Mysterious ultra-bright gamma-ray emissions in giant bubbles ejected by our galaxy may finally have an explanation.
The researchers used data from the gaia Y fermi space telescopes to search through the Fermi bubbles, a pair of colossal hourglass-shaped bubbles that extend from the poles of the Milky Way and span 50,000 light-years, to track the source of very bright gamma-ray emission points.
They found that one of the brightest of these spots, dubbed a “Fermi cocoon,” located in the southern bubble, was caused by emissions from rapidly spinning, dead stars called pulsars in the Milky Way’s satellite galaxy Sagittarius. The finding could shed light on how these collapsing dead stars act as cosmic particle accelerators, ejecting high-energy particles that cause gamma-ray emissions.
Related: Astronomers detect the brightest intergalactic pulsar yet beyond the Milky Way
Gamma rays have previously been highlighted as a possible result of the annihilation of dark matter. But if the gamma rays are the result of particles accelerated by pulsars, they may not be evidence of dark matter.
The Sagittarius Dwarf Satellite Galaxy is seen from Land through the Fermi bubbles and is marked by elongated streams of gas and stars that were ripped from the galaxy’s core when its tight orbit passed it through the Milky Way’s disk.
Gamma-ray emissions are believed to be created by young stars, by the annihilation of dark matter, or by millisecond pulsars. This violent removal of gas means that the Sagittarius Dwarf Galaxy is no longer forming. stars and it lacks stellar nurseries, so its gamma-ray emissions cannot be the result of young stars.
Furthermore, the shape of the Fermi cocoon closely matches the observed distribution of visible stars, ruling out dark matter as the source of the emissions. (If dark matter were present, its gravity would affect the shape of the cocoon.) The researchers therefore concluded that the only possible sources of this powerful radiation were a previously unknown population of millisecond pulsars, which are fast-rotating, ultra-dense stellar remnants that rotate hundreds of times per second.
“We are convinced there is only one possibility: rapidly rotating objects called ‘millisecond pulsars,'” the team at an Australian National University wrote. statement (opens in a new tab). “Millisecond pulsars in the Sagittarius dwarf were the ultimate source of the mystery cocoon, we found.”
Like all neutron stars, a pulsar forms when a star much more massive than the sun reaches the end of its life and can no longer carry out nuclear fusion at its core. As a result, it can no longer hold up against complete gravitational collapse. Accompanied by a massive supernova explosion, the gravitational collapse leaves behind a star the size of a city with a mass around that of the sun. This stellar remnant is made up of matter so dense that a teaspoon would weigh 4 billion tons.
Scientists believe that the rapid rotation of millisecond pulsars is caused by the buildup of matter from a binary companion star that adds angular momentum to the dead star, or “spins” it.
Due to their powerful magnetic fields, the poles of pulsars explode electrons and positrons (electrons antimatter equivalents). When electrons interact with low-energy photons that form the cosmic microwave background (CMB) — remnant radiation shortly after big Bang — the electrons impart some of their kinetic energy. This causes the CMB photons to become much more energetic gamma ray photons.
By showing that the gamma-ray cocoon is the result of pulsars, the team’s results suggest that gamma-ray emissions in Fermi bubbles are not the result of dark matter annihilation, the researchers said.
“This is significant because dark matter researchers have long believed that a gamma-ray observation from a dwarf satellite would be irrefutable evidence of dark matter annihilation,” said team co-leader Oscar Macias, a researcher at the University of Amsterdam. in a statement. (opens in a new tab) “Our study forces us to reassess the high-energy emission capabilities of dormant stellar objects, such as dwarf spheroidal galaxies, and their role as prime targets for dark matter annihilation searches.”
The team’s research was published online September 5 in the journal nature astronomy (opens in a new tab).
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