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Understanding the variable and transient non-thermal emission of Galactic sources

Final Report Summary - NONTHERMTRANSIENTS (Understanding the variable and transient non-thermal emission of Galactic sources)

High-energy astrophysics has entered a golden-era of understanding with the advent of current facilities. These instruments are revolutionising our understanding of non-thermal radiation from a plethora of astrophysical sources. In the past two decades a number of ground and space based observatories have opened up an exciting view of the universe from keV to TeV energies and astroparticle physics has become a mainstream field of research. Beyond 10 keV thermal blackbody radiation gives way to the dominant non-thermal emission produced from particle acceleration processes in diverse environments from neutron star magnetospheres to the formation of shocks and collimated jets. These are the most extreme and energetic environments in the universe, beyond anything that can be produced in laboratories on Earth.

The launch of the Fermi Gamma-ray Space Telescope in 2008 revealed a new view of the GeV sky, providing us with an instrument far more sensitive then previously flown, allowing us to survey the sky every 3 hours and search for transient eruptions from objects within our own Galaxy. Fermi has established that at GeV energies the sky is very dynamic, and includes a disparate set of Galactic source populations including gamma-ray binaries, the first detection of jetted emission from a microquasar at GeV energies and the unpredicted discovery of a gamma-ray nova. These sources all have distinct environments but are clearly capable of accelerating particles to high energies. The goal of this project is to advance our understanding of the physical properties and processes that are in operation in this diverse population of transient and variable Galactic sources. This in turn will advance the science of astrophysical particle acceleration and shed light onto the origin of cosmic-rays within our Galaxy.

Results
The research project has been highly successful over the duration of the project as the gamma-ray sky has been highly active providing ample opportunity to explore and analyse the behaviour of sources at high-energies. The goal of the project is to explore transient non-thermal radiation from a variety of astrophysical environments and to this end the project has been a success from the perspective that sources from multiple classes have been seen to switch on or off and have provided a wealth of high quality data.

Microquasars are so named as they are believed to be the small scale analogue to quasars, very energetic and distant active galactic nuclei. They are binary systems in which a compact object, either a black hole or a neutron star, is accreting material from its stellar companion and producing powerful collimated jets of material. This source population is well known at X-ray energies but prior to Fermi's launch there was no definitive detection of such a system at GeV energies. Through time-series analysis of high-energy gamma-ray emission coincident with a giant radio flare from Cygnus X-3 a direct link between the onset of jet emission and the production of gamma-rays has been established (Corbel et al. 2012, MNRAS, 421, 294). Linking the gamma-ray observations with contemporaneous data from other wavelengths identified that the gamma-ray emission preceded and followed the source transitioning into and out of a period when the radio emission is at its weakest and the X-ray emission is very soft. This behaviour can be explained by requiring that the GeV radiation be produced by accelerated particles at a 'sweet-spot' within the jet bounded by strong pair-production on thermal X-rays and a declining seed-photon density for inverse Compton scattering.

Gamma-ray binaries are also systems that contain a compact object and a stellar companion, however the source of their emission is generally still a mystery as the nature of the compact object is often unknown; they could be powered by microquasar jets or the relativistic wind of a pulsar. Of the five known gamma-ray binary systems all bar one has been detected by Fermi. An intensive analysis of the 'missing binary', HESS J0632+057, incorporating 3.5 years of Fermi data was performed. Despite innovative and rigorous background subtraction techniques, to enhance the sensitivity of the analysis, and despite the object being the nearest gamma-ray binary to Earth it is undetected by Fermi and unless it is variable we suspect that it is not luminous enough at GeV energies to be detected within the Fermi mission lifetime. As the source is detected at energies above 136 GeV this implies that the source spectrum must turn over sharply below this energy in order to be undetected by Fermi placing constraints on theoretical models explaining the origin of the very high-energy emission; Caliandro & Hill et al. 2013, MNRAS, 436, 740.

Fermi discovered bright transient activity from the millisecond pulsar (MSP) PSR J1028+0032. Simultaneous multi-wavelength observations revealed that the source had transitioned into a low-mass X-ray binary (LMXB) state. This is an exciting result as it has long been assumed that MSPs are a result of neutron stars being spun up by a binary companion in a LMXB. PSR J1028+0032 was thought to be the first system in which the transition from LMXB to MSP was seen (Archibald et al. 2009), and as this period in the evolutionary cycle is expected to be short, it was expected to be a very rare event. In 2013 the radio pulsations were seen to switch off and at the same time the gamma-ray emission drastically increased by a factor of ~4-5; this was interpreted as an accretion disk reforming around the pulsar causing the radio pulsations to be scattered and allowing the pulsar wind to shock against the accretion disk causing particle acceleration and producing the enhanced gamma-ray emission. This is the first time that an MSP has been seen to transition into an LMXB and implies that the previously hypothesised evolutionary track is not simply one way and likely doesn't happen only once in a given system (Stappers et al. 2014, ApJ, 790, 39). It has been proven that another Fermi source, XSS J12270-4859, is also a transitioning LMXB/MSP system as predicted by Hill et al. (2011). It has recently undergone a transition and the gamma-ray properties of this system are currently being analysed an prepared for publication.

One of the exciting early discoveries of Fermi was of GeV gamma-ray emissions from a nova in 2010, V407 Cyg. Emission at these high energies had never been predicted from such an event however we were able to construct a viable model for the emission based upon some unique characteristics of this rare type of symbiotic binary. However in 2012 and 2013 four more gamma-ray novae have been detected from traditional classical nova type systems and hence an entirely different model is needed to explain the emission from these systems. The analysis of the gamma-ray novae population has identified that they appear to behave very similarly in the gamma-ray band; they have similar shaped gamma-ray spectra and they are detectable for 2-3 weeks (see attached Figure). Despite this similarity at GeV energies they behave quite differently at other wavelengths and it is currently not possible to predict whether a new nova would be expected to be a gamma-ray source. Characterising this new population of high-energy sources is ongoing and will allow us to begin to construct one or more emission models to explain their behaviour. An initial analysis of the gamma-ray novae population was published in Ackerman, M. et al. 2014, Science, 345, 544.

Conclusions
The discovery of gamma rays produced in nova explosions is a very exciting result as it was not anticipated and the origin of this high-energy emission is still an outstanding question although the analysis of new gamma-ray novae has now led to the publication of a number of theoretical models. Consequently this is an active area of research that we are still pursuing. We have analysed and characterised all of the gamma-ray novae to date which has led to theoretical emission models to be constructed and tested against the known characteristics. Furthermore, we can now use the detected population of gamma-ray novae to simulate the detectability of novae at different distances within our galaxy, hence allowing us to estimate the 'gamma-ray horizon' and the fraction of the Galactic novae population that are capable of producing gamma-rays. This has directly feed into the global work of understanding the particle acceleration processes in operation within our galaxy and their relationship to the origin of cosmic rays.

In a similar vein the known binary systems that emit gamma-rays within our galaxy have been explored and analysed in an attempt to further understand the variety of particle acceleration processes in activity within these systems. The use of INTEGRAL and XMM-Newton data has resulted in new studies of Supergiant Fast X-ray transients in the ~10 keV energy domain where the transition from thermal to non-thermal radiation processes takes place. In the history of astronomy there has never been a set of instruments so sensitive to non-thermal radiation in operation at the same time. Currently the latest, deepest catalogues of the gamma-ray and hard X-ray sky as observed by Fermi and INTEGRAL have been completed and have been directly supported by this fellowship. The exploitation of this data to observe variable astrophysical objects within our galaxy is already revolutionising our understanding of the high-energy physics processes in action and providing new questions to be answered by future facilities such as the Cherenkov Telescope Array (CTA).