Dec. 8, 2009 Deep in our Galaxy, approximately 30,000 light-years from Earth, a small gravitational monster is sucking matter from a companion star, causing the infalling matter to violently radiate X-rays and occasionally be launched to form radio-wave-emitting jets that emanate close to the speed of light. This enigmatic binary star system, known as Cygnus X-3, has fascinated astronomers over four decades. It is thought to be either a small black hole or a neutron star and an ordinary, albeit massive star orbiting each other. Now, a team of researchers, including TKK's Metsähovi Radio Observatory, have made the first definitive detection of high-energy gamma rays from this system.
The findings may provide a new window on how Cygnus X-3 accelerates charged particles to enormous energies.
The study is scheduled to appear in an upcoming Nature.
Detecting the gamma rays, the most powerful type of electromagnetic radiation, is a feat in itself, and in this study their detection were made possible by sensitive detectors on-board italian gamma-ray satellite AGILE (Astro-rivelatore Gamma ad Immagini Leggero). From these observations an unexpected clockwork pattern of the gamma-ray emission was noted, which always seems to occur just before the onset of the powerful radio jets.
"Cygnus X-3 is a strange case indeed, being one of the brightest radio source in the Galaxy except when it descends into a radio quenched state. And now these extremely energetic gamma rays have been observed during this state. This may be indicating the preparation of the major radio flare, which follows just days after, when the source shoots up energetic radio jets from the core of the compact object," says researcher Karri Koljonen from Metsähovi Radio Observatory.
The new gamma-ray findings are expected to shed also light on 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. Because the emissions from microquasars vary on time scales of days to weeks rather than decades like quasar emissions, they present a convenient test bed for probing quasar activity.
The gamma rays observed by AGILE were in the form of flares at energies of about 100 million electron volts. Simultaneously the source was observed by AMILA (Arcminute Microkelvin Imager Large Array) and RATAN-600 radio telescopes from UK and Russia together with NASA's Swift and RXTE (Rossi X-ray Timing Explorer) X-ray satellites, which revealed that the flares preceded radio jets and occurred during a decline in high-energy X-rays from Cygnus X-3.
"The very complex behavior of Cygnus X-3 requires monitoring throughout the electromagnetic spectrum from radio through X-rays and now including also gamma-ray emission. Not until we have gathered data from all possible wavelengths we can start to form a unified picture of this enigmatic object. Microquasars have strong magnetic fields which can store enormous amount of energy. During these gamma-ray flares this stored energy can accelerate charged particles to observed high energies which prompts them to emit gamma rays. Then the magnetic gate opens, and radio-emitting blobs are pushed out of the system producing the major radio flares," Koljonen concludes.
Metsähovi will stay as a radio eyes for Cygnus X-3 along with other international radio, infrared, X-ray and gamma-ray facilities.
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- Tavani et al. Extreme particle acceleration in the microquasar Cygnus X-3. Nature, 2009; 462 (7273): 620 DOI: 10.1038/nature08578
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