“A Message From the Phantom Cosmos?”

Milky Way Galaxy


Did NASA scientists detect a dark matter signal from deep inside the Milky Way? In December of 2017, the Chandra X-Ray Observatory observed a strange and distinctive X-ray signal from deep inside the dynamic center of the Milky Way, the 3.5 keV line, that researchers believed could help them in proving the existence of the hidden, dark side of our universe.

Analyzing the energy spectrum, the astrophysicists found more X-ray photons with a particular energy than would be expected, unless, perhaps, they were generated by the decay of dark matter particles. The Milky Way, like other galaxies, is thought to be enveloped in a halo of dark matter.

The researchers found that the strength of the 3,500 electronvolts, the 3.5 keV signal, was consistent with data from Nasa’s X-ray satellite, the Nuclear Spectroscopic Telescope Array (NuStar). With no obvious interference within the satellite itself, the researchers concluded that the signal is unlikely to be caused by instrumental noise.

“This result is very exciting,” observed Kervork Abazajian, a professor of physics and astronomy, and Director of the Center for Cosmology at the University of California, Irvine, who was not involved in the research. “It makes it more likely that the line is due to dark matter, which makes up more than 80% of all the mass in the Universe. As its name suggests, it gives off no light, but reveals its presence through the gravitational pull it exerts on stars within galaxies.”

The Signal 

The research published in January of 2017, targeting relatively light particles of dark matter toward the galactic center, was carried out by Nico Cappelluti of the Yale Center for Astronomy and Astrophysics, and his colleague, Esra Bulbul with the Kavli Institute for Astrophysics and Space Research at MIT –the first scientist to spot an anomalous line at 3.5 keV, when looking at the X-ray spectra of large numbers of galaxy clusters in 2014. Their detection was consistent with previous measurements of this line toward the Galactic center that can be modeled as the result of sterile neutrino decay from the Milky Way for dark matter distribution.

Researchers elsewhere, they noted, have seen a line with the same energy in spectra from a variety of other objects, including the Andromeda and Milky Way galaxies..


“Dark Matter Particles as Big as a Galaxy”


The researchers were encouraged by the fact that four different satellites had seen the same signal, but were not ready to claim discovery of dark matter because it was still possible that their result is a statistical fluke – that the Chandra Observatory just happened to snare more X-rays with an energy of 3.5 keV than it did others. “As we collect more and more X-ray data, the evidence for the 3.5 keV line is growing and growing,” said Cappelluti.

Astrophysicists have been scanning the Milky Way for years for the photons generated when dark matter particles either annihilate with one another or decay. The so-called weakly-interacting massive particles (WIMPs) are believed by some researchers to be responsible for unusual emissions of gamma rays seen coming from the center of the Milky Way. While others think sources such as pulsars are probably the cause.

To establish whether dark matter could be the source, they compared Chandra’s spectra to those of X-rays from the center of the Milky Way that had been detected by the European Space Agency’s XMM-Newton satellite. As expected, they found the signal in the latter to be stronger, given that dark matter should be densest where there are more stars as found in the galactic center.

The researchers also ruled out a couple of alternative astrophysical sources for the signal: photons emitted either when very large black holes suck in material from their surroundings or when ions of sulfur take electrons from hydrogen in the center of galaxy clusters. “We found that our result is consistent with previous results if you assume the cause to be dark matter,” said Bulbul, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics whose research lies at the crossroads of astrophysics, cosmology, and particle physics centered on indirect searches of dark matter particle through its decay or annihilation in the X-ray band.

A word of caution, however, came from the author of The Dark Cosmos, Dan Hooper, a senior scientist and head of the Theoretical Astrophysics Group at Fermilab, who points out that a number of other studies have failed to see the line, including one by a group analyzing data from the Japanese space agency’s (Jaxa) ill-fated Hitomi X-ray satellite that malfunctioned just over a month after launch in February 2016, but managed to collect enough data to disprove a previously claimed sighting of the 3.5 keV line in the Perseus galaxy cluster (shown below). “The new paper claims a modest detection,” said Dr Hooper, “but it doesn’t sway me very strongly at this point.”


Perseus Cluster


Christoph Weniger, a theoretical astroparticle physicist at the University of Amsterdam, reported to the BBC that the new research “adds yet another piece to the 3.5 keV puzzle”. Observing that the signal might be due to a hypothetical particle known as the sterile neutrino, which would decay into an X-ray photon and a normal neutrino. But he stressed the need for more data to “confirm or reject the dark matter hypothesis”.

In December of 2018, Using XMM-Newton blank-sky observations to search for evidence of the 3.5 keV line consistent with arising from decaying dark matter within the ambient halo of the Milky Way a team headed by Christopher Dessert, Nicholas L. Rodd, Benjamin R. Safdi found the strongest limits to-date on the lifetime of dark matter in this mass range, strongly negating the possibility that the 3.5 keV line originates from dark matter decay.

X-ray observations of nearby clusters and galaxies have reported an unexpected X-ray line around 3.5 keV. This line has received significant attention due to its possible explanation through decaying dark matter; in particular, decaying sterile neutrinos, with a sterile neutrino mass around 7 keV, provide a good fit to the available data.

The image above  of the Perseus galaxy cluster contains clues that scientists thought could help them understand the nature of dark matter. It contains X-ray data from Chandra (blue) of the Perseus galaxy cluster, which has been combined with optical data from the Hubble Space Telescope (pink) and radio emission from the Very Large Array (red). In 2014, researchers detected an unusual spike of intensity, known as an emission line, at a specific wavelength of X-rays (3.5 keV) in the hot gas within the central region of the Perseus cluster. They also reported the presence of this same emission line in a study of 73 other galaxy clusters.

The Daily Galaxy, Max Goldberg, via NASA Chandra and BBC News

Image credit top of page: See “Warped & Twisted” –First 3D Map Unveils Milky Way’s Real Shape