Tag Archives: Fermi

Fermi proves that novae produce gamma rays

The Fermi Gamma-Ray Space Telescope has discovered that novae, small scale stellar explosions similar to some supernovae but far less powerful, also produce gamma rays when they explode.

A nova is a sudden, short-lived brightening of an otherwise inconspicuous star caused by a thermonuclear explosion on the surface of a white dwarf, a compact star not much larger than Earth. Each nova explosion releases up to 100,000 times the annual energy output of our sun. Prior to Fermi, no one suspected these outbursts were capable of producing high-energy gamma rays, emission with energy levels millions of times greater than visible light and usually associated with far more powerful cosmic blasts.

What is significant about this is that it demonstrates a solid link between novae and supernovae, since only recently have scientists shown that some supernovae also produce gamma ray bursts. It suggests that the two explosions are produced by somewhat similar processes, but at very different scales. This fact will have important ramifications in the study of stellar evolution and the death of stars. For example, some nova stars often go nova repeatedly. Other data suggest that some more powerful eruptions can be recurrent as well. Extending this recurrent pattern to supernova suggests many new theoretical possibilities.

NASA revealed Tuesday that last April the Fermi Gamma-Ray Telescope barely avoided a collision with an abandoned Russian satellite.

NASA revealed Tuesday that last April the Fermi Gamma-Ray Telescope barely avoided a collision with an abandoned Russian satellite.

Fermi mission scientists first learned of the space collision threat on March 29, 2012 when they received a notice that the space telescope and Cosmos 1805 would miss each other by just 700 feet (213.4 meters). The mission team monitored the situation over the next day and it became clear that the two spacecraft, traveling in different orbits, would zip through the same point in space within 30 milliseconds of one another, NASA officials said.

They used Fermi’s thrusters to shift its orbit enough so the two spacecraft missed each other by 6 miles.

Recent results from the Fermi Gamma-ray Space Telescope have found no evidence of dark matter, a result in some conflict with data obtained from several underground research detectors.

The uncertainty of science: Recent results from the Fermi Gamma-ray Space Telescope have found no evidence of dark matter, a result in some conflict with data obtained from several underground research detectors.

The mystery here is that there is no doubt that something causes the outer objects in galaxies to move faster than expected. Scientists have labeled this something as dark matter, guessing that some undetected and unknown mass exists in the outer reaches of galaxies, thereby increasing the gravity potential and hence the velocity in which objects move.

The problem is that they have yet to identify what that dark matter is.

Unknown objects in space

Fermi list of object types

NASA’s Fermi Gamma-Ray Telescope today released an updated catalog of the last two years of its survey of the sky at high energy emissions. All told, there are 1873 objects in the catalog, more than half of which are supermassive black holes at the center of distant galaxies. You can see this all-sky map below the fold.

Many of the objects are quite familiar, such as the Crab Nebula, the remnant of a supernova that exploded a little less than a thousand years ago.

For decades, most astronomers regarded the Crab Nebula as the steadiest beacon at X-ray energies. But data from several orbiting instruments — including Fermi’s Gamma-ray Burst Monitor — now show unexpected variations. Astronomers have shown that since 2008, the nebula has faded by 7 percent at high energies, a reduction likely tied to the environment around its central neutron star.

Since 2007, Fermi and the Italian Space Agency’s AGILE satellite have detected several short-lived gamma-ray flares at energies hundreds of times higher than the nebula’s observed X-ray variations. In April, the satellites detected two of the most powerful yet recorded. To account for these “superflares,” scientists say that electrons near the pulsar must be accelerated to energies a thousand trillion times greater than that of visible light — and far beyond what can be achieved by the Large Hadron Collider near Geneva, Switzerland, now the most powerful particle accelerator on Earth.

What I, and many astronomers, find even most interesting about this catalog, however, is the large number of completely mysterious objects scattered across the sky, objects that emit powerful gamma rays but are not visible in any other wavelengths. All told, these unidentified objects comprise almost one third of the entire catalog.
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The Crab Nebula erupts with flares six days

In mid-April the Crab Nebula erupted for six days, repeatedly emitting the most powerful flares ever recorded from the supernova remnant.

Scientists think the flares occur as the intense magnetic field near the pulsar undergoes sudden restructuring. Such changes can accelerate particles like electrons to velocities near the speed of light. As these high-speed electrons interact with the magnetic field, they emit gamma rays.

To account for the observed emission, scientists say the electrons must have energies 100 times greater than can be achieved in any particle accelerator on Earth. This makes them the highest-energy electrons known to be associated with any galactic source. Based on the rise and fall of gamma rays during the April outbursts, scientists estimate that the size of the emitting region must be comparable in size to the solar system.