Test proposed to understand the origin of solar mass black holes

A new study proposed an interesting and robust test that can be used to infer the origin of solar mass black holes. Solar mass black holes are not expected to form from the conventional stellar evolution but can be produced naturally via neutron star (NS) implosions from the capture of small primordial black holes (PBHs) or from the accumulation of dark matter (DM). The study reports that the mass distribution of such solar-mass BHs would be similar to that of the NSs. This would differ from the mass distribution of black holes in the solar-mass range predicted either by conventional stellar evolution or early universe PBH production.

An artist's impression of IGR J17091-3624: The stellar-mass black hole with the fastest Wind. Source: Chandra

Solar mass black holes should not be confused with stellar-mass black holes. A stellar-mass black hole is a black hole formed by the gravitational collapse of a star. They have masses between 5 to several tens of solar masses and have been observationally confirmed. Cygnus X-1 is the first confirmed stellar-mass black hole observed in 1964. Solar mass black holes, on the other hand, are completely hypothetical objects with no conclusive observational evidence. They are thought to be produced in the early Universe or when a neutron star engulfs dark matter or small primordial black holes. The discovery of gravitational waves (GW) has opened a new window for exploration of a black hole (BH) properties, in particular the distribution of their masses. Additionally, LIGO/VIRGO has confirmed the existence of collapsed objects with masses in the range of 1.5 to 2.5 solar masses. However, it is not yet clear what type of objects they are. They could either be neutron stars or solar mass black holes.

The authors proposed a simple test that can be used to understand the origin of solar mass black holes. The test works by fitting the solar-mass BH candidate events to the NS mass distribution by employing a Kolmogorov-Smirnov (K-S) test. For the null hypothesis, the authors assumed that the data follows the combined NS population distribution. After careful analysis of several events, the authors came to the conclusion that the null hypothesis is not true, and therefore the observed solar-mass BH candidate events are unlikely to be from neutron star-dark matter implosions. However, the authors state that more sophisticated analysis and further observations are required to improve the accuracy of this test. Nonetheless, the study is groundbreaking and has shown that mass distribution analysis of solar-mass BHs can be used as a powerful novel test of their origin.

Article Information: V. Takhistov, G. M. Fuller, A. Kusenko, "A Test for the Origin of Solar Mass Black Holes", arXiv:2008.12780

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