Record Gamma-Ray Flare Is Attributed to a Hypermagnetized Neutron Star in Our Galaxy

On 27 December 2004, gamma- and x-ray detectors aboard seven spacecraft were momentarily saturated by a 0.2-second gamma-ray pulse of unprecedented intensity. Another dozen orbiters detected the pulse with particle detectors activated by the sudden flood of high-energy photons. It was, in fact, the brightest transient event ever recorded by astronomical instruments. Appearing in the 28 April 2005 issue of Nature, the first five published papers ^1–5 on the giant flare make a strong case that its cause was a global rearrangement of the crust and magnetosphere of a neutron star, on the far side of the galaxy, whose magnetic field is a thousand times stronger than that of the more pedestrian neutron stars we know as radio pulsars.

In 1992, theorists Robert Duncan (University of Texas) and Christopher Thompson (University of Toronto) proposed the existence of such hyper-magnetized neutron stars, called them “magnetars,” and offered a scenario for their birth. ⁶ They argued that a magnetar in the Large Magellanic Cloud, just outside our galaxy, had been responsible for a giant gamma-ray flare, not unlike the recent event, in March 1979. No such giant flare had ever been recorded before. Indeed, it held the brightness record until it was displaced by the December 2004 burst, which was both closer and, intrinsically, a hundred times more luminous.

The theorists also invoked their magnetar model to explain the much less spectacular outbursts of soft gamma repeaters (SGRs), a handful of neutron stars in and near the galaxy that sporadically emit subsecond bursts of soft gammas a million times less luminous than the giant flares. The three recorded giant flares (there was also one in 1998) have all been unambiguously identified with previously or subsequently discovered SGRs. So it’s clear that SGR bursts and giant flares characterize the same class of relatively nearby objects—presumably magnetars—in different phases of activity.

Classic gamma-ray bursts

Do the giant magnetar flares have anything to do with the classic gamma-ray bursts (GRBs), several thousand of which have been recorded at cosmological distances over the past 30 years? No and yes. The answer is complicated because GRBs come in two distinct varieties: long- and short-duration. A typical long GRB puts out about 10⁵¹ ergs in tens of seconds. (An erg is 10⁻⁷ joules, and the luminosity of the Sun is 4 × 10³³ erg/s.) Long GRBs are intrinsically much more luminous than giant magnetar flares, which put out 10⁴⁵–10⁴⁷ ergs in a fraction of a second. After all, a GRB, unlike a giant magnetar flare, is presumed to obliterate its parent star. The magnetar lives on as an SGR. Its giant flares only appear more spectacular because we see them locally rather than in the very distant galaxies that host most GRBs.

The short GRBs are much more problematic. Because no host galaxies have been identified for those 20% of all GRBs, it hasn’t been possible to gauge their distances or intrinsic luminosities. But the fact that short GRBs and the initial spikes of giant magnetar flares have about the same subsecond duration is a tantalizing hint.

A subsecond gamma spike is difficult to reconcile with the supernova scenarios that have had considerable success in explaining the long GRBs. “I think the most important single thing we’ve learned from the record luminosity of the December 2004 giant flare,” says Duncan, “is that magnetar outbursts in other galaxies might well be bright enough to account for a significant fraction of the puzzling short GRBs—maybe all of them.”

At a cosmological distance of, say, 10⁸ light-years, one would see only the 0.2-s initial spike of a giant flare like the December 2004 event. But being only about 5 × 10 ⁴ light-years away from that flare, the orbiting detectors saw much more. Because the record flux of MeV gammas in the initial spike saturated the gamma-ray detectors, the spike’s energy had to be estimated from detectors designed to measure charged-particle fluxes. But after the initial overload, the gamma detectors were able to record a 6-minute-long fading tail of lower-energy gammas with a clear periodicity of 7.5 seconds, corresponding to the magnetar’s rotation. The 1979 and 1998 giant flares had shown similar tails.

Figure 1 shows the saturated spike and the tail as measured by Kevin Hurley (University of California, Berkeley) and collaborators with the soft-gamma detector (20–100 keV) aboard NASA’s RHESSI, an orbiter designed primarily to study solar flares. ¹ As measured with the burst-alert telescope aboard NASA’s recently launched Swift orbiter by David Palmer (Los Alamos National Laboratory) and coworkers, the tail accounts for 0.3% of the giant flare’s energy release. ² Figure 2 shows an unsaturated record of the spike, made by a particle detector aboard Geotail, a Japanese orbiter that studies Earth’s magnetosphere. ³

Pinpointing the source

From the arrival times of the spike at different spacecraft, the direction of the source was localized to the celestial position of SGR 1806–20, a soft-gamma repeater that has been known since 1979. It had been unusually active with soft-gamma bursts throughout 2004—as if rehearsing for the big one. Even when they’re not flaring, SGRs radiate quietly and continually in x rays, each repeater with its own rotation periodicity on the order of 5 or 10 seconds. That’s a thousand times slower than the periodicity of a typical young radio pulsar.

The well-measured period of SGR 1806–20 during quiescent emission is 7.47 s; that clinches its identification with the December giant flare. The presumed distance of SGR 1806–20 comes primarily from its directional coincidence with a tight cluster of particularly massive young stars. Two other SGRs are similarly situated, and the theory of magnetar provenance favors a nursery rich in such stars.

The distance is an important issue, in some dispute between the teams that have observed the giant flare’s radio afterglow. ^5,7 If SGR 1806–20 is at the presumed distance of 5 × 10⁴ light-years, the spike’s total energy, as determined by Hurley and company from charged-particle detectors aboard a number of spacecraft, is (4.2 ± 0.9) × 10⁴⁶ erg. ¹ But if it’s only half that far, the intrinsic luminosity of the record December flare is four times lower. The smaller distance cuts down the estimated fraction of short GBRs that might be attributable to magnetar flares in distant galaxies, and it reduces the maximum distance at which Swift can expect to see the spikes—and even the characteristic tails—of extragalactic magnetar flares. With its large gamma detector and the ability to slew its x-ray telescope toward the source of a gamma burst within seconds, Swift is the best observatory available for the task. Unhappily, its opportunity to study a giant flare up close just a month after launch was somewhat hampered by the fact that SGR 1806–20 was only 5° from the Sun on 27 December. ²

Magnetars

A typical young radio pulsar has a rotation period of tens of milliseconds and an external dipole field of 10¹² gauss. The field strength is estimated from the very small slowdown rate of the radio pulsation. But Chryssa Kouveliotou (NASA’s Marshall Space Flight Center) and coworkers discovered in 1998 that SGRs spin down much faster, which implies that their dipole fields are of order 10¹⁴–10¹⁵ G. Presumably, SGRs begin life with very short rotation periods like other neutron stars. But after about 10⁴ years, they’re spinning a thousand times slower. SGR ages are known from the expanded sizes of the supernova remnant shells in which some are found, and the magnetar theory explains why they don’t live much past 10 ⁴ years.

Duncan and Thompson sought to find out what determines the magnetic field strength of a neutron star formed by the collapse of a massive star’s iron core in a type-II supernova. The core’s sudden shrinkage to 20 km and nuclear density produces a 10⁵-fold increase in its spin and magnetic field. The neutrons that predominate in the nascent neutron star are in equilibrium with a smaller population of protons and electrons that make the spinning nuclear fluid a conductor and thus a dynamo.

The collapsed core’s spin is, at first, highly differential; shells farther from the center have higher angular velocity. The differential rotation, together with circulating convective cells in the hot fluid, makes for a complicated dynamo structure. The competition between spin and convection is decisive. If the convective turnover is faster than the spin, the theory predicts an incoherent sum of local dynamos yielding a magnetic field of only about 10¹² G. But if the spin is faster, one gets a coherent dynamo and fields of 10¹⁴–10¹⁵ G, that is, a magnetar. The theory’s crossover point is a spin period of about 5 ms for the newborn neutron star. Over the next 10⁴ years, magnetic braking slows the magnetar’s spin to a period of order 10 s.

The magnetar’s internal field is presumed to be even stronger than the external dipole field. At about 10¹⁶ G, “it’s the strongest magnetic field found anywhere in nature or in the laboratory,” says Duncan. Helical twists in the largely toroidal internal field subject the neutron star’s thin crust to enormous stresses. (A purely toroidal flux line would have only an azimuthal component.) The internal field tends to crack the crust, twisting it and the lines of the external dipole field it anchors. Twisting the dipole field raises its free energy and drives currents through the magnetosphere.

The frequent SGR outbursts are attributed to the energy released by local movements of the stressed crust and the consequent untwisting of magnetosphere flux lines. The rarer and much more powerful giant flares are thought to be global energy releases in the crust and magnetosphere.

Unlike radio pulsars, whose radiant energy comes largely from the kinetic energy of the neutron star’s spin, almost all of a magnetar’s radiation—giant flares, lesser SGR eruptions, and even its quiescent x radiation—is paid for by the energy stored in the star’s magnetic field. That’s about 10⁴⁹ ergs. So, in principle, a magnetar could afford several hundred giant flares in a lifetime. But as the magnetar cools with age, says the theory, the diffusion of flux lines through the interior and crust slows down, leaving a still strongly magnetized but now inactive relic after a few times ten thousand years.

Aftermath

In just 0.2 seconds, SGR 1806–20 radiated as much energy as the Sun does in 300 000 years. Its subsequent output over the next minutes and weeks was less showy, but nonetheless instructive. The 6-minute oscillating tail in soft gammas and x rays had a blackbody spectral temperature of about 10 keV. The spike’s blackbody temperature, by contrast, was several hundred keV. The tail is thought to emanate from a localized hot plasma of electron–positron pairs, in equilibrium with gammas, trapped by the magnetic field anchored to the surface.

As SGR 1806–20 rotates, the trapped fireball comes in and out of view. Hence the periodicity. All three giant flares have shown such oscillating tails. To the extent that Swift can detect the tails at intergalactic distances, one could readily distinguish distant magnetar flares from unrelated short gamma bursts.

What about longer-lasting afterglows? The radio-telescope team led by Bryan Gaensler (Harvard–Smithsonian Center for Astrophysics) has been monitoring the 27 December flare’s radio afterglow since 3 January with the Very Large Array in New Mexico and the Australia Telescope Compact Array. ⁴ They found a radio nebula, several hundred times brighter than that generated by the 1998 giant flare, expanding around SGR 1806–20 at about $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$ the speed of light. The nebula’s emission faded steadily until about three weeks after the flare, when it unexpectedly re-brightened for a week or so before resuming its wane. ⁸

Gaensler and company tentatively attribute the afterglow to moderately relativistic protons, ejected by the flare, that drive a shock front through ambient material surrounding the star. The radio signal would be synchrotron radiation from shock-accelerated electrons in the interstellar medium and the ejecta, spiraling around magnetic field lines.

The radio nebula’s relatively modest expansion velocity makes significant relativistic beaming of the giant flare’s radio or gamma-ray output unlikely. If the flare were narrowly beamed, serendipitously in our direction, one would have to reduce estimates of its total luminosity.

“The rebrightening of the radio nebula is particularly informative,” says Gaensler. “Because it manifests the deceleration of the ejecta after a coasting phase, it lets us estimate the total mass of material ejected by the giant flare.” That estimate, 10²⁴–10²⁵ grams, implies that something like the top 50 meters of the star’s crust were blown off. (The outer crust’s density, about 10⁸ g/cm³, is much lower than the nuclear density of the interior.) The crust ejection, he points out, had to be spotty. A uniform cover of ejecta would have created an atmosphere too opaque for the tail to be seen. ⁸

Magnetars promise a first look at the exotic physics of magnetic fields beyond the critical quantum-electrodynamic field strength B _c = m ² c ³/ħe = 4 × 10¹³ G, where m and e are the electron mass and charge. Above B _c, electrons gyrate relativistically even in their lowest Landau (cyclotron-orbit) states. The vacuum itself becomes strongly birefringent, and x-ray photons can split and merge without interacting with matter.