Fast Evolving X-ray Transients (FEXTs) are intense bursts of soft X-ray emissions that can last for minutes to hours with a wide range of luminosities. Due to their enigmatic nature, FEXTs have been a importance for scientists. Einstein Probe (EP) is one of the crucial space telescopes for the search and investigation of FEXTs. An international team of astronomers using the Einstein Probe reports the discovery of a new peculiar fast-evolving X-transient. The newfound transient exhibits an unprecedentedly long-lasting X-ray emission.

A Puzzling Long-Lived X-ray Transient

The finding was detailed in a paper published May 12 on the arXiv preprint server. According to the published paper, the newfound peculiar FEXT, dubbed EP241021a, was detected on Oct 21, 2024, with the help of EP’s wide-field X-ray telescope (WXT). EP241021a was identified by WXT as an intense flare, which lasted for approximately 92 seconds and reached a luminosity of about one quindecillion erg/s. The X-ray spectrum of the flare was found to be relatively hard with a photon index of 1.8.

The team of astronomers, led by Xinwen Shu of the Anhui Normal University in China, has concluded that EP241021a is an extremely unusual transient mainly due to its extended emission period. Follow-up observations of the flare up to 79 days after its detection revealed that its X-ray light curve shows a nearly plateau phase for the first 7 days, which is followed by a steep decline during the period of about 30 days. Afterward, the X-ray emission rapidly drops under the detection level.

The researchers also detected significant multiwavelength signals associated with EP241021a. About 1.8 days after the initial X-ray detection, optical emissions were also detected. This emission is likely connected to the flare’s afterglow.

Possible origins

The origin of FEXTs is puzzling. Astronomers try to explain their origin take into account several scenarios; for instance, stellar flares, supernova shock breakouts, or long gamma-ray bursts (GRBs).
Trying to explain the origin of EP241021a, the authors of the paper favor two scenarios. These are: a magnetar engine, involving a highly magnetized neutron star, or a jetted tidal disruption event (TDE), where a star is consumed by a black hole.