March 21, 2008

Blast called furthest object visible to naked eye

March 20, 2008
Courtesy NASA
and World Science staff

A gi­gantic stel­lar ex­plo­sion de­tected March 19 by has shat­tered the rec­ord for the fur­thest ob­ject vis­i­ble with the na­ked eye, sci­en­tists say—half­way across the known uni­verse.

Sad­ly, the show lasted only hours. But “if some­one just hap­pened to be look­ing at the right place at the right time, they saw the most dis­tant ob­ject ev­er seen by hu­man eyes with­out op­ti­cal aid” on record, said Ste­phen Hol­land of NASA’s God­dard Space Flight Cen­ter in Green­belt, Md.

The af­ter­glow of GRB 080319B was im­aged by Swift's X-ray Tel­e­scope (left) and Op­ti­cal/Ul­tra&s Tel­e­scope (right). (Cred­it: NA­SA/Swift/Ste­fan Imm­ler, et al.)

De­tected by the agen­cy’s Swift sat­el­lite, the ex­plo­sion was a gam­ma ray burst, a type of blast that usu­ally oc­curs when mas­sive stars run out of their nu­clear fu­el. Their cores col­lapse to form ex­tremely dense ob­jects known as black holes or neu­tron stars. In the pro­cess they re­lease a great burst of high-en­er­gy gam­ma rays and par­t­i­cle jets that rip through space at nearly light speed.

As the jets plow in­to sur­round­ing interstel­lar clouds, they heat the gas, of­ten gen­er­at­ing bright af­ter­glows. Gam­ma ray bursts are be­lieved to be the most lu­mi­nous ex­plo­sions in the uni­ver­se, and this one “was a whop­per,” said Swift prin­ci­pal in­ves­ti­ga­tor Neil Gehrels of the God­dard cen­ter. “It blows away ev­ery gam­ma ray burst we’ve seen so far.”

Swift’s Burst Alert Tel­e­scope pick­ed up the burst at 2:12 a.m. Eastern U.S. time and pin­pointed the co­or­di­nates in the con­stella­t­ion Bo├Âtes, re­search­ers said. Tel­e­scopes in space and on the ground quickly moved to catch the af­ter­glow. The burst is named GRB 080319B, be­cause it was the sec­ond gam­ma ray burst found that day.

Two oth­er Swift in­stru­ments al­so ob­served af­ter­glows. Sev­eral ground-based tele­scopes saw the af­ter­glow bright­en to vis­u­al mag­ni­tudes be­tween 5 and 6, in the scale used by as­tro­no­mers. The brighter an ob­ject is, the low­er its mag­ni­tude num­ber. From a dark loca­t­ion in the coun­try­side, peo­ple with nor­mal vi­sion can see stars slightly faint­er than mag­ni­tude 6.

Thus the af­ter­glow would have been dim, but vis­i­ble to the na­ked eye, said Hol­land, a mem­ber of the Swift sci­ence team.

Lat­er, the Very Large Tel­e­scope in Chil­e and the Hobby-Eberly Tel­e­scope in Tex­as meas­ured the burst’s red­shift at 0.94. A red­shift is a meas­ure of the dis­tance to an ob­ject. A red­shift of 0.94 trans­lates in­to a dis­tance of 7.5 bil­lion light years, mean­ing the ex­plo­sion took place 7.5 bil­lion years ago, a time when the uni­ver­se was less than half its cur­rent age and Earth had yet to form. The burst was seen oc­cur­ring in the dis­tant past because its light takes so long to reach us.

“No oth­er known ob­ject or type of ex­plo­sion could be seen by the na­ked eye at such an im­mense dis­tance,” said Hol­land.

GRB 080319B’s op­ti­cal af­ter­glow was 2.5 mil­lion times more lu­mi­nous than the most lu­mi­nous su­per­no­va, or stel­lar ex­plo­sion, ev­er rec­orded, sci­en­tists said. That would make it the most in­trin­sic­ally bright ob­ject ev­er ob­served by hu­mans. The most dis­tant pre­vi­ous ob­ject that could have been seen by the na­ked eye is the near­by gal­axy M33, a rel­a­tively short 2.9 mil­lion light-years from Earth.

Anal­y­sis of GRB 080319B is just get­ting un­der­way, so as­tro­no­mers don’t know why this burst and its af­ter­glow were so bright. One pos­si­bil­ity is the burst was more en­er­get­ic than oth­ers, per­haps be­cause of the mass, spin, or mag­net­ic field of the pro­gen­i­tor star or its jet, sci­ent­ists said. Or per­haps it con­cen­trat­ed its en­er­gy in a nar­row jet aimed di­rectly at Earth.

GRB 080319B was one of four bursts that Swift de­tected, a Swift rec­ord for one day—as it hap­pened, the same day ac­claimed sci­ence-fiction writ­er Ar­thur C. Clarke died. “Co­in­ci­den­tally, [his death] seems to have set the uni­ver­se ablaze with gam­ma ray bursts,” said Swift sci­ence team mem­ber Ju­dith Ra­cu­sin of Penn State Uni­ver­s­ity.

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