The First Naked-eye Superflare Detected from Proxima Centauri

The superflare reported here occurred during the three-month Pale Red Dot campaign, which first revealed the presence of Proxima b (Anglada-Escudé et al. 2016) using the HARPS spectrograph on the ESO 3.6 m at La Silla, Chile (Mayor et al. 2003). The HARPS spectrum was taken at 2016 March 18 8:59 UT, 27 minutes after the flare peak at 8:32 UT. This single 1200 s exposure captured the majority of the flare tail, including 20% of the total radiated flux.

Within the Evryscope's two-minute integration during the flare peak, Proxima reached an average flux of 38× the quiescent emission. By fitting a instantaneous-flux flare template (Davenport et al. 2014) we estimate the superflare's brightness on human-eye timescales to have reached 68× Proxima's flux. Proxima, an 11.4 g' magnitude star, thus briefly became a g' = 6.8 star, at the limit of visibility to trained observers at very dark sites (Weaver 1947; Schaefer 1990; Cinzano et al. 2001).

Calculation of the superflare's atmospheric impacts requires an estimate of the flare's energy in multiple bandpasses, from the far-UV (FUV) to the infrared. We measure the superflare energy in the g' Evryscope bandpass and subsequently convert into the bolometric energy using the energy partitions of Osten & Wolk (2015). We accomplish this by estimating the bolometric flare energy of a 9000 K flare blackbody with emission matching the measured Evryscope flux; the fraction of the bolometric energy found in the Evryscope g' bandpass is f_g' = 0.19. The canonical value of 9000 K provides a lower limit to the flare energy; a higher-temperature flare blackbody, as has been sometimes measured for larger flares (Kowalski et al. 2010), results in more short-wavelength energy. The energy seen in any bandpass Δλ is then given by the approximate relationship E_Δλ = f_Δλ × E_bol.

We obtain the quiescent flux in the Evryscope g' bandpass by scaling directly from the Evryscope-measured calibrated magnitude, and by weighting the flux-calibrated spectrum of Proxima used in Davenport et al. (2016) by the Evryscope response function and scaling using Proxima's distance (Lurie et al. 2014). Both methods measure Proxima's quiescent flux in the Evryscope bandpass to be L₀ = 10^28.6 erg s⁻¹, giving the superflare energy in the Evryscope bandpass of 10^32.8 erg, and a bolometric energy of 10^33.5 erg.

The Evryscope observed Proxima for a total of 1344 hr between 2016 January and 2018 March. We discovered 24 large flares (Figure 2), with bolometric energies from 10^30.6 to 10^33.5 erg. To obtain the cumulative FFD, we calculate the uncertainty in the cumulative occurrence rate for each Evryscope flare with a binomial 1σ confidence interval statistic (following Davenport et al. 2016). Errors in energy for high-energy flares are calculated using the inverse significance of detection; low-significance flares use the injection-and-recovery error estimate instead, to account for the possibility of correlated noise introducing bias.

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To sample both the rare high-energy events detectable by Evryscope and the frequent moderate-to-low-energy events detectable by MOST, we also include flares from the MOST sample (Davenport et al. 2016) with energy in the MOST bandpass greater than 10^30.5 erg (we exclude lower-energy MOST flares due to a possible knee in the FFD biasing the occurrence of superflares ∼1000X larger). We fit a cumulative power-law FFD to the MOST and Evryscope flares, and determine the uncertainty in our fit through 10000 Monte-Carlo posterior draws that are consistent with our uncertainties in energy and occurrence rates. We represent the cumulative FFD in the Evryscope bandpass (Figure 3) by a power law of the form $\mathrm{log}\nu =(1-\alpha )\mathrm{log}E+b$, where ν is the number of flares with an energy greater than or equal to E erg per day, α gives the frequency at which flares of various energies occur, and b is the y-intercept and crossover into the unphysical energy region E < 0.

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Evryscope constrains the expected occurrence of 10³³ erg bolometric events to be at least ${5.2}_{-3.0}^{+0.2}$ per year. It is evident from Figure 3 that it is difficult to fit a single power law that reproduces both the lower-energy flares and the Evryscope-observed superflare. This could mean that the probability of reaching superflare energies is higher than would be expected by a simple power-law extrapolation from lower energies; it could also be that the Proxima Superflare is just a statistically rare event. We therefore report two separate FFDs; the first excludes the Proxima Superflare, while the second includes it. For the no-superflare case, we report an FFD of $\mathrm{log}\nu =-{1.22}_{-0.003}^{+0.26}\mathrm{log}E+{38.1}_{-0.07}^{+8.4}$, displayed in Figure 3, in good agreement with both the Evryscope and MOST sample. Including the prior of the observed superflare, we obtain $\mathrm{log}\nu =-{0.98}_{-0.24}^{+0.02}\mathrm{log}E+{30.6}_{-7.6}^{+0.83}$. We note α_Evryscope is significantly steeper in our higher-energy flare sample in both cases than that for Proxima FFDs from previous studies, e.g., Walker (1981) and Davenport et al. (2016).

The spectrum of Proxima, in both the median quiescent and flare states, is shown in Figure 4. During the superflare, chromospheric metals and the Balmer series show sharply increased emission. A −30 km s⁻¹ splitting of the H_α, H_β, and He i lines is detectable, and is indicative of a flow of highly ionized plasma generated by the flare, most likely correlated to a hot stellar wind moving outward from the star (Pavlenko et al. 2017). No significantly blueshifted emission or anomalous emission lines are visible; the superflare spectrum is similar to other smaller flares recorded from Proxima and is therefore likely to be amenable to emission-line scaling relations to estimate FUV and particle fluxes (Section 3.5).

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FUV (912–1700 Å) photons are capable of photolyzing most molecules in planetary atmospheres. The Hubble Space Telescope (HST) archive contains 13.3 h of FUV spectrophotometry by the Space Telescope Imaging Spectrograph (STIS) of Proxima. From this data, we aggregated nine flares spanning FUV energies of 10^29.3–10^30.8 erg to construct a FUV energy budget for Proxima flares. We scale to a 9000 K blackbody and extreme ultraviolet (EUV) emission via a "fiducial flare" prescription (R. O. P. Loyd et al. 2018, in preparation) tailored to the Proxima data. The 10^32.5 erg FUV energy of the Proxima Superflare obtained by the fiducial flare prescription is found to be consistent with an independently obtained estimate using the quiescent scaling relations of Youngblood et al. (2017) applied to a measurement of the Ca ii K equivalent width in the HARPS spectrum (Section 2.1).

Coronal mass ejections (CMEs) are often assumed to accompany large flares (Yashiro et al. 2006). Youngblood et al. (2017) measured a relation for predicting >10 MeV proton fluxes based on the energy of the flare in Si iv. These particles can initiate nonthermal chemical reactions in the planetary atmosphere. From the Youngblood et al. (2017) relation and the HARPS spectrum, we estimate a proton fluence at Proxima b's 0.0485 au distance of 10^7.7 protons cm⁻².

We employ a 1D coupled photochemical and radiative-convective climate model to determine the effects of the observed flare activity on the potential habitability of Proxima b. We assume the planet to have an Earth-like atmosphere, but neglect the planetary magnetic field, which may be weaker than Earth's due to tidal locking (Grießmeier et al. 2009). The details of the coupled model can be found in Segura et al. (2010) and Tilley et al. (2017), which discovered that the results of only electromagnetic flaring cannot significantly drive O₃ column loss, while flares with proton events can rapidly destroy the O₃ column. Proton events lead to the dissociation of N₂ in the planet atmosphere into constituent, excited N-atoms, which then react with O₂ to produce NO and O. NO reacts with O₃ to produce NO₂. The NO_x species generated during the proton events therefore drive the evolution of the ozone column (see Tilley et al. 2017 for further details).

Using the superflare-included cumulative FFD measured in the present work and the scaling from Hawley et al. (2014), we generate a sequence of flares for a 5 year time span in the U-band energy range of 10^29.5–10^32.9 erg (scaled to represent the Evryscope-measured FFD). We assume a time-resolved UV superflare spectrum scaled from AD Leo to Proxima flares, following Tilley et al. (2017). This flare sequence drives the evolution of volatiles in an Earth-like atmosphere at a distance of 0.0485 au to an active M dwarf. Flares are selected at random to produce a proton event, with proton flux scaling with event amplitude. The probability for each flare to generate a planet-oriented energetic particle event was assumed to be a moderate P = 0.08, following Tilley et al. (2017).

The evolution of the O₃ column as a result of the impacting flares and proton events is shown in the top panel of Figure 5. At the end of the simulation, 846 of 10724 flares had generated a proton event that impacted the atmosphere of the planet, resulting in an O₃ column loss of 90%. The system does not appear to reach steady state with increasing time. We assess that it is likely that Proxima b has suffered extreme ozone loss. At Proxima's current activity rate, >99.9% of Proxima b's O₃ is likely to be lost within hundreds of kyr, leaving the planet's surface largely unprotected from UV light.

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A complete lack of O₃ would particularly affect the amount of germicidal UV-C reaching the surface. Although other volatiles capable of absorbing UV-C (i.e., O₂, H₂O) are not necessarily destroyed, they do not effectively block UV-C for wavelengths longer than ∼2500 Å. No significant Earth-like atmospheric gas except O₃ effectively absorbs in the UV-B and UV-C wavelengths ∼2450–2800 Å (Tilley et al. 2017). During the Proxima Superflare, the top-of-atmosphere receives ∼3.5 J cm⁻² of UV-C in the wavelength range 2400–2800 Å. Absent ozone, most of this reaches the surface.

We caution that our result assumes an Earth-like atmosphere and is exploratory in nature. Ozone loss depends upon indirect scaling relations between flare and particle flux, which exhibit 3–4 orders-of-magnitude of scatter (e.g., Cliver et al. 2012; Youngblood et al. 2017). Our primary result, shown in the top panel of Figure 5, assumes solar soft X-ray radiation (SXR)-particle scaling relations (Belov et al. 2005) and active M dwarf UV-SXR scaling relations (Mitra-Kraev et al. 2005), resulting in a median proton fluence of 5.5 × 10⁹ pr cm⁻². Even if we assume a 10⁵ times lower median proton fluence, a value from the lowest end of the proton-scaling distribution, we find that the ozone layer is severely depleted after 5 years. In this second run, we use the "fiducial flare" template spectrum of Section 3.5 and employ a planet-oriented energetic particle event probability of P = 0.25 and solar EUV-particle scaling relations, and then an M dwarf synthetic spectrum (Fontenla et al. 2016) to relate EUV to FUV (Youngblood et al. 2017) for a median proton fluence of 6.1 × 10⁴ pr cm⁻². We measure 40% ozone loss after 5 years. While the difference between these two models is large, it is unsurprising in light of the large scatter in the correlations between solar flare intensity and proton flux. Further work is needed to more rigorously constrain the energetic particle environments of these stars.

Figure 5 shows the UV-B and UV-C flux at the surface of Proxima b during the superflare, given the g' flux measured in the Evryscope light curve, the flare spectral modeling in Section 3.2, and assuming an orbital radius of 0.0485 au (Anglada-Escudé et al. 2016). We plot two flare regimes: (1) the expected fluxes for an intact Earth-like ozone layer (where essentially no UV-C reaches the surface, and ∼10% of UV-B reaches the surface; Grant & Heisler (1997) and references therein); and (2) an extreme ozone-loss scenario, where UV-C in the wavelength range ∼2450–2800 Å reaches the surface unimpeded by ozone or other atmospheric absorption (see Section 4), resulting in UV-B ≃ UV-C surface fluxes (this equivalence is coincidental: the lack of ∼2500 Å UV-C reaching the surface counters the proximity to the flare blackbody peak). We assume cloudless skies.

While UV-B accounts for only 5% of the solar UV radiation incident upon Earth, it has the largest impact upon terrestrial biology because shorter UV-C wavelengths are blocked by the Earth's atmosphere (Sliney & Wolbarsht 1980). During the Proxima Superflare, 3.5 J cm⁻² of UV-B reached the surface under the assumption of extreme ozone attenuation, which is below a lethal dose of 4 J cm⁻² for Deinococcus radiodurans but lethal for most UV-hardy organisms, even when protected by a shallow layer of freshwater. For example, in the top 50 cm of water, 1.5 J cm⁻² of UV-B will kill 50% of freshwater invertebrates (Hurtubise et al. 1998). Zooplankton's UV-B lethal dose is 0.5 J cm⁻² (Rautio & Korhola 2002).

UV-C is much more efficient at damaging DNA than UV-B (Rehemtulla et al. 1997; Batista et al. 2009). Although D. radiodurans is among the most UV-resilient organisms on Earth, its UV-C D90 dose (i.e., the amount of radiation required to kill 90% of the population) of 0.0553 J cm⁻² (Gascón et al. 1995) is a factor of 65× smaller than the 3.6 J cm⁻² reaching the surface during the Proxima Superflare, given no ozone. Recent results have suggested that more complex life such as lichens evolved for extreme environments, and with adaptations such as UV-screening pigments they may survive these radiation levels (Brandt et al. 2015). These results suggest that surface life on areas of Proxima b exposed to these flares would have to undergo complex adaptations to survive⁶ , even if the planetary atmosphere survives the long-term impact of the stellar activity.

Earth may have undergone significantly higher UV fluxes during the early evolution of life. Rugheimer et al. (2015) gave diurnally averaged values of the surface UV-B and UV-C flux on Earth during the pre-biotic (3.9 Ga ago) and early Proterozoic (2.0 Ga ago) epochs. Assuming the full-ozone-loss scenario, the surface UV-B flux during the Proxima superflare was an average of 2× higher than that 3.9 Ga ago and a factor of 3× higher 2.0 Ga ago, although between flares the UV flux was much lower than Earth's, because late M-dwarfs are far fainter in the UV than solar-type stars. The UV-C superflare flux was a factor of 7× higher than that 3.9 Ga ago and a factor of 1750× higher than 2.0 Ga ago; again, the UV-C flux potentially reaching Proxima's surface is the critical difference compared to Earth's environment.