A team of astronomers has found evidence for an intermediate-mass black hole in the spiral galaxy NGC 1313 (also known as the Topsy Turvy Galaxy).
This image shows the spiral galaxy NGC 1313. The very active state of this galaxy is very evident from the image, showing many star formation regions. A great number of supershell nebulae, that is, cocoon of gas inflated and etched by successive bursts of star formation, are visible. The green nebulosities are regions emitting in the ionized oxygen lines and may harbour clusters with very hot stars. This composite image is based on images obtained with the FORS1 instrument on one of the 8.2-m Unit Telescope of ESO’s Very Large Telescope. Astronomers determined that this galaxy harbors a black hole 5,000 times more massive than our Sun. Image credit: ESO.
Astronomers know that small black holes – black holes ranging from 10 times to 100 times the Sun’s mass – are the remnants of dying stars, and that supermassive black holes, more than 1,000,000 times the mass of the Sun, inhabit the centers of most galaxies.
But scattered across the Universe are a few apparent black holes of a more mysterious type – intermediate-mass black holes. Ranging from 100 to 10,000 solar masses, these black holes are so hard to measure that even their existence is sometimes disputed.
The new black hole candidate, NGC 1313 X-1, belongs to this rare class and has an estimated mass of 5,000 times that of our Sun.
It lies in NGC 1313, a spiral galaxy located approximately 13.5 million light-years away in the direction of the constellation Reticulum.
NGC 1313 X-1 is also classified as an ultraluminous X-ray source (ULX), and as such is among the brightest X-ray sources in the nearby Universe.
It has proven hard to explain exactly why ULXs are so bright. Some scientists suspect that they are intermediate-mass black holes actively drawing in matter, producing massive amounts of friction and X-ray radiation in the process.
Against this backdrop of haphazard X-ray fireworks created by NGC 1313 X-1, Dr Dheeraj Pasham of the Joint Space-Science Institute and co-authors identified two repeating flares, each flashing at an unusually steady frequency.
One flashed about 27.6 times per minute and the other about 17.4 times per minute. Comparing these two rates yields a nearly perfect 3:2 ratio.
The team also found this ratio in M82 X-1, the black hole they identified in August 2014, although the overall frequency of flashing was much higher in M82 X-1.
Although the astronomers are not yet sure what causes these steady flashes, the presence of a clockwork 3:2 ratio appears to be a common feature of stellar mass black holes and possibly intermediate-mass black holes as well.
“The flashes are most likely caused by activity close to the black hole, where extreme gravity keeps all surrounding matter on a very tight leash,” said Dr Pasham, lead author on the study published in the Astrophysical Journal Letters.
“The 3:2 ratios can also provide an accurate measure of a black hole’s mass. Smaller black holes will flash at a higher frequency, while larger black holes will flash less often,” he said.
The team hopes that identifying ULXs that exhibit the key 3:2 flashing ratio will yield many more intermediate-mass black hole candidates in the near future.
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Dheeraj R. Pasham et al. 2015. Evidence for high-frequency QPOs with a 3:2 frequency ratio from a 5,000 solar mass black hole. ApJ 811, L11; doi: 10.1088/2041-8205/811/1/L11
