Transients are short-lived cosmic events detected by telescopes. Recently, the Einstein Probe was able to detect one of these events in the X-ray range. However, scientists have still been unable to explain what kind of object could have produced it.
Multi-wavelength observations
Astronomers have reported the discovery of an unusual X-ray transient detected by the Einstein Probe, which does not fit into any known class of cosmic explosions. A study presenting its multi-wavelength analysis was published in Monthly Notices of the Royal Astronomical Society on June 13.
On March 5, 2024, the space telescope known as the Einstein Probe—designed to scan the sky for sudden X-ray flares—recorded a short-lived, never-before-seen source named EP240305a. It produced two brief X-ray bursts, one after another, separated by about 200 seconds of silence.
Researchers quickly directed multiple telescopes toward the source to collect additional data across X-ray, infrared, optical, and radio wavelengths.
They observed that the X-ray emission faded rapidly over the following days, while the radio emission declined much more slowly over several weeks, revealing signs of an evolving jet. A faint, fading near-infrared source was detected at the location, while no counterpart was observed in the optical band.
Ruled-out scenarios
To elucidate the nature of transient, the researchers compared its properties with several classes of known astrophysical phenomena that are accompanied by powerful short-lived outbursts. These include the tidal disruption of a star by a supermassive black hole, during which part of the stellar material forms an accretion disk and can generate a relativistic jet; outbursts of X-ray binaries, in which a black hole or neutron star strips material from a companion star; thermonuclear outbursts on the surface of neutron stars; giant magnetar outbursts; powerful stellar flares and gamma-ray bursts (GRBs).
They ruled out tidal disruption events and typical X-ray binary outbursts, since those usually fade over months, whereas EP240305a dimmed within just a few days. Even the short-duration subclass of X-ray binary flares, which can fade over weeks, does not produce the strong radio emission observed in this case. Thermonuclear bursts were also deemed unlikely, as the measured temperature was too low, and the radio signal persisted far longer than expected for such an event.
Giant magnetar flares were excluded as well, since they occur and fade in less than a second—far shorter than the two bursts of EP240305a, each of which lasted several minutes. Similarly, the radio emission lasted much longer than what is typically seen in stellar flares.
Was it a special gamma-ray burst?
Interestingly, its properties match several characteristics of energetic gamma-ray bursts (GRBs). The double-flare pattern resembles a known feature observed in some GRBs, often referred to as “double-peaked bursts.” The X-ray brightness profile also follows the typical pattern seen in GRBs. In addition, the gradually fading radio emission is consistent with what is expected from GRB afterglows.
But there was one problem: none of the gamma-ray bursts that would accompany such an event were detected. Astronomers call such events “gamma-ray dark” GRB-like transients. That is, they closely resemble gamma-ray bursts in their properties, but the gamma-ray burst itself is either not detected at all or is too weak.
There are several possible explanations for this behavior. One is that the relativistic jet was not pointed precisely towards Earth, so its gamma-ray emission barely reached our detectors. Another is a so-called “choked jet,” which failed to fully penetrate the surrounding matter and produce a powerful gamma-ray burst. It could also be a “dirty fireball,” a jet with a large amount of trapped matter, which reduces its speed and makes the gamma-ray emission much weaker.
A faint event
Given the available data, researchers cannot say with certainty what EP240305a is. “In the case of EP240305a, the current data do not allow us to confidently establish that it is a GRB origin, so we cautiously classify it as a gamma-ray–dark GRB-like transient or, more broadly, an extragalactic fast X-ray transient,” they conclude.
Faint events like this one are often overshadowed by brighter and more obvious explosions, making them easy to miss with less sensitive instruments. Researchers note that highly sensitive telescopes such as the Einstein Probe, combined with rapid follow-up observations across other wavelengths, are crucial for detecting and studying these dim and easily overlooked phenomena.
According to phys.org
