Launched on January 9, 2024, by the Chinese Academy of Sciences, the Einstein Probe has already made remarkable discoveries during its commissioning phase. In an interview with China Central Television last October, Yuan Weimin, the mission’s principal investigator, revealed that the X-ray observatory had identified approximately 60 exceptionally strong transient celestial phenomena, nearly 1,000 potential transient sources, and around 500 stellar flares. It also detected a gamma-ray burst originating from the early universe.

Among its findings was EP240408a, an unusual explosion that sparked extensive discussion among astronomers. Zhang and his team quickly utilized the Follow-up X-ray Telescope, the Einstein Probe’s second instrument, to observe this intriguing source just 1.8 days after its initial detection by the Wide-field X-ray Telescope.

In a remarkable collaborative effort, both research teams sought observations from various instruments on Earth and in space, covering a wide range of wavelengths. Nearly 20 different telescopes, beyond the Einstein Probe, were aimed at the new event, encompassing optical, radio, gamma-ray, ultraviolet, and near-infrared wavelengths.

Interestingly, most of these instruments detected nothing unusual, which stands out in the context of prior observations. Typically, known X-ray sources emit signals across multiple wavelengths. However, Zhang and his team only observed EP240408a shining in the X-ray spectrum, while O’Connor identified a faint optical counterpart, possibly a distant galaxy, as the origin of the signal.

EP240408a deviated from existing transient models in several ways. This unprecedented explosion produced X-rays for a period estimated between seven to 23 days, as determined when the Einstein Probe was directed toward it. Most fast X-ray bursts, caused by intense events, typically last seconds to a few minutes before disappearing, whereas longer-lived transients associated with galactic centers can persist for months to years. This intermediate duration of EP240408a is particularly noteworthy.

Moreover, the source emitted a brief flare lasting 12 seconds, which was 300 times more intense than its regular X-ray emission before tapering off.

Nicholas Zelati, a researcher at the Institute of Space Sciences and part of the Zhang team, noted that NASA’s Neutron star Interior Composition Explorer (NICER) was one of the few instruments capable of observing the new event’s characteristics.

“Once we recognized EP240408a as a significant new transient, we applied for NICER observations,” Zelati explained via email. The team leveraged the International Space Station-based observatory to analyze the X-ray properties of this phenomena closely and observe any rapid changes in its emissions.

NICER’s ability to detect this swift event stemmed from its high collecting area and flexible scheduling. As Zelati noted, “Many other observatories either lack the rapid scheduling or the sensitivity necessary for this event.” NICER’s quick response was crucial in monitoring the transient’s evolution.

Both teams also benefited from X-ray observations conducted by NASA’s Neil Gehrels Swift Observatory. This spacecraft contributed not only to measuring the signal but also to refining the location of the source. O’Connor utilized data on hydrogen, the universe’s fundamental building block, obtained from Swift observations to conclude that the explosion stemmed from outside the Milky Way. They inferred that hydrogen in the host galaxy was absorbing X-ray photons.

The researchers utilized various optical telescopes to further investigate the flare. At the Gemini South Observatory in Chile, O’Connor pinpointed a dim galaxy potentially associated with the event.

Is it a Unique Event or Something Entirely New?

After comparing their findings, the two independent research teams formulated differing theories. O’Connor’s group hypothesizes that EP240408a could be a tidal disruption event (TDE), a phenomenon occurring when a star ventures too close to a black hole and is torn apart due to its immense gravitational pull. Thus far, only about one hundred TDEs have been documented since the first one was observed in 1995.

In some rare instances, the tidal forces from a black hole may propel material outward, generating a high-velocity jet that emits radiation in both X-ray and radio wavelengths. However, only four known TDEs are associated with such jets, and EP240408a’s X-ray signature is akin to those few.

Nonetheless, radio observations from multiple telescopes did not yield any significant findings. As Zhang noted, “The absence of low-frequency emissions leads us to exclude jetted TDEs as a possibility regarding EP240408a.”

Zelati, however, remains cautious about ruling out jet-related activity. It’s feasible that radio emissions may occur later when the jet interacts with surrounding medium. “This potential radio-bright phase could arise weeks or even months post-event,” he added.

O’Connor and his colleagues expressed similar sentiments, suggesting that the jet might take time to decelerate, subsequently delaying the shockwave that triggers radio emissions. “Such delayed emission has been observed in numerous TDEs recently, across various timescales,” he noted.

The primary challenge, according to O’Connor, lies in the timescale. Although EP240408a shares visual similarities with other relativistic jetted TDEs, the decay rate of its X-ray signal is notably faster.

“A rapid decay could be explained by a relatively small black hole interacting with a dense star,” O’Connor asserted. “We posited the involvement of an intermediate-mass black hole disrupting a white dwarf.”

In contrast, Zhang and his research team propose that EP240408a may signify a novel category of astronomical phenomena.

They suggest in their research that, “EP240408a might represent a new variety of transient events with intermediate timescales of around ten days, which could have been overlooked in earlier surveys.”

Such intermediate X-ray transients may easily evade detection in studies concentrated on either long-lasting objects or extremely brief bursts, Zelati explained.

Additionally, O’Connor’s team does not dismiss the possibility of an entirely new transient class. They acknowledged in their research that the observed characteristics of EP240408a “do not neatly correspond with any established transient classifications,” implying that it could well represent a previously unknown type of transient.

“If EP240408a turns out to be a new class of intermediate X-ray transients, it would greatly enhance our understanding of the diverse phenomena occurring in the universe,” Zelati remarked. This discovery could bridge gaps in our classification of X-ray sources, potentially leading to the formulation of new theories and observational strategies to uncover similar events. “Ultimately, this would enrich our comprehension of high-energy astrophysical occurrences,” he noted.

As the mission of the Einstein Probe unfolds, it is highly likely that it will detect more such fascinating events, if it hasn’t already.

“Future discoveries by the Einstein Probe will help us collectively understand these occurrences,” O’Connor remarked. “I am excited about the unusual transients that the Probe will unveil in the future!”