Milky Way’s black hole seen as particle smasher
Mar 09,2007 00:00 by Bend_Weekly_News_Sources

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 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.

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)

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.”

Courtesy University of Arizona
and World Science staff