Astronomers have uncovered new clues about the origin of a rare and powerful cosmic X-ray flash known as a Fast X-ray Transient (FXT), linking the event to either the collapse of a massive star or the merger of two neutron stars.
The findings could help scientists better understand the physics behind some of the universe’s most extreme and short-lived phenomena.
FXTs are energetic, non-repeating bursts of low-energy X-rays associated with violent cosmic events. First identified about a decade ago, they typically last from a few minutes to several hours before fading rapidly, making them difficult to study and leaving their origins largely uncertain.
Over the years, astronomers have proposed several explanations for FXTs, including shock breakouts from core-collapse supernovae, highly magnetised neutron stars formed after binary neutron star mergers, and tidal disruption events involving white dwarfs and intermediate-mass black holes. While many FXTs have been linked to high-redshift long-duration gamma-ray bursts (GRBs), others show no gamma-ray counterpart, suggesting they may be low-luminosity GRBs or so-called “orphan” afterglows.
A new study led by Deepak Eappachen and Arvind Balasubramanian, postdoctoral fellows at the Indian Institute of Astrophysics (IIA), examined an FXT designated EP241107a, detected on November 7, 2024, by the Chinese-led Einstein Probe mission. The space observatory is designed to monitor the dynamic high-energy sky and detect transient cosmic events.
Using a multi-wavelength observational approach, the researchers identified a radio counterpart to the X-ray flash with the Karl G. Jansky Very Large Array in New Mexico, United States.
The study also drew on observations from several Indian facilities. Scientists used the Himalayan Chandra Telescope (HCT) and the GROWTH India Telescope (GIT) at the Indian Astronomical Observatory in Hanle, Ladakh, to monitor the event in visible light. The HCT is operated by the IIA, while the GIT is jointly managed by the IIA and the Indian Institute of Technology (IIT) Bombay.
Follow-up observations were conducted using the Upgraded Giant Metrewave Radio Telescope, operated by the National Centre for Radio Astrophysics. Additional data were obtained from the 10-metre Keck Observatory in Hawaii and the Southern Astrophysical Research Telescope, a 4.1-metre optical and near-infrared facility in Chile.
By comparing the optical and radio signatures of EP241107a with those of other extragalactic transients and analysing the characteristics of its host galaxy, the researchers concluded that the event was most likely associated with a gamma-ray-burst-like explosion triggered either by the collapse of a massive star or the merger of two neutron stars.
Detailed modelling of the afterglow suggested that the explosion generated a powerful jet with kinetic energy comparable to the amount of energy emitted by all the stars in the Milky Way over several months, assuming the energy was radiated equally in all directions.
The researchers concluded that EP241107a most likely originated from a gamma-ray burst despite the absence of a direct gamma-ray detection. Such events are known as “orphan afterglows” and represent some of the rarest transient phenomena studied in detail. The team suggested that EP241107a may belong to the lower-energy end of the known gamma-ray burst population.
The study has been published in the journal Monthly Notices of the Royal Astronomical Society. The authors include Deepak Eappachen, Arvind Balasubramanian, G.C. Anupama, D.K. Sahu and Sudhanshu Barway from the Indian Institute of Astrophysics, along with Vishwajeet Swain, V. Bhalerao, Tanishk Mohan and G. Waratkar from IIT Bombay.
The international collaboration also involved researchers from the California Institute of Technology, the University of North Carolina at Chapel Hill, and the Center for Astrophysics, Harvard & Smithsonian.
