Having launched on January 9, 2024 by the Chinese Academy of Sciences, the Einstein Probe detected several new events during its commissioning phase. Last October, Yuan Weimin, the spacecraft’s principle investigator, told China Central Television that the X-ray observatory had already discovered around 60 very strong transient celestial objects, close to a thousand potential transients, and nearly 500 stellar flares, along with a gamma-ray burst from the very early universe.
One of those detections was EP240408a, an unusual blast that lit up discussions between astronomers. Zhang and his colleagues immediately utilized the spacecraft’s second instrument, the Follow-up X-ray Telescope, to take observations of the new source 1.8 days after it was first spotted by its companion, the Wide-field X-ray Telescope.
Both teams of researchers sprung into action, requesting time on Earth– and space-based instruments in a multitude of wavelengths. Together, the two research groups pointed almost 20 different telescopes besides the Einstein Probe at the new incident, spanning optical, radio, gamma ray, ultraviolet and near-infrared wavelengths.
Most of those instruments saw nothing. And that’s unusual.
Known X-ray emitters tend to be of the multiwavelength sort, sending out signals in more than one regime. Zhang and his colleagues only saw the EP240408a shine in the X-ray, while O’Connor identified a possible optical counterpart: a small, faint galaxy that may be where the signal emerged.
That’s not the only way that EP240408a doesn’t fit with the existing transient models. The new explosion lit up X-rays for somewhere between seven and 23 days, an estimate based on when EP was pointed in its direction. Fast X-ray bursts, intense short explosions resulting from violent processes, flare anywhere from sub-seconds to hundreds of seconds before they disappear. Longer transients associated with galactic nuclei last anywhere from months to years. The unusual mid-range sets EP240408a apart.
Additionally, the new target fired off a 12-second flare 300 times brighter than the underlying X-ray emission before fading to the lower levels.
NASA’s Neutron star Interior Composition Explorer (NICER) was one of the few instruments able to catch a glimpse of EP’s newest hit.
“Once we realized that EP240408a was a compelling new transient, we requested NICER pointings,” Francesco Zelati, a researcher at the Institute of Space Sciences and part of the Zhang group, told Space.com by email. Zelati and his colleagues used the International Space Station-based X-ray observatory to better characterize the new event’s X-ray properties and capture any rapid changes in its emission.
NICER was one of the few instruments able to detect the brief event both because of its high collecting area and its flexible scheduling. “Many other observatories either lack the rapid scheduling or the sensitivity in the relevant energy ranges,” Zelati said. NICER’s quick response allowed it to obtain data that was “key to track the evolution of the transient,” he said.
Both teams also relied on detection by NASA’s Neil Gehrels Swift Observatory (Swift) in the X-ray. In addition to measuring the signal, Zhang said that the spacecraft helped to narrow down the location of the source. O’Connor used Swift’s measure of hydrogen, the primordial gas that is the building material for everything in the universe, to determine that the explosion came from outside the Milky Way. Their findings led them to infer hydrogen is absorbing the X-ray photons in the far-off host galaxy.
Both teams turned a variety of optical telescopes towards the flare. With the Gemini South Observatory, based in Chile, O’Connor identified a faint galaxy that could be home to the event.
A rare event or a brand new one?
The two independent teams of astronomers compared their measurements and came to slightly different conclusions.
O’Connor and his colleagues suspect that the distant explosion may be an event known as a tidal disruption event, or TDE. A TDE occurs when a star passes dangerously close to a black hole and is shredded by its gravitational forces. Only a hundred TDEs have been discovered since the first was spotted in 1995.
In extremely rare cases, the black hole’s tidal forces fire material outward in a high-velocity jet that interacts with nearby material, shining brightly in both X-ray and radio wavelengths. Only four TDEs to date are thought to have relativistic jets — and the X-ray signal is very similar to those four.
But observations in radio frequencies with multiple telescopes have come up empty. To Zhang, that omits jetted TDEs from consideration. “It is hard to explain EP240408a with jet [TDE] due to the lack of low-frequency — radio to near-infrared — radiation of the source,” he said.
His coauthor, Zelati, isn’t so quick to dismiss the results. It’s possible that radio or longer-wavelength emission would appear later, when the jet expands into the surrounding medium. “This radio-bright phase could emerge anywhere from weeks to many months after the initial event,” he said.
O’Connor and his colleagues had the same thought. They suspect that the jet may take some time to decelerate, delaying the shockwave that would spark radio emissions. “Such delayed late time radio emission has been observed in many past TDEs recently over a range of timescales,” he said. “It appears to be an almost ubiquitous property.”
It’s the timescale that is a challenge to explain, he said. While EP240408a has a similar appearance to other relativistic jetted TDEs, the X-ray signal decays more rapidly.
“Such a short timescale can be possible if the black hole is quite small and the star quite dense,” O’Connor says. “We favored an intermediate mass black hole disrupting a white dwarf.”
Zhang and colleagues suspect that the unusual signal may instead be an entirely new class of object.
“We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of about 10 days, that may have been missed in previous time-domain surveys,” they wrote in their paper.
That could be because intermediate-length X-ray transients could go unnoticed in surveys focusing on either very long-scale objects or extremely short bursts, Zelati said.
O’Connor and his coauthors don’t rule out a brand new class of object. Pointing out in their research that the observed properties of EP240408a “do not directly align with any known transient class,” they admit that a jetted TDE does not perfectly explain the observations. “The alternative is that EP240408a may represent a new, previously unknown class of transient,” they wrote.
“Discovering a new class of intermediate X-ray transients like EP240408a would significantly enhance our comprehension of the diverse and dynamic processes in the universe,” Zelati said. Such a discovery would fill in gaps of classifications of X-ray phenomena, potentially leading to the development of new theories and observational studies to find more like it. “Ultimately, it would broaden our understanding of high-energy astrophysical events,” he said.
It’s extremely likely that the Einstein Probe will most likely detect similar events throughout its mission, assuming it hasn’t already.
“Future detections of similar events by EP will help us figure this out as a community,” O’Connor wrote. “I am definitely looking forward to the weird transients EP will discover in the future!”
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