Bauer and his team thought they’d be lucky, because the Very Large Telescope (VLT) in Chile had also imaged this field that same night, about 80 minutes into the X-ray outburst. But they were disappointed — and intrigued — to find that no new optical source was visible.
The region in the sky where this event took place is part what is known as the Chandra Deep Field-South (CDF-S) — similar to the Hubble Deep Field images — where Chandra has taken several long and deep observations of this area, resulting in an exposure time equivalent to seven million seconds, or about 81 days. However, a look at archival Chandra data showed that no previous X-ray source had come from this area prior to October 2014.
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Other follow-up observations with other optical telescopes and ultimately the Hubble Space Telescope, covering a span of days to months, also came up empty, finding nothing associated with the X-ray transient.
"Ever since discovering this source, we've been struggling to understand its origin," Bauer said. "It's like we have a jigsaw puzzle but we don't have all of the pieces."
Bauer and his team said the lack of “smoking gun evidence” in other wavelengths rules out the usual suspects for what this bright X-ray object might be.
The first event that comes to mind is a gamma-ray burst (GRB), which is an extremely energetic explosion triggered either by the collapse of a massive star or by the merger of a neutron star with another neutron star or a black hole. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths (X-ray, ultraviolet, and optical, for example), but no other wavelengths were detected either before or after the X-ray burst.
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The researchers said two possible explanations for the CDF-S X-ray source are a GRB that is not pointed toward Earth, or a GRB that lies beyond the small host galaxy. A third possibility is that a medium-sized black hole shredded a white dwarf star.
"None of these ideas fits the data perfectly," said co-author Ezequiel Treister, also from Pontifical Catholic University, "but then again, we've rarely if ever seen any of the proposed possibilities in actual data, so we don't understand them well at all."
While they still don't have the final answers, the team continues to try to explain the object. It has begun highly targeted searches through the Chandra archive as well as through archives of the European Space Agency's XMM-Newton space observatory and NASA's Swift satellite, hoping to uncover more examples of this type of variable object, which has gone unnoticed until now.
Researchers are also looking toward future X-ray observations by Chandra and other X-ray telescopes that may also reveal the same phenomenon from other objects. If the X-ray source was caused by a GRB triggered by the merger of a neutron star with a black hole or another neutron star, then gravitational waves would also have been produced, so current and future gravitational wave observatories may lend clues.
“Unfortunately there is no ‘smoking gun’ evidence that favors one scenario over the other here,” Bauer wrote, “and thus our only remaining chance to understand the CDF-S transient is to find similar events in old or new data.”
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