In 2014 a team of American astronomers was studying the X-ray emissions from a series of galaxy clusters using data obtained by NASA’s Chandra and ESA’s XMM-Newton space-based X-ray observatories. Suddenly, a mysterious emission line from the center of the Perseus cluster , a swarm of galaxies some 250 million light-years from Earth and one of the most massive in the Universe, appeared in the records.
The astronomer who discovered it, Esra Bulbul of the Harvard Center for Astrophysics , has made it clear: “I couldn’t believe my eyes,” she said. “At first glance what we found is not explainable by known physics.” What makes it odd is that it’s immersed in a huge ‘atmosphere’ of superheated plasma.”It’s full of iron, sulfur, silicon ions…which we know are there from the lines they leave behind in the light spectrum.” Each atom, each ion, emits energy in a series of defined places of the spectrum that receive the name of spectral lines and that are characteristic of each one: they are the fingerprints of the chemical elements. But what caused the astrophysicist’s surprise was what she discovered when her team studied 17 days of Chandra data: they found a new spectral line where there should be none: “a line appeared at 3.56 keV that does not correspond to any known atomic transition “ . At first, Bulbul didn’t believe what he was seeing. “It took me a long time to convince myself that this line was not a detector error or a known atomic line.”
The existence of that line was later confirmed when Bulbul used the XMM-Newton satellite and found that same spectral signature in X-ray emissions from 73 other galaxy clusters. Just over a week later, a team of astronomers led by Alexey Boyarsky of Leiden University discovered it in our neighboring Andromeda galaxy . 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 in data collected by XMM-Newton.
The spectral line could not be emitted by any known type of matter so theorists began to play with its equations . “After submitting the paper, theorists proposed around 60 different types of dark matter that could explain this line.” He added with a laugh: “Some particle physicists jokingly dubbed it the ‘bulbulon’.”
But of all the possibilities there was one that was more popular: sterile neutrinos, a type of neutrino that is invisible to all the fundamental forces of nature except gravity.
The story got muddier when in February 2016, the Japanese launched the Hitomi X-ray observatory, specially designed to observe emission lines in X-rays from cosmic sources: it did not detect the elusive 3.56 keV emission. Unfortunately, he was unable to carry out new measurements because a month after its launch the satellite was lost.
But at that moment the plot took an unexpected turn: astronomers discovered that Hitomi was unable to separate the two components of X-ray emission that the Perseus cluster had : on the one hand, the diffuse component of hot gas that surrounds the enormous galaxy that occupies the center of the cluster; on the other, the emission of X-rays near the supermassive black hole of this galaxy. When the observations were corrected for this detail, Bulbul’s team was again shocked: instead of finding excess X-ray emission at 3.5 keV, they found that something in Perseus was absorbing X-rays along this line. . What was happening?
The conclusion reached by Bulbul’s team is that there is an absorption of X-rays when looking at the super black hole and an emission with the same energy when looking at the hot gas surrounding the galaxy. Interestingly, this behavior is well known by astronomers, since it appears when a star surrounded by a cloud of gas is observed: if we point at the star we will see an absorption line at a certain energy, and if only the gas is observed, that same line appears. line but broadcast. And as happens with stars that present this peculiar spectral pattern, the researchers believe that, in this case, the dark matter presents two energy states separated by 3.5 keV. But is it really so? Is this anomaly in the X-ray emission spectrum of the Perseus cluster caused by dark matter ? Is this the first detection of the elusive dark matter? Only time – and new observations – will tell.
Bulbul, E. et al (2014) “ Detection of an unidentified emission line in the stacked X-ray spectrum of galaxy clusters” , Astrophysical Journal, 789:13