, a 91探花 postdoctoral researcher in the Department of Astronomy and lecturer in the at 91探花Bothell, spends her days pondering X-rays.

As she and her colleagues report in published Feb. 12 in the Monthly Notices of the Royal Astronomical Society, they recently solved a mystery involving X-rays 鈥� a case of X-rays present when they shouldn’t have been. This mystery’s unusual main character 鈥� a star that is pretending to be a supernova 鈥� illustrates the importance of being in the right place at the right time.

Such was the case in May 2010 when an amateur South African astronomer pointed his telescope toward , a nearby galaxy. He discovered what appeared to be a supernova 鈥� a massive star ending its life in a blaze of glory.

“Most supernovae are visible for a short time and then 鈥� over a matter of weeks 鈥� fade from view,” said Binder.

After a star explodes as a supernova, it usually leaves behind either a black hole or what’s called a neutron star 鈥� the collapsed, high-density core of the former star. Neither should be visible to Earth after a few weeks. But this supernova 鈥� SN 2010da 鈥� still was.

“SN 2010da is what we call a ‘‘ 鈥� something initially thought to be a supernova based on a bright emission of light, but as a massive star that for some reason is showing this enormous flare of activity,” said Binder.

Many supernova impostors appear to be massive stars in a binary system 鈥� two stars in orbit of one another. Stellar astrophysicists think that the impostor’s occasional flare-ups might be due to perturbations from its neighbor.

For SN 2010da, the story appeared to be over until September 2010 鈥� four months after it was confirmed as an impostor 鈥� when Binder pointed NASA’s toward NGC300 and found something unexpected.

“There was just this massive amount of X-rays coming from SN 2010da, which you should not see coming from a supernova impostor,” she said.

Binder considered a variety of explanations. For example, material from the star’s corona could be hitting a nearby dust cloud. But that would not produce the level of X-rays she had observed. Instead, the intensity of the X-rays coming from SN 2010da were consistent with a neutron star 鈥� the dense, collapsed core remnant of a supernovae.

“A neutron star at this location would be surprising,” said Binder, “since we already knew that this star was a supernova impostor 鈥� not an actual supernova.”

In 2014, Binder and her colleagues looked at this system again with Chandra and, for the first time, the . They found the impostor star and those puzzling X-ray emissions. Based on these new data, they concluded that, like many other supernovae impostors, SN 2010da likely has a companion. But, unlike any other supernovae impostor binary reported to date, SN 2010da is probably paired with a neutron star.

“If this star’s companion truly is a neutron star, that would mean that the neutron star was once a giant, massive star that underwent its own supernova explosion in the past,” said Binder. “The fact that this supernova event didn’t expel the other star, which is 20 to 25 times the mass of our sun, makes this an incredibly rare type of binary system.”

To understand how this unusual binary system could form, Binder and her colleagues considered the age of the stars in this region of space. Looking at stellar size and luminosity, they discovered that most nearby stars were created in two bursts 鈥� one 30 million years ago and the other less than 5 million years ago. But neither SN 2010da nor its presumed neutron star companion could’ve been created in the older burst of starbirth.

“Most stars that are as massive as these usually live 10 to 20 million years, not 30 million,” said Binder. “The most massive, hottest stars can form, grow, swell, explode and leave a neutron star emitting X-rays in about 5 million years.”

Surveys of the galaxy as recently as 2007 and 2008 detected no X-ray emissions from the location of SN 2010da. Instead, Binder believes that the X-rays they first found in 2010 represent the neutron star 鈥渢urning on鈥� for the first time after its formation. The X-rays are likely produced when material from the impostor star is transferred to the neutron star companion.

“That would mean that this is a really rare system at an early stage of formation,” said Binder, “and we could learn a lot about how massive stars form and die by continuing to study this unique pairing.”

One mystery solved, Binder would like to keep looking at SN 2010da, seeing what else she can learn about its formation and evolution. Its home galaxy, which has yielded previously, is sure to keep her busy. She is also planning a follow-up study of other recent supernova impostors with the help of an undergraduate research assistant at 91探花Bothell.

Co-authors on the paper included 91探花astronomy professor , at the National Tsing Hua University, and Paul Plucinsky at the Harvard-Smithsonian Center for Astrophysics, at the University of Minnesota and at Raytheon. Their work was funded by NASA.

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For more information, contact Binder at 206-543-9590 or bbinder@uw.edu.

Grant number: G04-15088X.

They studied a binary star system called NGC 300 X-1, nestled within a in the constellation Sculptor. Starting in 2007, scientists reported that this system of two stars appeared to be a rare pairing of a massive black hole and a large, unstable star. Astronomers struggled to explain how these two mighty bodies with violent histories could remain so intimately joined. But as the 91探花researchers showed, they may not be paired at all.

“I thought this would be a quick follow-up project on this binary system with new observations,” said 91探花astronomer and lead author . “But what we found was a lot more interesting than I thought.”

Since its discovery, this system had puzzled astronomers. The black hole was unusually large. Most black holes 鈥� created after massive stars explode as a supernova 鈥� are about five to 10 times more massive than the sun, said Binder. Astronomers had reported that the black hole in NGC 300 X-1 was 20 to 30 times more massive than the sun. In addition, other groups reported that the black hole’s orbiting companion was an enormous, unstable star in the phase of its life. Only the largest stars go through this brief phase before they explode as supernovae. The pairing of two rare and violent stellar phenomena , said Binder.

“These really massive stars that form black holes and Wolf-Rayet stars are only one out of every 3 million stars,” said Binder. “So they’re incredibly rare to begin with, and then to see two of them together as a binary is even more rare.”

All previous observations of this “rare and exotic” system came from ground-based telescopes. But last year Binder obtained the first space-based views of NGC 300 X-1 from the and the . Binder, who divides her time between research and at 91探花Bothell, enlisted two , Jacob Gross and Daniel Simons, to help her analyze these more detailed datasets. She expected them to confirm ground-based observations. Instead, they found multiple inconsistencies.

“We basically showed that past groups could’ve gotten entirely wrong what this system is,” said Binder.

Gross and Simons first discovered a problem with the black hole: previous researchers may have inadvertently overestimated its mass.

“It might be one of these smaller and more normal black holes,” explained Binder.

In addition, Binder and her team realized that the Wolf-Rayet star may not be the black hole’s true companion. Using clearer, space-based observations of NGC 300 X-1, they discovered another star that may instead be orbiting the black hole. This alternative star is less massive and resembles what the sun will look like in its later life.

Binder and her team did not have enough observations from Chandra and Hubble to determine the black hole’s true mass, or identify which star is its companion. But if the black hole is smaller and paired with the alternative star, that would make this binary system much more ordinary. That may come as a relief to astronomers who could not explain why this system existed. The massive black hole was once a star that swelled and exploded as a supernova, and astronomers could not understand why this supernova hadn’t expelled the immense and unstable Wolf-Rayet star. If the Wolf-Rayet star is not the black hole’s true companion, then there is no need for head-scratching.

“People had to come up with a lot of new ideas about stellar evolution 鈥� especially for massive stars 鈥� in order to explain why this massive binary existed,” said Binder. “But if this actually turns out to be a more run of the mill, commonplace system, we may have had a better idea of stellar evolution and binary stars than we thought.”

Her team’s findings come on the heels of a new study that casts doubt on the composition of , the only other reported binary system containing a massive black hole and a Wolf-Rayet star. But, Binder stressed, astronomers must verify all of these findings 鈥� including her team’s 鈥� before writing the eulogy for this rare and perplexing type of binary system.

“We need to follow up on this system, get more X-ray coverage and measure the velocity of these stars,” said Binder.

She is requesting time on another NASA observatory to do just that.

91探花astronomy professor was also an author on the paper.

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For more information, contact Binder at 206-543-9590 or bbinder@uw.edu.