Sagittarius A*, or Sgr A*, is the enigmatic compact radio source surrounding the supermassive black hole in the center of our galaxy. The black hole, which has a mass 4 million times that of our sun, dominates this region with its extreme gravitational well. Many efforts have been carried out in an attempt to understand the population of stars and other stellar phenomena in the immediate vicinity of Sgr A*, but resolving features from such an extreme environment at a distance of 25,000 light-years is not easy.
Through measurements of X-ray spectra from the black hole’s neighborhood, however, astronomers are gradually adding some detail to Sgr A*’s landscape and in new research published in the journal Nature, it seems there’s a strange population of ‘dead’ stars accumulated in the black hole’s shadow.
Using data from NASA’s NuSTAR X-ray space telescope, high-energy physicist Kerstin Perez, of Haverford College, PA, and Columbia University, New York, and her team have been able to better resolve the signatures of various X-ray emissions and it appears that some sources are consistent with a large population of white dwarf stars.
White dwarfs are the stellar remnants of larger stars that have run out of hydrogen fuel and died. Our sun, for example, after exhausting its fuel in about 5 billion years, will puff up into a huge red giant and then rip itself to shreds, expelling huge quantities of solar matter through violent winds. This will cause chaos in the solar system, destroying planets and producing a vast planetary nebula of hot gas. In the center of all the mess will be a white, glowing ball of degenerate matter, a white dwarf.
From previous studies of white dwarfs elsewhere in our galaxy, astronomers have a good idea as to their X-ray emission profiles, so when studying the few parsecs surrounding Sgr A*, Perez’s team were surprised to find an abundance of white dwarf-like emissions. In short, it looks like they stumbled on a puzzling “mass grave” of dead stars.
“The Galactic Center region is dense with X-ray-emitting objects; it contains the supernova remnant Sagittarius (Sgr) A East, the colliding stellar winds surrounding Sgr A*, the hot plasma of the Sgr A East plume, dozens of magnetic X-ray filaments, and thousands of resolved and unresolved point sources that constitute the Galactic ridge X-ray emission,” the researchers write in their Nature Letters paper.
"Almost anything that can emit X-rays is in the galactic center," said Perez in a NASA Jet Propulsion Laboratory news release. "The area is crowded with low-energy X-ray sources, but their emission is very faint when you examine it at the energies that NuSTAR observes, so the new signal stands out."
Within this mess of X-rays are emissions that aren’t associated with other known emission sources, such as dense molecular gas or dust.
“A natural explanation for our emission is provided by the intermediate polar (a type of cataclysmic variable binary star), which has the hardest spectrum of all accreting magnetic white dwarfs.”
There are some other possible explanations — such as an anomalous population of pulsars (rapidly-spinning neutron stars) — but the most likely source appears to be a large number of white dwarfs orbiting within 10 parsecs (33 light-years) of our Milky Way’s supermassive black hole. The X-ray emission originates from these white dwarfs 'feeding' off binary partners' stellar gas.
If they are white dwarf stars, why are they there? The researchers aren’t too sure, but these observations provide a valuable insight to the stellar dynamics in close proximity to a galaxy’s supermassive black hole.
Read more at Discovery News
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