“I’ve never seen anything like this before in the local universe,” said Stephen Smartt, an astrophysicist at Queen’s University Belfast and a lead scientist for the Hawaii-based ATLAS survey, which first observed the object that has rocked astronomy since its discovery in June. “It popped up out of nowhere,” says Smartt.
The June 16 detection was first posted on the Astronomer’s Telegram, an online service for astronomers to rapidly report interesting new observations. Thanks to the site’s randomized three-letter naming system, the object has been dubbed AT2018cow, or “the cow” for short.
A spectacular stellar explosion, weirdly named “The Cow,” has offered an unprecedented window on to the collapse of a star that, contrary to the slow unfolding of a typical supernova, became unbelievably bright essentially overnight.
The two groups behind the latest papers, reports Nature, have arrived at the same conclusion: “that a ‘central engine’ has kept agitating the exploding star from the inside for months and that the energy must have come from either a newly formed black hole in the process of accreting matter, or the frenetic rotation of a neutron star.”
Black holes and neutron stars are both born when massive stars reach the end of their lives. Explosions such as ‘Cow’ could provide some of the most direct evidence of this type of birth, says Mansi Kasliwal, an astronomer at the California Institute of Technology (Caltech) in Pasadena. “I think this is telling us about how to understand the most extreme incarnations of massive-star explosions.”
Iair Arcavi, an astrophysicist at the University of California, Santa Barbara, says: “Pretty much everything about its emission is something we haven’t seen before.”
The story of how Cow came to be discovered begins on 16 June, when a colleague flagged to Smartt a bright star at a spot where there was nothing just days before.
At first, writes Google Scholar Davide Castelvecchi in Nature, Smartt discounted the effect as an unremarkable stellar flare in the Milky Way. But then, he realized that it was probably much farther off, in a galaxy called CGCG 137-068 known to be around 60 megaparsecs (200 million light years) away. “It was 11 o’clock on a Sunday night, and I said to myself, ‘I better tell everybody about this.’” He sent out an alert through the Astronomer’s Telegram, a service for reporting and commenting on transient astronomical observations.
The Cow was no ordinary supernova, reaching its peak brightness in days, not weeks. “Everybody put down what they were doing up to that point” and started following Cow, says Daniel Perley, an astrophysicist at Liverpool John Moores University, UK.
Perley and his collaborators commanded a robotic telescope on La Palma, one of Spain’s Canary Islands, to observe Cow nearly every night for a month and a half. They also used a number of other telescopes around the globe that belong to a network that Kasliwal designed just for this kind of follow-up study.
The evidence that the team gathered — mostly in the optical spectrum — seemed to point to an existing black hole tearing a star apart, an observation they posted online in August 3. But to get a full picture of the energetic events happening, the researchers needed to look more broadly at the spectrum of electromagnetic energy, from radio waves to γ rays.
Just days after Smartt’s discovery, Anna Ho, another astronomer at Caltech, moved quickly to observe Cow in the radio spectrum. In a stellar explosion, charged particles emit radio waves as they spiral inside strong magnetic fields, and their wavelengths stretch out as the material spreads out.
Ho realized that she might have a rare opportunity to observe short wavelengths — ones only one millimeter or less — as the material quickly spreads out, and so astronomers are unlikely to catch events early enough to see short-wavelength emissions.
Early observations in June by her group and others did find emissions in the sub-milllimeter range, so she made an emergency proposal to the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes, where observing time is extremely competitive.
Over the next several weeks, Ho and her collaborators watched the spectrum of the event’s millimeter emissions as it evolved. Their observations revealed that matter was expanding outwards as fast as one-tenth of the speed of light.
But unlike an ordinary supernova, this short-wavelength radiation lasted for weeks revealing the presence of a central engine — a black hole or a spinning neutron star. “We were able to show that it’s not consistent with any of the usual mechanisms,” Ho says.
Margutti and her colleagues, meanwhile, took advantage of a pre-approved proposal that Margutti had made to observe ‘transient’ events using NASA’s NuSTAR X-ray telescope, in order to observe Cow quickly.
Observations on NuSTAR and other telescopes led the team to conclude that the event was highly unusual. The X-ray spectra, in particular, showed that it was being reheated from the inside. This, too, points to a black hole or neutron star powering the event — though it’s too soon to conclude which. “We have seen the formation of a compact object in real time,” she says.
Astronomers have discovered a special kind of neutron star with no stellar companion for the first time outside of the Milky Way galaxy shown at the top of the page, using data from NASA’s Chandra X-ray Observatory and the European Southern Observatory’s Very Large Telescope (VLT) in Chile. The composite image of the supernova 1E0102.2-7219 contains X-rays from Chandra (blue and purple), visible light data from VLT’s MUSE instrument (bright red), and additional data from Hubble (dark red and green). A neutron star, the ultra dense core of a massive star that collapses and undergoes a supernova explosion, is found at its center.