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

Hubble Chandra Image: The Ring Galaxy NGC 922

Credits: X-ray: NASA/CXC/SAO/A. Prestwich et al; Optical: NASA/STScI.

Astronomers are studying the number of black holes in galaxies with different compositions.

One of these galaxies, the ring galaxy NGC 922, is seen in this composite image containing X-rays from NASA's Chandra X-ray Observatory (red) and optical data from the Hubble Space Telescope (pink, yellow and blue).

NGC 922 was formed by the collision between two galaxies - one seen in this image and another located outside the field of view.

This collision triggered the formation of new stars in the shape of a ring. Some of these were massive stars that evolved and collapsed to form black holes.

Most of the bright X-ray sources in Chandra's image of NGC 922 are black holes pulling material in from the winds of massive companion stars. Seven of these are what astronomers classify as "ultraluminous X-ray sources" (ULXs).

These are thought to contain stellar-mass black holes that are at least ten times more massive than the sun, which places them in the upper range for this class of black hole.

They are a different class from the supermassive black holes found at the centers of galaxies, which are millions to billions of times the mass of the sun.

Theoretical work suggests that the most massive stellar-mass black holes should form in environments containing a relatively small fraction of elements heavier than hydrogen and helium, called "metals" by astronomers.

In massive stars, the processes that drive matter away from the stars in stellar winds work less efficiently if the fraction of metals is smaller.

Thus, stars with fewer of these metals among their ingredients should lose less of their mass through winds as they evolve. A consequence of this reduced mass loss is that a larger proportion of massive stars will collapse to form black holes when their nuclear fuel is exhausted.

This theory appeared to be supported by the detection of a large number (12) of ULXs in the Cartwheel galaxy, where stars typically contain only about 30% of the metals found in the sun.

To test this theory, scientists studied NGC 922, which contains about the same fraction of metals as the sun, meaning that this galaxy is about three times richer in metals than the Cartwheel galaxy.

Perhaps surprisingly, the number of ULXs found in NGC 922 is comparable to the number seen in the Cartwheel galaxy.

Rather, the ULX tally appears to depend only on the rate at which stars are forming in the two galaxies, not on the fraction of metals they contain.

One explanation for these results is that the theory predicting the most massive stellar-mass black holes should form in metal poor conditions is incorrect.