Astronomers detected a new superflare of an ultracool star

The event appears to be one of the most powerful flares on ultracool stars so far discovered.

Using Xinglong observatory’s Ground-based Wide Angle Cameras (GWAC), astronomers from China and France have found a new event called GWAC 181229A. They have detected a new superflare of an ultracool star known as SDSS J013333.08+003223.7. This newly detected superflare appears to be one of the most powerful flares on ultracool stars so far discovered.

Identifying new flares of UCDs and examining them in detail is essential to more readily comprehend the origin of these events and the interaction between the magnetic fields and surfaces of ultracool stars. One of the instruments that could be useful in identifying new flares on UCDs is GWAC. It is one of the leading ground offices of the planned Space Variable Objects Monitor (SVOM) satellite—a joint China-France satellite mission devoted to detecting and investigating gamma-ray blasts (GRBs).

A team of astronomers led by Li-Ping Xin of National Astronomical Observatories of the Chinese Academy of Sciences (CAS) reports the discovery of a powerful flare on SDSS J013333.08+003223.7—a UCD of spectral type M9, located some 471 light-years away. GWAC 181229A turns out to be more powerful than typical bursts usually detected on UCDs.

Astronomers noted, “In this paper, we report the detection and follow-ups of a super stellar flare GWAC181229A with an amplitude of ∆R ∼9.5 mag on an M9 type star by SVOM/GWAC and the dedicated follow-up telescopes.”

The study suggests that the newly detected superflare had bolometric energy between 55.6 and 92.5 decillion ergsm hence one of the most powerful flares on ultracool stars. Its magnetic strength was estimated to be at a level of 3.6-4.7 kG. The total duration of the flare from the onset to the quiescent flux level was calculated to be about 14,465 seconds.

The observations show that the superflare of GWAC181229A promptly rises from the quiescent flux level to the peak time in about 50 seconds, and then returns to a decay. The scientists found that this star’s blackbody temperature is about 5,340 K, suggesting filling factors of the flare of around 30 and 19 percent at the peak time and 54 minutes after the first detection, respectively.

Astronomers noted, “Thanks to the large field of view and the high survey cadence, GWAC is well-suited for the detection of white-light flares. We have hitherto detected more than ∼ 130 white-light flares with an amplitude of more than 0.8 mag. More GWAC units are planned to work in the next two years, aiming to increase the detection rate of high-amplitude stellar flares by monitoring more than 5,000 square degrees simultaneously.”

Journal Reference:
  1. A ∆R ∼ 9.5 mag Super Flare of An Ultracool Star Detected by SVOM/GWAC System, arXiv:2012.14126 [astro-ph.SR]

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