More than 160 years ago, on September 1, 1859, the English astronomers Richard C. Carrington and Richard Hodgson independently observed a sudden brightening on the surface of the Sun. This phenomenon was named “flare” by the solar physicists.

From the light curves of solar flares, it can be seen that a flare has two distinct phases: the rise phase and the decay phase. The rise phase generally represents a rapid release of magnetic field energy through a magnetic reconnection process, while the decay phase generally demonstrates a prolonged term comprising the whole cooling process. The timescales of the flare rise phase and decay phase have the important physical significance to flare research.

Akin to solar flares, the flare phenomenon was also discovered on the stars other than the Sun. Although the study of stellar flares also has a long history, only very few stellar flare cases had been identified, and even fewer for Sun-like stars.

In 2009, the Kepler space telescope was launched. This mission has observed the light curves of a large volume of stars. These light-curve data provide a massive number of stellar flares. An example stellar flare light curve observed by Kepler is shown in Figure 1.

Figure 1: An example stellar flare light curve observed by Kepler (Credit: YAN et al., MNRAS, 2021)

Based on the light-curve data of Kepler, a research team led by Prof. HE Han at National Astronomical Observatories, Chinese Academy of Sciences (NAOC) has revealed the characteristic time of stellar flares on Sun-like stars. The study was published in Monthly Notices of the Royal Astronomical Society on June12. Dr. YAN Yan and Prof. HE Han of NAOC are the corresponding authors of the paper.

The research selected star samples that have the stellar parameters approximate to the Sun and identified 184 stellar flares from the SC light curves of the Sun-like stars. The duration times of the flare rise phase and decay phase were determined for each flare sample based on the flare light-curve profile (see Figure 1), and then a statistical analysis was performed on the obtained rise times and decay times of the flare samples.

The result shows that, for the stellar flares on Sun-like stars, the median values of the flare rise time and decay times are 5.9 min and 22.6 min, respectively. These time values for stellar flares are similar to the timescale of solar flares, which supports the idea that stellar flares and solar flares have the same physical mechanism.

It is also found that both the rise time and the decay time of the stellar flares follow a lognormal distribution, showing a peak-shaped head and a long tail (see Figure 2). The distribution passes the Kolmogorov–Smirnov test at a confidence level of 0.95. This result opens up an innovative approach for analyzing the characteristic time of stellar flares. The statistical results obtained for Sun-like stars can be a benchmark of flare characteristic times when compared with other types of stars.

In addition, stellar flare radiation is a key factor in the habitability of exoplanets within a stellar system. The result obtained in this work can act as an important input element for analyzing the impact of stellar flares to the atmosphere, space environment, and habitability of exoplanets.

Figure 2: Left panels: histograms (blue) and fitted lognormal distribution curves (red) for the flare rise times (upper) and decay times (lower); Right panels: normal distribution diagrams for the logarithm of the rise times and decay times. (Credit: YAN et al., MNRAS, 2021)

The paper can be accessed at https://academic.oup.com/mnrasl/article-abstract/505/1/L79/6297375