NASA’s Chandra X-Ray Observatory has observed very strange and distinctive X-ray signal coming from our Milky Way Galaxy that researchers believe can help them in proving the existence of the mysterious dark side of the universe. That is the claim of US scientists who analyzed the energy spectrum of X-rays.
The data was gathered by NASA’s Chandra satellite who found more X-ray photons with a particular energy than would be expected if they were generated by the decay of dark matter particles, say the researchers. This is not the first time that scientists have seen extra photons with an energy of about 3,500 electronvolts (3.5 keV) in the spectra recorded by X-ray satellites.
The researchers found that the strength of their 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," said Dr Abazajian, a cosmologist 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 latest research, which targets relatively light particles of dark matter, has been carried out by Nico Cappelluti of the Yale Center for Astronomy and Astrophysics in Connecticut, US, and colleagues.
His colleague, Esra Bulbul of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT) was 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. Researchers elsewhere have seen a line with the same energy in spectra from a variety of other objects, including the Andromeda and Milky Way galaxies, which are thought to be enveloped in a bubble of dark matter.
The researchers are heartened by the fact that four different satellites have now seen the same signal but are not ready to claim discovery of dark matter because, they say, it is 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 sulphur take electrons from hydrogen in the centre of galaxy clusters. "We found that our result is consistent with previous results if you assume the cause to be dark matter," said Bulbul.
Others, however, urge caution. Dan Hooper, a particle theorist at Fermilab near Chicago, points out that a number of other studies have failed to see the line, including one by a group analysing 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.
"The new paper claims a modest detection," said Dr Hooper, "but it doesn't sway me very strongly at this point."
Christoph Weniger, a theoretical astroparticle physicist at the University of Amsterdam, is a little more upbeat, reorting to the BBC that the new research "adds yet another piece to the 3.5 keV puzzle".
He said 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".
Via DailyGalaxy