Underground telescopes may catch black holes in the making

Physics and astronomy researchers at the State University of New York (SUNY; Stony Brook, NY) believe that an underground detector array consisting of almost 10,000 photomultiplier tubes may help to observe the formation of black holes in collapsing proto-neutron stars.

A proto-neutron star is what is left when the core of a massive star collapses after a supernova explosion. The proto-neutron star consists of an equal number of protons and neutrons but only lasts for about ten seconds. That's how long it takes for the dying star to give off about 99% of its binding energy in the form of neutrinos and to become a cold neutron star, consisting almost entirely of neutrons. Within the first minute after the supernova, however, the neutron star may collapse further into a black hole, if negatively charged, strongly interacting particles such as quarks, hyperons or a Bose condensate happen to be lurking in the original proton-neutron mix.

The "quark matter" contributes to a softening process that facilitates post supernova compression and enables the proto-neutron remnants of a sufficiently massive star to collapse further into a black hole, driven by the force of its own gravity, according to the researchers.¹ The signal that this is happening would be the abrupt cessation of neutrino flux that accompanies black hole formation. Observing such changes in neutrino flux, however, would require the use of relatively new technology for neutrino detection, such as the Sudbury Neutrino Observatory (SNO; Ontario, Canada).

"Observing the neutrinos is the only way we can observe the proto-neutron star," according to James Latimer, a member of the SUNY research team, which expects its recently published calculations to help sensitive neutrino detectors, such as the SNO or the Super-Kamiokande in Japan, analyze neutrino flux from supernovae and perhaps observe black holes in the process of formation.

Originally designed to differentiate the relatively intense electronic neutrinos from more subtle neutrino "flavors," such as muon and tau, that also contribute to the solar neutrino flux, the SNO, designed and built by researchers at Lawrence Berkeley National Laboratory (Berkeley, CA), is more sensitive then the Super-Kamiokande, which only detects electronic neutrinos (see Laser Focus World, October 2000, p. 77).

The SNO is located in an underground cave where more than a mile of solid stone provides a natural shield from the noisy cosmic rays that might otherwise mask the stealthy transit of neutrinos, which would normally pass through thousands of miles of lead undisturbed and undetected.² The SNO, which was completed in April 1999 and achieved first light that summer, consists of 9500 photomultiplier tubes that fill the surface area of an 18-m-diameter geodesic sphere.

The spherical photo detector detects faint blue-light emissions from the actual neutrino detector, which is a 12-m-diameter acrylic sphere suspended within the geodesic sphere in a bath of purified water. One thousand tons of heavy water within the acrylic sphere provides a medium that actually interacts with the neutrinos (which are the second most numerous particles in the universe, after photons) yielding photons that can then be detected by the photomultiplier tubes.

Of course, the prospect of observing the process of black hole formation generates a reasonably high degree of excitement among astronomers. Madappa Prakash, head of the SUNY research group likened the possibility of observing such an event to "catching a thief in the act."

REFERENCES

1. J. A. Pons, A. W. Steiner, M. Prakash, J. M. Latimer, Phys. Rev. Lett. 86(23), 5223 (June 4, 2001).
2. R. Kaltschmidt, Berkeley Lab Res. Rev. (Photojournal) (Fall 1998).