A noninteracting low-mass black hole–giant star binary system

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Science  01 Nov 2019:
Vol. 366, Issue 6465, pp. 637-640
DOI: 10.1126/science.aau4005

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A black hole hiding in a binary star

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.

Science, this issue p. 637


Black hole binary systems with companion stars are typically found via their x-ray emission, generated by interaction and accretion. Noninteracting binaries are expected to be plentiful in the Galaxy but must be observed using other methods. We combine radial velocity and photometric variability data to show that the bright, rapidly rotating giant star 2MASS J05215658+4359220 is in a binary system with a massive unseen companion. The system has an orbital period of ~83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicating star spots and tidal synchronization. Constraints on the giant’s mass and radius imply that the unseen companion is 3.30.7+2.8 solar masses, indicating that it is a noninteracting low-mass black hole or an unexpectedly massive neutron star.

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