Acr620412707714472-18110.tmp.pdf

create a black hole, a jet forms and burns

its way through the star’s outer layers

in seconds. The jet then rams into the

surrounding gas cocoon left behind by

the dead star’s winds, pushing electrons

to relativistic speeds. These electrons

race through surrounding magnetic

fi elds, releasing their pent-up energy as

synchrotron radiation.

But no matter how fast these elec-

trons spiral, they can’t radiate away the

73 billion and 95 billion electron volts

the two photons carried.

One study’s lead author, Alessandro

Maselli (INAF-IASF Palermo, Italy), sug-

gests that the same electrons that created

the photons in the fi rst place could pass

on an extra punch of energy if they later

collide with their progeny.

But a coauthor on another study,

Charles Dermer (Naval Research Labo-

ratory), says that if the GRB made its

photons in two diff erent ways, we should

see a jump in the number created at

diff erent energies (for example, many

more gamma rays than X-rays). Instead,

the whole spectrum from visible light to

ultrahigh-energy gamma rays is smooth,

suggesting all the photons come from

one mechanism.

The teams propose everything from

snapping magnetic fi eld lines to scorch-

ing-hot gas around the forming black

hole as solutions. They haven’t ruled out

the synchrotron model, but it’s clear that

something extra is needed. ✦

■ MONICA YOUNG

STELLAR I Monster Burst Challenges Theories

Observations of one of the most power-

ful exploding stars ever recorded suggest

that the standard model for gamma-ray

bursts might be missing a piece of the

puzzle, scientists report in papers pub-

lished online November 21st in Science.

The gamma-ray burst GRB 130427A

set off an alarm on NASA’s Fermi

Gamma-ray Space Telescope on April 27,

2013. The Swift spacecraft, an array of
      ground-based robotic telescopes called
      RAPTOR, the CARMA millimeter-wave
      observatory, and NASA’s NuSTAR X-ray
      telescope also joined in on the action.
      In the end, the explosion fl ooded 58
      ground- and space-based observatories
      with its photons.
      These observations show that GRB
      130427A is the longest, most energetic
      such explosion on record. Although the
      main burst lasted just 20 seconds (a
      typical duration for this kind of “long”
      GRB), stray gamma rays kept pouring in
      for another 20 hours.
      Two of the thousands of gamma-ray
      photons Fermi collected during that
      time are problematic. The fi rst appeared
      19 seconds after the burst began; the
      next appeared 3¾ minutes later. Both
      packed a serious punch: 73 and 95 bil-
      lion electron volts, the highest-energy
      photons ever recorded from a GRB.
      According to the current scenario,
      these photons shouldn’t have existed.
      In the standard picture, long GRBs like
      130427A herald the collapse of very mas-
      sive stars. As the star’s core implodes to

IN BRIEF

News Notes

Black Hole Spews Atoms. Astronomers

know that black holes launch jets along their

twisted magnetic fi eld lines, but they haven’t

known where that material comes from or,

consequently, what it’s made of. Now, María

Díaz Trigo (ESO) and colleagues report in

the December 12th Nature that they have

detected ionized iron and nickel in the jet

from a stellar-mass black hole gobbling mate-

rial from its companion star. The evidence

for atoms strongly favors the idea that the jet

material comes from the black hole’s accre-

tion disk and is funneled out along magnetic

strands that the disk itself has threaded

through the black hole. This scenario makes

sense: jets usually show up when a sizable

corona of ionized gas grows around a black

hole’s disk. (The alternate explanation is that

the material originates from the electrical

current generated just outside the black hole,

which would instead fi ll the jet with electrons

and their antimatter counterparts, positrons.)

The detection is a big deal, but both mecha-

nisms might still ultimately contribute.

■ CAMILLE M. CARLISLE

Oddball Pulsar Origin. A few rapidly

whirling neutron stars might get their start

as white dwarfs, Paulo Freire (Max Planck

Institute for Radio Astronomy, Germany)

and Thomas Tauris (Argelander Institute

for Astronomy, Germany) suggest in the

Monthly Notices of the Royal Astronomical

Society. Millisecond pulsars generally form

in binary systems, in which a neutron star

spins itself up by siphoning material off its

companion star. But in Freire and Tauris’s

hypothesis, the spun-up stellar corpse is ini-

tially a white dwarf. The dwarf spins itself up

so fast that, in lieu of collapsing, the object

survives beyond its usual mass limit of 1.

solar masses. When accretion shuts off and

the dwarf slows down a bit, the stellar corpse

can’t delay its death any longer, and it col-

lapses directly into a rapidly spinning pulsar.

This scenario might explain two millisecond

pulsars found in elongated orbits with their

companions, which don’t match the circular

orbits the neutron star scenario creates.

■ CAMILLE M. CARLISLE

NASA GSFC

16 March 2014 sky & telescope

Forming black hole

Dying star

Jet

Collision-induced shock

waves create radiation

J

NN layout.indd 16 12/23/13 11:38 AM