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Astronomy

Blasts from the radio heavens

There is no coherent explanation for newly observed salvos of radio waves emanating from a direction near the Galactic Centre. Are they from a new type of stellar object? The search is on for similar radio emitters.

For thousands of years, we self-important humans have interpreted transient heavenly events as omens. The Chinese emperor was the Son of Heaven and paid a retinue of astronomers to keep careful track of comets and other ‘guest stars’ (novae and supernovae) to predict earthly catastrophes. Modern astronomers are mostly paid for different reasons, yet continue to discover new kinds of transients, which have delivered handsome dividends in our understanding of stellar death and corpses (white dwarfs, neutron stars and black holes). The terms supernova, nova, X-ray transient, γ-ray burst and magnetar have crept into the lexicon of most readers of Nature.

On page 50 of this issue, Scott Hyman et al.1 report a bright bursting radio source near (in projection at least) the centre of our Galaxy. They suggest that the object (dubbed GCRT J1745–3009) is a prototype of a new class of particularly bright, coherently emitting radio transients. Because the distance and precise position of the source are as yet unknown, more mundane explanations are still possible. But the manner of its discovery, and the potentially exciting interpretation, will inspire more dedicated searches for radio transients.

Now the essential facts. While observing the central region of our Galaxy at radio wavelengths, Hyman and colleagues discovered five strong bursts, each lasting about 10 minutes, and separated by about 77 minutes, coming from the same 10-arcsecond region of the sky. No emission, steady or otherwise, is seen in subsequent (and archival) searches or between the bursts.

The source is not well enough localized to identify counterparts at other wavelengths, so its distance is unknown. GCRT J1745–3009 could, like most stars in its direction, be near the centre of our Galaxy (about 24,000 light years or 8,000 parsecs distant), in which case its radio luminosity is around an impressive one-hundredth that of the Sun. But because the centre of our Galaxy is so interesting, astronomers tend to stare there more than anywhere else. So it is possible that the source is much nearer (say, 300 light years or 100 parsecs), less luminous, and only coincidentally projected near the Galactic Centre.

The duration of the burst limits the size of the source to less than the distance travelled by light over the burst duration. Armed with this knowledge, we can compute the equivalent ‘black-body’ temperature of the emitter. Cosmic radio sources have a natural thermostat2 that normally restricts this brightness temperature to less than 1012 kelvin. But the brightness temperature of GCRT J1745–3009, the radio source observed by Hyman et al., exceeds this if it does indeed lie farther from Earth than 100 parsecs. In some cases, temperatures higher than the 1012-kelvin thermostat are seen through a form of trickery3 involving special relativity: when emitting matter is racing towards Earth at nearly the speed of light, much higher apparent brightness temperatures will be inferred.

Galactic examples of such astronomical tricksters are black holes4 and neutron stars5 in binary systems accreting mass from a companion star. GCRT J1745–3009 could be one of those objects. However, the known examples have prominent X-ray emission, whereas no X-ray emission from GCRT J1745–3009 has been reported in other studies (the RXTE, ROSAT and ASCA space missions). On the other hand, if the roughly 77-minute interval between the source's radio bursts is an orbital period in an accreting binary system, only the smallest star or a white dwarf can fit in the tiny orbit. The accretion rate in such a binary system would be low, and the accretion might also be radiatively inefficient6,7, so it could hide well below the X-ray limits. It is possible that the radio source, modulated by absorption of the radio waves in a stellar wind, is an ‘X-ray quiet, radio-loud’ X-ray binary, similar to certain types of active galactic nuclei6, but with stellar mass.

Instead, the radio source could also genuinely beat the 1012-kelvin thermostat by emitting its radiation coherently. Coherent emission requires organized electrons. Terrestrial examples include radio stations, masers and lasers. Coherent emission when seen in heavenly objects is always a source of wonder. Our Solar System has some coherent emitters: cyclotron masers in planetary aurorae, long-wavelength radio emission from the Io– Jupiter system, and solar flares. However, the impressive luminosity of GCRT J1745–3009, even if it is as near as 100 parsecs, is not compatible with such systems. Outside the Solar System, we know of only three types of coherent emitter: masers (ruled out for GCRT J1745–3009 because it does not have the appropriate narrow bandwidth), magnetically active ‘flare’ stars, and — most spectacularly — pulsars.

Radio emission from flare stars is usually (but not always) highly polarized8. The dwarf star or brown dwarf would also be seen at optical wavelengths. There are potential candidates within the uncertain position of GCRT J1745–3009, so this remains a possible, though unlikely, explanation.

Could GCRT J1745–3009 be a pulsar? Hyman et al. note that the five bursts they record appear in rapid succession with a period of about 77 minutes. The rotational energy lost by a normal neutron star (or white dwarf) rotating this slowly would be inadequate to power the observed radio bursts. Thus, by a process of elimination, Hyman and colleagues argue that they have uncovered a new class of coherent emitter.

In our opinion, the claim of a new class is plausible but not beyond doubt. As discussed above, the bursts could still be incoherent emission from an accreting binary star with a whittled-down companion and a relativistic jet but suppressed X-ray emission. If the source turns out to be nearer than the Galactic Centre, it could be one of several previously known types of coherent radio source, including an isolated or binary flaring (brown) dwarf star or magnetized white dwarf, or a nulling radio pulsar (a pulsar that broadcasts pulses only sporadically).

This last seems to us to be the most plausible conventional alternative. PSR 0826–34, for example, is a pulsar that can shut itself off for periods ranging from tens of minutes to eight hours9. PSR J1752+2359 is characterized by 45-second bursts of emission that appear roughly every five minutes10, like GCRT J1745–3009 but speeded up by an order of magnitude.

GCRT J1745–3009 will cause a stampede of further observations: searches for pulsations and quiescent emission in radio, infrared and X-ray bands. But perhaps even more important is the possibility that the radio heavens contain other fast radio transients (which, in anticipation of a trove of discoveries, we nickname ‘burpers’). Sensitive radio telescopes and arrays currently lack large fields of view. Fortunately, the construction of several new radio facilities with wider fields of view are being contemplated, and one is already funded11. Radio astronomy is poised to deliver new bursts of excitement.

References

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    Hyman, S. D. et al. Nature 434, 50–52 (2005).

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    Readhead, A. C. S. Astrophys. J. 426, 51–59 (1994).

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    Rees, M. J. Nature 211, 468–470 (1966).

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    Mirabel, I. F. & Rodríguez, L. F. Annu. Rev. Astron. Astrophys. 37, 409–443 (1999).

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    Fender, R. et al. Nature 427, 222–224 (2004).

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    Rees, M. J., Phinney, E. S., Begelman, M. C. & Blandford, R. D. Nature 295, 17–21 (1982).

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    Esin, A. A., McClintock, J. E. & Narayan, R. Astrophys. J. 489, 865–889 (1997).

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    Lewandowski, W. et al. Astrophys. J. 600, 905–913 (2004).

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    LOFAR http://www.lofar.org

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