NuSTAR discovers impossibly bright dead star - First ultraluminous pulsar

High-energy X-rays streaming from a rare and mighty pulsar (magenta), the brightest found to date, can be seen in this new image combining multi-wavelength data from three telescopes. 

The bulk of a galaxy called Messier 82 (M82), or the 'Cigar galaxy,' is seen in visible-light data captured by the National Optical Astronomy Observatory's 2.1-meter telescope at Kitt Peak in Arizona. 

Starlight is white, and lanes of dust appear brown. Low-energy X-ray data from NASA's Chandra X-ray Observatory are colored blue, and higher-energy X-ray data from NuSTAR are pink. 

Credit: NASA/JPL-Caltech/SAO/NOAO

Astronomers working with NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), led by Caltech's Fiona Harrison, have found a pulsating dead star beaming with the energy of about 10 million suns.

The object, previously thought to be a black hole because it is so powerful, is in fact a pulsar, the incredibly dense rotating remains of a star.

"This compact little stellar remnant is a real powerhouse. We've never seen anything quite like it," says Harrison, NuSTAR's principal investigator and the Benjamin M. Rosen Professor of Physics at Caltech.

"We all thought an object with that much energy had to be a black hole."

Dom Walton, a postdoctoral scholar at Caltech who works with NuSTAR data, says that with its extreme energy, this pulsar takes the top prize in the weirdness category. Pulsars are typically between one and two times the mass of the sun.

This new pulsar presumably falls in that same range but shines about 100 times brighter than theory suggests something of its mass should be able to.

"We've never seen a pulsar even close to being this bright," Walton says. "Honestly, we don't know how this happens, and theorists will be chewing on it for a long time."

Besides being weird, the finding will help scientists better understand a class of very bright X-ray sources, called ultraluminous X-ray sources (ULXs).

Harrison, Walton, and their colleagues describe NuSTAR's detection of this first ultraluminous pulsar in a paper that appears in the current issue of Nature.

"This was certainly an unexpected discovery," says Harrison. "In fact, we were looking for something else entirely when we found this."



This animation shows a neutron star, the core of a star that exploded in a massive supernova. 

This particular neutron star is known as a pulsar because it sends out rotating beams of X-rays that sweep past Earth like lighthouse beacons. 

Credit: NASA/JPL-Caltech

Earlier this year, astronomers in London detected a spectacular, once-in-a-century supernova (dubbed SN2014J) in a relatively nearby galaxy known as Messier 82 (M82), or the Cigar Galaxy, 12 million light-years away.

Because of the rarity of that event, telescopes around the world and in space adjusted their gaze to study the aftermath of the explosion in detail.

Besides the supernova, M82 harbours a number of other ULXs. When Matteo Bachetti of the Université de Toulouse in France, the lead author of this new paper, took a closer look at these ULXs in NuSTAR's data, he discovered that something in the galaxy was pulsing, or flashing light.

"That was a big surprise," Harrison says. "For decades everybody has thought these ultraluminous X-ray sources had to be black holes, but black holes don't have a way to create this pulsing."

More information: An Ultraluminous X-ray Source Powered by An Accreting Neutron Star, Nature, dx.doi.org/10.1038/nature13791189

ESA Integral catches dead star exploding in a Type 1a Supernova

Astronomers studying SN2014J, a Type Ia supernova discovered in January 2014, have found proof that this type of supernova is caused by a white dwarf star reigniting and exploding.

This finding was made by using ESA’s Integral observatory to detect gamma rays from the radioactive elements created during the explosion.

This sequence of artist's impressions shows some of the steps leading up to and following the explosion.

A white dwarf, a star that contain up to 1.4 times the mass of the Sun squeezed into a volume about the same size as the Earth, leeches matter from a companion star (image 1).

The Integral measurements suggest that a belt of gas from the companion star builds up around the equator of the white dwarf (image 2). 

This belt detonates (image 3) and triggers the internal explosion that becomes the supernova (image 4). 

Material from the explosion expands (image 5) and eventually becomes transparent to gamma rays (image 6).

Astronomers using ESA’s Integral gamma-ray observatory have demonstrated beyond doubt that dead stars known as white dwarfs can reignite and explode as supernovae.

The finding came after the unique signature of gamma rays from the radioactive elements created in one of these explosions was captured for the first time.

The explosions in question are known as Type Ia supernovae, long suspected to be the result of a white dwarf star blowing up because of a disruptive interaction with a companion star.

However, astronomers have lacked definitive evidence that a white dwarf was involved until now.

The ‘smoking gun’ in this case was evidence for radioactive nuclei being created by fusion during the thermonuclear explosion of the white dwarf star.

“Integral has all the capabilities to detect the signature of this fusion, but we had to wait for more than ten years for a once-in-a-lifetime opportunity to catch a nearby supernova,” says Eugene Churazov, from the Space Research Institute (IKI) in Moscow, Russia and the Max Planck Institute for Astrophysics,in Garching, Germany.

Although Type Ia supernovae are expected to occur frequently across the Universe they are rare occurrences in any one galaxy, with typical rates of one every few hundred years.