ASTRONOMERS CLOSING IN ON BLACK HOLES
TORONTO, Ont.--Measurements made with the Japanese-NASA satellite ASCA and othersatellites imply the reality of the ``event horizon,'' the one-waymembrane around black holes that is predicted by Einstein's theory ofrelativity. A report presented at the America Astronomical Societymeeting here today by Drs. Ramesh Narayan, Jeffrey McClintock andMichael Garcia of the Harvard-Smithsonian Center for Astrophysics,Cambridge, Mass., offers the most direct evidence yet that black holesreally exist.
The black hole is perhaps the most fascinating prediction to come outof Einstein's Theory of General Relativity. When any mass becomesmore compact than a certain limit, its own gravity is so strong thatthe object collapses in on itself to a singular point, a black hole.The event horizon is a sort of one-way membrane which surrounds thesingularity. Matter and energy can fall into the black hole throughthe event horizon, but nothing, not even light, can get out.
The announcement by Narayan, McClintock and Garcia differs in afundamental way from other recent discoveries of black holecandidates. These other studies have proved the existence of objectswhich are so dense that modern physics tells us they cannot be stablestars. The most straightforward interpretation is that they are blackholes. Narayan, McClintock and Garcia have found that some of theserecently discovered black holes are not only ultra-dense, but thatthey actually ``vacuum up'' energy from their surroundings throughtheir event horizons. This makes the case for black holes stronger.
The new work involves a class of X-ray emitting binary stars called``X-ray novae.''
McClintock, who is a Staff Astrophysicist at theCenter for Astrophysics, has discovered some of the best examples. AnX-ray nova consists of two stars orbiting each other, with gas fromone being transferred steadily to the other. The gas flows onto thesecond star by a process called ``accretion,'' and, in the process,the gas heats up and radiates X-rays. This highly energetic radiationshows that the accreting (or mass receiving) star must be very compactand therefore a potential black hole candidate. However, one cannotbe sure that it really is a black hole. For instance, it might be a``neutron star,'' which is almost as compact as a black hole, or itmight be something even more bizarre.
To prove that an object is a black hole, one would like to show thatit has an event horizon, which is one of the fundamental, definingfeatures of a black hole. Such a proof is now at hand according toNarayan, McClintock, and Garcia.
A theoretical study in 1994 by Narayan and Dr. Insu Yi (now at theInstitute for Advanced Study in Princeton) first revealed thepossibility of a new mode of accretion around black holes, which theynamed an ``advection-dominated accretion flow,'' or ADAF.
[Similarwork was done independently by another group of scientists in Europeand Japan, led by Dr. Marek Abramowicz of Goteborg University inSweden.]
Whenever gas flows onto a compact star via accretion, a large amountof energy is released. Prior to the work on ADAFs, astronomersbelieved that the accreting gas will radiate most of this energy. Incontrast, an ADAF stores most of the energy as heat and liberates onlya small fraction as radiation. Indeed, the accreting gas becomes soextraordinarily hot
that the temperature reaches up to a million timesa million degrees.
When the superheated gas reaches the center, it disappears through theevent horizon carrying with it the enormous thermal energy. Sincemost of the energy is lost into the black hole, the object appearsvery dim, much dimmer than if that energy had been radiated outwardinto space.
Narayan and Yi showed that the ADAF mode occurswhenever the rate of accretion of gas is relatively low. The X-raynovae are excellent for testing this model, since these objects have avariable rate of accretion. Although they occasionally accrete a lotof mass and become very bright, most often they accrete only a smallamount and are dim. In this dim state, ADAFs are expected.
Narayan and colleagues have analysed a particular X-raynova called "V404 Cyg," in the constellation Cygnus at adistance of about ten thousand light years. What they find is thatthe X-ray emission from this object has a spectral form which isexactly as predicted by the theory of ADAFs. (See Figure 1.) Thestriking agreement between the observations and the theoretical modelstrongly suggests that V404 Cyg possesses an ADAF.
``This star seems to be swallowing nearly a hundred times as muchenergy as it radiates, and the only way this can happen is if the staris a true black hole.
This is the most direct evidence scientistshave had that black holes are real,'' says Narayan, who is Professorof Astronomy at Harvard University and Associate Director of theCenter for Astrophysics.
What will happen if the star in V404 Cyg is not a black hole, but aneutron star with a surface? In this case, when the hot accretingmaterial reaches the center, the thermal energy cannot disappear. Thehot gas will heat up the star, and the energy will come out asradiation from the star's surface.
Thus, the object will be muchbrighter. (See Figure 2.) It will also have a very differentspectrum than in the black hole case.
Can one look for the predicted difference between a black hole and astar with a normal surface? Indeed, yes, since nature has been kindenough to make two distinct kinds of X-ray novae: those with blackholes (like V404 Cyg and several others) and those with neutron stars.
Garcia, McClintock, and Narayan have analysed X-ray data on nine X-raynovae, four with candidate black holes and five with neutron stars.They find that the black hole systems are all systematically dimmerthan the neutron star systems. (See Figure 3.) According to theirinterpretation, both kinds of systems are accreting via ADAFs. In thecase of the black holes, nearly all the energy disappears through theevent horizon and only a very small fraction is radiated. The objectsare therefore extremely dim. In the case of the neutron stars,however, energy falling on the star is re-radiated from its surfaceand the sources are brighter. (See the attached artist's conception.)
``The observed difference is what we expected, and confirms that blackholes do have event horizons,'' says Garcia, who is a StaffAstrophysicist at the Center for Astrophysics.
The universe has large numbers of black holes in a variety of sizesand masses. If the work reported at Toronto is right, the vastmajority of these black holes are accreting via ADAFs, and they willprovide numerous opportunities in the years ahead to test the realityof event horizons around black holes. If the tests continue to verifyEinstein's theory, there will be compelling evidence that each blackhole is the ultimate ``cosmic thief'' --- silently guzzling tons ofmatter through its event horizon each second, matter that never will be seen againby the rest of the universe.
This work was supported by grants from NASA, the National ScienceFoundation, and the Smithsonian Institution.
Ramesh Narayan: (617) 496-9393 (email: email@example.com)
Jeffrey E. McClintock: (617) 495-7136 (email: firstname.lastname@example.org)
Michael R. Garcia: (617) 495-7169 (email: email@example.com)
Harvard-Smithsonian Center for Astrophysics
60 Garden Street
Cambridge, MA 02138
Related Information: Artist's Conception of X-ray Novae (Color)
Artist's Conception of X-ray Novae (Line Drawing)
Scientific Paper on V404 Cyg (Postscript)
Scientific Paper on X-ray novae (Postscript)