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Milky Way’s black hole seen as particle smasher

Feb. 27, 2007
Courtesy University of Arizona
and World Science staff

Sci­en­tists were star­tled to find in 2004 that the cen­ter of our gal­axy is emit­ting gam­ma rays, the highest-en­er­gy form of light. Now as­t­ro­phys­i­cists say they’ve dis­cov­ered what might pro­duce these.

A graph­ic il­lus­trat­ing the idea that the black hole at the cen­ter of the Milky Way is like an ex­treme­ly pow­er­ful par­t­i­cle ac­cel­er­a­tor, rev­ving up pro­tons in the sur­round­ing mag­net­ic plas­ma and sling­ing them in­to lower-en­er­gy pro­tons with such en­er­gy that high-en­er­gy gam­ma rays re­sult from the col­li­sion. The yel­low line de­picts a high-en­er­gy pro­ton flung in­to a lower-en­er­gy pro­ton in a cloud of hy­dro­gen gas. The green ar­row rep­re­sents the high-en­er­gy gam­ma ray that re­sults from the col­li­sion. (Cred­it: Sar­ah Bal­lan­tyne)


A black hole be­lieved to lurk in that place, they pro­pose, could be a cos­mic form of par­t­i­cle ac­cel­er­a­tor—a ma­chine built to smash sub­a­to­mic par­t­i­cles to­geth­er in or­der to un­der­stand their com­po­nents.

The black hole, ac­cord­ing to this view, would rev up par­t­i­cles known as pro­tons, parts of the cores of or­di­nary atoms, and smash them at near-light speeds in­to low­er en­er­gy pro­tons. The col­li­sions would pro­duce gam­ma rays.

“It’s si­m­i­lar to the same kind of par­t­i­cle phys­ics ex­per­i­ments that the Large Had­ron Col­lid­er,” a next-generation ac­cel­er­a­tor in Switz­er­land, will per­form, said Da­vid Bal­lan­tyne of the Uni­ver­si­ty of Ar­i­zo­na in Tuc­son, Ariz. That ma­chine is due to start op­er­at­ing this year.

Bal­lan­tyne and col­leagues wrote a pa­per on the find­ings pub­lished in the March is­sue of As­t­ro­phys­i­cal Jour­nal Let­ters, a re­search pub­li­ca­tion.

The Large Had­ron Col­lider is ex­pect­ed to be able to ac­cel­er­ate pro­tons to sev­en tril­lion elec­tron­volts, a meas­ure of en­er­gy. Our gal­ax­y’s black hole whips pro­tons to up to 100 tril­lion elec­tron­volts, ac­cord­ing to the new stu­dy. That’s all the more im­pres­sive be­cause “Our black hole is pret­ty in­ac­tive com­pared to mas­sive black holes sit­ting in oth­er galax­ies,” Bal­lan­tyne said.

A black hole is an ob­ject so tight­ly com­pressed that its own weight cre­ates grav­i­ty that sucks in an­y­thing with­in a cer­tain range, in­clud­ing light. Most ga­lax­ies are thought to har­bor cent­ral, huge black holes dubbed su­per­mas­sive black holes.

Pow­er­ful, cha­ot­ic mag­net­ic fields ac­cel­er­ate pro­tons and oth­er par­t­i­cles near our black hole to ex­treme­ly high en­er­gies, Bal­lan­tyne’s team ar­gued. 

“Our gal­ax­y’s cen­tral supermas­sive ob­ject has been a con­stant source of sur­prise ev­er since its dis­cov­ery some 30 years ago,” said the Uni­ver­si­ty of Ar­i­zo­na’s Ful­vio Melia, a col­lab­o­ra­tor in the study. “S­lowly but sure­ly it has be­come the best stud­ied and most com­pel­ling black hole in the uni­verse. Now we’re even find­ing that its ap­par­ent qui­et­ness over much of the [light] spec­trum be­lies the real pow­er it gen­er­ates a mere breath above its event hori­zon—the point of no re­turn” past which no­thing can es­cape its grip.

The Milky Way black hole “is one of the most en­er­get­ic par­t­i­cle ac­cel­er­a­tors in the gal­axy, but it does this by proxy,” Melia said. It ca­joles a mag­net­ized plas­ma, or e­lec­tri­cal­ly charged gas that’s “hap­less­ly trapped with­in its clutches, in­to sling­ing pro­tons to un­earth­ly speeds.”


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Scientists were startled to find in 2004 that the center of our galaxy is emitting gamma rays, the highest-energy form of light. Now astro physicists say they’ve discovered what might produce these. A black hole believed to lurk in that place could be a cosmic form of particle accelerator—the machines scientists build to smash particles together and understand their fundamental components, the researchers say. The black hole, according to this view, would rev up particles known as protons, which sit at the heart of ordinary atoms, and smash them at incredible speeds into lower energy protons. The collisions would produce gamma rays. “It’s similar to the same kind of particle physics experiments that the Large Hadron Collider,” a next-generation accelerator in Switzerland, will perform, said David Ballantyne of the University of Arizona in Tucson, Ariz. The machine is due to start operating this year. Ballantyne and colleagues at the Los Alamos National Laboratory in Los Alamos, N.M., and the University of Adelaide, Australia, wrote a paper on the findings published in the March issue of Astro physical Journal Letters, a professional publication. When complete, the Large Hadron Collider in Switzerland will be able to accelerate protons to seven trillion electronvolts, a measure of energy. Our galaxy’s black hole whips protons to energies as much as 100 trillion electronvolts, according to the new study. That’s all the more impressive because “Our black hole is pretty inactive compared to massive black holes sitting in other galaxies,” Ballantyne said. Powerful, chaotic magnetic fields accelerate protons and other particles near the black hole to extremely high energies, the group argued. A black hole is an object so tightly compressed that its own weight creates gravity that sucks in anything within a certain range, including light. “Our galaxy’s central supermassive object has been a constant source of surprise ever since it’s discovery some 30 years ago,” said the University of Arizona’s Fulvio Melia, a collaborator. “Slowly but surely it has become the best studied and most compelling black hole in the universe. Now we’re even finding that its apparent quietness over much of the [light] spectrum belies the real power it generates a mere breath above its event horizon—the point of no return.” The Milky Way black hole “is one of the most energetic particle accelerators in the galaxy, but it does this by proxy, by cajoling the magnetized plasma [electrically charged gas] haplessly trapped within its clutches into slinging protons to unearthly speeds,” Melia said.