Fast X-ray Transients (FXTs) are some of the most enigmatic high-energy events in astrophysics. They show extraordinary soft X-ray (0.3–10 keV) brightness, with durations from minutes to hours, followed by decay to background levels.
Some theories for the origin of FXTs link them to astrophysical processes at high-energy scales that accompany stellar core collapse, jet formation, or intermittent black hole accretion. However, the exact origin remains a mystery. This viable progenitor could be from gamma-ray bursts (GRBs), unusual flares associated with obscured black holes, or yet-unidentified mechanisms.
Astronomers have recently detected the probable production mechanism of a certain Fast X-ray Transient (FXT). The event, discovered on Nov. 7 of last year, is thought to result from the core collapse of a massive star or a collision of two neutron stars. This sudden release of energy provides observational constraints on the extreme physics that powers these ultra-high-energy events.
Recent advancements include the launch of a new wide-field X-ray survey mission, called Einstein Probe (EP), that has greatly improved detection sensitivity. The EP identifies these transients in real-time and begins follow-up observations as soon as possible, producing key data for transient astronomy.
One of its most recent detections, EP241107a, has been a main target for multi-wavelength studies. EP241107a was first discovered as a rapid X-ray flare and then soon observed across the optical and radio bands using both optical telescopes and radio arrays. Astronomers detected the radio counterpart of an FXT for the first time, making it possible to confirm or constrain the physical properties of that (and other) such events.
The research was carried out by Deepak Eappachen and Arvind Balasubramanian, postdoctoral fellows at the Indian Institute of Astrophysics (IIA), an autonomous institution under the Department of Science and Technology, Government of India. The team used a worldwide array of observational facilities to study the transient X-ray emission.
In the U.S., astronomers used the Karl G. Jansky Very Large Array in New Mexico. In India, the event was tracked from Ladakh with the Himalayan Chandra Telescope (HCT) and the GROWTH India Telescope (GIT). For radio follow-up, they also used the Upgraded Giant Metrewave Radio Telescope (uGMRT).
The group also added data from the 10-meter Keck Observatory in Hawaii and the Southern Astrophysical Research (SOAR) Telescope in Chile, a 4.1-meter instrument proficient in optical and near-infrared observations.
Comparing optical and radio observations of EP241107a and other distant events showed clear similarities to gamma-ray bursts (GRBs). Scientists say that gamma-ray bursts happen after the collision of two massive stars that form black holes or when neutron stars merge, creating narrow, fast-moving jets.
When scientists studied the jet from EP241107a using detailed models, they found it had the same kind of energy usually seen in gamma-ray bursts (GRBs). The jet itself was narrow, with a clear opening angle that lined up with our view. Additionally, the viewing angle matches the observed brightness. Thus, EP241107a has a structure similar to a GRB viewed at a small angle.
By studying these events together, scientists can sort FXTs into groups, learn about the galaxies they originate from, and narrow down the possible causes.
The evidence so far suggests that at least some FXTs are directly linked to gamma-ray bursts (GRBs), while others may be driven by different cosmic processes. With each new detection, theoretical models are being refined, steadily narrowing the range of possible explanations for these powerful flashes.
Journal Reference:
- D Eappachen, A Balasubramanian, Vishwajeet Swain, G C Anupama, D K Sahu et al. Characterizing EP241107a: multiwavelength observations of an Einstein Probe-detected fast X-ray transient. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf2062