Perseus Cluster
Credit: NASA/CXC/Univ. of Cambridge/C. Reynolds et al.
Astronomers using NASA's Chandra X-ray Observatory
have made one of the first experimental tests of string theory, a set
of models intended to tie together all known forces, particles, and
interactions. As described in our latest press release,
researchers used Chandra to look for signs of an as-yet undetected
particle predicted by string theory. The lack of a detection in these
Chandra observations helps rule out some versions of string theory.
The team looked for extraordinarily low-mass "axion-like" particles
in the Perseus galaxy cluster, shown in a Chandra image in the main
panel of this graphic (red, green and blue colors are low, medium and
high X-ray energies respectively). Galaxy clusters,
the largest structures in the Universe held together by gravity, offer
an excellent opportunity to search for these particles. In a galaxy
cluster, X-ray photons from an embedded or a background source can travel through a large amount of hot gas permeated with magnetic field
lines. Some of the X-ray photons may undergo conversion into axion-like
particles, or the other way around, along this journey. A simplified
illustration shows this process, with shorter wavelength X-ray photons
(in blue) converting into axion-like particles (yellow) and back to
photons, as they travel across magnetic field lines (grey) in the
cluster. Longer wavelength X-ray photons (red) are converting into
axion-like particles, but not back into photons. Such conversions would
cause a distortion in the X-ray spectrum (the amount of X-rays at
different energies) of a bright or embedded source of X-rays.
Astronomers obtained a long Chandra observation, lasting over five days, of the central supermassive black hole in the center of the Perseus galaxy cluster (shown in the inset.)
The spectrum of the region around the black hole showed no distortions,
allowing the team to rule out the presence of most types of axion-like
particles in the relatively low mass range their search was sensitive
to.
Here the Chandra spectrum (red) of Perseus' central black hole shows
the intensity of X-rays as a function of X-ray energy, along with an
example (black) of a model X-ray spectrum predicted if axion-like
particles were actually being converted from and into photons. To
highlight the distortions that could have been detected, the data
divided by the example model are also shown.
Chandra Spectrum
Credit: NASA/CXC/Univ. of Cambridge/C. Reynolds et al.
One possible interpretation of this work is that axion-like particles
do not exist. Another possible interpretation is that the particles
undergo conversion from and into photons less easily than some particle
physicists have expected. They also could have higher masses than probed
with the Chandra data.
There has been a surge of interest in studies of these particles in
recent years for three reasons: First, despite a lot of work, there
continues to be no detection of Weakly Interacting Massive Particles
(WIMPs), either with gamma-ray observations, or earth-based experiments that could explain the nature of dark matter.
These particles are predicted to interact with normal matter only via
the weak force, and have been considered to be one of the strongest
candidates for dark matter. Secondly, scientists have realized that
axions and axion-like particles are predicted by string theory. Finally,
there are a large number of experiments or observations that can be
done to search for these particles.
A paper describing these results appeared in the February 10th, 2020 issue of The Astrophysical Journal and is available online.
The authors are Christopher Reynolds (University of Cambridge, UK),
David Marsh (Stockholm University, Sweden), Helen Russell (University of
Nottingham, UK), Andrew C. Fabian (University of Cambridge), Robyn
Smith (University of Maryland in College Park, Francesco Tombesi
(University of Rome, Italy), and Sylvain Veilleux (University of
Maryland).
NASA's Marshall Space Flight Center manages the Chandra program. The
Smithsonian Astrophysical Observatory's Chandra X-ray Center controls
science and flight operations from Cambridge and Burlington,
Massachusetts.
Fast Facts for Perseus Cluster:
Scale: Main image is about 8 arcmin (550,000 light years) across. Inset image is about 11 arcsec (13,000 light years) across.
Category: Groups & Clusters of Galaxies, Black Holes
Coordinates (J2000): RA 03h 19m 47.60s | Dec +41° 30´ 37.00"
Constellation: Perseus
Observation Date: Main image: 25 pointings between Sep 1999 and Dec 2009; Inset: 15 pointings between Jun 2017 and Dec 2017
Observation Time: Main Image: 17 days 8 hours 37 minutes; Inset: 5 days 16 hours 30 minutes
Obs. ID: Main Image: 502, 503, 1513, 3209, 3404, 4289, 4946-4953, 6139, 6145, 6146, 11713-11716, 12025, 12033, 12036, 12037; Inset: 20449-20451, 20823-20827, 20837-20844
Instrument: ACISAlso Known As: Abell 426
References: Reynolds, C.S., et. al., 2020, ApJ, 890, 59; arXiv:1907.05475
Color Code: Red = 0.5-1.2 keV, Green = 1.2-2.0 keV, Blue = 2.0-7.0 keV
Distance Estimate: About 240 million light years
Source: NASA’s Chandra X-ray Observatory
