Supermassive black holes are not picky eaters. They placidly sit at the center of galaxies, but if material gets too close, they’ll voraciously gorge on it. Stars have been disrupted and eaten too when they got too close. Now, researchers report that they have caught a supermassive black hole in the act of eating a white dwarf, the last stage of evolution for stars like our Sun – but the white dwarf is not giving up without a fight.
Astronomers have been keeping a close eye on supermassive black hole 1ES 1927+654 since 2011. This close attention paid off when the gargantuan object dramatically changed in 2018: The corona of X-rays that surrounded the black hole disappeared completely – but stranger things were yet to come.
By 2021, the corona and the usual emission appeared to have been restored, only for weird fluctuations of X-rays in 2022 to be spotted by XMM-Newton. With periods varying between 400 and 1,000 seconds, the X-ray emission was increasing and decreasing by around 10 percent. These are known as quasi-periodic oscillations, and they are very difficult to see in supermassive black holes.
“This was our first indication that something strange was going on,” lead author Megan Masterson, a graduate researcher at Massachusetts Institute of Technology, said in a statement.
The best explanation for these oscillations is the presence of an object in the accretion disk surrounding the black hole. The material in the disk and the massive object were doomed to be swallowed. As the object moved closer and closer to the black hole, the frequency of the oscillations increased.
According to the calculations, the likely object was a white dwarf about 10 percent the mass of the Sun. It was moving about one-third of the speed of light, completing a 100 million kilometers (61 million miles) orbit in just 18 minutes, in the early days.
White dwarfs are dense, and supermassive black holes are even denser. Their interaction releases energy in the form of gravitational waves, which would eventually force the white dwarf on a collision course with the black hole. Thanks to the oscillation changes, the team was able to estimate when the white dwarf was doomed to be eaten: January 4, 2024.
“I’ve never in my career been able to make a prediction that precisely before,” added Erin Kara, Massachusetts Institute of Technology, and Megan’s PhD supervisor.
Somebody must have forgotten to tell the white dwarf, because in March 2024, it was still orbiting the black hole – now even faster, doing one orbit in just 7 minutes. The density of the white dwarf might give it an advantage, it is not easily disrupted by a black hole weighing about one million times the mass of the Sun.
Still, the X-ray observations of the last year don’t make sense. The whole system challenges what we think is happening around the black hole, which means that we are missing some pieces. There might be different configurations, so the gravitational waves emitted are not what is estimated by the team. Or maybe the X-ray oscillations are not all due to the massive object.
The good news is that there is a way to test the right hypothesis. The bad news is that we need to wait until the LISA mission is launched in a decade. This will be a space-based gravitational wave observatory and the team is confident that the gravitational waves emitted by this system are in the right frequency range for LISA.
“The one thing I’ve learned with this source is to never stop looking at it because it will probably teach us something new,” Masterson told MIT News. “The next step is just to keep our eyes open.”
The paper has been accepted to the journal Nature and can be found on the preprint repository arXiv. The research was presented at the 245th American Astronomical Society meeting.
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