Giant flashes from soft γ-ray repeaters are spectacular but rare events — only three have ever been observed in our Galaxy. The suspicion is that we have been missing some from farther afield.
On 27 December 2004, virtually all of the γ-ray detectors in orbit were triggered by the brightest flash of γ-rays ever seen. Two similar flares had previously been detected from different sources of the same class during 30 years of observations1,2 — on 5 March 1979 and 27 August 1998. The 2004 flare, however, must be regarded as unique: it outshone both the preceding events by two orders of magnitude, releasing in its first fraction of a second as much energy as the Sun releases in a quarter of a million years. Five papers3,4,5,6,7 in this issue provide an observational overview of this exceptional event.
The source of the outburst is known as SGR 1806–20, a ‘soft γ-ray repeater’ (SGR) lying in our Galaxy at an estimated distance of 15 kiloparsecs (almost 50,000 light years) from the Solar System (Fig. 1). An SGR is an extremely highly magnetized neutron star8, or ‘magnetar’, that produces recurrent bursts of low-energy (‘soft’) γ-rays — that is, high-energy photons, or electromagnetic radiation. The flare from SGR 1806–20 was characterized by an initial spike that lasted less than a second and contained most of the energy of the burst3,4 as well as the highest-energy, or ‘hardest’, photons. This spike of the flare was followed by an exponential tail with a duration of some 400 seconds, oscillating with the period (7.56 seconds) at which SGR 1806–20 is known — from measurements of its much dimmer X-ray emission during quiescence — to rotate.
The characteristics of the SGR 1860–20 flare can be explained as the outcome of a readjustment of the huge magnetic field — up to 1015 times stronger than that at Earth's surface — anchored to what is a relatively young (about 5,000-year-old) neutron star8. Such a readjustment releases a sizeable fraction of the internal energy of the field, stored in a hot ‘plasma’ of radiation and electron–positron pairs, and generates the bright initial spike of the flare.
The flux of photons in the spike of SGR 1860–20 was so large that it saturated most detectors3, making it difficult to characterize its properties. Terasawa et al.5 (page 1110), however, report an oscillatory modulation, with a period of around 60 milli-seconds, in the number of photons detected in the spike. They suggest that the periodicity of these ‘humps’ in the flare's profile indicates repeated injections of energy into the system, the origin of which is unclear.
The pulsed emission in the flare's tail comes from a fraction of the plasma that initially remains confined by the magnetic field lines anchored to the neutron star. As the star rotates, it produces a ‘lighthouse’ effect, resulting in the periodic oscillations in brightness.
On a timescale of days after the initial flare, X-ray9 and radio emissions6,7 originating from SGR 1860–20 were also observed. Gaensler et al.6 (page 1104) report that the expanding radio nebula had a luminosity more than 500 times greater than the only other similar object detected (after the flare of August 1998). In some ways the afterglow was similar to that of long γ-ray bursts (GRBs). GRBs are another class of bright γ-ray flashes that are known to originate from galaxies on the edge of the visible Universe. Long GRBs last for tens of seconds and are followed by a dimmer emission tail lasting weeks to months. Cameron et al.7 (page 1112), however, point out general problems in reconciling the spectrum and light curve of the SGR 1860–20 radio emission with standard models of long GRBs.
The most striking property of the SGR 1860–20 flare is its extraordinary luminosity. This raises the question of whether this giant flare and its two predecessors1,2 are related to a class of mysterious short GRBs, detected in large numbers by BATSE (the Burst and Transient Source Experiment), a soft γ-ray detector that flew on board NASA's Compton observatory in the 1990s. No afterglow emission has so far been identified from these GRBs. However, if we rescale the properties of the SGR 1806–20 flare to a distance of several megaparsecs (around a thousand times farther away), an instrument such as BATSE would indeed have seen only the initial bright spike of the event, with a timescale of several hundred milliseconds and a spectrum consisting almost entirely of hard photons. Such a description matches that of the short GRBs detected. Hurley et al.3 (page 1098) estimate that the rate of giant flares that should have been detectable by BATSE is about 30 per year, which could account for up to 40% of the short GRBs actually found.
There are three ways to identify the presence of SGR flares in the BATSE catalogue of short bursts. First, they should be much closer than cosmological GRBs, and therefore associated with bright galaxies. The position in the sky is known with the required accuracy for only five short GRBs; but no bright host galaxy can be associated with any of these five10, limiting the possible proportion of SGR flares to less than 20% in the BATSE short-burst catalogue. Second, SGRs should produce a periodic signal in the 200 seconds following the burst; a search for such a signal has so far been unsuccessful. Third, SGR candidates in the BATSE catalogue can be identified by their spectrum. SGR flares typically emit a thermal ‘black-body’ spectrum (indicative of an optically thick medium), whereas GRBs are characterized by broad power-law spectra (emitted by optically thin material) with the radiation spread over many orders of magnitude in frequency. The SGR 1806–20 flare had an average temperature of 2×109 kelvin, which should be easily identifiable among non-thermal GRB spectra; but a search for high-temperature thermal spectra performed over about 100 BATSE short GRBs has been unsuccessful.
These factors constrain the percentage of possible SGR flares among short GRBs to 5% at most, close to an order of magnitude less than expected. So, where are the extragalactic SGR flares?
Several factors may explain the apparent discrepancy. First, the distance to SGR 1806–20 could be inaccurate. If the flare were at only half the distance assumed, its luminosity would be reduced by a factor of four, limiting the volume of space in which extragalactic SGRs could be detected by a factor of eight. A possible smaller distance to SGR 1806–20 has been proposed. An upper limit of 9.8 kiloparsecs, two-thirds of the previous working assumption11 of 15 kiloparsecs, is suggested by Cameron et al.7.
Another factor in the calculation is the rate of occurrence of giant flares in our Galaxy. SGR 1806–20 is the only flare of its scale to be seen in 30 years of observations, and this is our best estimate of how often they occur. Of course, we may merely have been lucky, and the true rate could be lower: we might just happen to live in an epoch when one of these flares went off randomly.
Whatever the answers to these questions, the hunt for extragalactic flares is on. In their analysis, Palmer et al.4 (page 1107) use data from NASA's Swift satellite, which was launched in 2004 expressly to solve the GRB mystery, and the rapid follow-up capabilities of this mission should facilitate further discoveries. The importance of such detections for our understanding of an extreme phenomenon that is represented so far only by the example of 27 December 2004 cannot be overestimated.
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Terasawa, T. et al. Nature 434, 1110–1111 (2005).
Gaensler, B. M. et al. Nature 434, 1104–1106 (2005).
Cameron, P. B. et al. Nature 434, 1112–1115 (2005).
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