The object's curious properties make it a good match for a supermassive black hole ejected from its home galaxy after merging with another giant black hole, US space agency said in a statement.

But astronomers cannot yet rule out an alternative possibility. The source, called SDSS1133, may be the remnant of a massive star that erupted for a record period of time before destroying itself in a supernova explosion.

"One exciting discovery is that the brightness of SDSS1133 has changed little in optical or ultraviolet light for a decade which is not something typically seen in a young supernova remnant," explained lead researcher Michael Koss, an astronomer at ETH Zurich (Swiss Federal Institute of Technology).

The source has brightened significantly in visible light during the past six months, a trend that, if maintained, would bolster the black hole interpretation.

"We suspect we are seeing the aftermath of a merger of two small galaxies and their central black holes," said co-author Laura Blecha, an Einstein Fellow in the University of Maryland's Department of Astronomy.

If SDSS1133 is not a black hole, then it might have been a very unusual type of star known as a Luminous Blue Variable (LBV).

These massive stars undergo episodic eruptions that cast large amounts of mass into space long before they explode.

Interpreted in this way, SDSS1133 would represent the longest period of LBV eruptions ever observed, followed by a terminal supernova explosion whose light reached Earth in 2001.

To analyze the object in greater detail, the team is planning ultraviolet observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope in October 2015.

The study was published in the journal Monthly Notices of the Royal Astronomical Society.

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