It sounds like something from a bad sci-fi movie, but a NASA space telescope has detected what might be the X-ray “screams” from dead (zombie) stars. The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in 2012, has discovered a mysterious glow of high-energy X-rays which might be the “howls” of dead stars as they feed on stellar companions.
“We can see a completely new component of the center of our galaxy with NuSTAR’s images,” said Kerstin Perez of Columbia University in New York. “We can’t definitively explain the X-ray signal yet – it’s a mystery. More work needs to be done.” Perez is the lead author of a new report on the findings in the journal Nature.
The center of the Milky Way galaxy is filled with both young and old stars, black holes, and other types of dead stars. They all orbit around a supermassive black hole called Sagittarius A* at the heart of the galaxy.
NuSTAR is the first telescope capable of imaging the galactic center in high-energy X-rays in crisp detail. Surprisingly, astronomers found a haze of high-energy X-rays which dominated over the rest of the stellar background in the new images, which cover a region about 40 light-years across.
“Almost anything that can emit X-rays is in the galactic center,” said Perez. “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.”
Various theories have been offered for the unexpected finding. In one, a “stellar zombie” called a pulsar might be responsible. Pulsars are the collapsed remains of stars which ended their lives by exploding in a massive supernova blast. They emit beams of intense radiation as they spin extremely fast. They have been commonly referred to as “cosmic lighthouses,” and were thought to possibly be intelligent signals sent from another civilization when they were first discovered.
“We may be witnessing the beacons of a hitherto hidden population of pulsars in the galactic center,” said co-author Fiona Harrison of the California Institute of Technology (Caltech) in Pasadena, and principal investigator of NuSTAR. “This would mean there is something special about the environment in the very center of our galaxy.”
Such collapsed, dead stars which are in pairs, or binaries, can suck in matter from their companion stars, a sort of “zombie-like” feeding process, which can result in the release of X-rays.
Another theory says that a different kind of stellar corpse might be involved: dwarf stars. They are also the collapsed and burned-out remains of dead stars, but those stars were not massive enough to explode in a supernova. Our own Sun will meet that fate in an estimated 5 billion years or so. White dwarfs can produce even more high-energy X-rays, due to their extreme density and strong gravity.
In a different theory, the X-rays may come from small black holes which, in zombie-style, are slowly feeding off of their companions. X-rays are emitted as material from the companion star is swallowed into the deep, dark void.
However, the odd X-rays might not have dead stars involved at all. According to a fourth theory, the X-rays may originate from a diffuse haze of charged particles called cosmic rays. The cosmic rays themselves would still originate from the supermassive black hole at the galaxy’s center. X-rays would be emitted as the cosmic rays interact with other dense gas in the region.
But at the moment, none of the theories agree with what is already known from previous research, which leaves astronomers still scratching their heads.
“This new result just reminds us that the galactic center is a bizarre place,” said co-author Chuck Hailey of Columbia University. “In the same way people behave differently walking on the street instead of jammed on a crowded rush hour subway, stellar objects exhibit weird behavior when crammed in close quarters near the supermassive black hole.”
More observations are planned, which may help to solve the mystery, but could also open up new questions.
“Every time that we build small telescopes like NuSTAR, which improve our view of the cosmos in a particular wavelength band, we can expect surprises like this,” said Paul Hertz, the astrophysics division director at NASA Headquarters in Washington.
Whatever the explanation turns out to be, it is clear that the center of our galaxy is a very chaotic and bizarre place, seemingly haunted by cosmic zombies.
These observations from NuSTAR are part of a two-year primary mission phase. NuSTAR will also take a census of collapsed stars and black holes of different sizes by surveying regions surrounding the center of own Milky Way Galaxy, performing deep observations of the extragalactic sky, and mapping recently synthesized material in young supernova remnants to understand how stars explode and how elements are created and try to understand what powers relativistic jets of particles from the most extreme active galaxies hosting supermassive black holes. In addition, NuSTAR will offer opportunities for a broad range of science investigations, ranging from probing cosmic ray origins to studying the extreme physics around collapsed stars to mapping microflares on the surface of the Sun, as well as respond to targets of opportunity including supernovae and gamma-ray bursts.
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif., for NASA’s Science Mission Directorate in Washington. More information about the NuSTAR mission is available here.
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