A S T R O P H Y S I C S

Startling superflaresStars that are just like our Sun have flares more than a million times more energetic than the biggest flare ever seen on the Sun. The Kepler satellite has allowed these superflares to be studied in detail for the first time. See Letter p.478

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A superflare on a Sun-like star is a bright-ening that has an energy of from 1033 to more than 1039 erg and lasts from

minutes to days. The Sun has frequent flares that are caused by magnetic effects above sun-spots, regions that are cooler than the Sun’s typical surface temperature. However, the largest flare ever observed1 on the Sun — the 1859 Carrington event — had a total energy of about 1032 erg. With Sun-like stars being the epitome of constancy, it is startling, evocative

and exciting that they can have superflares as energetic as 1039 erg. On page 478 of this issue, Maehara et al.2 report the emissions from 365 superflares, measured by the awesome Kepler satellite, which was launched in 2009*.

Over the past 120 years, four dozen super-flares have been reported in the literature3–5. But these events were always ignored as iso-lated anomalies. Only in 1989 were all these reports put together in recognition that the events represent a coherent phenomenon, *This article and the paper2 under discussion were published online on 16 May 2012.

energies, even though it is manifestly broken in the low-energy Universe in which we live.

The non-zero mass of the W boson is inti-mately connected with the Higgs boson, with the origins of mass in general and with our understanding of physics in terms of quan-tum field theories. It is a quantity well worth measuring precisely — just as the Tevatron experimenters1 have done.

This was a hugely challenging analysis. The Tevatron (Fig. 1), which recently ceased oper-ation, was a high-energy collider that stored protons and antiprotons, accelerated them to high energies — almost 1 teraelectronvolt (1012 eV) — and forced them into head-on collisions. The energy and frequency of the collisions were sufficient to produce large numbers of W bosons. W bosons decay rap-idly, and their decay products could generally be detected in the Collider Detector at Fermi-lab (CDF), or in the rival detector D0, which is also located at the Tevatron. In the CDF analy-sis, the researchers used 1,094,834 W-boson decays to measure the W-boson mass.

The W boson can decay in many different ways, but those decays that produce an elec-tron or a muon — a short-lived particle similar to the electron — are the most useful for meas-uring the W boson’s mass because electrons and muons can be reliably detected. However, an electrically neutral particle called a neutrino that is hard to detect is also produced in these decay events. This is problematic because the neutrino’s momentum is needed to determine the W boson’s mass; however, this momentum can be deduced only indirectly from an analy-sis of all the other particles produced in the decay event.

The CDF is a cylinder constructed such that proton and antiproton beams enter at either end and collide in the centre. Although the neu-trino cannot be detected, its presence — and the component of its momentum transverse to the beam — can be deduced by applying the law of conservation of momentum to all the other particles produced in the collision. In addition to the electron and the muon, this includes composite particles known as hadrons, which are generated when elementary particles called quarks and gluons are scattered from the colliding particles and then combine.

Detailed analyses of all of these components led the CDF Collaboration to obtain a value for the W boson’s mass of 80,387 MeV with an error of 19 MeV, a precision of about two parts in 10,000. This value is consistent with that obtained from an experiment performed with the D0 detector, which found3 a mass of 80,367 MeV with an error of 26 MeV.

The W boson and the top quark, the heaviest of all known elementary particles, contribute to many particle-production and scattering processes that have been accurately meas-ured in particle-physics experiments. In these processes, the particles enter quantum loops as virtual particles with fleeting existence but measurable effect — at least, if the measure-ment is precise enough. If it exists, the Higgs boson must also appear in these loops. By com-bining these measurements with their value of the W boson’s mass, the authors were able to conduct a precise test of the symmetry struc-ture of the standard model.

Knowledge of the W boson’s mass has imposed limits on the range of possible mass values for the Higgs boson. This range has

been further curtailed4,5 by data from the direct searches for the Higgs at the LHC. And yet there is still a region of overlap. If the hints seen at the LHC do turn out to be the Higgs, then the particle’s mass is consistent with that inferred from standard-model calcula-tions, using the W boson’s mass, of an array of particle-physics processes. This consistency is built into the quantum loops and symmetries of the standard model. A theorist, or even a mathematician, might call this a highly non-trivial consistency test of the theory. I call it beautiful. ■

Jonathan Butterworth is in the Department of Physics and Astronomy, University College London, London WC1E 6BT, UK. e-mail: j.butterworth@ucl.ac.uk

1. Aaltonen, T. et al. Phys. Rev. Lett. 108, 151803 (2012).2. ‘t Hooft, G. & Veltman, M. Nucl. Phys. B 44, 189–213

(1972).3. Abazov, V. M. et al. Phys. Rev. Lett. 108, 151804

(2012).4. Aad, G. et al. Phys. Lett. B 710, 49–66 (2012).5. Chatrchyan, S. et al. Phys. Lett. B 710, 26–48

(2012).

Figure 1 | The Tevatron collider. Aaltonen et al.1 have used the Tevatron — a 6.3-kilometre-long circular particle accelerator and collider — at Fermilab in Batavia, Illinois, to measure the mass of the W boson to high precision.

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extending to all types of normal stars3–5, and the name superflares was coined to distinguish them from ordinary flares such as those that happen on the Sun5.

Superflares occur on single, middle-aged stars that are rotating slowly and are pow-ered by the fusion of hydrogen in their core. Such stars, which are technically known as main-sequence stars of spectral type F8 to G8, include the closest known ‘twins’ of our Sun. The similarity of superflares to solar flares suggests that superflares arise from magnetic effects. However, the now-default model6–9 for these events involves a magnetic field that connects the star to an orbiting ‘hot Jupiter’ — a planet that has a mass compa-rable to, or larger than, that of Jupiter but that is much closer to its host star than Jupiter is to the Sun (Fig. 1).

The limitation of pre-Kepler observations is that they are an inhomogeneous collection of data. Some data are from X-ray satellites, whereas others are from spectroscopic obser-vations, multi colour photometry and even visual observations. Such an assorted data set makes it impossible to calculate rates of occurrence, or to seek correlations between observed features or do other statistical analy-ses. With the average recurrence timescale for the brightest events being much longer than a decade, it was unfeasible to set up any realistic programme to observe superflares, and so the field stagnated.

Maehara and colleagues2 now break this deadlock by analysing data from the Kepler satellite, which provides continuous monitor-ing of the brightness of more than 100,000 stars for years on end with an accuracy of 10 parts per million. The authors have looked at some 83,000 G-type main-sequence stars and pro-vide light curves — plots showing the evolu-tion of an object’s brightness over time — for 365 superflares on 148 stars. Suddenly, we have a wealth of data, we can do statistical analyses, and we know exactly which stars to watch.

The Kepler superflares have durations of 1–12 hours, brightness increases of 0.1–30% and total energies of 1033–1036 erg. A typical star in the Kepler data set has 1035-erg flares every 100 days. Interestingly, Maehara et al. find that the observed superflares all occur on stars that have large starspots, as evidenced by quasi-periodic modulation of the star’s quiescent brightness. This finding crucially ties the superflares to starspots and hence to magnetic fields. Another insight is that the superflare stars apparently have no planetary transits — that is, no planets have been found to pass in front them. However, roughly 10% of the superflare stars should have such transits if they are all associated with hot Jupiters. So the mechanism underlying superflares remains unclear.

With Maehara and colleagues’ results, theorists have a rich field to investigate. And

I can think of many new paths that have been opened up for observers. Measurement of the radial velocity of the superflare stars might reveal Jupiter-like planets. The brightest of the superflare stars are bright enough to allow the stars’ magnetic fields to be determined. High-resolution spectra of the most active stars (which have superflares every 9 days) could be obtained with a view to detecting changes in the shape of the calcium spectral lines (the H and K lines) simultaneous with a Kepler superflare. Such changes would pro-vide information about the velocity, tempera-ture and energy of the superflares. The Kepler data could be further scrutinized to investigate whether flares occur in preferred phases of the star’s brightness modulation and whether the energies and time intervals of successive flares are correlated. Maehara and colleagues’ work could also be extended to stars of all types. Indeed, this task of searching for flares is per-fect for pursuits such as the Planet Hunters programme of citizen science, which is part of the Zooniverse project10.

The possibility that the Sun has superflares is not realized. Historical and geophysical records show that the Sun has not had any superflares in the past two millennia, and no superflares with more than roughly 1036 erg for perhaps a billion years5. Maehara et al. show that only 0.2% of Sun-like stars have superflares, so it is unlikely that the Sun has such events. With their average rate of occurrence (once every 100 days for 1035-erg flares) and their observed size distribution (with a power-law index of roughly −2.0), the expected frequency of 1032-erg flares on all superflare stars should be very high. In stark contrast to this, the Sun has one 1032-erg event roughly every 450 years1,11 and so is completely different from superflare stars. This is all readily understood within the

default model, because the Sun does not have a hot Jupiter.

Superflares have implications far beyond being just a challenge for stellar physics. If a superflare’s energy is linked to the orbital energy of a hot Jupiter, then three events a year on the star would make its planetary companion spiral in towards it on a timescale of a billion years. The huge energy output of superflares could make any planets around the star uninhabitable for far-future human colonization, and astrobiologists will have to consider the effect of the superflares on possible alien life. Superflares might pro-vide the high-energy radiation required to create organic molecules, so perhaps super-flare systems are a good place to look for alien life that has evolved to avoid the effects of the huge flares. ■

Bradley E. Schaefer is in the Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA. e-mail: schaefer@lsu.edu

1. Tsurutani, B. T., Gonzalez, W. D., Lakhina, G. S. & Alex, S. J. Geophys. Res. 108, 1268 (2003).

2. Maehara, H. et al. Nature 485, 478–481 (2012).3. Schaefer, B. E. Astrophys. J. 337, 927–933

(1989).4. Schaefer, B. E. Astrophys. J. 366, L39–L42 (1991).5. Schaefer, B. E., King, J. R. & Deliyannis, C. P.

Astrophys. J. 529, 1026–1030 (2000).6. Rubenstein, E. P. & Schaefer, B. E. Astrophys. J. 529,

1031–1033 (2000).7. Cuntz, M., Saar, S. H. & Musielak, Z. E. Astrophys. J.

533, L151–L154 (2000).8. Ip, W.-H., Kopp, A. & Hu, J.-H. Astrophys. J. 602,

L53–L56 (2004).9. Lanza, A. F. Astron. Astrophys. 487, 1163–1170

(2008).10. www.zooniverse.org 11. Shea, M. A., Smart, D. F., McCracken, K. G.,

Dreschhoff, G. A. M. & Spence, H. E. Adv. Space Res. 38, 232–238 (2006).

Figure 1 | Magnetic connection. One idea to explain the superflares observed by Maehara et al.2 invokes the presence of intense magnetic fields that connect the star with a Jupiter-like planet in very close orbit around the star. The magnetic-field lines will become twisted and amplified by the orbital motion of the planet, and at some time the lines will be strained and twisted to the point of breaking. The broken lines will accelerate particles to very high energy and release this energy in an explosive event, similar to what happens in ordinary solar flares seen on the Sun.

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