Final Report Summary - RADIOTRANSIENTS (Detecting and characterizing radio transients)
It has recently been found that violent, extremely energetic astrophysical events are far more common than previously thought. These extreme events --- nova eruptions, supernova explosions, magnetar outbursts, neutron star mergers, and relativistic jets ejected from accreting black holes --- provide an opportunity to study the laws of physics operating in conditions of density, temperature, pressure, velocity, and gravity well beyond anything achievable in a laboratory.
Optical emission is easily attenuated by interstellar dust. This fact, combined with their typically narrow fields of view, means that optical telescopes do not commonly discover extreme events. Furthermore, although many of these events are detected by their bright X-ray and gamma-ray emission, wide-field instruments operating in these bands suffer from limited sensitivity, and their poor angular resolution severely complicates the correct identification of high-energy sources at other wavelengths. Consequently, ground-based radio telescopes are the facilities best suited for finding the most violent events in the universe --- radio emission is ubiquitously associated both with particles accelerated to relativistic energies, and with regions where particles are induced to move and emit coherently. Until very recently, radio telescopes were also only capable of delivering narrow fields of view, so that past radio surveys have been too slow to discover rare and rapid events. However, the revolutionary new generation of facilities being developed as pathfinders for the Square Kilometer Array (SKA) is now changing this. Vast technological advances mean that it has become possible to connect thousands of small detectors, resulting in sufficiently wide fields of view to compete even with all-sky X-ray and gamma-ray monitors, but, importantly, with the added advantages of greater sensitivity and arcsecond angular resolution.
By discovering transient radio sources and identifying their physical origins through optical follow-up, as well as by monitoring known sources at radio, optical, and X-ray wavelengths, we will be able to study normally dormant super-massive black holes, probe relativistic accretion, investigate the mechanisms of jet formation, measure kinetic feedback into the ambient medium, and test theoretical models of the evolution of black hole and neutron star binaries. We might in addition identify coherent radio bursts at cosmological distances, and use them as probes of the intergalactic medium.
An ambitious program to detect and characterize a large number and variety of transient radio sources is ongoing at the University of Southampton in the UK, in partnership with the University of Cape Town in South Africa. At the heart of this is the new generation of radio telescopes, in particular MeerKAT in South Africa, and LOFAR in Europe. The purpose of my project is to exploit these new telescopes, as well as the infrastructure being created to deal efficiently with the large volume of data, to study high-energy astrophysical processes. I have been implement a program of multi-wavelength follow-up of newly discovered sources and simultaneous monitoring of known objects. The multi-wavelength observations that I will coordinate are crucial for securing the astrophysical interpretation of new transients and for the study of jets in known sources.
• summary description of the project objectives
For the complete incoming as well as return phase, the project objectives are:
– Refine target lists and observing strategy; write telescope proposals.
– Installation and commissioning of the MeerKAT optical monitor.
– Detailed studies of the most interesting individual jet sources.
– Detailed study of a tidal disruption event.
– Comparison of a large set of white dwarf jets.
– Comparison of jets in accreting WDs and XRBs.
– Study of the rate and properties of TDEs.
– Constrain binary evolution theory using Galactic Bulge LMXB sample.
• description of the work performed since the beginning of the project
– Refine target lists and observing strategy; write telescope proposals (2 months).
I have spent some time becoming familiar with transient detection algorithms. Furthermore, I have been involved in a project, led by PhD student Gosia Pietka, aimed at classifying radio transients based on their rise and decay times. We are now in a position to obtain rapid multi-wavelength follow-up of transients, as soon as the radio data starts being taken.
– Installation and commissioning of the MeerKAT optical monitor (3 months)
Funding has now been obtained for the MeerKAT optical monitor, MeerLICHT. The design is finalized, and data reduction software is in place. All problems regarding data storage and transfer have been solved. Commissioning observations will be taken before the end of this calendar year, meaning that this facility will be ready to provide scientifically valuable, simultaneous optical coverage of all the radio observations.
I have taken part in regular meetings with colleagues in Nijmegen and Cape Town working on the development of MeerLICHT.
– Detailed studies of the most interesting individual jet sources (3 months)
The study of jets is an important part of my project, and a subject outside of my area of expertise. This means that learning about jets were one of the main researcher training activities foreseen in the project. I have been immersed in a leading group in the area of jets, and have gained valuable knowledge on the subject. As soon as TunderKATs observations start, I will investigate the behaviour of jets and the coupling between accretion and jet ejection in a sample of black hole, neutron star, and white dwarf systems.
– Detailed study of a tidal disruption event (8 months)
Since the TunderKATs key science project has not started taking data, we have yet to detect a tidal disruption event. We expect to detect a large number of these events with MeerKAT, we will obtain the first multi-wavelength follow-up observations using Director's Discretionary Time proposals submitted after the radio discovery observations (since these are relatively slowly evolving transients).
– Comparison of a large set of white dwarf jets (4 months)
One of the key questions we want to address with our monitoring observations is how common jets are in accreting WDs, since no systematic study has yet been done. Jets have to date only been detected in a handful of white dwarfs, but the future study of jets in these systems is promising, since our collaborators, Elmar Koerding and Deanne Coppejans in Nijmegen, have recently detected an additional 4 cataclysmic variables in the radio band. We will investigate the presence and properties of jets over a range of accretion rates (both in individual varying sources, and over a range of object classes with different typical accretion rates). This requires a large dataset,which will only be available later on in the project.
• description of the main results achieved so far
In collaboration with Koji Mukai, I investigated the Galactic intermediate polar population detected in soft gamma-rays, and found evidence for a faint, short-period population that is significantly under-represented in existing observational samples. This provides a likely explanation for the evolutionary fate of long-period intermediate polars.
In a project led by Don Hoard, we found excess emission (over the expectation for a theoretical steady state accretion disk) at mid-infrared wavelengths in a sample of 12 nova-like variables. We investigated the origin of this excess in terms of emission from bremsstrahlung or circumbinary dust, facilitated by the mass outflows, and found that either mechanism is capable of explaining existing data, and that stricter observational constraints are needed.
Together with collaborators at Texas Tech University, I have explored the duty cycles of the common optical transients, dwarf novae, and constrained their space density in the Galactic Bulge direction. We found a quantitative empirical relation between the duty cycle of DNe outbursts and the X-ray luminosity of the system in quiescence. Our space density estimate is the first not based on a solar neighbourhood sample. We also discussed the likely detection frequency of such systems by the future Large Synoptic Survey Telescope in the light of these results.
Finally, I have been involved in a project led by a PhD student in our group, Aidan Glennie, regarding the discovery of two new X-ray transients in archival Chandra data. The physical nature of both transients remain uncertain, but one is consistent with tidal disruption of a white dwarf by an intermediate-mass black hole. The second was likely a bright flare from a nearby late M or early L dwarf. The project illustrates the use of archival data in the study of transient phenomena.
• the expected final results and their potential impact and use
The results achieved to date are summarized above. They include work on X-ray transients, as well as the population and multi-wavelength properties of certain types of interacting binaries. This has been presented in refereed papers.
All of this work has important implications for the future study of transient events at all wavelengths, including predictions of the rates of different types of transients in specific upcoming surveys.
I expect that the final results of the project will be of great value in the study of both radio transients, as well as transients detected at other frequencies.
Optical emission is easily attenuated by interstellar dust. This fact, combined with their typically narrow fields of view, means that optical telescopes do not commonly discover extreme events. Furthermore, although many of these events are detected by their bright X-ray and gamma-ray emission, wide-field instruments operating in these bands suffer from limited sensitivity, and their poor angular resolution severely complicates the correct identification of high-energy sources at other wavelengths. Consequently, ground-based radio telescopes are the facilities best suited for finding the most violent events in the universe --- radio emission is ubiquitously associated both with particles accelerated to relativistic energies, and with regions where particles are induced to move and emit coherently. Until very recently, radio telescopes were also only capable of delivering narrow fields of view, so that past radio surveys have been too slow to discover rare and rapid events. However, the revolutionary new generation of facilities being developed as pathfinders for the Square Kilometer Array (SKA) is now changing this. Vast technological advances mean that it has become possible to connect thousands of small detectors, resulting in sufficiently wide fields of view to compete even with all-sky X-ray and gamma-ray monitors, but, importantly, with the added advantages of greater sensitivity and arcsecond angular resolution.
By discovering transient radio sources and identifying their physical origins through optical follow-up, as well as by monitoring known sources at radio, optical, and X-ray wavelengths, we will be able to study normally dormant super-massive black holes, probe relativistic accretion, investigate the mechanisms of jet formation, measure kinetic feedback into the ambient medium, and test theoretical models of the evolution of black hole and neutron star binaries. We might in addition identify coherent radio bursts at cosmological distances, and use them as probes of the intergalactic medium.
An ambitious program to detect and characterize a large number and variety of transient radio sources is ongoing at the University of Southampton in the UK, in partnership with the University of Cape Town in South Africa. At the heart of this is the new generation of radio telescopes, in particular MeerKAT in South Africa, and LOFAR in Europe. The purpose of my project is to exploit these new telescopes, as well as the infrastructure being created to deal efficiently with the large volume of data, to study high-energy astrophysical processes. I have been implement a program of multi-wavelength follow-up of newly discovered sources and simultaneous monitoring of known objects. The multi-wavelength observations that I will coordinate are crucial for securing the astrophysical interpretation of new transients and for the study of jets in known sources.
• summary description of the project objectives
For the complete incoming as well as return phase, the project objectives are:
– Refine target lists and observing strategy; write telescope proposals.
– Installation and commissioning of the MeerKAT optical monitor.
– Detailed studies of the most interesting individual jet sources.
– Detailed study of a tidal disruption event.
– Comparison of a large set of white dwarf jets.
– Comparison of jets in accreting WDs and XRBs.
– Study of the rate and properties of TDEs.
– Constrain binary evolution theory using Galactic Bulge LMXB sample.
• description of the work performed since the beginning of the project
– Refine target lists and observing strategy; write telescope proposals (2 months).
I have spent some time becoming familiar with transient detection algorithms. Furthermore, I have been involved in a project, led by PhD student Gosia Pietka, aimed at classifying radio transients based on their rise and decay times. We are now in a position to obtain rapid multi-wavelength follow-up of transients, as soon as the radio data starts being taken.
– Installation and commissioning of the MeerKAT optical monitor (3 months)
Funding has now been obtained for the MeerKAT optical monitor, MeerLICHT. The design is finalized, and data reduction software is in place. All problems regarding data storage and transfer have been solved. Commissioning observations will be taken before the end of this calendar year, meaning that this facility will be ready to provide scientifically valuable, simultaneous optical coverage of all the radio observations.
I have taken part in regular meetings with colleagues in Nijmegen and Cape Town working on the development of MeerLICHT.
– Detailed studies of the most interesting individual jet sources (3 months)
The study of jets is an important part of my project, and a subject outside of my area of expertise. This means that learning about jets were one of the main researcher training activities foreseen in the project. I have been immersed in a leading group in the area of jets, and have gained valuable knowledge on the subject. As soon as TunderKATs observations start, I will investigate the behaviour of jets and the coupling between accretion and jet ejection in a sample of black hole, neutron star, and white dwarf systems.
– Detailed study of a tidal disruption event (8 months)
Since the TunderKATs key science project has not started taking data, we have yet to detect a tidal disruption event. We expect to detect a large number of these events with MeerKAT, we will obtain the first multi-wavelength follow-up observations using Director's Discretionary Time proposals submitted after the radio discovery observations (since these are relatively slowly evolving transients).
– Comparison of a large set of white dwarf jets (4 months)
One of the key questions we want to address with our monitoring observations is how common jets are in accreting WDs, since no systematic study has yet been done. Jets have to date only been detected in a handful of white dwarfs, but the future study of jets in these systems is promising, since our collaborators, Elmar Koerding and Deanne Coppejans in Nijmegen, have recently detected an additional 4 cataclysmic variables in the radio band. We will investigate the presence and properties of jets over a range of accretion rates (both in individual varying sources, and over a range of object classes with different typical accretion rates). This requires a large dataset,which will only be available later on in the project.
• description of the main results achieved so far
In collaboration with Koji Mukai, I investigated the Galactic intermediate polar population detected in soft gamma-rays, and found evidence for a faint, short-period population that is significantly under-represented in existing observational samples. This provides a likely explanation for the evolutionary fate of long-period intermediate polars.
In a project led by Don Hoard, we found excess emission (over the expectation for a theoretical steady state accretion disk) at mid-infrared wavelengths in a sample of 12 nova-like variables. We investigated the origin of this excess in terms of emission from bremsstrahlung or circumbinary dust, facilitated by the mass outflows, and found that either mechanism is capable of explaining existing data, and that stricter observational constraints are needed.
Together with collaborators at Texas Tech University, I have explored the duty cycles of the common optical transients, dwarf novae, and constrained their space density in the Galactic Bulge direction. We found a quantitative empirical relation between the duty cycle of DNe outbursts and the X-ray luminosity of the system in quiescence. Our space density estimate is the first not based on a solar neighbourhood sample. We also discussed the likely detection frequency of such systems by the future Large Synoptic Survey Telescope in the light of these results.
Finally, I have been involved in a project led by a PhD student in our group, Aidan Glennie, regarding the discovery of two new X-ray transients in archival Chandra data. The physical nature of both transients remain uncertain, but one is consistent with tidal disruption of a white dwarf by an intermediate-mass black hole. The second was likely a bright flare from a nearby late M or early L dwarf. The project illustrates the use of archival data in the study of transient phenomena.
• the expected final results and their potential impact and use
The results achieved to date are summarized above. They include work on X-ray transients, as well as the population and multi-wavelength properties of certain types of interacting binaries. This has been presented in refereed papers.
All of this work has important implications for the future study of transient events at all wavelengths, including predictions of the rates of different types of transients in specific upcoming surveys.
I expect that the final results of the project will be of great value in the study of both radio transients, as well as transients detected at other frequencies.