Hefty black hole weighs in at 33 Suns

Researchers have confirmed that a black hole lurking in the dwarf galaxy IC 10, and dubbed IC 10 X-1, weighs in at up to 33 Suns, double the previous record for a "single star" black hole held by M33 X-7 which tips the scales at 16 Suns.

IC 10 X-1, lying 1.8 million light years from Earth, was first spotted back in 2007 by the US's Harvard-Smithsonian Centre for Astrophysics in Cambridge, New Scientist reports. Researchers estimated its weight at between 24 and 33 times the weight of our own Sun, but the result was speculative.

The researchers calculated the weight "based on the apparent orbit of a nearby companion star" deduced from "repeating dips in the brightness of X-rays coming from the black hole's vicinity" as the orbiting companion passed in front of a disc of hot, X-ray spewing material being sucked into the black hole.

Although they had no concrete evidence to back their figures, astronomers Jeffrey Silverman and Alexei Filippenko from the University of California at Berkeley have used spectra taken by the 10 metre Keck I telescope in Hawaii to confirm the result at between 23 and 33 solar masses.

The leeway in the black hole's possible weight is because the mass of its companion is unknown, NS explains.

While black holes can be "supermassive" monsters weighing billions of Suns, there's a limit on the size of black holes formed from the death of a single star - dubbed "stellar-mass" objects - depending on the star's mass and chemical make-up.

All stars lose material into space over their lifetimes, an amount determined by their chemical composition, and even the biggest bodies at up to 100 solar masses "can barely produce a 16-solar-mass black hole if they are born with a composition similar to the Sun's".

A more heavyweight prospect comes from stars "low in elements heavier than hydrogen and helium", according to astrophysicist Stanford Woosley of the University of California in Santa Cruz. These "cast off material less efficiently and could therefore have up to 42 solar masses available to form a black hole when they die".

Significantly, galaxy IC 10 "contains very few 'heavy' elements", NS notes.

IC 10 X-1 offers a possible explanation for "puzzlingly bright" X-ray sources spotted in other galaxies, so-called "ultra-luminous" sources which are 10 to 100 times brighter in X-rays than your average stellar-mass black holes. Some have suggested these are "intermediate mass black holes" weighing "hundreds or thousands of times" the mass of the Sun, which emit X-rays as they ingest matter.

However, Roy Kilgard of Wesleyan University in Middletown, Connecticut, reckons large stellar-mass black holes like IC 10 X-1, "undergoing especially bright outbursts of X-rays", could be the cause.

Kilgard suggests that there may be "many other stellar-mass black holes in this mass range that have eluded detection until now because they are farther away and harder to study", and concludes: "I expect in the coming years we'll identify many more of these things." ®

Note for commentators

Regarding the weight of IC 10 X-1 in Reg standard units, the Sun is reckoned to tip the scales at 1.9891 × 10³⁰ kg. A Jub is, of course, 4.2kg.

We'll leave you to do the rest, and suggest the answer is probably best expressed in megaJubs.