Observing optical flares from Proxima Centauri at the highest timescales

Deirdre Ní Chonchubhair 1 , Aaron Golden 1

  • 1 University of Galway, Galway

Abstract

Proxima Centauri, the nearest stellar neighbour to Earth beyond the Sun, at a distance of 1.3pc, is an active M5.5 dwarf star with one confirmed and two candidate exoplanets. We present results from high time resolution U-band observations of Proxima Centauri using the Sutherland High Speed Optical Camera (SHOC) on the 1m telescope at the South African Astronomical Observatory. These data were taken over seven contiguous nights in April 2019 as part of a multiwavelength campaign using the MeerKAT radio telescope to identify and characterise white light flare events and to coincide with TESS (Transiting Exoplanet Survey Satellite) observations of Proxima Centauri at this epoch. Using SHOC’s superior cadence, varying from 0.1 to 3 seconds, and comparing to the 120 second TESS cadence we explore flare incidence and frequency rates compared to that determined from TESS. We detect several flares with bolometric energies ranging from 10^28 to 10^30 erg in our SHOC data which proceeds the small but energetic flare on 1st May 2019 as described by MacGregor et al. (2021), which has a bolometric energy of 10^31.2 erg (Vida et al., 2019). We focus on the implications our observations have on the viability of the solar paradigm in the context of white light flare activity and how this contributes towards a better understanding of the space weather environment in the nearest exoplanetary system.

References:
MacGregor, M. A., Weinberger, A. J., Loyd, R. O. P., et al. 2021, ApJL, 911, L25
Vida, K., Oláh, K., Kővári, Z., et al. 2019, ApJ, 884, 160

Observations

Proxima Centauri, the nearest stellar neighbour to Earth beyond the Sun, at a distance of 1.3pc, is an active M5.5 dwarf star with one confirmed and two candidate exoplanets. We observed Proxima Centauri for 7 nights in April 2019 using the Sutherland High Speed Optical Camera (SHOC, Figure 1) on the 1.9m telescope at the South African Astronomical Observatory (SAAO). The Transiting Exoplanet Survey Satellite (TESS, Figure 2) also observed Proxima Centauri in two sectors (Sector 11 and 12) in 2019. This provided nearly 2 months of observations at 120 second cadence. The SHOC observations were taken in the U-band filter and with a cadence of between 0.1 and 3 seconds. Our SHOC observations were coincident with some of the TESS observations.

Figure 1: SHOC on the 1.9m Telescope at SAAO.

Figure 2: TESS space telescope.

Results

SHOC Flares

Figures 3 and 4 show examples of both classical and complex flares we detected using SHOC. The SHOC relative flux is calculated using aperture photometry of the target, comparison star and a background aperture. We detected ~30 flares in our dataset and the bolometric energy of those range from \(6.2\times10^{28}\) to \(5.3\times10^{30}\ erg\).

Figure 3: A SHOC flare at 0.2 s cadence.

Figure 4: Largest energy SHOC flare at 0.12 s cadence.

Simultaneous SHOC & TESS Flare

Figure 5 shows ~11 days of observations of Proxima Centauri using TESS. AltaiPony (Ilin et al., 2021) was used to identify flares in the TESS dataset which are shown in red. The orange bar shows the SHOC dataset as it overlaps with TESS and the simultaneous flaring within. One such flare is identified by the blue arrow and discussed further in Figure 6.

Figure 5: Approx 11 days of TESS observations with flares marked in red. Orange bar shows the timing of the SHOC observations.

The top and bottom panel of Figure 6 show one simultaneous flare (blue arrow in Figure 5) as seen by both TESS (top panel) and SHOC (bottom panel). The SHOC cadence here is 0.25 s, compared with 120 s for TESS, which shows the complex flare structure in the SHOC data. Where TESS sees one small flare, with the high speed SHOC data we see two distinct flares and the complex structure within. We calculate a bolometric energy of \(9.7 \times 10^{29}\ erg\) for the first SHOC flare and \(8.7 \times 10^{29}\ erg\) for the second SHOC flare, compared to \(3.6 \times 10^{30}\ erg\) for the combined TESS flare.

Figure 6: Simultaneous flare observed by TESS (top) and SHOC (bottom).

Flare Frequency Distribution

Figure 7 shows a flare frequency distribution plot for Proxima Centauri using ~56 days of TESS observations and 6 nights of SHOC data. Fitting both sets of data with a power law distribution gives an alpha of \(1.55 ± 0.12\) which is similar to the values of alpha from Vida et al. (2019) and Davenport et al. (2016). In Figure 7, the purple square marks the TESS flare shown marked in Figure 5 (and top panel of Figure 6) while the blue star and red triangle show the first and second SHOC flares from the bottom panel of Figure 6.


Figure 7: Flare frequency distribution of both SHOC and TESS flares.

Effect on Exoplanet Habitability

Figure 8 shows an artist’s impression of a stellar flare on a red dwarf star and an orbiting exoplanet. Stellar flares are much more powerful than solar flares. Many of these exoplanets are within the stars’ habitable zone but the question remains, are they still ‘habitable’? Proxima Centauri's exoplanet, Proxima Centauri b is a 1.27 Earth mass planet within the habitable zone. We detect multiple smaller flares from Proxima Centauri with bolometric energies ranging from \(10^{28}\) to \(10^{30}\ erg\) in our SHOC dataset. We show that with higher time resolution observations a small TESS flares can be resolved into smaller but still energetic multiple flare events. Work by Roettenbacher & Kane (2017) and Vida et al., (2017, 2019), showed that strong frequent flares can have a greater effect on exoplanet habitability than single large flare events. Our observations and flares precede the short duration but multiwavelength flare as seen by MacGregor et al. (2021) and highlight the need for high speed optical combined with mutilwavelength observational campaigns to fully understand stellar flares and their implications for exoplanet habitability.

Figure 8: Flaring on a red dwarf star.

References

Davenport, J. et al., 2016, ApJL, 829, L31

Ilin, E. et al., 2021, A&A, 645, A42

MacGregor, M. et al., 2021, ApJL, 911, L25

Roettenbacher, R. & Kane, S., 2017, ApJ, 851, 77

Vida, K. et al., 2017, ApJ, 841, 124

Vida, K. et al., 2019, ApJ, 884, 160

Image Credit

Figure 1: https://www.saao.ac.za/astronomers/shoc/. Accessed: 2025-03-24.

Figure 2: https://science.nasa.gov/mission/tess/. Accessed: 2024-03-24.

Figure 8: Roberto Molar Candanosa/Carnegie/NASA. Accessed: 2024-03-24.