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