A Tiny Flare from a Tiny Star

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Title: The Mouse That Squeaked: A Small Flare from Proxima Cen Observed in the Millimeter, Optical, and Soft X-ray with Chandra and ALMA
Authors: Ward S. Howard et al.
First Author’s Institution: University of Colorado, Boulder
Status: Published in ApJ

Most stars like to flare. Stellar flares are rapid, short-duration increases in brightness that are particularly common in M-dwarf stars — a class of stars that is very likely to host Earth-like planets. Studying the variety of stellar flares from M dwarfs is important to understand their effects on the atmospheres of planets around these stars. Today’s article describes detailed observations of a flare in a nearby M-dwarf star: Proxima Centauri.

What Are M-dwarf Flares?

M-dwarf stars have strong magnetic fields and convective envelopes. The magnetic field lines are dragged around due to convective motion in the envelope. This magnetic activity can cause a sudden energy release through a mechanism known as magnetic reconnection. This burst of energy causes the star to flare and emit a pulse of radiation across the electromagnetic spectrum. Several flares in M-dwarf stars have been studied to date, but only the most energetic flares have been studied at multiple wavelengths. The lower-energy flares (or “squeaks,” as the authors call them) have received comparatively lesser attention. Understanding these low-energy flares is crucial because they are expected to be far more common than their more energetic counterparts, and they are thus expected to have significant effects on the planets that orbit the star.

The Flare from Proxima Centauri

The authors conducted a campaign to monitor an M-dwarf star at multiple wavelengths across the electromagnetic spectrum to search for low-energy flares. For the subject of this study, they chose the star Proxima Centauri — the closest M-dwarf star to Earth. They monitored this star with X-ray, optical, and radio telescopes, and on 6 May 2019, they detected a flare!

On this day, the authors observed the star with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Chandra X-ray Observatory. ALMA is a radio telescope operating at millimeter wavelengths, while Chandra is a space telescope that operates at X-ray wavelengths. Both telescopes detected a flare from Proxima Centauri. In X-rays, Chandra witnessed a 40-minute-long burst in the soft X-ray band, which has energies of ~1,000–10,000 eV (for comparison, the energy of a visible photon is ~1 eV). The X-ray flare shows a complex structure with a rapid rise followed by a slow second peak and a final third peak on the decline. The ALMA telescopes also detected the flare at a wavelength of 1.3 mm (for comparison, the wavelength of visible light is on the order of 10-4 mm!). Unlike the X-ray flare, the millimeter flare lasted only for a few seconds and showed only two peaks — coincident with the final two peaks seen in X-rays. It turns out that during the first X-ray peak, ALMA suffered a calibration glitch, which is why it did not detect the first peak.

In addition to X-ray and millimeter wavelengths, the flare was also detected at optical wavelengths (i.e., visual light) by the Las Cumbres Observatory Global Telescope Network telescopes. These telescopes clearly detected the flare in the U band (~300 nm wavelength) for a duration of ~30 mins.

The profile of the flare at different wavelengths is shown in Figure 1. The shorter duration of the flare in the millimeter bands compared to the optical and X-ray bands isn’t very surprising. The millimeter emission traces the sudden initial acceleration of charged particles, which heats up the outer layers of the star. These hot layers then subsequently emit at X-ray and optical wavelengths over a longer time duration.

Plots of the time variation of the flux in three wavelength ranges during the flare

Figure 1: X-ray (top), optical (middle), and millimeter (radio; bottom) profiles of the observed flare from Proxima Centauri. [Adapted from Howard et al. 2022]

What Did We Learn from This Flare?

From their multiwavelength observations, the authors calculate that the total energy released in this flare was about 1026 ergs. While this is a tremendous amount of energy (the energy released by an atomic bomb explosion is ~1021 ergs), it is still small compared to M-dwarf flares that have been studied in the past, which have energies of ~1034 ergs. Such low-energy flares have been extensively studied for the Sun, which underwent 175 such flares during its last 11-year solar cycle. The observations of this flare thus provide a unique opportunity to compare solar flares to M-dwarf flares.

The authors find that the ratios of both the millimeter to X-ray flux and the optical to X-ray flux are much larger for the Proxima Centauri flare than solar flares. However, several properties such as the temperature and relative timing of the flare in different wavelengths are similar to those of solar flares. This suggests that the flare emission properties are similar across a broad range of flare energies. If the flare emission properties are the same for M-dwarf and solar flares, this observation could suggest that millimeter emission should be present in all M-dwarf flares as well. This is important because millimeter emission helped the authors understand the nature of the plasma in the M-dwarf envelope.

Motivated by these observations, the authors are continuing their multiwavelength campaign to search for additional flares from other M-dwarf stars with a variety of ages and activity levels. These multi-wavelength observations will help us to understand similarities of these flares with their solar counterparts, the nature of the plasma in their envelopes, and their effects on the stars’ orbiting planets.

Original astrobite edited by Benjamin Cassese.

About the author, Viraj Karambelkar:

I am a second-year graduate student at Caltech. My research focuses on infrared time-domain astronomy. I study dusty explosions and dust enshrouded variable stars using optical and infrared telescopes. I mainly work with data from the Zwicky Transient Facility and the Palomar Gattini-IR telescopes. I love watching movies and plays, playing badminton and am trying hard to improve my chess and crossword skills.