A rapid rise in radiocarbon around AD 775 measured in tree rings is attributed to an increase of cosmic-ray intensity in a Nature paper this week.
The only known events that can produce a 14C spike are floods of γ-rays from supernova explosions or proton storms from giant solar flares. But neither seems likely, Miyake says, because each should have been large enough to have had other effects that would have been observed at the time.
Gamma-rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. In early November 1572, observers on Earth witnessed the appearance of a "new star" in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It's often called "Tycho's supernova" after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA's Fermi Gamma-Ray Space Telescope reveal that the shattered star's remains shine in high-energy gamma rays.
Cosmic rays are thought to originate from a variety of sources, and the specific cause of this cosmic-ray event remains to be determined, but this study suggests that neither a solar flare nor a local supernova is likely to have been responsible.
Rises in the carbon-14 (14C) content of trees in the past 3,000 years have been detected in previous studies, but none have shown increases on the timescale of around one year. Achieving this resolution, Fusa Miyake and colleagues report radiocarbon measurements in annual rings of Japanese cedar trees that demonstrate a rapid increase in 14C content, around 12‰ (1.2%), from AD 774 to AD 775.
Tree rings
Credit: Wikipedia
The only known events that can produce a 14C spike are floods of γ-rays from supernova explosions or proton storms from giant solar flares. But neither seems likely, Miyake says, because each should have been large enough to have had other effects that would have been observed at the time.
Credit: Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS
A massive supernova, for example, should have been bright enough to produce a 'new' star visible even in the daytime, as was the case for two known supernovae in ad 1006 and ad 1054. Such an explosion would have needed to be brighter than either of these, Miyake says, because those events were not large enough to leave traces in the 14C record.
It is possible, he says, that the proposed event might have occurred in the far southern skies, where astronomers of the era wouldn't have seen it. But still, he says, if it did happen, today's X-ray and radio astronomers should have found signs of a "tremendously bright" remnant of the explosion.
But Miyake’s team say that the cosmic whack of 774-775 cannot be attributed to the Schwabe cycle of the time—and it is far bigger than any known flare from the Sun.
The other possibility is a supernova, or a star that explodes at the end of its life in a welter of gamma radiation.
It burns brightly for a few years before cooling into a remnant that can glow sullenly for centuries.
But there is no documented record in the northern hemisphere of a supernova at around 775.
Recent surveys of cosmic radiation show that, at this time, there were the remains of two nearby supernovae called Cassiopeia A and Vela Jr.
These findings are consistent with data from North American and European trees and from Antarctic ice cores, indicating that elevated 14C resulted from an increase in cosmic-ray intensity.
The authors have ruled out solar flares as the cause because the 14C spike is around 20 times larger than expected from this solar activity. Likewise, the rapid increase in 14C is not consistent with a supernova explosion, and no such events have been recorded for this time.
The authors have ruled out solar flares as the cause because the 14C spike is around 20 times larger than expected from this solar activity. Likewise, the rapid increase in 14C is not consistent with a supernova explosion, and no such events have been recorded for this time.
A massive supernova, for example, should have been bright enough to produce a 'new' star visible even in the daytime, as was the case for two known supernovae in ad 1006 and ad 1054. Such an explosion would have needed to be brighter than either of these, Miyake says, because those events were not large enough to leave traces in the 14C record.
It is possible, he says, that the proposed event might have occurred in the far southern skies, where astronomers of the era wouldn't have seen it. But still, he says, if it did happen, today's X-ray and radio astronomers should have found signs of a "tremendously bright" remnant of the explosion.
But Miyake’s team say that the cosmic whack of 774-775 cannot be attributed to the Schwabe cycle of the time—and it is far bigger than any known flare from the Sun.
The other possibility is a supernova, or a star that explodes at the end of its life in a welter of gamma radiation.
It burns brightly for a few years before cooling into a remnant that can glow sullenly for centuries.
But there is no documented record in the northern hemisphere of a supernova at around 775.
Recent surveys of cosmic radiation show that, at this time, there were the remains of two nearby supernovae called Cassiopeia A and Vela Jr.
Cassiopeia A, the supernova remnant that was Chandra's "First Light" image that has been observed ever since.
Credit: NASA/CXC/SAO/D.Patnaude et al.
But they were probably too far away or not powerful enough to be responsible for the carbon-14 burst on Earth.
“With our present knowledge, we cannot specify the cause of this event,” Miyake admits.
“However, we can say that an extremely energetic event occurred around our space environment in AD 775 ... (but) neither a solar flare nor a local supernova is likely to have been responsible.”
The team is delving deeper into the mystery. They intend to fine-tune the search for the source by looking at telltale traces of beryllium and nitrate isotopes.
They also plan a wider search of historical documents to see if, 1,237 years ago, anyone noted a strange flare in the sky.
Contacts and sources:
Fusa Miyake
But they were probably too far away or not powerful enough to be responsible for the carbon-14 burst on Earth.
“With our present knowledge, we cannot specify the cause of this event,” Miyake admits.
“However, we can say that an extremely energetic event occurred around our space environment in AD 775 ... (but) neither a solar flare nor a local supernova is likely to have been responsible.”
The team is delving deeper into the mystery. They intend to fine-tune the search for the source by looking at telltale traces of beryllium and nitrate isotopes.
They also plan a wider search of historical documents to see if, 1,237 years ago, anyone noted a strange flare in the sky.
Contacts and sources:
Fusa Miyake
Nagoya University, Japan
Citation: Miyake, F., Nagaya, K., Masuda, K. & Nakamura, T. Nature http://dx.doi.org/10.1038/nature11123(2012).
Citation: Miyake, F., Nagaya, K., Masuda, K. & Nakamura, T. Nature http://dx.doi.org/10.1038/nature11123(2012).
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