Mystery in the Perseus Cluster
 
July 24, 2014:  The Universe is a big place, full of unknowns.  Astronomers
using NASA's Chandra X-ray Observatory have just catalogued a new one.
 
"I couldn't believe my eyes," says Esra Bulbul of the Harvard
Center for Astrophysics.  "What we found, at first glance, could not
be explained by known physics."
 
https://www.youtube.com/watch?v=3439YtdQZ1Y
 
A new ScienceCast video explores the mystery signal coming from the heart
of the Perseus Cluster.  Play it
 
Together with a team of more than a half-dozen colleagues, Bulbul has been
using Chandra to explore the Perseus Cluster, a swarm of galaxies
approximately 250 million light years from Earth.   Imagine a cloud of gas
in which each atom is a whole galaxy-that's a bit what the Perseus cluster
is like.  It is one of the most massive known objects in the Universe.
 
The cluster itself is immersed in an enormous 'atmosphere' of superheated
plasma-and it is there that the mystery resides.
 
Bulbul explains:  "The cluster's atmosphere is full of ions such as Fe
XXV, Si XIV, and S XV.  Each one produces a 'bump' or 'line' in the x-ray
spectrum, which we can map using Chandra. These spectral lines are at
well-known x-ray energies."
 
Yet, in 2012 when Bulbul added together 17 day's worth of Chandra data, a
new line popped up where no line should be.
 
"A line appeared at 3.56 keV (kilo-electron volts) which does not
correspond to any known atomic transition," she says.  "It was a
great surprise."
 
At first, Bulbul herself did not believe it. "It took a long time to
convince myself that this line is neither a detector artifact, nor a known
atomic line," she says. "I have done very careful checks.  I have
re-analyzed the data; split the data set into different sub groups; and
checked the data from four other detectors on board two different
observatories. None of these efforts made the line disappear."
 
http://chandra.si.edu/index.html
 
For more results from NASA's flagship X-ray observatory, visit the Chandra
Home PageIn short, it appears to be real.  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.  Those data were gathered by
Europe's XMM-Newton, a completely independent X-ray telescope.
 
Moreover, about a week after Bulbul team posted their paper online, a
different group led by Alexey Boyarsky of Leiden University in the
Netherlands reported evidence for the same spectral line in XMM-Newton
observations of the Andromeda galaxy.  They also confirmed the line in the
outskirts of the Perseus cluster.
 
The spectral line appears not to come from any known type of matter, which
shifts suspicion to the unknown: dark matter.
 
"After we submitted the paper, theoreticians came up with about 60
different dark matter types which could explain this line. Some particle
physicists have jokingly called this particle a 'bulbulon'," she
laughs.
 
The menagerie of dark matter candidates that might produce this kind of
line include axions, sterile neutrinos, and "moduli dark matter"
that may result from the curling up of extra dimensions in string theory.
 
Solving the mystery could require a whole new observatory.  In 2015, the
Japanese space agency is planning to launch an advanced X-ray telescope
called "Astro-H." It has a new type of X-ray detector, developed
collaboratively by NASA and University of Wisconsin scientists, which will
be able to measure the mystery line with more precision than currently
possible.  
"Maybe then," says Bulbul, "we'll get to the bottom of this."
 
Credits:
Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:
Science{at}NASA
 
 
Regards,
 
Roger

--- D'Bridge 3.99