Astronomers have traced the mechanism behind a mysterious, infrequent celestial flash of X-rays called Fast X-ray Transients (FXTs) that was discovered on November 7 last year. Studying the X-ray flashes they linked to the collapse of a massive star or the merger of two neutron stars could help us better understand the physics of these extreme events.
FXTs are energetic, non-recurring flashes of X-rays associated with violent processes in the universe. They are a new class of transient sources in the sky, first discovered about a decade ago. These mysterious events appear as sudden bursts of low-energy X-rays, lasting from a few minutes to several hours, before quickly fading from view.
Due to their short duration, FXTs have long been difficult to study, making their origins uncertain. Over the years, astronomers have proposed several possible mechanisms to explain these mysterious flashes. Proposed progenitors of these events include supernova shock explosions from collapsing supernovae at their cores, neutron stars with intense magnetic fields rotating every few milliseconds that form following the merger of binary neutron stars, and tidal disruption events involving white dwarfs and intermediate-mass black holes.
Many FXTs are associated with long-period high-redshift gamma-ray bursts (lGRBs), while others lack any gamma-ray counterpart, indicating low-luminosity GRBs or “orphan” flares.
A new study led by Deepak Epachen and Arvind Balasubramanian, both postdoctoral fellows at the Indian Institute of Astrophysics, an autonomous institution under the Department of Science and Technology (DST), focuses on one of the FXTs called EP241107a, which was discovered on November 7, 2024 by the Einstein Probe mission. Designed to survey the dynamic, high-energy skies, this Chinese space mission is transforming the study of short-lived cosmic events.
Using a multi-wavelength approach, the team detected a radio counterpart to the X-ray flash using the Karl J. Janski Very Large Array, located in New Mexico, USA.
To augment this multi-wavelength study, the researchers also used several powerful ground-based facilities in India. At the Indian Astronomical Observatory in Hanle, Ladakh, the Chandra Himalayan Telescope (HCT) and the GROWTH India Telescope (GIT) took advantage of clear skies to observe the event in visible light. HCT is run by the Indian Institute of Astrophysics (IIA) and GIT is jointly run by IIA and IIT Bombay. The upgraded Giant Metrewave radio telescope, operated by the National Center for Radio Astrophysics, was also used to follow up on the observations. In addition, the team used the 10-meter Keck Observatory in Hawaii, along with the Southern Astrophysical Research Telescope, a 4.1-meter optical and near-infrared facility located in Chile.
fig: This image shows the isotope detection (marked by arrows) of the fast X-ray transient EP241107a detected at the GROWTH India Optical Telescope (left) and at the Radio-Very Large Array (right).
By comparing optical and radio observations of EP241107a with those of extragalactic transits, and by analyzing the properties of its host galaxy, the researchers suggest that this event was associated with a gamma-ray burst-like explosion, caused either by the collapse of a massive star or by the merger of two neutron stars. Detailed modeling of the afterglow suggests that the explosion produced a powerful jet with kinetic energy comparable to the total energy emitted by all the stars in the Milky Way over several months, if it is assumed to be emitted equally in all directions.
The researchers concluded that EP241107a most likely has a gamma-ray burst origin. EP241107a represents one of the rarest transient events studied in detail: a burst that has not been directly detected in gamma rays, yet is clearly linked to the origin of a gamma ray burst, sometimes referred to as an “orphan aurora.” They explain that this event could represent a gamma-ray burst at the lower-energy end of the known gamma-ray burst spectrum.
This study, published in the Monthly Notices of the Royal Astronomical Society, was authored by Deepak Epachin, Arvind Balasubramanian, J.C. Anupama, D.K. Sahu, and Sudhanshu Barwai of the Indian Institute of Astrophysics, and Vishwajit Swain, V. Bhalerao, Tanishk Mohan, and J. Waratkar from the Indian Institute of Technology, Bombay.
The international team also includes researchers from the California Institute of Technology, the University of North Carolina at Chapel Hill, the Center for Astrophysics at Harvard University and the Smithsonian.
Link to the paper: https://academic.oup.com/mnras/article/545/1/staf2062/8339698
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