Astronomers think a bright X-ray source in the galaxy NGC 1313 is a mid-size black hole.

galaxy NGC 1313
This VLT color composite reveals the central parts of the starburst galaxy NGC 1313. Successive bursts of star formation have inflated and carved out many glowing cocoons of gas. Near the galaxy's dynamical center (at least, from our line of sight) lies an X-ray-emitting object that astronomers suspect is an intermediate-mass black hole.

Of all the black holes we observe in the universe, only a handful fall into the range known as intermediate-mass. These are the anti-Goldilockses of black holes: too massive to be born from a single star’s death but too puny to be one of the supermassive beasts skulking in galactic centers, intermediate-mass black holes (IMBHs) are “just wrong.”

Most IMBH candidates are iffy contenders. They might instead be smaller black holes — or even neutron stars — that are binge-eating, making themselves look brighter (and, therefore, more massive) than they really are.

One way to potentially tell the nuggets from the fool’s gold is to use what are called quasi-periodic oscillations (QPOs), cyclic blips in a black hole’s X-ray signal. Astronomers think these variations might be caused by some sort of hotspot deep in the blazing-hot disk of gas feeding the black hole. If so, then a QPO’s cyclic frequency is the time it takes this source to orbit the black hole — and that depends on the black hole’s mass.

For stellar-mass black holes, QPOs sometimes come in pairs, with a frequency ratio of 3:2. These linked frequencies are also related to the black hole’s mass, so astronomers can use them to estimate how big the black hole is.

Last year, Dheeraj Pasham (NASA Goddard) and colleagues used this QPO relation to estimate the mass of the object producing an X-ray signal in the galaxy M82. They calculated a mass of roughly 400 solar masses.

Now, the team has looked at another QPO-bearing X-ray signal, this time in the galaxy NGC 1313. They estimate that the object has a mass between 3,700 and 6,300 Suns. That would easily put it in the IMBH family, whose members (we expect) range from a few hundred to a hundred thousand solar masses.

If the result holds up, it adds to growing evidence that black holes do indeed exist on all scales. But it’s important to remember that NGC 1313 X-1’s mass is an extrapolation from a signal whose nature we don’t fully understand. You can read more about the study in the University of Maryland’s press release, or in the team’s paper, which appears in the September 20th Astrophysical Journal Letters.


Reference: D. R. Pasham et al. "Evidence for High-frequency QPOs with a 3:2 Frequency Ratio from a 5000 Solar Mass Black Hole." Astrophysical Journal Letters. September 20, 2015.

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