The Enigmatic Class of Gamma-ray Binaries

The majority of star systems are comprised not of a single star but of two or more stars orbiting about their common center of gravity. However, only a handful of these emit high-energy gamma rays. Most X-ray binary systems detected in gamma rays belong to the class of systems known as high mass X-ray binaries (HMXBs); these systems are comprised of a massive (>10 solar masses) star and either a neutron star or a black hole. For the majority of gamma-ray binaries, the nature of the compact object is still a mystery. Gamma rays are believed to be produced in these systems by particles accelerated either within a relativistic jet, or by a pulsar wind colliding with the stellar wind and/or the outflowing equatorial disk of the massive star. But there are still many questions about the details of how binaries produce gamma-ray emission.

Microquasars

Believed to be the small-scale version of active galactic nuclei, microquasar systems are the pairing of a black hole in orbit around a main sequence star. This pairing gives rise to relativistic jet structures as the black hole accretes material from its companion. While many microquasars are known in in the X-ray and radio regime, these systems have been very difficult to detect in the gamma-ray band. The only such system known in Fermi's Large Area Telescope (LAT) catalog is Cygnus X-3. Multiple gamma-ray flares, coincident with radio flares, have now been detected by the LAT, directly linking the gamma rays to a relativistic jet. This is the only confirmed jet activity from a Galactic object that Fermi has detected. + Learn More

Left: An artists conception of a microquasar system and its associated jets. Right: The Fermi-LAT gamma-ray count map from the Cygnus X-3 (source circled) region. The brighter sources in the map are known gamma-ray pulsars.

Pulsar Binaries

Fundamentally different from microquasar systems, pulsar binaries are the pairing of an energetic pulsar and a main sequence star. As opposed to microquasars, where the non-thermal emission is powered by relativistic jets, the emission from pulsar binaries is driven by the interaction between the pulsar wind and the stellar wind from a main sequence companion. The best known example of a pulsar binary system in the Fermi-LAT catalog is PSR B1259-63, a known pulsar in a 3.4 year orbit around a massive Be star. The system only emits gamma rays during periastron passage as the pulsar passes through the Be star disk. The LAT saw the source switch on during the 2010/11 periastron passage and generate an intense, unexpected flare. Other gamma-ray binaries such as LS I +61 303 and LS 5039 are also detected by the LAT, and are believed to be pulsar binaries, although further observations are needed to confirm their nature. + Learn More

An artist's conception of the pulsar binary B1259-63 as the pulsar traverses its orbit around the Be star.

Gamma-ray Novae

One of the most surpsiing discoveries from Fermi-LAT has been in the study of the subclass of binary systems known as novae.

The gamma-ray count maps of 4 new gamma-ray novae seen by Fermi-LAT.

In March 2010 Fermi made an unprecedented discovery; the detection of gamma rays from an optical nova in the symbiotic binary V407 Cyg. Novae are massive eruptions caused by a runaway thermonuclear explosion on the surface of a white dwarf which has been accreting hydrogen from its binary companion. While novae are well studied, Fermi-LAT was the first instrument to establish novae as gamma-ray sources. Furthermore, Fermi-LAT has now seen GeV emission from multiple "classsical" novae as well: the systems V959 Mon, V339 Del, and V1324 Sco. While in V407 Cyg the gamma-ray emission is believed to be produced by the nova blast interacting with the dense wind of its red giant companion, the GeV emission from classical novae is believed to take place from shocks interacting with ejected material. Both of these processes are similar to what occurs within supernovae, but in novae the process can be seen to evolve over a few weeks rather than thousands of years. + Learn More

The optical (AAVSO) and gamma-ray light curves of the nova event in V339 Del show that the high-energy GeV emission lags the optical emission by several days, due to the travel time necessary for the nova blast to leave the system and interact with surrounding media.