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In Colliding Galaxies, Pasadena Researchers Find a Pipsqueak Shining Bright

Published on Thursday, February 21, 2019 | 5:49 am
Bright green sources of high-energy X-ray light captured by NASA's NuSTAR mission are overlaid on an optical-light image of the Whirlpool galaxy (in the center of the image) and its companion galaxy, M51b (the bright greenish-white spot above the Whirlpool), taken by the Sloan Digital Sky Survey.Credit: NASA/JPL-Caltech, IPAC

Scientists working with the NuSTAR mission, a NASA Small Explorer mission led by Caltech and managed by the Jet Propulsion Laboratory, have revealed a new study showing that a much smaller object is competing with the Whirlpool galaxy and its companion galaxy, M51b, in the emission of the most luminous X-ray sources in sight, JPL said in a news release Wednesday.

The Whirlpool galaxy’s most stunning features are two long, star-filled “arms” curling around the galactic center like ribbons. The much smaller M51b clings like a barnacle to the edge of the Whirlpool. The two galaxies, collectively known as M51, are in the process of merging.

Caltech and JPL staff on the NuSTAR (Nuclear Spectroscopic Telescope Array) mission are saying a “neutron star,” an incredibly dense nugget of material left over after a massive star explodes at the end of its life, is remarkably competing with Whirlpool – officially known as M51a – and M51b, in the emission of bright X-rays.

Both Whirlpool and M51b has a supermassive black hole at their center that’s millions of times more massive than the Sun. The galactic merger should push huge amounts of gas and dust into those black holes and into orbit around them. In turn, the intense gravity of the black holes should cause that orbiting material to heat up and radiate, forming bright disks around each that can outshine all the stars in their galaxies.

But neither black hole is radiating as brightly in the X-ray range as scientists would expect during a merger. Based on earlier observations from satellites that detect low-energy X-rays, the scientists believed that layers of gas and dust around the black hole in the larger galaxy were blocking extra emission. The new study, published in the Astrophysical Journal, used NuSTAR’s high-energy X-ray vision to peer below those layers and found that the black hole is still dimmer than expected.

“I’m still surprised by this finding,” said Murray Brightman, a researcher at Caltech in Pasadena, who’s lead author of the study. “Galactic mergers are supposed to generate black hole growth, and the evidence of that would be strong emission of high-energy X-rays. But we’re not seeing that here.”

Brightman thinks the most likely explanation is that black holes “flicker” during galactic mergers rather than radiate with a more or less constant brightness throughout the process.

“The flickering hypothesis is a new idea in the field,” said Daniel Stern, a research scientist at JPL and the project scientist for NuSTAR. “We used to think that the black hole variability occurred on timescales of millions of years, but now we’re thinking those timescales could be much shorter. Figuring out how short is an area of active study.”

The neutron star they’re looking at is millions of times smaller than either black hole at Whirlpool or M51b and yet is shining with equal intensity, JPL said.

A typical neutron star is hundreds of thousands of times smaller in diameter than the Sun – only as wide as a large city – yet has one to two times the mass. A teaspoon of neutron star material would weigh more than 1 billion tons.

Despite their size, neutron stars often make themselves known through intense light emissions. The neutron star found in M51 is even brighter than average and belongs to a newly discovered class known as “ultraluminous neutron stars.”

Brightman said some scientists have proposed that strong magnetic fields generated by the neutron star could be responsible for the luminous emission; a previous paper by Brightman and colleagues about this neutron star supports that hypothesis. Some of the other bright, high-energy X-ray sources seen in these two galaxies could also be neutron stars.

Caltech leads the NuSTAR mission, and JPL manages it for NASA’s Science Mission Directorate in Washington.

Developed in partnership with the Danish Technical University and the Italian Space Agency (ASI), the spacecraft was built by Orbital Sciences Corporation in Dulles, Virginia (now part of Northrop Grumman).

NuSTAR’s mission operations center is at UC Berkeley.

For more information about NuSTAR, visit

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