively resolved, as little is known about the origin (or physical plausibility) of the local, highly inhomogeneous scattering screens that need to be invoked to explain the observed scintillation properties of these sources.

The four-epoch MASIV survey (Lovell et al. Reference Lovell2003, Reference Lovell2008) detected variability in 56% of the 482 sources observed but the characteristics of the variability changed from epoch to epoch. While able to illustrate this ISS intermittency, the MASIV survey itself, because of its limited duration and cadence, could not address many of the resulting questions, notably the extent to which the intermittency results from changes in the ISM properties or changes in the sources. In order to address the cause of the intermittency, a survey must have the following characteristics. It must observe a large number of sources widely distributed on the sky so as to have robust statistics and to be able to search for correlations expected for ISS (e.g. a Galactic latitude dependence or with nearby structures in the ISM). The survey must measure both the total and linear polarisation intensity of the sources—ISM-driven intermittency should result in the linear polarisation scintillation tracking that of the total intensity, while source-driven intermittency should result in the linear polarisation and total intensity scintillating nearly independently of each other. The survey must be conducted with close to daily cadence to capture the scintillation variations, and over a multi-year timescale in order to separate changes in source structure or scattering medium from changes associated with the annual cycle resulting from the Earth's orbital velocity.

In addition to determining the cause of ISS intermittency and mapping the scattering properties of the local ISM, such a survey would address the existence of AGN emission above the inverse Compton brightness limit and potentially constrain the baryonic content of the IGM as a function of redshift.

The cause of ISS intermittency: By continuously observing a large sample of sources over a long period, VAST will comprehensively determine the intermittency properties of scintillating AGNs. It will discriminate between ISM-related and source-driven effects via polarisation variations, as discussed above. A complication arises because annual cycles in the variability timescale (caused by the Earth's velocity vector changing with respect to the scattering screens) need to be distinguished from true IDV intermittency. However, these two phenomena can be separated by making regular (daily) observations over the course of at least 2 years. The variations should be dissimilar between epochs separated by a year if the scintillation is intermittent. Intermittency, if caused by the ISM, points to the physical nature of the turbulent patches responsible for IDV.

AGN emission above the inverse Compton brightness limit: In some cases, fast scintillation may be attributable to the appearance of microarcsecond structure within a source, associated with an outburst (Macquart & de Bruyn Reference Macquart and de Bruyn2007). The rise and fade times of the manifestation of fast variations allow us to determine the longevity of microarcsecond components associated with IDV. The rate at which the AGN supplies energy to power the bright emission that causes the variability, and the rate at which energy losses cause its eventual decay. These rates can then be compared with specific AGN processes to identify the mechanism associated with the super-Compton radio emission.

Baryonic content of the intergalactic medium: The MASIV survey showed that few sources above z∼2 are variable (Lovell et al. Reference Lovell2008). If this apparent suppression of ISS as a function of source redshift is confirmed by VAST, there are a number of possible explanations: (1) intrinsic source evolution; (2) gravitational microlensing; or (3) scattering in the turbulent ionised IGM. If the last is at least partially responsible, then the angular broadening is caused by the cumulative effect of all the baryons in the ionised IGM. The sheer number of sources VAST can monitor will enable us to probe in detail the evolution of structure in the ionised IGM as a function of redshift.

One potential complication with scintillation studies is that the largest amplitude IDV is typically observed at frequencies near 5 GHz. At ASKAP frequencies, it is possible that the strength of scattering, even towards high Galactic latitudes, and intrinsic source diameters will result in large-amplitude scintillations being quenched (Dennison & Condon Reference Dennison and Condon1981), although lower amplitude refractive scintillations may still be exhibited (e.g. Dennison et al. Reference Dennison1987).

2.3.1 Intrinsic Variability in Active Galactic Nuclei

As discussed in Section 2.2.3, Fiedler et al. (Reference Fiedler1987a) observed a sample of 33 AGNs, with flux densities of a few Janskys, every day for more than 6 years. All sources were variable over this time span with typically smooth variations in flux with quasi-periods of months to years. VAST will produce light curves and detect this sort of variability for more than 10⁴ sources over the whole sky. The Fermi gamma-ray satellite observes the whole sky every 3 h and has detected blazar variability on timescales as short as 6 h (Abdo et al. Reference Abdo2010). Simultaneous measurements of the gamma-ray and radio variability in blazars will provide us with a powerful tool for understanding the physics of the central engines in AGNs.

Beyond single black holes in AGNs, a number of binary supermassive black holes (SMBHs) are known or suspected (e.g. Komossa Reference Komossa2006; Rodriguez et al. Reference Rodriguez, Taylor, Zavala, Peck, Pollack and Romani2006; Smith et al. Reference Smith, Shields, Bonning, McMullen, Rosario and Salviander2010; Burke-Spolaor Reference Burke-Spolaor2010). The evidence for these varies, but it is clear that with the mergers of galaxies the central SMBHs could also merge. The act of inspiralling may lead to radio transients accessible to ASKAP. This is because the SMBHs will likely move through a static magnetic field provided by an accretion disc, and this should drive electromagnetic jets (Palenzuela, Lehner, & Liebling Reference Palenzuela, Lehner and Liebling2010; Lyutikov Reference Lyutikov2011; Moesta et al. Reference Moesta, Alic, Rezzolla, Zanotti and Palenzuela2012). The detailed emission processes are unclear, but it is plausible that there could be a radio signature accessible to ASKAP before (O’Shaughnessy et al. Reference O’Shaughnessy, Kaplan, Sesana and Kamble2011) or during (Kaplan et al. Reference Kaplan, O’Shaughnessy, Sesana and Volonteri2011) the merger. Numerous other electromagnetic mechanisms have also been proposed; see Schnittman (Reference Schnittman2011) for a detailed discussion. Combining the radio observations with observations at other wavelengths or even with GWs would significantly improve our understanding of galaxy mergers (e.g. Sesana et al. Reference Sesana, Gair, Berti and Volonteri2011).

2.3.2 Tidal Disruption Events

Tidal disruption events (TDEs) occur when a star wanders too close to a black hole at the centre of a galaxy or star cluster. The black hole tidal forces tear the star apart, but the debris continue on ballistic orbits around the star and are accreted as they approach periastron on the following orbit. The resulting sudden increase in the black hole accretion rate may be directly detected at optical and X-ray frequencies. After the initial flare up, the luminosity decays as t ^−5/3 (Rees Reference Rees1990). Several candidates of such TDEs have been detected serendipitously through all-sky X-ray and ultraviolet surveys (Grupe, Thomas, & Leighly Reference Grupe, Thomas and Leighly1999; Komossa & Bade Reference Komossa and Bade1999; Komossa & Greiner Reference Komossa and Greiner1999; Esquej et al. Reference Esquej, Saxton, Freyberg, Read, Altieri, Sanchez-Portal and Hasinger2007, Esquej et al. Reference Esquej2008; Gezari et al. Reference Gezari2006, Reference Gezari2008, Reference Gezari2009, Reference Gezari2012; Lin et al. Reference Lin, Carrasco, Grupe, Webb, Barret and Farrell2011; Cenko et al. Reference Cenko2012a; Saxton et al. Reference Saxton, Soria, Wu and Kuin2012).

Crucially, black hole accretion is often accompanied by relativistic jets, with a mechanical power often comparable to the radiative luminosity. The jet can be a source of beamed radio emission and hard X-rays in a way similar to a blazar. To a hard X-ray telescope such as the Burst Alert Telescope (BAT) on the Swift, it will initially resemble a GRB, but it will be distinguished by its much longer duration. Swift J1644+57 is the first observed example of such a TDE candidate, at a redshift of z=0.35. It was discovered as a luminous X-ray transient and was observed across the electromagnetic spectrum, enabling multi-band analysis and modelling of the event (Bloom et al. Reference Bloom2011; Levan et al. Reference Levan2011; Zauderer et al. Reference Zauderer2011; Berger et al. Reference Berger, Zauderer, Pooley, Soderberg, Sari, Brunthaler and Bietenholz2012; Wiersema et al. Reference Wiersema2012). The mass of the central black hole responsible for this event was derived to be ≲10⁷ M_⊙ and the ejecta Lorentz factor ≳10. More recently, another TDE candidate has also been detected by the Swift in hard X-rays, Swift J2058.4+0516 (Cenko et al. Reference Cenko2012b).

The main difference between AGN and TDE jets is that the former propagate to Mpc distances, and create characteristic cocoon/lobe structures where their kinetic power is dissipated. In contrast, TDE jets are impulsive ejections that produce a forward shock/reverse shock structure as they interact with the ISM (Sari & Piran Reference Sari and Piran1995). As there is no pre-existing cavity around the previously quiescent black hole, TDE jets have to propagate through a much denser environment and decelerate to sub-relativistic speed at distances of ≲1 pc. Most of their kinetic power is expected to be dissipated in the reverse shock (Giannios & Metzger Reference Giannios and Metzger2011). Typical TDE radio flares from the disruption of a solar-mass star by a 10⁷ solar mass black hole are expected to peak in brightness after ∼1 year, reaching a flux density of ∼2 mJy at 1.4 GHz, for a distance of 1 Gpc corresponding to z=0.2 (not including the effect of relativistic boosting for jets oriented along our line of sight). At peak brightness, the expected radio spectrum is inverted (self-absorbed) with F _ν∼ν⁻² below ∼2 GHz, and F _ν∼ν^−1/3 above that (Giannios & Metzger Reference Giannios and Metzger2011).

Radio afterglows of TDEs would be a powerful tool to understand jet physics and source geometry, through measurement of the flux evolution and polarisation. The expected degree of polarisation for these events is low if they are collimated, similar to GRB afterglows. Recent IR and radio polarisation measurements have been found to be consistent with theoretical expectations (Metzger, Giannios, & Mimica Reference Metzger, Giannios and Mimica2012; Wiersema et al. Reference Wiersema2012).

Radio detections of TDE jets hold the key to other unsolved astrophysical problems (e.g. Krolik & Piran Reference Krolik and Piran2012). It is well known that not all accreting black holes have jets, but it is not clear why. Jets are formed only at certain ranges of accretion rate, but other parameters such as the black hole spin may also be important. Finding what fraction and what type of X-ray-detected TDEs have radio jets will help address this question, and will shed light on the evolution of nuclear black holes in galaxies. Furthermore, the timescale for the formation of a radio-bright TDE jet will help us understand how quickly a collimated magnetic field is generated inside the transient accretion disc.

The predicted brightness of TDEs at gigahertz frequencies suggests that VAST should be able to detect about a dozen events per year. A systematic survey of the radio sky would be able to constrain the rate of TDEs and reduce the collimation uncertainty significantly. In particular, the discovery of an entirely new population of orphan TDEs would allow us to place solid constraints on the occurrence rate of this phenomenon and therefore test the theoretical prediction that we should see ∼10⁻⁴ yr⁻¹ per galaxy (Rees Reference Rees1990).

2.3.3 Accreting Neutron Stars, Black Holes and Microquasars

Exploring the connection between accretion discs and jets in XRBs requires correlated radio and X-ray observations (Fender Reference Fender2006). The VAST survey will reveal and subsequently measure the radio emission from a large number of XRBs and allow us to search for new radio outbursts. This, coupled with the recent availability of large field-of-view instruments like Swift-BAT and Fermi- Gamma-ray Burst Monitor (GBM), and with the early results (from 2013) of eROSITA's all-sky survey, will provide a large sample of XRBs with active jets, and will allow us to do statistical studies of their time variability properties and X-ray/radio correlation.

VAST will yield information on the population of XRBs showing quasi-steady-state radio jets, which are mildly relativistic and are associated with non-thermal, radiatively inefficient accretion states (Fender et al. Reference Fender2004; McClintock & Remillard Reference McClintock and Remillard2006). For these persistent jets, fundamental properties such baryonic content, energy budget, emission mechanism, and the role played by the spin of the compact object are not well constrained (Markoff et al. Reference Markoff2005; Narayan Reference Narayan1996; Yuan et al. Reference Yuan2005), and regular radio monitoring will allow us to investigate them. In addition, we will determine the duty cycle of the radio-loud phases and test whether radio state transitions (changes in the kinetic power) are always associated with X-ray transitions (changes in the radiative power). We will also investigate what physical mechanism causes major radio flares and the ejection of highly relativistic, optically thin bullets, often associated with the state transition from the hard/non-thermal to soft/thermal state.

In addition to stellar mass and SMBHs, it is predicted that VAST will detect jets from intermediate-mass black holes (IMBHs) with masses between ∼100 and 1 00 000 M_⊙. IMBHs may provide a pathway for the formation of SMBHs (e.g. Ebisuzaki et al. Reference Ebisuzaki2001) and also have important connotations for other areas of astrophysics (e.g. Fornasa & Bertone Reference Fornasa and Bertone2008; Wang et al. Reference Wang, Wang, Xiang, Wang, Hao and Yuan2010), but until recently their existence was highly disputed. The discovery of the object known as HLX-1 located in the galaxy ESO 243−49, with an X-ray luminosity of ∼1 000 times greater than the Eddington limit for a 10-M_⊙ black hole, currently provides very strong evidence for the existence of IMBHs (Farrell et al. Reference Farrell, Webb, Barret, Godet and Rodrigues2009), with independent studies constraining the mass to 9 000 M_⊙ < M _BH < 90 000 M_⊙ (Davis et al. Reference Davis, Narayan, Zhu, Barret, Farrell, Godet, Servillat and Webb2011; Servillat et al. Reference Servillat, Farrell, Lin, Godet, Barret and Webb2011; Godet et al. Reference Godet2012; Webb et al. Reference Webb2012). Recently, transient radio emission was detected from HLX-1 following a transition from the low/hard to high/soft spectral states, consistent with the ejection of relativistic jets from an IMBH (Webb et al. Reference Webb2012).

Objects such as HLX1 may be very rare (only one known within 100 Mpc), but there are hundreds of ultraluminous X-ray sources (ULXs; see Feng & Soria Reference Feng and Soria2011 for a recent review) in the local Universe with luminosities exceeding the classical Eddington limit of stellar-mass black holes. Most of them are likely to be explained through other mechanisms such as anisotropic emission and/or super-Eddington accretion, but we cannot rule out that a few of them may be IMBHs. In addition, there may be hundreds of low-state or quiescent IMBHs lurking in the core of globular clusters, in the nuclei of dwarf galaxies, or inside recently merged galaxies that may occasionally become active. The detection of jet emission from these objects would allow us to infer the black hole mass, as the radio and X-ray luminosities are correlated as a function of mass (e.g. Fender Reference Fender2006). Steady jets would be undetectable for all but the closest of these objects. However, transient radio flares have been observed from XRBs associated with jet ejection events with radio luminosities a factor of 10–100 times brighter than the non-flaring radio emission (e.g. Körding, Falcke, & Corbel Reference Körding, Falcke and Corbel2006). Such flares should be detectable by VAST out to distances of ∼5–15 Mpc from 1 000-M⊙ black holes, and out to ∼23–72 Mpc from 10 000-M⊙ black holes (S. A. Farrell, in preparation). Thus, we will be able to survey the local Universe for flaring IMBHs, placing constraints on the number density and therefore testing their validity as a pathway for SMBH formation. Conversely, if ULXs are super-Eddington accretors, the detection of radio emission will prove that even at those extreme accretion rates, a substantial fraction of accretion power can come out as kinetic power, and will improve our understanding of black hole feedback at high accretion rates.

2.3.4 Neutron Star Mergers

The merger of two compact objects (neutron stars) is expected to generate strong GWs (Section 2.1.3) but they will also reveal themselves through electromagnetic radiation. The relativistic, beamed outflows detected from short–hard GRBs are thought to originate from such mergers (e.g. Fong et al. Reference Fong2012). VAST is not expected to fortuitously observe such beamed events. However, merger simulations also show that substantial mass can be ejected from these systems quasi-isotropically and at sub- or trans-relativistic velocities, and with energies of 10⁴⁹-10⁵⁰ erg (Piran, Nakar, & Rosswog Reference Piran, Nakar and Rosswog2012).

Consider a mildly relativistic blast wave (β₀∼0.8) moving out with an energy of ∼10⁴⁹ erg into the surrounding medium. The shock wave sweeps the surrounding medium and heats it to relativistic temperatures by converting bulk kinetic energy of the incoming material into thermal energy of the shocked material. The electrons in the shock-heated plasma are accelerated in the post-shock magnetic field to radiate synchrotron radiation. This would eventually be optically thin in the gigahertz regime, peaking on a timescale of about a couple of months when the blast wave starts to decelerate. The optimum waveband for detecting these transients, which are also supposed to be prime sources of GWs, is around gigahertz frequencies (Nakar & Piran Reference Nakar and Piran2011). A merger within 300 Mpc would shine as a radio transient of about 1-mJy brightness at ∼1 GHz (A. Kamble & Kaplan Reference Kamble and Kaplan2013), which would be detectable to the VAST and, potentially, also to Advanced-LIGO. Both the peak flux density and the time to peak are uncertain since we lack accurate estimates of the ejecta velocity and energy, and we expect the circumburst density to vary from 1 to 10⁻⁴ cm⁻³.

Using the ‘best-bet’ neutron star merger detection rates predicted for Advanced LIGO (Abadie et al. Reference Abadie2010), we estimate that up to about half a dozen of these transients should be detectable by ASKAP at any given time, although there are significant uncertainties involved. Piran et al. (Reference Piran, Nakar and Rosswog2012) independently estimate ASKAP's sensitivity to these events and obtain similar results. These are comparable to the rate estimated from observations of short GRBs which have been proposed to originate in similar mergers (Coward et al. Reference Coward2012). Metzger & Berger (Reference Metzger and Berger2012) emphasise the challenges related to uncertain GW event localisations, which are significant. As discussed in Section 2.1.3, the large instantaneous sky coverage of ASKAP may be a significant advantage in this situation.

2.3.5 Flaring and Pulsing Neutron Stars

Of the known neutron stars, the vast majority are pulsars that have been detected through their pulsed radio emission. VAST will not be sensitive to radio pulsations on timescales of ≲1 s, but will instead probe neutron star populations that are not selected in pulsar surveys. We will be sensitive to intermittent or deeply nulling radio pulsars as well as radio pulsars with large variations in their average pulsed flux.

Magnetars will be detectable through their rare flaring events such as the giant flare and expanding radio nebula produced by SGR 1806−20 (Gaensler et al. Reference Gaensler2005; Cameron et al. Reference Cameron2005). In addition, some magnetars are known to turn on as radio sources. For example, XTE J1810−197 was initially detected as a transient radio source (Halpern et al. Reference Halpern2005) before its radio pulsations were identified (Camilo et al. Reference Camilo2006). We may also be able to detect bright intermittent pulses from the enigmatic rotating radio transients (RRATs; McLaughlin et al. Reference McLaughlin2006). VAST will thus reveal a broader census of the Galactic neutron star population than a standard pulsation search, with subsequent implications for the overall SN event rate in our Galaxy.

The radio source FIRST J1023+0038 presents an interesting case study. Initially, it was classified as a Galactic cataclysmic variable (CV; Bond et al. Reference Bond, White, Becker and O’Brien2002) because of its optical spectrum and variability, but its radio flaring behaviour appeared to be anomalous. The classification was called into question because of the changes in the spectrum over time (e.g. Thorstensen & Armstrong Reference Thorstensen and Armstrong2005) and it was eventually re-identified as a transitional object where a recycled radio pulsar had turned on after a low-mass XRB phase (Archibald et al. Reference Archibald2009, Reference Archibald, Kaspi, Bogdanov, Hessels, Stairs, Ransom and McLaughlin2010). The actual transition event had an extremely short duration, with the accretion disc apparently dissipating between 2001 December and 2002 May. The radio pulsar was probably absent in 1998 (upper limits of 1.8 and 3.4 mJy at 1.4 GHz) but detected in a survey in mid-2007 with a flux density of ∼14 mJy at 1.6 GHz.

2.3.6 Classical Novae

Novae occur in binaries (CVs) that consist of a white dwarf that is accreting matter from its companion star. A nova is a thermonuclear explosion in the layer of accreted matter on the surface of a white dwarf. A shell of matter is ejected during the explosion and this shell might produce radio emission. The radio emission can persist for a few years after the explosion, and in the case of FH Ser it reached ∼10 mJy at 2.7 GHz about 1 year after the outburst. In the late stages when the ejecta is optically thin, radio emission is generated by the entire ejecta and can therefore be used to estimate the total ejecta mass (see e.g. Hjellming Reference Hjellming, Cassatella and Viotti1990 for a review).

2.3.7 Flare Stars, Pre-Main-Sequence Stars, and Rotation-Driven Stellar Activity

There are several types of stars that display large flares that can be observed in the radio band. The most common are cool dwarf stars of spectral type M or later including brown dwarfs. The flares have typical flux densities above ∼1 mJy, but there is also quiescent emission at the sub-mJy level between the flares (e.g. Pestalozzi et al. Reference Pestalozzi, Benz, Conway and Güdel2000). For these types of systems, it has been argued that radio observations are the best way to obtain information on the stellar magnetic field strength (Berger Reference Berger2006). The number of flaring events that are potentially detectable is unclear, but Berger (Reference Berger2006) obtained a detection rate of ∼10% for objects of later spectral class than M7. They also find that the radio to X-ray luminosity ratio increases over at least three orders of magnitude for these objects compared to earlier spectral classes. VAST will undertake a census of such stars in the local (10 pc) neighbourhood.

In AM Her-like systems (cataclysmic variables in which the white dwarf has a surface magnetic field of ∼10³-10⁴ T), the M dwarfs are forced to corotate with the binary, which has an orbital period of a few hours. The dwarf star then becomes more active; flares in AM Her itself have been observed to be as bright as 10 mJy at 5 GHz (Dulk, Bastian, & Chanmugam Reference Dulk, Bastian and Chanmugam1983).

Another group of stars that can produce radio flares are the RS CVn systems (Gunn et al. Reference Gunn, Spencer, Abdul Aziz, Doyle, Davis and Pavelin1994). These are close detached binaries consisting of two main-sequence or evolved stars of spectral type F or later that are tidally locked. The rapid spin of the stars leads to strong magnetic activity, which is manifested as large spots on the stars and also as flaring activity that can be observed both in X-rays and at radio wavelengths.

Flare-like outbursts are also observed in pre-main-sequence stars, but it is not clear whether the physical mechanism is really that of a stellar flare or related instead to the accretion flow onto the star. Such outbursts have been observed in both naked T Tauri stars, which do not show signs of extended accretion discs, and in systems with an IR excess, which indicates the presence of a disc (Osten & Wolk Reference Osten and Wolk2009).

A magnetic field can convert the rotational energy of a star into an outflow and electromagnetic radiation. The most well-known examples of this are the classical radio pulsars, but a similar phenomenon can arise in less extreme stars. Radio observations have detected two fully circularly polarised radio pulses per spin period from the Ap star CU Vir, which is also spinning down in a way similar to a pulsar (e.g. Trigilio et al. Reference Trigilio, Leto, Umana, Buemi and Leone2008, Reference Trigilio, Leto, Umana, Buemi and Leone2011; Lo et al. Reference Lo2012).

Another example of activity driven by rotation is the white dwarf in the cataclysmic variable AE Aqr. The spin period of the white dwarf is 33 s, so rapid that the white dwarf does not accrete the matter that streams over from the companion K dwarf. Instead, the matter is ejected from the system. AE Aqr can be observed over a wide range of frequencies from X-rays to radio, and pulsed emission is observed at optical and X-rays, but it has not yet been detected at radio wavelengths (De Jager et al. Reference De Jager, Meintjes, O’Donoghue and Robinson1994; Bookbinder & Lamb Reference Bookbinder and Lamb1987; Bastian, Dulk, & Chanmugam Reference Bastian, Dulk and Chanmugam1988).

4.1.1 VAST Wide

To detect rare events such as GRBs and SNe, and to map out the gas density distribution of the ISM across the whole sky, a large survey area is required. To fully sample the ESE light curves, a regular (preferably daily) cadence is desirable. Two years of data are required to discriminate between an annual cycle and intrinsic variability for IDV sources.

Hence, our planned survey will cover 10 000 deg² (comprised of 400 pointings, each of 30 deg²). Integrating for 40-s per pointing will give an expected RMS sensitivity of 0.5 mJy beam⁻¹, with a total observing time (including overheads) of 6 h day⁻¹. This survey will be repeated daily for 2 years, with a total time of 4 380 h. Figure 3 shows that it will be a substantial improvement over existing blind gigahertz surveys.

4.1.2 VAST Deep

In our Deep survey we plan to observe 10 000 deg², with 400 pointings each observed for 1 h. This would achieve an expected RMS sensitivity of 50 μJy beam⁻¹. Each of the fields will be observed by the Evolutionary Map of the Universe (EMU) collaboration (see Section 4.2) for their continuum survey (Norris et al. Reference Norris2011). While both the Deep and the Wide surveys observe the same amount of the sky, they are complementary in cadence and sensitivity.

One field, coinciding with a field observed by the EMU collaboration, will be repeated daily for 1 year (Deep Single field). The other fields will be repeated eight times at irregularly spaced intervals (Deep Multi-field). This is the optimal survey to detect as-yet unknown source classes (see Bower et al. Reference Bower2007) and detect GRBs and SNe to larger distances.

4.1.3 VAST Galactic

A third component of VAST is a survey of 750 deg² of the Galactic plane. This is a total of 30 pointings, plus an additional three pointings, to cover the Large and Small Magellanic Clouds. Each pointing will be observed for 16 min, achieving an expected RMS sensitivity of 0.1 mJy beam⁻¹. The Galactic plane survey will require 9 h of observing in a single day, repeated weekly for 1 year and supplemented by additional epochs at longer intervals.

5.5.1 Archival Classification

We approached the classification problem using the standard methodology of supervised machine learning with 10-fold cross-validation. First, we extracted a set of features from each light curve. These include time-domain features which are simply the flux measurements at each time point (tme), statistical features (stat) such as the mean, standard deviation, and skew, following Richards et al. (Reference Richards2011); statistical (stat) features built cumulatively (stat-cum) after observing a fixed number of observations in multiples of k (k, 2k, etc.); periodic features derived from the Lomb–Scargle periodogram (lsp); and wavelet features derived from the Haar wavelet (wlet). We then created two more feature sets that are concatenations of the above. Feature set all-reps is the concatenation of stat-cum, lsp, and wlet, and feature set all concatenates tme, stat-cum, lsp, and wlet. Details of all seven feature sets are discussed by Rebbapragada et al. (Reference Rebbapragada, Lo, Wagstaff, Reed and Murphy2012).

We used these features as input to a set of machine-learning algorithms, which, when given a training set of examples, will learn the discriminative features for each source type. Using the Weka toolkit (Hall et al. Reference Hall, Frank, Holmes, Pfahringer, Reutemann and Witten2009), we experimented with three classification algorithms: Support Vector Machines (SVM; Cortes & Vapnik Reference Cortes and Vapnik1995), Random Forests (Breiman Reference Breiman2001), and the J48 Decision Tree (Quinlan Reference Quinlan1986). Figure 7 shows the classification accuracy in the archival setting. The Random Forest is the best performing classifier when used with the combined feature sets, achieving 93% accuracy. This suggests that archival classification using light curves alone is feasible with a reasonable degree of accuracy, given the large number of sources. However, the performance on real light curves is likely to be lower since our simulated light curves have ideal Gaussian noise and are aligned to the starting time of the event.

Figure 7.

Comparison of overall classification performance (accuracy percentage) by feature set (tme, stat, stat-cum, wlet, lsp, all-reps, and all) and classification algorithm (J48, Random Forest, and SVM), with results grouped by classification algorithm.

5.5.2 Online Classification

The aim of online classification is to identify sources of interest as soon as possible after new observations arrive. The earlier a classification is made, the more likely that a follow-up observation can be scheduled, and so online classification should run continuously as part of the real-time transient detection pipeline. We evaluated online classification using the first 30 d of simulated light curves. With so few observations, it is not possible to extract periodogram or wavelet features; hence, we were limited to the statistical feature set and the time-series values themselves.

Figure 8 shows the confusion matrix for our best performing algorithm: the J48 classifier built with stat features extracted from 30 d of observations. The overall classification accuracy achieved was 50% across eight source types. This is better than the baseline expected from random classification (12% accuracy) but probably not sufficient to be useful in scheduling follow-up observations. Accuracy was improved by grouping source types with similar light-curve characteristics together (grouping SNe and novae, BG, and ESE yielded an accuracy of 70%). Finally, accumulating more data led to higher accuracy (∼80% accuracy with 100 d of observations).

Figure 8.

Online classification confusion matrix shown as a heat map (red = higher accuracy, blue = lower accuracy). The x-axis is the classified output class and the y-axis is the actual input class. We used 200 sources of each type and the number in each cell represents how these sources are classified. Overall accuracy at 30 d is 50%.

5.5.3 Other Machine-Learning Approaches

Considerable insight into the classification problem can be gained from the current efforts on classification of transients in optical synoptic sky surveys (Bloom et al. Reference Bloom, Starr, Butler, Nugent, Rischard, Eads and Poznanski2008; Donalek et al. Reference Donalek, Mahabal, Djorgovski, Marney, Drake, Glikman, Graham and Williams2008; Mahabal et al. Reference Mahabal2008a, Reference Mahabal and Bailer-Jones2008b, Reference Mahabal, Djorgovski, Donalek, Drake, Graham, Williams, Moghaddam, Turmon, Mizumoto, Morita and Ohishi2010, Reference Mahabal2011; Djorgovski et al. Reference Djorgovski2011). We experimented with several approaches, using the data from the Palomar-Quest and Catalina Real-Time Transient Surveys, as well as selected data sets from the literature.

Similar to the work described in Section 5.5.1, we parameterised observed optical light curves as feature vectors and applied various machine-learning techniques (e.g. decision trees) to their classification. A set of decision trees was trained using the feature vectors for various combinations of classes. To reduce the dimensionality of the input space, we applied a forward feature selection strategy that consisted of selecting a subset of features from the training set that best predicts the test data by sequentially selecting features until there is no improvement in prediction. Each tree was built using the Gini diversity index as criterion for choosing the split; the splitting stopped when there is no further gain that can be made. To avoid over-fitting we used a 10-fold cross-validation approach. The decision trees were pruned in order to choose the simplest one within one standard error of the minimum. When tested on a data set consisting of light curves of blazars, cataclysmic variables, and RR Lyrae stars, we achieved completeness in the range of ∼83%–97% and contamination in the range of ∼4%–13% (C. Donalek et al., in preparation). In future work, we will apply this to the classification of radio light curves.

Another approach we experimented with used two-dimensional distributions of magnitude changes for different time baselines for all possible epoch pairs in the data set (Djorgovski et al. Reference Djorgovski2011; B. Moghaddam et al., in preparation). These two-dimensional (Δm, Δt) histograms can be viewed as probabilistic structure functions for the light curves of different types. Template distributions for different kinds of transients and variables are constructed using the reliably classified data with the same survey cadences, S/N, etc. For any newly detected variable or a transient, corresponding (Δm, Δt) histograms are accumulated as the new data arrive, and we used a variety of metrics to compute the effective probabilistic distances from different templates. Initial testing indicates that we can obtain a high classification accuracy using this approach. We plan to generalise it to include triplets or even higher order sets of data points for multi-dimensional histograms.

One important lesson so far is that existing archival and contextual data will play a critical role in classification. This includes a spatial context (i.e., what is near the observed event on the sky—a possible host galaxy, a cluster, an SN remnant, etc.), the multi-wavelength context (has it been detected at other wavelengths, what is its broad spectral energy distribution), and the temporal context (what was its flux variability or a detection history in the previously obtained data). Some of this information can be readily extracted from the archives (e.g. flux measurements from different wavelengths, light curves at that location), but some—spatial context in particular—require a human judgement, e.g., is the apparent proximity to other objects (galaxies, clusters, etc.) likely to be relevant, and if so, what does it imply about the transient? Human inspection of vast numbers of transients does not scale to the massive data streams such as those contemplated here. We are currently experimenting with crowdsourcing approaches to harvesting of the relevant human pattern recognition skills and domain expertise, and their translation to machine-processable algorithms.

5.6.1 Very Long Baseline Interferometry

As noted earlier, radio sources that are highly variable on short timescales are necessarily intrinsically compact. ASKAP's angular resolution of ∼10 arcsec will be high enough to localise variable sources on the sky and measure variability, but will be orders of magnitude too low to resolve the evolving structures that give rise to the variations in integrated flux. In order to resolve the structure and evolution of the radio source, much higher angular resolution imaging is required, provided by Very Long Baseline Interferometry (VLBI).

Alerts from the VAST pipelines will be used to trigger observations using the Australian Long Baseline Array (LBA), now including telescopes from Western Australia (ASKAP) to New Zealand (Warkworth), allowing milliarcsecond-scale imaging. Due to the very large number of VAST alerts, targets for VLBI follow-up will have to be chosen selectively and will need to be restricted to the most interesting or rare classes of transient and variable sources. The Australian LBA is very well suited to this type of follow-up, as evidenced by previous program that have taken triggers for transient radio sources from Molonglo Observatory Synthesis Telescope (MOST) or the Australia Telescope Compact Array (ATCA) (Tingay et al. Reference Tingay1995; Miller-Jones et al. Reference Miller-Jones2012).

The current LBA has a sensitivity of 60 μJy beam⁻¹ for a 1-h integration at 2.3 and 8 GHz. This is comparable to the sensitivity of ASKAP at 1.4 GHz. With ASKAP incorporated as an element of the LBA, the sensitivity at 1.6 GHz would be 40 μJy beam⁻¹ for a 1-h integration. Hence, a 12-h observation with the LBA would be able to detect any unresolved source found by the VAST surveys.

The recent advent of real-time eVLBI on the LBA means that the VLBI observations can be rapidly processed and analysed. Thus, subsequent observations can be adapted based on the initial results. This facility will benefit studies of new transients that will be detected by ASKAP.