A supernova checkup

NGC 337

A barred spiral galaxy on a dark background. The galaxy’s central region is a pale colour due to older stars, contains some pale reddish threads of dust, and is brighter along a broad horizontal bar through the very centre. Off the bar come several stubby spiral arms, merging into the outer region of the disc. It is a cool blue colour and contains some bright sparkling blue spots, both indicating young hot stars. Credit: ESA/Hubble & NASA, C. Kilpatrick

The subject of this NASA/ESA Hubble Space Telescope Picture of the Week is the spiral galaxy NGC 337, located about 60 million light-years away in the constellation Cetus (The Whale).

This image combines observations made at two wavelengths, highlighting the galaxy’s golden centre and blue outskirts. The golden central glow comes from older stars, while the sparkling blue edges get their colour from young stars. If Hubble had observed NGC 337 about a decade ago, the telescope would have spotted something remarkable among the hot blue stars along the galaxy’s edge: a brilliant supernova

The supernova, named SN 2014cx, is remarkable for having been discovered nearly simultaneously in two vastly different ways: by a prolific supernova hunter, Koichi Itagaki, and by the All Sky Automated Survey for SuperNovae (ASAS-SN). ASAS-SN is a worldwide network of robotic telescopes that scans the sky for sudden events like supernovae.

Researchers have determined that SN 2014cx was a Type IIP supernova. The “Type II” classification means that the exploding star was a supergiant at least eight times as massive as the Sun. The “P” stands for plateau, meaning that after the light from the supernova began to fade, the level reached a plateau, remaining at the same brightness for several weeks or months before fading further. This type of supernova occurs when a massive star can no longer produce enough energy in its core to stave off the crushing pressure of gravity. SN 2014cx’s progenitor star is estimated to have been ten times more massive than the Sun and hundreds of times as wide. Though it has long since dimmed from its initial brilliance, researchers are still keeping tabs on this exploded star, not least through the Hubble observing programme which produced this image.

Source: ESA/Hubble/potw

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Mistaken identity: A presumed supernova is actually something much rarer

Screenshot of the NASA-produced animation showing a giant star being slowly devoured as it orbits the galaxy’s central black hole. Image is courtesy of NASA’s Goddard Space Flight Center.

Pasadena, CA—In a case of comic mistaken identity, an international team of astronomers revealed that what they once thought was a supernova is actually periodic flaring from a galaxy where a supermassive black hole gives off bursts of energy every 114 days as it tears off chunks of an orbiting star.

Six years after its initial discovery—reported in The Astronomer’s Telegram by Carnegie’s Thomas Holoien—the researchers, led by Anna Payne of University of Hawai’i at Mānoa, can now say that the phenomenon they observed, called ASASSN-14ko, is a periodically recurring flare from the center of a galaxy more than 570 million light-years away in the southern constellation Pictor.

Their findings—based on 20 instances of regular outbursts—will be published in The Astrophysical Journal and presented by Payne at the American Astronomical Society’s annual meeting.

Active galaxies, such as the host of ASASSN-14ko, have unusually bright and variable centers. These objects produce much more energy than the combined contribution of all their stars. Astrophysicists think this is due to gravitational and frictional forces heating up a swirling disk of gas and dust that accumulates around the central supermassive black hole. The black hole slowly consumes the material, which creates low-level, random changes in the light emitted by the disk.

This is the first unambiguous example of such clockwork behavior from an active galaxy. Periodically recurring flares, such as those from ASASSN-14ko, could be evidence of observationally elusive cosmic phenomena that have been previously predicted by theorists.

“Knowing the schedule of this extragalactic Old Faithful allows us to coordinate and study it in more detail,” Payne said.

ASASSN-14ko was first detected by the All-Sky Automated Survey for Supernovae (ASAS-SN), a global network of 20 robotic telescopes headquartered at The Ohio State University (OSU) in Columbus. When Payne examined all the ASAS-SN data on the phenomenon, she noticed a series of 17 regularly spaced flares.

Based on this discovery, the astronomers predicted that the galaxy would experience another burst on May 17 of last year and coordinated ground- and space-based facilities to make observations. They have since successfully predicted and witnessed flares on September 7 and December 26.

“ASAS-SN is designed to probe the physics of our universe by looking for transient and variable events.” Holoien said. “It’s exciting that the luminous object we originally thought was a violent supernova explosion—which would be interesting in its own right, but more commonplace—turned out to be a long-sought-after cosmic event.”

Swift, TESS Catch Eruptions From an Active Galaxy

So, what causes the repeated flares? The team considered several possible explanations, but think the most likely is what’s called a partial tidal disruption event.

Tidal disruption events, or TDEs, occur when a star gets too close to a supermassive black hole, which tears it to shreds. Some of its material gets flung out into space and the rest falls back onto the black hole, forming a disk of hot, bright gas as it is consumed.

In this instance, instead of a star being obliterated by interaction with the black hole, it would be slowly stripped during each orbit.  The flares occur when the lost material—equal to three times the mass of Jupiter at each pass—falls in towards the black hole.

The astronomers are unsure how long the flares will persist. The star can’t lose mass forever, and while scientists can estimate the amount of mass it loses during each orbit, they don’t know how much it had originally.

“We plan to keep predicting and observing these bursts or as long as we can,” said second author Benjamin Shappee, also of UH Mānoa (and a Carnegie alumnus). “This rare find could reveal new details about black hole physics.”

Source: Carnagie Science/News

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