image: The blue curve in the image illustrates the evolution of EP240408a's X-ray flux over time, with purple solid dots marking discrete measurements. A pronounced, brief flare is clearly visible around 0.2 days, signaling a sharp, intense flux increase. The inset at the top right provides a closer view of the flare.
Credit: ©Science China Press
On April 8, 2024, the Einstein Probe's Wide Field X-ray Telescope (WXT) picked up an unusual event designated as EP240408. This previously unseen source emitted X-rays consistently in the 0.5-4 KeV energy range, followed by a brief, intense soft X-ray flare that was 300 times brighter and lasted 12 seconds, featuring three equal-sized peaks, and was quickly followed by a fast decrease in X-ray radiation. Emissions weakened to one-tenth of their original strength by the seventh day and disappeared by day 10. Scientists suggest that the duration of its X-ray activity likely ranges between 7 and 23 days, as it was not observed in WXT data from 13 days earlier.
Astronomers have made use of space telescopes such as EP's FXT, Swift's XRT, and NICER to meticulously scrutinize the X-ray emissions from EP240408a. Despite thorough searches across the electromagnetic spectrum using different telescopes such as GROND, NOT, GSP, MASTER, BOOTES-6, Swift's Ultraviolet/Optical Telescope (UVOT), ATCA, and GECAM-B, no signals beyond the X-ray band were detected. The unique timing and light curves of EP240408a do not correlate with traditional X-ray sources, and the absence of visible light makes it difficult to measure its redshift. Nevertheless, despite these challenges, the scientific team endeavored to compare the changing characteristics and light patterns of EP240408a with other known transient sources, which could be linked to it, in the hope of obtaining useful insights.
Dr. Wenda Zhang, lead author and a researcher at the National Astronomical Observatory, stated, “EP240408a's behavior and spectrum are unlike any known X-ray transients, which is intriguing. It's like nothing we've seen before.”
The X-ray pattern of EP240408a is different from what we usually see. It has a spectrum inconsistent with that from gas with a hot temperature, which sets it apart from regular Tidal Disruption Events (TDEs), which are transient phenomena caused by a supermassive black hole devouring a nearby star. Unlike a specific subclass of TDEs that accelerate gas to nearly the speed of light, EP240408a fades away more quickly and gives off less light in the optical, radio, and near-infrared bands. It also has a bright flare lasting only 12 seconds, similar to the quick flashes we see in long Gamma-Ray Bursts (GRBs) that are chactererized by short but strong bursts and are associated with the deaths of massive stars. However, its steady X-ray light behaves differently from the usual afterglow seen in GRBs. It probably isn't an X-ray Binary (XRB), a system that ocassionally becomes bright from radio to X-rays due to a black hole or neutron star gravitationally attracting matter from its stellar companion, either because its fast fading and weaker light don't match what we expect from XRBs. In terms of X-ray strength and how it changes over time, this object is somewhat like the Fast Blue Optical Transient (FBOT)—a rare type of fast transients originally detected in optical with blue colors, but it is not as bright in the optical range, which makes it unlikely to be an FBOT.
Dr. Weimin Yuan, Principal Investigator of the EP mission, highlighted, “EP240408a's discovery might point to a new type of transient source. These short-term X-ray flares have a time scale of about 10 days and may have been missed in previous time-domain X-ray surveys. This finding demonstrates EP’s power to make new discoveries and helps us understand the variety of extreme physical processes in the universe.”
The Einstein Probe, now referred to as “Tianguan” in Chinese, launched on January 9, 2024, is a team effort by the Chinese Academy of Sciences and international partners including ESA, MPE, and CNES. Only two months after commencing its first light, EP240408a was discovered, and to date, no other sources similar to it have been found in the seven months since.
This finding was reported in "Science China Physics, Mechanics & Astronomy" on October 30th.
See the article:
Einstein Probe discovery of EP240408a: A peculiar X-ray transient with an intermediate timescale, https://doi.org/10.1007/s11433-024-2524-4
Journal
Science China Physics Mechanics and Astronomy