“We use special telescopes to catch X-ray light in the sky, and while looking at these X-rays, the telescopes noticed an unexpected feature and captured a spectrum of light, which is not produced by any known atomic emission,” said University of Miami astrophysicist Nico Cappelluti about a signal first detected in 2014. “This emission line is now called the 3.5 kiloelectron volt (keV). One interpretation of this emission line is that it’s produced by the decay of dark matter.”
Unidentified –”Decaying Dark Matter?”
“This 3.5 keV emission line is unidentified. We truly don’t know what it is,” said Esra Bulbul, an astrophysicist currently with the Max Planck Institute for Extraterrestrial Physics and co-author in Cappelluti’s study. “But one theory is that it could be a sterile neutrino, which is also known as decaying dark matter. What is truly interesting about Cappelluti’s study is that he found this 3.5 keV line within our own galaxy.”
In 2014, a team of astronomers led by Bulbul discovered a noticeable spike in intensity at a very specific energy level. While studying the hot gas within the Perseus galaxy cluster (image above), the Chandra and XMM-Newton observatories revealed an unexpected spike, or emission line, corresponding to an energy of 3.5 kiloelectron volts (keV). This wavelength is very difficult to explain, as it cannot be described by previously observed – or even predicted – astronomical objects.
“Are We Living in a Bubble of Dark Matter?”
“If confirmed, this will tell us what dark matter is and could be one of the major discoveries in physics,” said Cappelluti. “We know that the Milky Way is surrounded by dark matter. Think of it as if we are living in a bubble of dark matter. But we also want to have the statistical certainty of our detection, so now we are putting together a Sterile Neutrino Task Force.”
Cappelluti is intrigued by the cosmic phenomena of supermassive black holes, the nature of dark matter, and active galactic nuclei, which is the very bright light source found at the center of many galaxies. His recent published findings could give insight on a subject scientists and astrophysicists have been investigating for decades: What is dark matter and where does it come from?
The Source of Light is Unfamiliar and Unrecognizable
Cappelluti’s study, published in The Astrophysical Journal and entitled, “Searching for the 3.5 keV line in the deep fields with Chandra: the 10 MS observations,” examines an interesting light source that was captured by four different telescopes, each pointing in a different direction in the sky. The source of light is unfamiliar and unrecognizable to scientists and has caused quite a stir in the world of astrophysics.
The four telescopes that captured the 3.5 keV emission were NASA’s NuSTAR telescope, the European Space Agency’s (ESA) XMM-Newton telescope, the Chandra telescope, and the Suzaku telescope from Japan.
“We now have a better knowledge of the instrumental background of Chandra,” Cappelluti told The Daily Galaxy, “thanks to the work of my student Dominic Sicilan where we analyzed the whole Chandra archive to search for the 3.5 keV line . While we did not detect any significant signal we cannot still rule out the sterile neutrino scenario.”
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, 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.
“The Nature of the Signal Remains to be Unknown”
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.”
“The nature of the signal remains to be unknown despite the intense efforts in the community. We will need a high-resolution micro calorimeter, for example XIFU instrument on board the next generation X-ray telescope Athena, to solve this mystery once for all,” wrote Bulbul in an email to The Daily Galaxy.