Gravitational wave event likely signaled creation of a black hole

May 31, 2018
Credit: NASA/CXC/Trinity University/D. Pooley et al. Illustration: NASA/CXC/M.Weiss

The spectacular merger of two neutron stars that generated gravitational waves announced last fall likely did something else: birthed a black hole. This newly spawned black hole would be the lowest mass black hole ever found.

A new study analyzed data from NASA's Chandra X-ray Observatory taken in the days, weeks, and months after the detection of gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) and gamma rays by NASA's Fermi mission on August 17, 2017.

While nearly every telescope at professional astronomers' disposal observed this source, known officially as GW170817, X-rays from Chandra are critical for understanding what happened after the two collided.

From the LIGO data astronomers have a good estimate that the mass of the object resulting from the star merger is about 2.7 times the mass of the Sun. This puts it on a tightrope of identity, implying it is either the most massive neutron star ever found or the lowest ever found. The previous record holders for the latter are no less than about four or five times the Sun's mass.

"While neutron stars and black holes are mysterious, we have studied many of them throughout the universe using telescopes like Chandra," said Dave Pooley of Trinity University in San Antonio, Texas, who led the study. "That means we have both data and theories on how we expect such objects to behave in X-rays."

The Chandra observations are telling, not only for what they revealed, but also for what they did not. If the neutron stars merged and formed a heavier neutron star, then astronomers would expect it to spin rapidly and generate a very strong magnetic field. This, in turn, would have created an expanding bubble of high-energy particles that would result in bright X-ray emission. Instead, the Chandra data show levels of X-rays that are a factor of a few to several hundred times lower than expected for a rapidly spinning, merged neutron star and the associated bubble of high-energy particles, implying a black hole likely formed instead.

If confirmed, this result shows that a recipe for making a black hole can sometimes be complicated. In the case of GW170817, it would have required two supernova explosions that left behind two neutron stars in a sufficiently tight orbit for gravitational wave radiation to bring the neutron stars together.

Credit: NASA/CXC/M.Weiss
"We may have answered one of the most basic questions about this dazzling event: what did it make?" said co-author Pawan Kumar of the University of Texas at Austin. "Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now."

A Chandra observation two to three days after the event failed to detect a source, but subsequent observations 9, 15 and 16 days after the event, resulted in detections. The source went behind the Sun soon after, but further brightening was seen in Chandra observations about 110 days after the event, followed by comparable X-ray intensity after about 160 days.

By comparing the Chandra observations with those by the NSF's Karl G. Jansky Very Large Array (VLA), Pooley and collaborators explain the observed X-ray emission as being due entirely to the shock wave—akin to a sonic boom from a supersonic plane—from the merger smashing into surrounding gas. There is no sign of X-rays resulting from a neutron star.

The claims by Pooley's team can be tested by future X-ray and radio observations. If the remnant turns out to be a neutron star with a , then the source should get much brighter at X-ray and radio wavelengths in about a couple of years when the bubble of high energy particles catches up with the decelerating shock wave. If it is indeed a black hole, astronomers expect it to continue to become fainter that has recently been observed as the shock wave weakens.

"GW170817 is the astronomical event that keeps on giving," said J. Craig Wheeler, a co-author on the study also from the University of Texas. "We are learning so much about the astrophysics of the densest known objects from this one event."

If follow-up observations find that a heavy neutron star has survived, such a discovery would challenge theories for the structure of neutron stars and how massive they can get.

"At the beginning of my career, astronomers could only observe neutron stars and in our own galaxy, and now we are observing these exotic across the cosmos," said co-author Bruce Gossan of the University of California at Berkeley. "What an exciting time to be alive, to see instruments like LIGO and Chandra showing us so many thrilling things nature has to offer."

Explore further: Image: Black hole bounty captured in the center of the Milky Way

More information: David Pooley et al. GW170817 Most Likely Made a Black Hole, The Astrophysical Journal (2018). DOI: 10.3847/2041-8213/aac3d6 ,

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4.3 / 5 (4) May 31, 2018
You'd think a science journalist would realise that "last fall" is a very hemisphere specific time designation. I could have sworn it was last spring.
1 / 5 (1) May 31, 2018
The bastards probably did it in meters instead of godly intended cubits!

I wonder would this mean the masses of awesomeness "fall in"? or "spring in"?
5 / 5 (3) May 31, 2018
AllStBob, you have a point, but a small one. All the contributors mentioned in the article were associated with universities in the USA, so a northern hemisphere seasonality is a very reasonable assumption.
not rated yet Jun 01, 2018
Seems like a potentially false dichotomy and a bit of an assumption. There are only two options? Do we know that for sure? If we don't see the usual signs of the one, then it must necessarily be the other? Really?
1 / 5 (4) Jun 01, 2018
'The spectacular merger of two neutron stars that generated gravitational waves announced last fall' is taken as the Nobel sealed starting assumption for the two adopted alternative explanations for the merged object although both alternatives are against so far proven concepts. The third alternative is to doubt the 2011 and 2017 Nobel 'confirmations' of Dark Energy and NS merge GW results of the outdated GRT and QM theories 100 years ago. The 1995 Dynamic Universe (DU) bounce replacement of BB and GRT in physics foundations is supported by the math inventions of advanced estimation theory in terms of unified matrix and tensor calculus. They have explained the flaws in theoretical GW and practical LIGO systems foundations. They question the NS merge GW data interpretation as an additional GRT based mistake in the waste basket of 5-10 other 'proofs' of GRT and traditional quantum theories, see Suntola DU literature at his PFS web site and quantum/string theory of photogrammetry.
5 / 5 (3) Jun 02, 2018
Why how clever of you, Anonym, to concatenate a random number after your handle to defeat those of us who wish to ignore your crank garbage. What on earth is wrong with you? Maybe you should get out more.
1 / 5 (3) Jun 02, 2018
No matter this is beyond the constraints of BH theory, I'm sure something dark was involved though.
1 / 5 (2) Jun 04, 2018
Anonym. you are on to something there
1 / 5 (2) Jun 05, 2018
In the light of Suntola's repeated papers countering the GRT based interpretations of 1998 SN1a data with convincing side-by-side comparisons with the DU model (see e.g. 2004-2005 papers) without any need of DE, the 2011 Nobel blessing of 'DE confirmation' appears premature. The 2017 Nobel of 'direct detection of GW' was awarded only days and weeks after latest disputed explanations of the theoretical and engineering foundations to explain the reported assumed GW data events in terms of Mach and DU energy balance principle and in terms of similar past mistakes in the early days of range and image sensing technologies of geodesy and photogrammetry as explained in my 208 blogs of WSU Master Class discussions since fall 2014. Glad to see recent articles about potential triggers of GRB and GW sources to explain the assumed (fake?) NS merge just weeks/days before the more than premature 10/2017 Nobel decision. Galileo's days are here again!
1 / 5 (2) Jun 05, 2018
Yes anonym, it was fake alright

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