A gamma-ray glow at the center of the Milky Way is most likely caused by pulsars – the super dense, rapidly spinning cores of collapsed ancient stars that were up to 30 times larger than our sun.
Researchers used NASA’s Fermi Gamma-ray Space Telescope, which has been orbiting Earth since 2008, to make their observations.
The new findings cast doubts on previous theories that the signal was a potential sign of dark matter – a form of matter that accounts for 85 per cent of all matter in the universe but has so far not been detected.
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An excess of gamma-rays coming from the center of the Milky Way is likely due to a population of pulsars – rapidly spinning, very dense and highly magnetized neutron stars that emit ‘beams’ of gamma rays like cosmic lighthouses. The pulsars’ location in the oldest region of the galaxy suggests that they leach energy from companion stars, which prolongs the pulsars’ lifetime. This image shows the galactic center as seen by NASA’s Chandra X-ray Observatory
WHAT IS A PULSAR?
Pulsars are collapsed stars that emit beams of gamma rays.
These stars are made of matter much more densely packed than normal and which gives the entire star a density comparable to an atomic nucleus.
Neutron stars, a type of collapsed star, have extremely strong magnetic fields which accelerate charged particles.
The fields give off radiation in a cone shaped beam which sweeps across the sky like the light from a lighthouse as the star rotates.
When the beam sweeps over Earth, it becomes visible as a pulsar, producing light that cycles every few seconds to just a few milliseconds.
Their rotational period is so stable that some astronomers use it to calibrate instruments and have proposed using it to synchronise clocks.
The international team of astrophysicists, including researchers from the Department of Energy’s SLAC National Accelerator Laboratory at Stanford University, used NASA Fermi’s Large Area Telescope (LAT), a sensitive ‘eye’ for gamma rays, the most energetic form of light.
The Telescope was made at SLAC, which also hosts its operation center.
The researchers came to the conclusion that the source of the gamma-rays were from pulsars, and not from dark matter colliding as previously thought.
Most of the Milky Way’s gamma rays originate in gas between stars that is lit up by cosmic rays, which are charged particles produced when stars explode, called supernovae.
But gamma rays are also produced by supernova remnants, pulsars – collapsed stars that emit beams of gamma rays – and other objects that appear as points of light.
‘Our study shows that we don’t need dark matter to understand the gamma-ray emissions of our galaxy,’ said Dr Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and SLAC.
‘Instead, we have identified a population of pulsars in the region around the galactic center, which sheds new light on the formation history of the Milky Way.’
Dark matter makes up most of the mass of the universe, but it can’t directly be observed, not does it emit light or energy.
Researchers know that dark matter exists because it bends light from distant galaxies and affects how they rotate, but they don’t know what it’s made of.
Most researchers believe it’s not made of protons, neutrons and electrons, and instead is made of particles that have yet to be discovered that almost never interact with regular matter other than through gravity, which makes it difficult to detect them.
One way scientists may observe dark matter particles either decay or collide and destroy each other.
‘Widely studied theories predict that these processes would produce gamma rays,’ said Dr Seth Digel, head of KIPAC’s Fermi group.
‘We search for this radiation with the LAT in regions of the universe that are rich in dark matter, such as the center of our galaxy.’
Previous research has shown that there are more gamma rays coming from the galactic center than expected, which fueled reports suggesting the signal might have originated from dark matter particles.
However, gamma rays are produced in other types of cosmic processes too, which must be ruled our first before any conclusions about dark matter can be drawn.
This is challenging because the galactic center is extremely complex and researchers don’t know the details of everything happening in that region.
‘Two recent studies by teams in the US and the Netherlands have shown that the gamma-ray excess at the galactic center is speckled, not smooth as we would expect for a dark matter signal,’ said KIPAC’s Dr Eric Charles, who contributed to the new analysis.
‘Those results suggest the speckles may be due to point sources that we can’t see as individual sources with the LAT because the density of gamma-ray sources is very high and the diffuse glow is brightest at the galactic center.’
This new study takes these earlier analyses to a new level, showing that the speckled gamma-ray signal is consistent with pulsars.
‘Considering that about 70 percent of all point sources in the Milky Way are pulsars, they were the most likely candidates,’ Dr Di Mauro said.
‘But we used one of their physical properties to come to our conclusion.
Pulsars have very distinct spectra – that is, their emissions vary in a specific way with the energy of the gamma rays they emit.
When astrophysicists model the Milky Way’s gamma-ray sources to the best of their knowledge, they are left with an excess glow at the galactic center. Some researchers have argued that the signal might hint at hypothetical dark matter particles. However, it could also have other cosmic origins
‘Using the shape of these spectra, we were able to model the glow of the galactic center correctly with a population of about 1,000 pulsars and without introducing processes that involve dark matter particles.’
The researchers are planning follow-up studies with radio telescopes to determine whether the sources are emitting their light as a series of brief pulses, the tell-tale sign gives pulsars their name.
Discoveries in the halo of stars around the center of the galaxy – the oldest part of the Milky Way – also reveal details about its evolution, just as ancient remains teach archaeologists about human history.
‘Isolated pulsars have a typical lifetime of 10 million years, which is much shorter than the age of the oldest stars near the galactic center,’ Dr Charles said.
‘The fact that we can still see gamma rays from the identified pulsar population today suggests that the pulsars are in binary systems with companion stars, from which they leach energy.
‘This extends the life of the pulsars tremendously.’
The new study also adds to other data that challenge the idea that the gamma-rays come from dark matter signals.
Dr Digel said that if the signal were due to dark matter, we would expect to see it also at the centers of other galaxies.
An excess of gamma rays coming from the center of the Milky Way has fueled hopes the signal might stem from hypothetical dark matter particles that collide and destroy each other (left). The radiation could also be produced by pulsars – rapidly rotating neutron stars with strong magnetic fields (right)
‘The signal should be particularly clear in dwarf galaxies orbiting the Milky Way. These galaxies have very few stars, typically don’t have pulsars and are held together because they have a lot of dark matter,’ said Dr Digel.
‘However, we don’t see any significant gamma-ray emissions from them.’
The researchers believe that a recently discovered strong gamma-ray glow at the center of the Andromeda galaxy, the major galaxy closest to the Milky Way, may also be caused by pulsars rather than dark matter.
Having said these things, the Fermi-LAT team studied a large area of 40 degrees by 40 degrees around the Milky Way’s galactic center (the diameter of the full moon is about half a degree).
But the extremely high density of sources in the innermost four degrees makes it very difficult to see individual ones and rule out a smooth, dark matter-like gamma-ray distribution, leaving limited room for dark matter signals to hide.