For more than a decade, astronomers have been puzzled by an unexplained surplus of gamma ray signals—the highest energy form of light—that originate at the center of our galaxy, the Milky Way. Known as the Galactic Center Excess (GCE), this diffuse glow has prompted many exciting hypotheses, including the idea that it may be driven by collisions between particles of dark matter, an enigmatic substance that makes up most of the mass in the universe.
Now, scientists led by Anuj Gautam, a Masters student at the Australian National University, have presented new evidence that the GCE is powered by a population of rapidly spinning dead stars, known as millisecond pulsars, that are individually invisible, but could together produce the strange gamma-ray glow, which stretches for about 5,000 light years in every direction around the galactic center.
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The team suggests that these extreme stellar corpses could “naturally reproduce the morphology, spectral shape and intensity of the GCE signal” and, “as a bonus,” may also be responsible for a “mysterious microwave ‘haze’ from the inner galaxy,” according to a study published on Thursday in Nature Astronomy.
“Gamma-ray data from the Fermi Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge that peaks near ~2 [giga-electron volts] with an approximately spherical intensity profile,” Gautam and his colleagues said in the study. “The origin of this ‘Galactic Center excess’ (GCE) has been debated, with proposed sources prominently including self-annihilating dark matter and a hitherto undetected population of millisecond pulsars (MSPs).”
“While there is no secure detection of individual MSPs in the Galactic bulge or center, such a population is not unanticipated,” the team added.They also note that “a completely independent argument” for the pulsar origin “is that its morphology traces the stars of the inner galaxy better than it traces the radially symmetric distribution expected for dark matter.”
Previous studies have challenged the pulsar explanation on many grounds. For instance, these pulsars are predicted to undergo extreme “natal kicks,” which are high-powered expulsions from their environment fueled by their births. Whereas all pulsars are the collapsed cores of large stars that exploded eons ago, the unusually fast spins of millisecond pulsars are thought to be fueled by material pulled off a companion star, an interaction that super-charges their rotational speeds. Millisecond pulsars get their names from these incredibly swift spins, with each rotation taking fewer than ten milliseconds, as well as the luminous pulses that erupt out of their poles in clockwork patterns.
It stands to reason, then, that if these MSPs are forged at the galactic center, many of them would get flung far from their birthplaces. This process would leave the Milky Way’s center depleted of these systems, casting doubt on them as a source of the GCE, and it also “may not accommodate the close spatial correspondence between the GCE signal and stars in the bulge,” the researchers noted in the study.
However, Gautam and his colleagues have now produced a new model that overcomes these past critiques by envisioning a special population of giant MSPs hewn from another type of dead star, called an oxygen-neon white dwarf.
If one of these white dwarfs fed off of a companion star, it could collapse into a heavier version of an MSP, in a process called accretion-induced collapse (AIC). These systems would have different dynamics and emission patterns compared to the more familiar pulsars forged from large live stars.
“Because little mass is lost in the collapse, the binary is not disrupted, nor does the system receive a large natal kick,” the team explained in the paper. “Thus, MSPs born via AIC are likely to remain trapped in the bulge gravitational potential, allowing for a large population to build up over its history and proffering an explanation of the detailed match between bulge stellar and GCE morphologies.”
In other words, a population of about 100,000 of these special AIC-born MSPs would be able to hang around the galactic center, thereby explaining the strange gamma ray signals at the center of the galaxy. In addition, such a population may also be responsible for a hazy glow in microwave light that has likewise perplexed scientists for years.
The researchers note that the debate over whether the GCE is caused by dark matter particles or MSPs should be resolved by the upcoming Cherenkov Telescope Array, which “will look at the sky” at high energies “and is expected to spend at least 500 hours observing the inner Galaxy,” according to the study. These unparalleled observations should be able to at last detect individual MSPs, if they exist, which could finally resolve this deep space enigma.
