|Jeremy Heyl [Clippings]||
Here are some clippings from some recent magazine articles about my work. Links to the original magazine articles are provided but unfortunately some of these may require a subscription. I have also linked to entries in my bibliography.
Black holes are such a staple of astronomical theory, it's difficult to remember that nobody has ever seen one. But in July, astrophysicists Jeremy Heyl and Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics confirmed one of the strangest qualities of black holes: They have no surface, just an event horizon that marks the point of no return for anything falling into them.
Most suspected black holes are surrounded by discs of hot gas drawn by the hole's intense gravity. These discs, rather than the hole itself, are what astronomers see. Except for the speed at which the gas orbits, however, a black hole is largely indistinguishable from a neutron star, a less extreme type of compact object. A neutron star is the remnant of a brilliant star that burned out and collapsed into a ball about 12 miles across, twice the diameter of an equivalent black hole. But a neutron star, unlike a black hole, has a well-defined surface. Heyl and Narayan zeroed in on that difference.
Gas from a stellar companion can fall onto a neutron star's surface, pile up, and explode in a brilliant thermonuclear blast. "Typically, when neutron stars accrete mass, this occurs every day or so," Heyl says. When gas lands on a black hole, in contrast, it should cross the event horizon and disappear forever. Heyl and Narayan compared theoretical models of the two kinds of objects with observed X-ray emissions from a dozen black-hole candidates and nearly 100 neutron stars. All of these objects accrete large quantities of gas, but only the neutron stars exhibited explosive flashes. The others showed no such behavior, indicating that they lack a surface where gas could accumulate. "We can't avoid the conclusions that they really are black holes," Heyl says.
Astronomers reach the event horizon
10:17 12 July 02
Black holes really do imprison matter and light, and sap energy from light that narrowly escapes their grip. Until now, these were only predictions of Einstein's theory of gravity, but astronomers peering at suspected black holes have at last found compelling evidence that this does actually happen.
Black hole theory says that if a very large star explodes at the end of its life and leaves behind a core weighing more than about three times the mass of the Sun, the core will collapse to a point under its own gravitational pull.
So strong would be the gravity of the resulting "singularity" that it would prevent matter and even light escaping from a region around it bounded by the so-called event horizon.
By definition, it's impossible to see black holes directly. But astronomers have located around a dozen black hole candidates in our Galaxy, because orbiting telescopes can see the X-rays emitted by a black hole's accretion disc - the disc of hot matter swirling towards it.
However, there has been no concrete proof that any of these objects really has an event horizon - until now. Jeremy Heyl of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts and his colleague Ramesh Narayan think they have found conclusive evidence.
It comes from studies of neutron stars, dense remnants of supernova explosions that are not heavy enough to collapse into black holes. Instead, they collapse into neutron-rich balls about 12 kilometres across with solid surfaces made of iron nuclei.
Most of them occasionally eject flares of X-rays called type 1 bursts that last up to 15 minutes. The bursts are thought to occur because matter trickling onto a neutron star's surface gradually piles up and then burns in a nuclear fusion explosion.
Narayan and Heyl have calculated that if very heavy objects do not collapse to a point with an event horizon but instead have a surface, they would eject as many type 1 bursts as neutron stars. But to date, we have not seen a single burst from an object thought to be a black hole.
"Since they don't burst, we can argue beyond reasonable doubt that they don't have a surface - it's pretty compelling," says Heyl.
In a separate study, scientists led by Jane Turner of the University of Maryland, Baltimore County, have confirmed that light narrowly escaping from a black hole loses energy as it emerges - the second of Einstein's predictions.
They were following up earlier work on the X-ray spectrum of a black hole accretion disc, which revealed a broad "fingerprint" generated by iron. It had a smeared-out pattern of frequencies, suggesting that X-rays near the event horizon were losing energy as they escaped from the black hole's gravitational pull.
However, critics argued that the pattern could be due to jostling electrons in the hot gases colliding with the X-rays. But now the Maryland team has proven the critics wrong.
The astronomers studied a supermassive black hole with a mass 23 million times that of the Sun. They looked at fine detail in the broad spectral fingerprint of iron using NASA's Chandra X-ray satellite and the European Space Agency's XMM-Newton satellite.
The pattern of frequencies was precisely what Einstein's theory predicts for light climbing out of an accretion disc, rather than the result of the chaotic jostling of electrons.
The researchers believe that the X-ray features they looked at are coming from two very bright "hot spots" within the black hole's accretion disc. If so, tracking the hot spots could allow astronomers to measure how fast the black hole inside is spinning.
"This research shows the possibility of watching an individual hot spot as it spirals toward the event horizon," says Fred Baganoff, an astronomer at MIT. "That would be a tremendous advance."
Robin Orwant and Hazel Muir
New Scientist vol 172 issue 2314 - 27 October 2001, page 12
Flickering X-rays could reveal Earth-like weather systems
NEUTRON stars may have weather systems like those on Earth. This novel idea may explain why some neutron stars emit mysterious flickering X-rays.
Neutron stars are the collapsed cores of massive stars. They pack roughly one-and-a-half times the mass of our Sun into a space about the diameter of a city. In 1996, Tod Strohmayer of NASA's Goddard Space Flight Center in Maryland noticed that the X-ray bursts coming from some neutron stars flickered several hundred times a second.
Strohmayer reasoned that the effect was related to the stars' rotation, but didn't know how. "The question is, what would allow you to see the star spinning?" he says. Now Jeremy Heyl from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, suggests that weather systems on the surface of the stars could be causing the flicker.
On Earth, the primary weather system is the westerly jet stream, resulting from the Earth's rotation relative to its atmosphere. The rotation also causes planetary waves, called Rossby waves, which move westwards and modulate the jet stream.
A similar effect may occur on neutron stars, Heyl told astronomers attending the Chandra Symposium at the centre last week. The stars pull in matter from a normal neighbouring star, and when this pours onto the neutron core it creates an iron crust and an atmosphere of hydrogen and helium. The gases produce X-rays when they burn through nuclear fusion.
Rossby waves in the atmosphere would make some parts burn brighter, Heyl says. "As the star rotates you see light and dark spots." His calculations show that the spinning rate of a neutron star combined with the speed of the Rossby waves across its surface exactly reproduce the pattern of X-ray flicker that astronomers have observed.
This is the first real insight into what the surface of a neutron star might look like, Strohmayer says. "It's a nice piece of work."