FAST Detecting Auroras
FAST Detecting Auroras On Brown Dwarfs

Brown dwarfs are known as “failed stars” owing to an absence of central hydrogen burning. They bridge the gap between planets and stars.

Some brown dwarfs are found to maintain kilogauss magnetic fields and produce flaring radio emissions, similar to auroras on magnetized planets in the solar system, arousing astronomers’ curiosity about their field properties and dynamos.

Radio emissions from brown dwarfs reflect their magnetic activity. For solar-type stars, radio, optical, and X-ray emissions are all used as magnetic indicators, while for brown dwarfs, optical and X-ray emissions decrease dramatically, with radio emissions becoming the most efficient probe.

Dr. TANG Jing and her colleagues from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) carried out a statistical analysis of the radio-flaring brown dwarf population. The analysis has helped quantify the potential for finding such objects in Five-hundred-meter Aperture Spherical radio Telescope (FAST) surveys.

This study was published in Research in Astronomy and Astrophysics, a peer-reviewed international journal in astronomy and astrophysics supported by NAOC and the Chinese Astronomical Society.

Traditionally, scientists selected a number of brown dwarfs and tracked them for several hours to catch possible flares, which is a very expensive process. As a result, less than 20 flaring brown dwarfs have been detected so far. However, the so-called Commensal Radio Astronomy FasT Survey (CRAFTS) promises to increase this number by almost one order of magnitude, according to the study.

Led by Dr. LI Di, chief scientist of FAST, CRAFTS utilizes a novel and unprecedented mode to realize simultaneous data acquisition for pulsar and fast radio burst (FRB) searches, Galactic neutral hydrogen (HI) mapping, and HI galaxy studies. It is designed to cover 60% of the sky in drift-scan mode.

For FAST, the most significant problem is to resolve discrete sources due to confusion caused by large beam size. However, flaring radio emissions are highly circularly polarized, distinguished from the confusion noise. Circular polarization can be calculated from the orthogonally polarized outputs, independent of system fluctuation, and is a good method for searching for flares.

If the researchers find a highly circularly polarized signal in the survey, they can cross match the archival optical/infrared counterpart for identification. FAST is expected to detect flaring brown dwarfs as far away as 180 pc.

Most flaring brown dwarfs are detected at high frequencies. Although some efforts have been made at low frequencies, flaring emissions in the L band have not been detected yet. FAST may fill in this gap. If successful, it also bodes well for FAST’s potential to discover exoplanets with strong magnetic fields.