Using data from NASA’s Chandra and ESA’s XMM-Newton space telescopes, astronomers have found a pair of remarkable sources that dramatically flare in X-rays.
This Chandra image shows the location in the elliptical galaxy NGC 5128 of a remarkable source that dramatically flares in X-rays unlike any ever seen. Low, medium, and high-energy X-rays are colored red, green, and blue. Image credit: NASA / CXC / UA / J. Irwin et al.
The two mysterious sources were both found in elliptical galaxies, NGC 5128 (a huge elliptical in the constellation Centaurus, approximately 12 million light-years away) and NGC 4636 (a giant elliptical in the constellation Virgo at a distance of 47 million light-years).
These sources brighten in X-rays by a factor of 100 in about a minute before returning to previous level in about an hour.
At their peak, the objects qualify as ultraluminous X-ray sources (ULXs) that give off hundreds to thousands of times more X-rays than typical binary systems where a star is orbiting a black hole or neutron star.
Dr. Jimmy Irwin from the University of Alabama and co-authors looked at the X-ray variation of several thousand X-ray sources in Chandra observations of 70 nearby galaxies.
“We found two ultraluminous flaring sources in globular clusters or ultracompact dwarf companions of parent elliptical galaxies,” they wrote in a paper published last week in the journal Nature.
The source associated with NGC 4636 flared once to a peak luminosity of 9×1040 erg per second; the other, in NGC 5128, flared five times to 1040 erg per second.
The rise times of all of the flares were less than one minute, and the flares then decayed over about an hour.
“We’ve never seen anything like this. Astronomers have seen many different objects that flare up, but these may be examples of an entirely new phenomenon,” Dr. Irwin said.
“These flares are extraordinary. For a brief period, one of the sources became one of the brightest ULX to ever be seen in an elliptical galaxy,” added co-author Dr. Peter Maksym, from the Harvard-Smithsonian Center for Astrophysics.
“When not flaring, the sources appear to be normal accreting neutron-star or black-hole X-ray binaries, but they are located in old stellar populations, unlike the magnetars, anomalous X-ray pulsars or soft gamma repeaters that have repetitive flares of similar luminosities,” the scientists said.
While the nature of these X-ray flares is unknown, the team has begun to search for answers.
One idea is that the flares represent episodes when matter being pulled away from a companion star falls rapidly onto a black hole or neutron star. This could happen when the companion makes its closest approach to the compact object in an eccentric orbit.
Another explanation could involve matter falling onto an intermediate-mass black hole, with a mass of about 800 times that of the Sun for one source and 80 times that of the Sun for the other.
“Now that we’ve discovered these flaring objects, observational astronomers and theorists alike are going to be working hard to figure out what’s happening,” said co-author Dr. Gregory Sivakoff, from the University of Alberta.
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Jimmy A. Irwin et al. 2016. Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies. Nature 538, 356-358; doi: 10.1038/nature19822
