“I couldn’t believe my eyes,” said Esra Bulbul of the Harvard Center for Astrophysics in July of 2014 of a still unfolding story about a strange signal that appeared not to come from any known type of matter detected in the x-ray spectrum in the Perseus Cluster –one of the most massive known objects in the universe. “What we found, at first glance, could not be explained by known physics.”
Imagine a cloud of gas in which each atom is a whole galaxy
Together with a team of more than a half-dozen colleagues, Bulbul used the Chandra X-Ray Observatory to explore the Perseus Cluster, a swarm of galaxies approximately 250 million light years from Earth. Imagine a cloud of gas in which each atom is a whole galaxy—that’s a bit what the Perseus cluster is like. The cluster itself is immersed in an enormous ‘atmosphere’ of superheated plasma—and it is there that the mystery resides.
“The cluster’s atmosphere is full of ions such as Iron (Fe XXV), Silicon (Si XIV), and Sulfur (S XV). Each one produces a ‘bump’ or ‘line’ in the x-ray spectrum, which we can map using Chandra,” Bulbul explains. “These spectral lines are at well-known x-ray energies.”
A new line popped up where no line should be
Yet, in 2012 when Bulbul added together 17 day’s worth of Chandra data, a new line popped up where no line should be. “A line appeared at 3.56 keV (kilo-electron volts) which does not correspond to any known atomic transition,” she said. “It was a great surprise.”
At first, Bulbul herself did not believe it. “It took a long time to convince myself that this line is neither a detector artifact, nor a known atomic line,” Bulbul said. “I have done very careful checks. I have re-analyzed the data; split the data set into different sub groups; and checked the data from four other detectors on board two different observatories. None of these efforts made the line disappear.”
In short, it appeared to be real. The reality of the line was further confirmed when Bulbul’s team found the same spectral signature in X-ray emissions from 73 other galaxy clusters. Those data were gathered by the European space agency (ESA) XMM-Newton observatory, a completely independent X-ray telescope.
Then, about a week after the Bulbul team posted their paper online, a different group led by Alexey Boyarsky of Leiden University in the Netherlands reported evidence for the same spectral line in XMM-Newton observations of the Andromeda galaxy. They also confirmed the line in the outskirts of the Perseus cluster.
Shifts suspicion to the unknown
The spectral line appears not to come from any known type of matter, which shifts suspicion to the unknown: dark matter. “After we submitted the paper, theoreticians came up with about 60 different dark matter types which could explain this line. Some particle physicists have jokingly called this particle a ‘bulbulon’,” she laughed.
The menagerie of dark matter candidates that might produce this kind of line include axions, sterile neutrinos, and “moduli dark matter” that may result from the curling up of extra dimensions in string theory.
Solving the mystery could require a whole new observatory. In 2015, the Japanese space agency launched an advanced X-ray telescope called “Astro-H.” It has a new type of X-ray detector, developed collaboratively by NASA and University of Wisconsin scientists, which will be able to measure the mystery line with more precision than currently possible.
Fast forward to December 2017
Astronomers may have uncovered a hint about what dark matter is: A new finding involved an explanation for a set of results made with NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton and Hitomi, the Japanese-led X-ray telescope. If confirmed with future observations, this may represent a major step forward in understanding the nature of dark matter.
“We expect that this result will either be hugely important or a total dud,” said Joseph Conlon of Oxford University who led the new study. “I don’t think there is a halfway point when you are looking for answers to one of the biggest questions in science.”
The story of this work started in 2014 when the team of astronomers led by Bulbul (Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.) got the ball rolling when she found the spike of intensity at a very specific energy in Chandra and XMM-Newton observations.
A dark matter origin
This spike, or spectroscopic emission line, is at an energy of 3.5 kiloelectron volts (keV). The intensity of the 3.5 keV emission line is very difficult if not impossible to explain in terms of previously observed or predicted features from astronomical objects, and therefore a dark matter origin was suggested. Bulbul and colleagues also reported the existence of the 3.56 keV line in a study of 73 other galaxy clusters using XMM-Newton.
The plot of this dark matter tale thickened when only a week after Bulbul’s team submitted their paper a different group, led by the aforementioned Alexey Boyarsky at Leiden University, reported evidence for an emission line at 3.56 keV in XMM-Newton observations of the galaxy M31 (aka the Andromeda galaxy) and the outskirts of the Perseus cluster, confirming the Bulbul result.
However, these two results were controversial, with other astronomers later detecting the 3.56 keV line when observing other objects, and some failing to detect it.
The debate seemed to be resolved in 2016 when Hitomi, especially designed to observe detailed features such as line emission in the X-ray spectra of cosmic sources, failed to detect the 3.56 keV line in the Perseus cluster.
An interesting plot twist occurred
“One might think that when Hitomi didn’t see the 3.56 keV line that we would have just thrown in the towel for this line of investigation,” said co-author Francesca Day, also from Oxford. “On the contrary, this is where, like in any good story, an interesting plot twist occurred.”
Conlon and colleagues noted that the Hitomi telescope had much fuzzier images than Chandra, so its data on the Perseus cluster are actually comprised of a mixture of the X-ray signals from two sources: a diffuse component of hot gas enveloping the large galaxy in the center of the cluster and X-ray emission from near the supermassive black hole in this galaxy. The sharper vision of Chandra can separate the contribution from the two regions. Capitalizing on this, Bulbul and collaborators.isolated the X-ray signal from the hot gas by removing point sources from their analysis, including X-rays from material near the supermassive black hole.
They Find Something Surprising
In order to test whether this difference mattered, the Oxford team re-analyzed Chandra data from close to the black hole at the center of the Perseus cluster taken in 2009. They found something surprising: evidence for a deficit rather than a surplus of X-rays at 3.56 keV. This suggests that something in Perseus is absorbing X-rays at this exact energy. When the researchers simulated the Hitomi spectrum by adding this absorption line to the hot gas’ emission line seen with Chandra and XMM-Newton, they found no evidence in the summed spectrum for either absorption or emission of X-rays at 3.56 keV, consistent with the Hitomi observations.
The challenge is to explain this behavior: detecting absorption of X-ray light when observing the black hole and emission of X-ray light at the same energy when looking at the hot gas at larger angular distances on the sky away from the black hole.
Detected at the supermassive black hole at the center of Perseus
The latest work shows that absorption of X-rays at an energy of 3.56 keV is detected when observing the region surrounding the supermassive black hole at the center of Perseus.
Could provide a path to identify the true nature of dark matter
This suggests that dark matter particles in the cluster are both absorbing and emitting X-rays. If the new model turns out to be correct, it could provide a path for scientists to one day identify the true nature of dark matter. For next steps, astronomers will need further observations of the Perseus cluster and others like it with current X-ray telescopes and those being planned for the next decade and beyond. (NASA/CXC/M. Weiss)
In fact, such behavior is well known to astronomers who study stars and clouds of gas with optical telescopes. Light from a star surrounded by a cloud of gas often shows absorption lines produced when starlight of a specific energy is absorbed by atoms in the gas cloud. The absorption kicks the atoms from a low to a high energy state. The atom quickly drops back to the low energy state with the emission of light of a specific energy, but the light is re-emitted in all directions, producing a net loss of light at the specific energy—an absorption line—in the observed spectrum of the star. In contrast, an observation of a cloud in a direction away from the star would detect only the re-emitted, or fluorescent light at a specific energy, which would show up as an emission line.
Dark matter particles may be like atoms in having two energy states separated by 3.56 keV
The Oxford team suggests in their report that dark matter particles may be like atoms in having two energy states separated by 3.56 keV. If so, it could be possible to observe an absorption line at 3.56 keV when observing at angles close to the direction of the black hole, and an emission line when looking at the cluster hot gas at large angles away from the black hole.
“This is not a simple picture to paint, but it’s possible that we’ve found a way to both explain the unusual X-ray signals coming from Perseus and uncover a hint about what dark matter actually is,” said co-author Nicholas Jennings, also of Oxford.
To write the next chapter of this story, astronomers will need further observations of the Perseus cluster and others like it. For example, more data is needed to confirm the reality of the dip and to exclude a more mundane possibility, namely that we have a combination of an unexpected instrumental effect and a statistically unlikely dip in X-rays at an energy of 3.56 keV. Chandra, XMM-Newton and future X-ray missions will continue to observe clusters to address the dark matter mystery.
Image credit: APOD/NASA
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