Tau.Neutrino said:
paper
A noninteracting low-mass black hole–giant star binary system
https://science.sciencemag.org/content/366/6465/637
“As material falls toward a black hole, it heats up and emits x-rays. Almost all black holes are discovered by this x-ray emission. Thompson et al. observed light from a giant star that is Doppler shifted, indicating an orbit around a binary companion. The companion object must weigh more than 2.6 solar masses, but it emits no light, including x-rays. This indicates the presence of a black hole that is not currently consuming any material. There may be a population of similarly hidden black holes that have been missed by x-ray observations.”
So, looking for planets and didn’t find one. Unless there’s a huge calculation error, this does look like quiescent black hole. Let’s look at the alternatives.
- High inclination – that would mean the mass was more than 2.6 solar, not less.
- White dwarf – the heaviest can’t be more than about 1.4 solar masses.
- Neutron star – the heaviest can’t be much more than 2.1 solar masses.
- Multiple objects or eccentric orbits – possible but unlikely.
- Pulsating star – hmm, that would give similar doppler behaviour, but unless the star was on the instability strip it’d be unlikely.
How do they know it’s rapidly rotating? From sunspots or from spectral lines? If it’s a giant star with a 2MASS designation, that makes it a red giant or AGB star. Because 2MASS is an infrared survey and most giant stars shine more brightly in visible light than in IR.
Consider for instance two neutron stars 60 degrees apart (Lagrangian relationship), then that would look in doppler effect exactly like one object larger by a factor of sqrt(3) I think 2.1 * sqrt(3) > 2.6. So a pair of heavy orbits in lockstep is possible, I think, but I wouldn’t call it likely.