Some 30,000 light-years from Earth, a tiny gravitational monster is tearing material from a companion star, blasting X-rays into space and sporadically hurling out jets of radio-wave-emitting blobs at close to the speed of light.
Known as Cygnus X-3, this mercurial star system — thought to be either a small black hole or a neutron star orbiting an ordinary partner — has fascinated astronomers for more than four decades with its surprisingly bright X-ray emissions. Now, two teams of researchers have made the first definitive detection of high-energy gamma rays, the most powerful type of electromagnetic radiation, from this small but nearby stellar system.
The findings may provide a new window on how this beast accelerates charged particles to enormous energies, researchers reported in early November at the 2009 Fermi Symposium in Washington, D.C.
Detecting the gamma rays from Cygnus X-3 was a feat in itself, made possible by sensitive detectors on two flying observatories, the researchers note. But both teams note that they are most excited about the unexpected clockwork pattern of the gamma-ray emission, which always seems to occur during a lull in high-energy X-rays and just before the onset of the powerful radio jets.
The gamma rays, generated by the acceleration of charged particles to extreme energies in the system, may be signaling “the preparation, the storage of energy for the major radio flares,” says Marco Tavani of Italy’s Space Astrophysics and Cosmic Physics Institute and the University of Rome, Tor Vergata. “Just one day after the gamma-ray flare — boom! It makes this very major radio flare,” says Tavani, who led one of the two studies. He and his collaborators have seen the pattern three times since April 2008.
The new gamma-ray findings are expected to shed light not only on how Cygnus X-3 accelerates particles to enormous energies but also how distant quasars, powered by supermassive black holes, pump even greater amounts of energy into space. “Microquasars such as Cygnus X-3 are the ideal laboratory for studying the jet phenomena that dominate the most luminous quasars’ emission,” comments X-ray astronomer Josh Grindlay of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Because the emissions from microquasars vary on time scales of days to weeks rather than decades like quasar emissions, systems such as Cygnus X-3 “are the test bed of choice” for probing quasar activity, he says.
Tavani’s team, which has used the Italian Space Agency’s AGILE spacecraft to monitor gamma-ray emissions from Cygnus X-3 for the past two years, has also posted its findings online. The study is scheduled to appear in an upcoming Nature.
Several members of the other team, which used the Fermi Gamma-ray Space Telescope to observe Cygnus X-3, declined to comment on their work because it’s scheduled to be published in Science. The Fermi team’s findings “are completely consistent” with those recorded by AGILE and show a similar pattern, Tavani says.
The gamma rays observed by AGILE were in the form of flares at energies of about 100 million electron volts. Follow-up radio observations by Tavani’s team, along with comparisons with X-ray observations recorded by NASA’s Swift satellite revealed that the flares preceded radio jets and occurred during a decline in high-energy X-rays from Cygnus X-3.
“This is a complete change from previous models,” Tavani asserts. Neutron stars and black holes (both thought to power microquasars) have strong magnetic fields, and Tavani envisions a mechanism in which a magnetic field stores an enormous amount of energy. This stored energy first accelerates charged particles and prompts them to emit gamma rays. Then the magnetic gate opens, and radio-emitting blobs are pushed out of the system. “The radio jets are the manifestation of what happened before” with the gamma rays, he suggests.
In addition, the high-resolution Fermi observations show that the intensity of the gamma rays varies on a 4.8-hour cycle, known from years of X-ray observations to be the time it takes for the ultradense member of the Cygnus X-3 system to orbit its partner star. The 4.8-hour signature confirms that the gamma rays are coming from Cygnus X-3 rather than from another source in the same patch of sky.
Both the radio jets and the gamma-ray flares are infrequent, notes Tavani. That could explain why telescope observations of the Cygnus region in the 1980s revealed gamma rays at energies of 10 trillion eV but were never confirmed, he says. It now seems possible that these telescopes detected something real that was associated with strong but fleeting flares, says Grindlay.
The study by Tavani and his colleagues plausibly argues “that there are gamma-ray flares associated with the switching on of the radio emission, presumably the jet, and showing that it is apparently related to the processes that energize the jet,” comments Tod Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Md. “It is an interesting result, but I also think that it’s still not clear in detail how the gammas are produced,” he adds. “Nevertheless, it is giving us another tool to study these extremely energetic beasts, and that’s exciting.”
