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Studies of jets from Galactic black holes and neutron stars

Final Report Summary - GALACTIC JETS (Studies of jets from Galactic black holes and neutron stars)

Background

The aims of this project were to improve our understanding of how black holes produce relativistic jets. Unfortunately, studying the jet properties from extragalactic quasars can be difficult as there can be large uncertainties associated with measuring, for example, their distance and mass. Extragalactic jets can also be >10,000 light years across, so changes in the jet can take much longer than the life-time of the researcher! Therefore, this project aimed to study the much closer Galactic Jets, as a tool that will implicitly produce insights into the inner workings of super-massive black holes.

Galactic Jets can be produced by X-ray binaries (XRBs), which are formed from the remnant of a star that has collapsed to form a compact object (e.g. a neutron star or black hole) and has remained gravitationally bound to its binary companion star. Radio emission from XRBs produces non-thermal spectra and the polarisation properties are consistent with self-absorbed synchrotron radiation, which is indicative of a jet. For a few XRBs, high-resolution radio observations have directly resolved a transient jet. We have produced a basic understanding of the relationship between the X-ray emission and the Galactic jet during different X-ray spectral states. As the X-ray luminosity increases, so does the bulk Lorentz factor of the jet (Γ) and the jet luminosity. When the X-ray spectral colour changes from a hard-state to a soft-state (i.e. becomes dominated by lower-energy X-ray photons), shocks form in the jet and the outflow is quenched.

Radio emission is also produced from the supermassive black hole at the Galactic Centre known as Sgr A*. It is believed that most or all galaxies contain a supermassive black hole at their centre and through powerful relativistic jets they regulate the formation of the surrounding host galaxy, hence play an important role within the universe. Like XRBs, jets from supermassive black holes are thought to be transient and are produce by changes in the accretion flow. Whilst the accretion flow around Sgr A* is thought to be very low, it is not known if the radio emission is still produced by a jet (as seen in active galactic nuclei) or if the emission comes directly from the accretion disk. Solving this mystery is important to our understanding of how black holes work at very low accretion rates.

Aims and methods

This project aimed to study Galactic jets by taking new measurements with high-resolution radio interferometers. As these sources are transient in nature, further monitoring produced new insight to their behaviour and different sources go into outburst. Also, recent upgrades to observatories such as the Very Large Array (VLA) in New Mexico, USA, have significantly increased the instrumental sensitivity of radio telescopes.

We therefore submitted proposals to the VLA, VLBA, and EVN radio telescopes to observe XRBs when they went into outburst. Our strategy was to trigger observations when hard X-rays dominated the luminosity. We then monitored a few individual sources over a period of a few days/weeks to detect changes in the structure and lightcurve.

During this project, we also had a unique opportunity to monitor Sgr A* as a gas cloud approached our Galactic supermassive black hole. Infrared observations showed that a gas cloud, known as “G2”, would come within close proximity of Sgr A* and could potentially fall into the black hole. We therefore submitted further proposals to monitor the Galactic centre at all wavelengths, including within the radio band.

Results

Our main results successfully caught the radio and X-ray outburst of a stellar-mass black hole. The XRB known as XTE J1908+904 was detected to enter a hard X-ray state at the end of 2013 and we triggered a series of radio observations using the EVN and VLBA instruments (see attached figure). The source was found to eject two knots of plasma that appeared to expand; this was the first time we have measured the expansion of particles from a Galactic black hole.

We also caught the outburst of the luminous Galactic low-mass XRB Cygnus X-2. A directly resolved ejection event has been discovered while the source was on the horizontal branch of the Z-track.

We successfully monitored Sgr A* during the encounter with G2. Although no changes in the radio emission was detected a series of useful results were found. The multi-wavelength monitoring had shown the gas cloud probably contained a dense stellar core as the cloud did not disintegrate into the black hole and the cross-sectional area for G2 was less than 2 × {10}29 cm2. Also, small flares from Sgr A* seemed to show a frequency dependent-lag in the radio band, suggesting some of the radio emission could be from a jet.

We also detected an accreting neutron star near to Sgr A* (known as AX J1745.6-2901) which went into outburst where winds (traced by the same Fe K absorption features) are observed only during the accretion-disc-dominated soft states, and disappear during the hard states characterized by jet emission. We also re-processed some of the observations of Sgr A* to measure a pulsar in the Galactic centre. We showed that the velocity and position are consistent with a bound orbit originating in the clockwise disk of massive stars orbiting Sgr A* and a natal velocity kick of <~ 500 km s-1. These results support the hypothesis that Galactic centre pulsars will originate from the stellar disk and deepen the mystery regarding the small number of detected Galactic centre pulsars.

Finally, we triggered observations of a series of XRBs that place upper limits on their radio luminosity. These observations put important limits on jet production during the different phases of the accretion process.

Conclusions

This project successfully advanced our understanding of Galactic jets. We now know the apparent opening angle of a relativistic Galactic jet, which places a strong constraint on the maximum velocity of the relativistic particles. We also detected and placed upper limits of the radio emission on a series of neutron star XRBs. Whilst unfortunately we did not detect any major outburst from the Galactic Centre, we showed that G2 did not significantly interact with the black hole and we accurately measured the proper-motion of the closest pulsar. Finally, whilst our results further suggest Sgr A* comes from a Galactic jet, further measurements are required to conclusively confirm this hypothesis.

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