Astronomers using NASA's Hubble Space Telescope have made a unique measurement that indicates a jet, plowing through space at speeds greater than 99.97% the speed of light, was propelled by the titanic collision between two neutron stars.

The explosive event, named GW170817, was observed in August 2017. The blast released energy comparable to that of a supernova explosion. It was the first combined detection of gravitational waves and gamma radiation from a binary neutron star merger.

This was a major watershed in the ongoing investigation of these extraordinary collisions. The aftermath of this merger was collectively seen by 70 observatories around the globe and in space, across a broad swath of the electromagnetic spectrum in addition to gravitational wave detection. This heralded a significant breakthrough for the emerging field of Time Domain and Multi-Messenger Astrophysics, the use of multiple "messengers" like light and gravitational waves to study the universe as it changes over time.

Scientists quickly aimed Hubble at the site of the explosion just two days later. The neutron stars collapsed into a black hole whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated jets moving outward from its poles. The roaring jet smashed into and swept up material in the expanding shell of explosion debris. This included a blob of material through which a jet emerged.

While the event took place in 2017, it has taken several years for scientists to come up with a way to analyze the Hubble data and data from other telescopes to paint this full picture.

The Hubble observation was combined with observations from multiple National Science Foundation radio telescopes working together for very long baseline interferometry (VLBI). The radio data were taken 75 days and 230 days after the explosion. 

"I'm amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by powerful radio VLBI telescopes spread across the globe," said Kunal P. Mooley of Caltech in Pasadena, California.

The authors used Hubble data together with data from ESA's (the European Space Agency) Gaia satellite, in addition to VLBI, to achieve extreme precision. "It took months of careful analysis of the data to make this measurement," said Jay Anderson of the Space Telescope Science Institute in Baltimore, Maryland.

By combining the different observations, they were able to pinpoint the explosion site. The Hubble measurement showed the jet was moving at an apparent velocity of seven times the speed of light. The radio observations show the jet later decelerated to an apparent speed of four times faster than the speed of light.

In reality, nothing can exceed the speed of light, so this "superluminal" motion is an illusion. Because the jet is approaching Earth at nearly the speed of light, the light it emits at a later time has a shorter distance to go. In essence, the jet is chasing its own light. In actuality, more time has passed between the jet's emission of the light than the observer thinks. This causes the object's velocity to be overestimated — in this case seemingly exceeding the speed of light.

"Our result indicates that the jet was moving at least at 99.97% the speed of light when it was launched," said Wenbin Lu of the University of California, Berkeley.

The Hubble measurements, combined with the VLBI measurements, announced in 2018, greatly strengthen the long-presumed connection between neutron star mergers and short-duration gamma-ray bursts. That connection requires a fast-moving jet to emerge, which has now been measured in GW170817.

This work paves the way for more precision studies of neutron star mergers, detected by the LIGO, Virgo, and KAGRA gravitational wave observatories. With a large enough sample over the coming years, relativistic jet observations might provide another line of inquiry into measuring the universe's expansion rate, associated with a number known as the Hubble constant.

At present, there is a discrepancy between Hubble constant values as estimated for the early universe and the nearby universe — one of the biggest mysteries in astrophysics today. The differing values are based on extremely precise measurements of Type Ia supernovae by Hubble and other observatories, and Cosmic Microwave Background measurements by ESA's Planck satellite. More views of relativistic jets could add information for astronomers trying to solve the puzzle.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

Puzzling image from the James Webb Space Telescope explained in two new studies

A bizarre image of the distant star known as WR140 surrounded by concentric geometric ripples, captured by the James Webb Space Telescope (JWST) in July, has baffled astronomers worldwide – even triggering frenzied internet speculation that it might be evidence of alien megastructure light-years across.

The puzzling image was captured shortly after JWST started science operations and released its first full batch of images. It quickly provoked spirited discussion online, with the wild corners of the internet theorizing the giant ripples might have alien origins. Mark McCaughrean, a senior adviser for science and exploration at the European Space Agency and a member of the James Webb Space Telescope Science Working Group, described the image as “bonkers”.

But in two companion studies, two Australian astronomers explain that the 17 concentric rings seen girdling the star are actually a series of mammoth dust shells created by the cyclic interaction between a pair of hot stars, one of them a dying Wolf-Rayet, locked together in a tight orbit.

“Like clockwork, WR140 puffs out a sculpted smoke ring every eight years, which is then inflated in the stellar wind like a balloon,” said Professor Peter Tuthill from the Sydney Institute for Astronomy at the University of Sydney, a co-author in both papers. “Eight years later, as the binary returns in its orbit, another ring appears, the same as the one before, streaming out into space inside the bubble of the previous one, like a set of giant nested Russian dolls.”

The WR140 binary is comprised of a huge Wolf-Rayet star and an even bigger blue supergiant star, gravitationally bound in an eight-year orbit. And while all stars generate stellar winds, those from Wolf-Rayet stars can be more likened to a stellar hurricane. Elements such as carbon in the wind condense out as soot, which remains hot enough to glow brightly in the infrared. Like smoke captured by wind, the dust clouds give telescopes something to observe, following the flow.

Because the two stars are in elliptical rather than circular orbits, dust production turns on and off as WR140’s binary companion nears and then departs the point of closest approach. Based on data collected with other telescopes since 2006, Professor Tuthill and his former student Yinuo Han – now at the University of Cambridge’s Institute of Astronomy – created a three-dimensional model of the dust plume’s geometry.

That model turned out to perfectly explain the bizarre results obtained by the JWST in July. Thanks to this and other contributions, both Han and Professor Tuthill also became co-authors of the Nature Astronomy paper with the new Webb data.

What’s more, in their Nature paper, Han and Professor Tuthill showed – for the first time – direct evidence of intense starlight driving into the matter and accelerating it, after tracking titanic plumes of dust generated by the violent interactions between two colossal stars over 16 years. 

It’s known that starlight carries momentum, exerting a push on the matter known as ‘radiation pressure. Astronomers often see the aftermath of this in the form of matter coasting at high speed around the cosmos, but have never caught the process in the act. Direct observation of acceleration due to forces other than gravity is rarely witnessed, and never in a stellar environment like this. 

“It's hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over,” said Han. “To witness acceleration at the level that it becomes measurable, the material needs to be reasonably close to the star or the source of the radiation pressure needs to be extra strong. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data.”

Using imaging technology known as interferometry, which was able to act like a zoom lens for the 10-meter mirror of the Keck telescope in Hawaii, the Australians were able to recover sufficiently sharp images of WR140 for the study. 

They discovered that the dust does not stream out from the star with the wind forming a hazy ball, as had been thought. Instead, the dust condenses adjacent to where the winds from the two stars collide, on the surface of a cone-shaped shock front between them. Because the orbiting binary star is in constant motion, the shock front also rotates. The sooty plume gets wrapped into a spiral, in the same way, that droplets form a spiral in a garden sprinkler.

“In the absence of external forces, each dust spiral should expand at a constant speed,” said Han. “We were puzzled at first because we could not get our model to fit the observations until we finally realized that we were seeing something new. The data did not fit because the expansion speed wasn’t constant, but rather that it was accelerating. We’d caught that for the first time on camera.”

Once they added the acceleration of dust by starlight into their three-dimensional model of the WR140 binary, it explained their observational data perfectly. And also ended up explaining the strange concentric rings later spotted with JWST. 

“In one sense, we always knew this must be the reason for the outflow, but I never dreamed we’d be able to see the physics at work like this,” said Professor Tuthill. “When I look at the data now, I see WR140’s plume unfurling a like giant sail made of dust. When it catches the photon wind streaming from the star, like a yacht catching a gust, it makes a sudden leap forward.”

With JWST now in operation, researchers will be able to learn much more about WR140 and similar systems. “The Webb telescope offers new extremes of stability and sensitivity,” said Dr Ryan Lau, Assistant Astronomer at the U.S. National Optical-Infrared Astronomy Research Laboratory and lead author of the JWST study. “We’ll now be able to make observations like this much more easily than from the ground, opening a new window into the world of Wolf-Rayet physics.”