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Defining a new research topic in astrophysics: Rapidly changing magnetic fields as the missing link to the physics of relativistic jets

Final Report Summary - RELATIVISTIC JETS (Defining a new research topic in astrophysics: Rapidly changing magnetic fields as the missing link to the physics of relativistic jets)

In our Galaxy, the fastest naturally occurring flows of matter known are collimated jets launched close to black holes and neutron stars. These are extremely compact objects, and their strong gravitational field pulls in gas from a nearby star, if such a star is close enough in a tight orbit with the compact object. These objects are X-ray binaries (XBs), and they are the bright objects in the X-ray sky. Processes that are not fully understood accelerate some of the matter into two continuous jets that escape the system at close to the speed of light. In this project the process of accretion onto compact objects, and the jets this process produces, are studied using astronomical data and analysed, shedding light on the jet formation process.
The physical process of jet formation is, after 30 years of research, still poorly understood. The main objective of the project is to use new, novel techniques on state of the art multi-wavelength data sets of XBs, which reveal the properties of these jets in finer detail than was ever possible previously. In particular, time-resolved optical/infrared polarimetry studies reveal the conditions near the jet base, probing the magnetic field structure. Careful analysis of new data provides tight constraints for models and simulations of jet formation.
By achieving the main objectives, the aim is to uncover the physical conditions near the base of relativistic jets, and how these conditions change with the configuration or rate of the matter falling towards the compact object; how the 'accretion disc' feeds the jet. What differences are there between black hole and neutron star jets, and how much energy do these jets release into space? This project therefore helps to answer Astronet's Science Vision question 'How do black hole accretion, jets and outflows operate?' The results are applicable to jets launched from X-ray binary systems in our galaxy (microquasars), supermassive black holes at the centres of galaxies, and gamma-ray bursts, all of which harbour compact objects.
The Researcher has exploited this new field in astrophysical research by combining his experience and knowledge of jets from compact objects with the expertise of variability and polarisation of compact objects by several scientists in the host institution, the Instituto de Astrofisica de Canarias (IAC). Training of the Researcher was accomplished at the IAC via meetings with the Scientist in charge, group meetings, specific data reduction training from telescope instrument specialists at the IAC, and discussions with colleagues at international conferences and institutes. Defining and pioneering this new research field at the host institution has also directly helped his career development, enabling him to achieve a university tenure-track position.
Over the length of the two-year project (1st November 2011 until 1st November 2013), the Researcher has carried out different activities, pursued a number of projects, and published and disseminated the results, with the aim of completing the objectives and promoting EU science.
The work plan included gathering a wealth of multiwavelength data of XB outbursts in order to reveal the jet spectrum. By studying the evolution of the jet spectrum, in particular how the characteristic break frequency varies with luminosity, the total power of the jets can be revealed, and this helps to answer one of the two fundamental questions, “how powerful are the jets?”. By collecting multiwavelength data in the literature and making new observations, I was able to identify the characteristic features in the broadband spectra of jets in a number of black hole X-ray binaries (BH XBs). The results have revealed that BH XBs have a range of different magnetic field strengths, jet base radii and jet powers, and in one object, the jet spectrum is seen to evolve dramatically when the accretion state is changing, revealing how the jet power decreased/increased during this transitional phase of its outburst. This evolution solidified a link between the inflow and outflow (accretion disc and jet), which has implications for the conditions required for jets to be produced, and hence their launching process, which are some of the key objectives. This work resulted in two publications in astrophysical journals led by the Researcher.

The Researcher and Scientist in charge have both acquired and analysed infrared and optical polarimetric data of numerous XBs. These data are essential in allowing us to achieve one of the main aims of the project – to measure the properties of jet magnetic fields, specifically how tangled they are, and how much they vary. For one project, we acquired new near-IR and mid-IR polarimetric measurements of the BH XB Cygnus X-1 with telescopes on La Palma, Spain. These include the first ever mid-IR polarimetry measurements of an XB. The Researcher took radio, UV, X-ray and gamma-ray flux and polarimetric data of Cyg X-1 from the literature, combined it with our new IR data, and developed a theoretical model that is able to reproduce the observed radio to gamma-ray spectrum of Cyg X-1 and its polarisation spectrum. The model requires that the magnetic field is highly ordered in the jet for this object. This is astonishing and differs to studies of some other X-ray binaries, in which the magnetic field is turbulent and variable. This work has been accepted for publication.

Other polarimetry and variability observations have led to a number of results and advances in the field. We have discovered the first direct evidence for a jet emitting in optical/IR in a quiescent XB (at very low mass accretion rate), and the first time its timing properties have been studied. The weak jet in quiescence is highly variable, and polarised at a level of about ~8% (from data acquired during an observing trip by the Researcher to the WHT telescope on La Palma in February 2013), implying a moderately tangled magnetic field.

We have also collected numerous infrared and optical observations of XBs, both black hole and neutron star sources, which we are compiling into a database which will be published as a survey of polarisation measurements of XBs. In almost all cases, intrinsic polarisation is at a level of up to a few per cent, and in some cases there is evidence for variations on minute-timescales. We can conclude that in general, the magnetic fields in jets in XBs may be ubiquitously tangled near their bases (except in the case of Cyg X-1; see above). This essentially answers one of the main questions we aimed to answer in the work program – “What are the physical conditions near the base of relativistic jets?” A highlight of other projects include the result that current measurements of jet power and BH spin (how fast a BH rotates) in XBs do not favour a strong link between them, which gives us clues about the jet launching process.

The Researcher disseminated the work of the project by giving oral presentations at conferences in Spain, Turkey and the USA, and at astronomical institutes/universities during collaborative trips. In total the Researcher gave 12 talks in 7 countries covering Europe, North America and Asia during the period of the project. The publications resulting from the Project also reach a wider astronomical community, and all were posted on the popular pre-print server arXiv (astro-ph) to increase scientific impact. The full impact of the project will be known in years to come, but already researchers are acting on the new information revealed regarding jet physics, due to the results of this project, which promotes EU scientific advancement.

Contact details:

David Russell
New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates