Huge Cosmic Explosions That Produce Platinum, Silver and Gold May Be More Common Than Previously Thought

A source with remarkable similarities to GW170817, the first source identified to emit gravitational waves and light, has been discovered. NASA/CXC/GSFC/UMC/E. Troja et al.; Optical and infrared: NASA/STScI

In October 2017, scientists announced a groundbreaking discovery: the first simultaneous detection of light and gravitational waves from the same source—an event referred to as GW170817. In this case, the origin was the merger of two neutron stars, which produced an immense cosmic explosion, known as a kilonova, which had never been definitively observed before.

Now, a team led by researchers from the University of Maryland (UMD) have identified another event with remarkable similarities, according to a study published in the journal Nature Communications, suggesting such phenomena may occur more often than previously thought.

The newly described event—known as GRB150101B—was first observed in 2015 as a brief burst of gamma rays (GRB)—extremely energetic short-lived flashes observed in distant galaxies that are the brightest electromagnetic events known to occur in the universe.

"We see many short gamma-ray bursts every year", Geoffrey Ryan, a co-author of the study from UMD, told Newsweek. "We think many, if not all, come from neutron star mergers, but until GW170817 there were few ways to know for sure. There has been tantalizing evidence for kilonovae in past bursts, but not enough to be rock solid."

"At the time, [GRB150101B] seemed an odd type of cosmic explosion and we could not figure out the origin of its unusual look," Eleonora Troja, another astronomer from UMD and lead author of the study, told Newsweek.

Follow-up observations with NASA's Chandra X-ray Observatory, the Hubble Space Telescope (HST) and the Discovery Channel Telescope (DCT) indicate that this event may be a direct relative of the neutron star merger reported in 2017—known as GW170817—which was discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO).

"Thanks to the joint detection of gravitational waves and light in 2017, we now have the genetic markers of neutron star collisions," said Troja. "Just like a CSI episode, we compared the features of the neutron star merger GW170817 with those of the unsolved case of GRB150101B and found a match. We think that GRB150101B was indeed produced by the collision of two neutron stars which, as we know, are bright sources of gravitational waves.

"LIGO was not operative when this explosion happened but, even if it was, it would have probably missed it as its distance is too far from us," she said. "Luckily we had our telescopes ready to catch its light. This is extremely exciting for astronomers. Gravitational waves are teaching us what these fierce collisions look like, and we can use our telescopes to find them in the most remote parts of the universe."

According to Troja, the discovery tells us that events like GW170817 and GRB150101B could represent a whole new class of erupting objects which might actually be relatively common.

"We probably observed similar explosions in the past, but did not realize what we were looking at," Troja said. "Like in the case of GRB150101B we could not figure out its identity and classified it as a 'weird' explosion until we saw another explosion of the same family, GW170817."

Neutron stars are incredibly dense, compact objects that are left behind when massive stars explode as supernovae (provided that the exploding star's mass is insufficient to produce a black hole). Sometimes, these stars collide, or merge, as is the case with GW170817 and GRB150101B.

Both of these mergers produced long, narrow jets of high-energy particles, resulting in intense gamma-ray bursts lasting just a few seconds (although these were unusually faint compared to most GRBs).

The two events were also similar in other ways. They both produced bright blue optical light and long-lasting emissions of X-rays. Furthermore, their host galaxies—which are both elliptical and contain stars a few billion years old—are also surprisingly alike.

"We have a case of cosmic look-alikes," Ryan said in a statement. "They look the same, act the same and come from similar neighborhoods, so the simplest explanation is that they are from the same family of objects."

The emission of blue optical light from GRB150101B indicated that this event, like GW170817, was another example of a kilonova—immense cosmic explosions that occur when two neutron stars, or a neutron star and a black hole, merge, producing large quantities of important elements.

"A kilonova is a flash of light produced by the radioactive debris of a neutron star collision," Troja said. "During the collision a lot of neutron-rich stuff is spewed into space at a velocity that is 20-30 percent the speed of light. In these extreme conditions something very special happens: neutron star matter turns into gold, silver, platinum, uranium... all the metals heavier than iron are forged in these cosmic collisions."

"[The research] is telling us that the gravitational wave event GW170817 was not a space oddity," she said. "Last year we were astounded by how different it was from all the cosmic explosions previously detected and could not understand why we had never seen a similar kilonova before. Maybe not all these stellar collisions could produce silver and gold, and we had been really lucky to spot it at our first attempt. Now we know that there are other members of the same family and they are also accompanied by luminous kilonovae."

According to Ryan, because GW170817 had many new and strange traits, it was hard to tell which of these were quirks of this one object and which may be shared by the population as a whole.

"Finding more events and comparing their properties allows us to find their commonalities and learn more about these objects, he said. "Do all binary neutron star mergers produce kilonova? Do all kilonova look the same? How much of the heavy elements in the universe are produced in these events? When do binary neutrons star mergers produce black holes, and when do they leave behind a single massive neutron star? Now that we are getting a better idea of what to look for, with more observations we may be able to address these questions."

Despite the similarities between GW170817 and GRB150101B, the two events are very far apart in space: GW170817 is located around 130 million light-years from Earth, while GRB150101B lies much farther away—about 1.7 billion light-years away. The two events also differ because there is currently no gravitational wave data for GRB150101B, unlike GW170817.

"Finding these signatures in observations without gravitational waves is also very important, Ryan said. "There is a lot of universe out there, and LIGO is an incredible machine but its range for the moment is limited. With just the regular electromagnetic spectrum, from gamma-rays to radio, we can serve as an independent check on LIGO events, inform and refine each other's results, and potentially find more sources than with gravitational waves alone."

Without the gravitational wave data, however, scientists cannot calculate the masses of the two merging objects. Until then, the researchers can't rule out that the possibility that GRB150101B involved the merger of a black hole and a neutron star, rather than two neutron stars.

Nevertheless, the astronomers hope that more events which provide both gravitational wave data and observations in the electromagnetic spectrum, like GW170817, will be discovered soon.

"Surely it's only a matter of time before another event like GW170817 will provide both gravitational wave data and electromagnetic imagery," Alexander Kutyrev, another astronomer from UMD, said in the statement. "If the next such observation reveals a merger between a neutron star and a black hole, that would be truly groundbreaking. Our latest observations give us renewed hope that we'll see such an event before too long."

Vik Dhillon, a researcher from the Department of Physics & Astronomy at the University of Sheffield, U.K., who was not involved in the latest study, described the results as "interesting."

"GW170817 was obviously a major discovery in astrophysics, and a remarkable triumph not just for the people behind LIGO/VIRGO but also the theoretical astrophysicists who correctly predicted what such a neutron-star merger would look like in electromagnetic waves," he told Newsweek.

"The question this paper is trying to answer is—can we use the properties of the GW170817 event in electromagnetic waves to try to identify other similar events in archival data, for which no simultaneous gravitational-wave data exists," he said. "By detecting a second object similar to GW170817, the authors are able to further constrain the mechanisms leading to the electromagnetic-wave emission and speculate on how many more such [events] are likely to be detectable by future X-ray satellites".

This article has been updated to include additional comments from Eleonora Troja, Geoffrey Ryan and Vik Dhillon.