How Low Can a Black Hole Go?

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Science  19 Dec 2003:
Vol. 302, Issue 5653, pp. 2035
DOI: 10.1126/science.302.5653.2035a

Black holes have been detected in many systems, and the range of their estimated masses is thought to reflect different mechanisms of formation. Stellar mass black holes (those with masses of 3 to 15 solar masses) are the remnants of supernovae, and any star with a mass greater than about 3 solar masses will collapse and form a black hole; on the other hand, lower mass stars will form neutron stars. Supernova models suggest that lower mass black holes should be more abundant than higher mass black holes and that there should be a continuum from neutron stars to stellar mass black holes. Observations, however, reveal a gap at 3 to 5 solar masses, where no black holes have been identified.

Gelino et al. may have filled this gap with a lightweight black hole candidate. Based on optical and infrared photometry of the soft x-ray transient GRO J0422+32, they estimate that the primary object in this binary system has a mass of 3.97 solar masses. This object falls within the mass gap and is the lowest estimated mass for a black hole so far. Thus, lightweight black holes might actually be abundant (consistent with supernova models), just very difficult to detect. Further observations of this and other lightweight black holes will help to refine supernova models that rely on theories about the fundamental physics and equation of state of collapsing stars to hyperdense objects. — LR

Astrophys. J., in press (astro-ph/0308490).

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