Superflare

Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions which should make them solar analogues, and would be expected to be stable over very long time scales.
The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler space telescope, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun.
The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. However, no such planet has shown up as a Kepler transit and this theory has been abandoned.
All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields. This suggests that superflares only differ in scale from solar flares.
Attempts have been made to detect past solar superflares from nitrate concentrations in polar ice, from historical observations of auroras, and from those radioactive isotopes that can be produced by solar energetic particles. Although three events and a few candidates have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with superflare events.
Solar superflares would have drastic effects, especially if they occurred as multiple events. Since they can occur on stars of the same age, mass and composition as the Sun this cannot be ruled out, but no indication of solar superflares have been found for the past ten millennia. However, solar-type superflare stars are very rare and are magnetically much more active than the Sun; if solar superflares do occur, it may be in well-defined episodes that occupy a small fraction of its time.

Superflare stars

A superflare star is not the same as a flare star, which usually refers to a very late spectral type red dwarf. The term is restricted to large transient events on stars that satisfy the following conditions:

The star is in spectral class F8 to G8
It is on or near the main sequence
It is single or part of a very wide binary
It is not a rapid rotator
It is not exceedingly young

Essentially such stars may be regarded as solar analogues.
Originally nine superflare stars were found, some of them similar to the Sun.

Original superflare candidates

The original paper identified nine candidate objects from a literature search:

Star	Type	V	Detector	Flare Amplitude	Duration	Energy
Groombridge 1830	G8 V	6.45	Photography	ΔB = 0.62 mag	18 min	E_B ~ 10³⁵
Kappa1 Ceti	G5 V	4.83	Spectroscopy	EW = 0.13Å	~ 40 min	E ~ 2 × 10³⁴
MT Tauri	G5 V	16.8	Photography	ΔU = 0.7 mag	~ 10 min	E_U ~ 10³⁵
Pi1 Ursae Majoris	G1.5 Vb	5.64	X-ray	L_X = 10²⁹ erg/s	>~ 35 min	E_X = 2 × 10³³
S Fornacis	G1 V	8.64	Visual	ΔV ~ 3 mag	17 - 367 min	E_V ~ 2 × 10³⁸
BD +10°2783	G0 V	10.0	X-ray	L_X = 2 × 10³¹ erg/s	~ 49 min	E_X >> 3 × 10³⁴
Omicron Aquilae	F8 V	5.11	Photometry	ΔV = 0.09 mag	~ 5 - 15 day	E_BV ~ 9 × 10³⁷
5 Serpentis	F8 IV-V	5.06	Photometry	ΔV = 0.09 mag	~ 3 - 25 day	E_BV ~ 7 × 10³⁷
UU Coronae Borealis	F8 V	8.86	Photometry	ΔI = 0.30 mag	>~ 57 min	E_opt ~ 7 × 10³⁵

Type gives the spectral classification including spectral type and luminosity class.
V means the normal apparent visual magnitude of the star.
EW is the equivalent width of the 5875.6Å He I D3 line seen in emission.
The observations vary for each object. Some are X-ray measurements, others are visual, photographic, spectroscopic or photometric. The energies for the events vary from 2 × 10³³ to 2 × 10³⁸ ergs.

Kepler discoveries

The Kepler spacecraft is a space observatory designed to find planets by the method of transits. A photometer continually monitors the brightness of 150,000 stars in a fixed area of the sky to detect changes in brightness caused by planets passing in front of the stellar disc. More than 90,000 are G-type stars on or near the main sequence. The observed area corresponds to about 0.25% of the entire sky. The photometer is sensitive to wavelengths of 400–865 nm: the entire visible spectrum and part of the infrared. The photometric accuracy achieved by Kepler is typically 0.01% for 30 minute integration times of 12th magnitude stars.

G-type stars

The high accuracy, the large number of stars observed and the long period of observation make Kepler ideal for detecting superflares. Studies published in 2012 and 2013 involved 83,000 stars over a period of 500 days. The stars were selected from the Kepler Input Catalog to have T_eff, the effective temperature, between 5100 and 6000 K to find stars of similar spectral class to the Sun, and the surface gravity log g > 4.0 to eliminate sub-giants and giants. The spectral classes range from F8 to G8. The integration time was 30 minutes in the original study. The studies found 1,547 superflares on 279 solar-type stars. The most intense events increased the brightness of the stars by 30% and had an energy of 10³⁶ ergs. White-light flares on the Sun change the brightness by about 0.01%, and the strongest flares have a visible-light energy of about 10³² ergs. Most events were much less energetic than this: flare amplitudes below 0.1% of the stellar value and energies of 2 × 10³³ ergs were detectable with the 30 minute integration. The flares had a rapid rise followed by an exponential decay on a time scale of 1–3 hours. The most powerful events corresponded to energies ten thousand times greater than the largest flares observed on the Sun. Some stars flared very frequently: one star showed 57 events in 500 days, a rate of one every nine days. For the statistics of flares, the number of flares decreased with energy E roughly as E⁻², a similar behaviour to solar flares. The duration of the flare increased with its energy, again in accordance with the solar behaviour.
Some Kepler data is taken at one minute sampling, though inevitably with lower accuracy. Using this data, on a smaller sample of stars, reveals flares that are too brief for reliable detection with 30-min integrations, allowing detection of events as low as 10³² ergs, comparable with the brightest flares on the Sun. The occurrence frequency as a function of energy remains a power law E⁻ⁿ when extended to lower energies, with n around 1.5. At this time resolution some superflares show multiple peaks with separations of 100 to 1000 seconds, again comparable to the pulsations in solar flares. The star KIC 9655129 showed two periods, of 78 and 32 minutes, suggesting magnetohydrodynamic oscillations in the flaring region. These observations suggest that superflares are different only in scale and not in type to solar flares.
Superflare stars show a quasi-periodic brightness variation, which is interpreted as evidence of starspots carried around by stellar rotation. This allows an estimate of the rotation period of the star; values range from less than one day up to tens of days. On the Sun, radiometer monitoring from satellites shows that large sunspots can reduce the brightness by up to 0.2%. In superflare stars the most common brightness variations are 1–2%, though they can be as great as 7–8%, suggesting that the area of the starspots can be very much larger than anything found on the Sun. In some cases the brightness variations can be modelled by only one or two large starspots, though not all cases are so simple. The starspots could be groups of smaller spots or single giant spots.
Flares are more common in stars with short rotation periods. However, the energy of the largest flares is not related to the period of rotation. Stars with larger variations also have much more frequent flares; there is as well a tendency for them to have more energetic flares. Large variations can be found on even the most slowly rotating stars: one star had a rotation period of 22.7 days and variations implying spot coverage of 2.5% of the surface, over ten times greater than the maximum solar value. By estimating the size of the starspots from the amplitude variation, and assuming solar values for the magnetic fields in the spots, it is possible to estimate the energy available: in all cases there is enough energy in the field to power even the largest flares observed. This suggests that superflares and solar flares have essentially the same mechanism.
In order to determine whether superflares can occur on the Sun, it is important to narrow the definition of Sun-like stars. When the temperature range is divided into stars with T_eff above and below 5600 K, stars of lower temperature are about twice as likely to show superflare activity as those in the solar range and those that do so have more flares: the occurrence frequency of flares is about five times as great in the late-type stars. It is well known that both the rotation rate and the magnetic activity of a star decrease with age in G-type stars. When flare stars are divided into fast and slow rotators, using the rotation period estimated from brightness variations, there is a general tendency for the fastest-rotating stars to show a greater probability of activity: in particular, stars rotating in less than 10 days are 20–30 times more likely to have activity. Nevertheless, 44 superflares were found on 19 stars with similar temperatures to the Sun and periods greater than 10 days ; four superflares with energies in the range 1-5 × 10³³ ergs were detected on stars rotating more slowly than the Sun. The distribution of flares with energy has the same shape for all classes of star: although Sun-like stars are less likely to flare, they have the same proportion of very energetic flares as younger and cooler stars.