Astronomers Baffled by Black Hole Jets Igniting Star Explosions

By Space Telescope Science Institute September 28, 2024

Collected at: https://scitechdaily.com/astronomers-baffled-by-black-hole-jets-igniting-star-explosions/

Nova Explosions in Double Star Systems Doubled Near Black Hole Jet

The supermassive black hole in the core of the giant elliptical galaxy M87 shoots out a blazing bright jet of plasma racing across space at nearly the speed of light. It makes the “Death Star” beam in the Star Wars trilogy look like a wimpy candle flame lighter.

Hubble astronomers have found that it seems to be dangerous just getting near to the energetic plasma jet. Stars seem to explode more frequently in the vicinity of the jet’s 3,000-light-year-long trajectory. The stars, called novae, erupt in double-star systems where an aging, swelled-up, normal star spills hydrogen onto a burned-out white dwarf companion star. As the hydrogen accumulates on the surface of the dwarf, it reaches a tipping point where it explodes like a hydrogen bomb. Novae frequently pop off throughout the giant galaxy of 1 trillion stars, but those near the jet seem to explode more frequently.

Astronomers guessed that something about the jet is either enhancing the fueling process and rate of explosions, or birthing new nova binaries in its vicinity. But once astronomers “crunched the numbers,” both hypotheses failed. So it remains anybody’s guess why black hole jets enhance the rate of nova eruptions.

Hubble Space Telescope Finds that a Black Hole Beam Promotes Stellar Eruptions

In a surprise finding, astronomers using NASA’s Hubble Space Telescope have discovered that the blowtorch-like jet from a supermassive black hole at the core of a huge galaxy seems to cause stars to erupt along its trajectory. The stars, called novae, are not caught inside the jet, but apparently in a dangerous neighborhood nearby.

The finding is confounding researchers searching for an explanation. “We don’t know what’s going on, but it’s just a very exciting finding,” said Alec Lessing of Stanford University, lead author of the paper published on September 27 in The Astrophysical Journal. “This means there’s something missing from our understanding of how black hole jets interact with their surroundings.”

The Mystery of Nova Eruptions

A nova erupts in a double-star system where an aging, swelled-up, normal star spills hydrogen onto a burned-out white dwarf companion star. When the dwarf has tanked up a mile-deep surface layer of hydrogen that layer explodes like a giant nuclear bomb. The white dwarf isn’t destroyed by the nova eruption, which ejects its surface layer and then goes back to siphoning fuel from its companion, and the nova-outburst cycle starts over again.

Hubble found twice as many novae going off near the jet as elsewhere in the giant galaxy during the surveyed time period. The jet is launched by a 6.5-billion-solar-mass central black hole surrounded by a disk of swirling matter. The black hole, engorged with infalling matter, launches a 3,000-light-year-long jet of plasma blazing through space at nearly the speed of light. Anything caught in the energetic beam would be sizzled. But being near its blistering outflow is apparently also risky, according to the new Hubble findings.

This video plots the location of Hubble Space Telescope observations of novae – exploding stars – in the giant elliptical galaxy M87. The galaxy’s most notable feature is a 3,000-light-year-long jet of plasma ejected from a supermassive central black hole. Hubble observations identified numerous novae in the jet’s vicinity. These are identified by red circles that turn white when a nova explodes. Stars seem to explode more frequently in the jet’s vicinity. The jet may be pushing interstellar hydrogen toward neighboring stars, enhancing the stellar fueling process that increases the rate of explosions. When the entire galaxy is photometrically surveyed fewer novae appear farther away from the jet. Credit: NASA, ESA, Joseph DePasquale (STScI)

The Role of the Jet in Nova Frequency

The finding of twice as many novae near the jet implies that there are twice as many nova-forming double-star systems near the jet or that these systems erupt twice as often as similar systems elsewhere in the galaxy.

“There’s something that the jet is doing to the star systems that wander into the surrounding neighborhood. Maybe the jet somehow snowplows hydrogen fuel onto the white dwarfs, causing them to erupt more frequently,” said Lessing. “But it’s not clear that it’s a physical pushing. It could be the effect of the pressure of the light emanating from the jet. When you deliver hydrogen faster, you get eruptions faster. Something might be doubling the mass transfer rate onto the white dwarfs near the jet.” Another idea the researchers considered is that the jet is heating the dwarf’s companion star, causing it to overflow further and dump more hydrogen onto the dwarf. However, the researchers calculated that this heating is not nearly large enough to have this effect.

Enhanced Stellar Activity and Hubble’s Observations

“We’re not the first people who’ve said that it looks like there’s more activity going on around the M87 jet,” said co-investigator Michael Shara of the American Museum of Natural History in New York City. “But Hubble has shown this enhanced activity with far more examples and statistical significance than we ever had before.”

Shortly after Hubble’s launch in 1990, astronomers used its first-generation Faint Object Camera (FOC) to peer into the center of M87 where the monster black hole lurks. They noted that unusual things were happening around the black hole. Almost every time Hubble looked, astronomers saw bluish “transient events” that could be evidence for novae popping off like camera flashes from nearby paparazzi. But the FOC’s view was so narrow that Hubble astronomers couldn’t look away from the jet to compare with the near-jet region. For over two decades, the results remained mysteriously tantalizing.

Compelling evidence for the jet’s influence on the stars of the host galaxy was collected over a nine-month interval of Hubble observing with newer, wider-view cameras to count the erupting novae. This was a challenge for the telescope’s observing schedule because it required revisiting M87 precisely every five days for another snapshot. Adding up all of the M87 images led to the deepest images of M87 that have ever been taken.

Hubble found 94 novae in the one-third of M87 that its camera can encompass. “The jet was not the only thing that we were looking at — we were looking at the entire inner galaxy. Once you plotted all known novae on top of M87 you didn’t need statistics to convince yourself that there is an excess of novae along the jet. This is not rocket science. We made the discovery simply by looking at the images. And while we were really surprised, our statistical analyses of the data confirmed what we clearly saw,” said Shara.

This accomplishment is entirely due to Hubble’s unique capabilities. Ground-based telescope images do not have the clarity to see novae deep inside M87. They cannot resolve stars or stellar eruptions close to the galaxy’s core because the black hole’s surroundings are far too bright. Only Hubble can detect novae against the bright M87 background.

Novae are remarkably common in the universe. One nova erupts somewhere in M87 every day. But since there are at least 100 billion galaxies throughout the visible universe, around 1 million novae erupt every second somewhere out there.

Reference: “A 9 Month Hubble Space Telescope Near-UV Survey of M87. II. A Strongly Enhanced Nova Rate near the Jet of M87” by Alec M. Lessing, Michael M. Shara, Rebekah Hounsell, Shifra Mandel, Nava Feder and William Sparks, 27 September 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad70b7