X-Ray Binaries & Microquasars


Artist's impression of the microquasar GRO J1655-40. Image credit: NASA/STScI

Binary Star Systems as Particle Physics Laboratories

The study of known and/or new binary systems containing compact objects at VHE is very important because the complexity of these systems enables us to probe several important physical processes. These systems are extremely efficient accelerators. Such a high efficiency is not easy to explain in the present theoretical framework of these objects, and could eventually force a revision of particle acceleration theory. Currently just a handful of VHE gamma-ray emitters are known to be binary systems, consisting of a compact object, such as a neutron star or black hole, orbiting a massive star. Whilst many questions about the gamma-ray emission from such systems are still open (in some cases it is not even clear if a pulsar-driven nebula around a neutron star or accretion onto a black hole is the energy source) it is evident that they offer a unique chance to “experiment” with cosmic accelerators.

Along the eccentric orbits of the compact objects around their companion stars, the environment (including the radiation field) changes periodically. When the system geometry is known, these orbital variations can be exploited as a means to gain a better understanding of the nature of the accelerated particles and conditions of the environment.

For example, gamma-ray binaries such as LS I +61 303 and LS 5039 are found to be periodic at GeV and TeV energies, although the emission at the two energies is anti-correlated. A cut-off in the spectrum is observed at a few GeV, which was completely unexpected. The large energy coverage of CTA is essential for disentangling pulsed and continuous emission, and exploring the processes leading to the GeV-TeV spectral differences we observe.

Microquasars and Microblazars

Some binary systems in our own Galaxy, known as microquasars, produce relativistic jets. The physical processes in these objects resemble those occurring around supermassive black holes in distant active galaxies. In the microquasars, the emission timescales are much reduced, providing insights into the emission mechanisms at work that could take months or years to achieve using active galaxies. CTA can also search for microblazars – binary systems in which the jet is pointing directly towards Earth.

The improved angular separation at high energies afforded by CTA will provide opportunities for the study of the way in which microquasars interact with their surroundings, particularly if their jets contain a sizeable fraction of relativistic hadrons. While the inner engines will remain unresolved with CTA, microquasar jets and their interaction with the ISM might become resolvable, leading to the separation of the emission from the central object (which may be variable) from that of the jet-ISM interaction (which may be stable).

Catching Rapid Flares

It is known that black holes display different spectral states in X-ray emission, with  transitions between a low/hard state, where a compact radio jet is seen, to a high/soft state, where the radio emission is reduced by large factors or not detectable at all.

Gamma-ray emission via the inverse Compton effect is expected when flares occur in the radio to X-ray region due to synchrotron radiation from relativistic electrons and radiative, adiabatic and energy-dependent escape losses in fast-expanding radio clouds, or plasmoids.

Long-term monitoring of key objects such as Cygnus X-1 using sub-arrays of CTA would provide an instrument with the sensitivity of current gamma-ray telescope arrays. Flares lasting less than 1hour with a flux of 10% of the Crab could be detected from as far away as the Galactic Centre. Exploiting this flexibility of CTA allows us to monitor variable sources and provide triggers for observations either with the full CTA observatory or with other instruments.

For short flares, energy coverage in 10-100 GeV band is not possible with current instruments as satellite-based instruments lack the required instantaneous sensitivity. At present, good coverage at higher energies is also impossible, due to the lack of sensitivity of the current generation of telescopes. CTA will provide improved access to both regions of the spectrum.


Further Reading

Paredes, High Energy Gamma-ray Emission in AGNs and Microquasars; http://arxiv.org/abs/astro-ph/0609168

Romero, Gamma-ray Emission from Pulsar/Massive-Star Binaries, Highlights of Astronomy, 15, p. 126-130; http://arxiv.org/abs/0908.3616

Hill et al., Fermi Results on Gamma-ray Binaries;  http://arxiv.org/abs/1008.4762

Pittard, Models of the Non-Thermal Emission from Early-Type Binaries;  http://arxiv.org/abs/0905.3315