Team Of Swift And Hubble Nabs “kilonova” Blast

Hubble pictures taken in infrared light of the fading afterglow left after the merger of two neutron stars in a distant galaxy in Leo. Credit: NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick)

Astronomers using the Hubble Space Telescope have spotted the afterglow of an enormous gamma-ray fireball from a merging pair of stars in a galaxy almost 4 billion light years away.

Dubbed a “kilonova”, the blast occurs when two neutron stars the size of a city like Duluth, Minn. merge into one. “Kilo” in kilonova refers to the explosion being 1,000 times brighter than a nova, an eruption on the surface of a white dwarf star. A nearby nova in our home galaxy might become as bright as a 1st magnitude star; a similar kilonova would shine 6 times brighter than Venus.

This all-sky map shows the locations of Swift’s 500 gamma-ray bursts through 2010, color coded by the year in which they occurred. In the background, an infrared image shows the location of our galaxy. The bursts come from all around, mostly outside out galaxy. Credit: NASA/Swift/Francis Reddy

For years, gamma ray bursts coming from random directions in space have confounded scientists. What causes them? Where do they originate?

The blasts come in two varieties – short (lasting less than 2 seconds) and long (more than 2 seconds). Abundant evidence points to the collapse of extremely massive stars in supernova explosions as the origin of the long bursts. Big blasts pour out radiation of every kind, while biggest blasts create gamma-rays, which pack 10,000 times more energy than sunlight.

Illustration showing how two compact stars merge to make a kilonova. Radioactive material released in the merger heats up and expands, emitting a burst of light called a kilonova as well as a slew of gamma-rays. While kilonovas are 1,000 times brighter than a nova, they’re 1/10th to 1/100th the brightness of a typical supernova. Credit: NASA, ESA, and A. Field (STScI)

Prior to Hubble’s recent discovery, astronomers suspected the short blasts might originate from the merger of two extremely tiny, dense neutron stars. Neutron stars are born in the fires of a supernova when a massive star runs out of fuel, collapses and then explodes. Sometimes the massive star’s core, now compressed to incredible density by the implosion, remains after the blast to live on as a neutron star.

Put two neutron stars in a tight binary system and they’ll eventually spiral together into one or take the next step and snap into a black hole. As the whirl in an every tighter orbit, the stars emit gravitation radiation – literally ripples in the fabric of space-time – causing them to approach one another ever more closely until they finally meld into one. At that moment the fireworks begin.

Gamma rays are the most energetic form of radiation in the universe, produced in extreme heat and violent stellar explosions. Luckily, our atmosphere protects us from getting hit. Credit: NASA

NASA’s Swift space telescope detected the initial gamma ray blast on June 3. The explosion lasted only 1/10 of a second but was 100 billion times brighter than the subsequent kilonova flash. On June 12-13 Hubble swung its 94-inch eye at the remote galaxy SDS J112848.22+170418.5 in Leo and discovered a rosy red afterglow at the source of the gamma-ray burst.

Hubble spotted the explosion site in near-infrared light (a deeper, heat-radiating “red” light just beyond the color red) which faded away in early July. While this sounds unremarkable, it’s exactly what you’d expect if two neutron stars merged, shot out a blast of gamma rays and then settled back into relative peace as a newly minted black hole. The fading fireball and debris cloud temporarily blocks our view of the system but glows in telltale infrared until the smoke clears.

Like a thief caught red-handed, the team of Swift and Hubble nabbed the system in an act of merger and camouflage, providing astronomers with the hard data they needed to lock up an explanation for a most puzzling phenomenon.