In a study recently accepted in The Astrophysical Journal, an international team of scientists has utilized AstroSat’s full multi-wavelength capability, featuring all four co-aligned payloads, to paint a comprehensive portrait of an X-ray binary system hosting a black hole. By capturing soft and hard X-ray emissions with its three X-ray payloads and far ultraviolet radiation with its UV telescope, AstroSat has unveiled a treasure trove of insights into both the near and distant regions surrounding the black hole in the X-ray binary system MAXI J1820+070. Additionally, optical data from Las Cumbres Observatory and soft X-ray data from NASA’s NICER mission further bolstered AstroSat’s findings. This study marks a pivotal achievement in the history of AstroSat as the first instance where its full multi-wavelength capabilities have been harnessed. The collaborative team includes researchers from India, the United Kingdom, Abu Dhabi, and Poland.

The Universe is full of mysteries, and photons are the main clues to unlock them. The higher the energy of photons, the more violent the circumstances in which they originate. The most energetic are gamma-ray photons that have wavelengths smaller than the diameter of an atomic nucleus. On Earth, gamma rays are produced by nuclear fusion/fission, lightning, and radioactive decay. In the Universe, they originate in the most extreme and explosive astrophysical environments, such as those associated with neutron stars and pulsars, supernova explosions, and regions around black holes.

Two IUCAA astronomers, Prof. Vaidehi S. Paliya and Prof. Dhruba J. Saikia, have recently identified a gamma-ray emitting collisional ring galaxy system, “Kathryn′s Wheel”, located at a mere 30 million light years from the Milky Way. Such an enigmatic structure forms when a smaller ‘bullet’ galaxy pierces through another larger galaxy close to its center. The produced shock wave sweeps up and kicks the interstellar gas out of the system, leaving behind a ring of star-forming regions and a gas-poor galaxy. The best example of such a system is the Cartwheel galaxy (www.youtube.com/watch?v=GoD1MSveHgA&t=88s). The research finding by Prof. Paliya and Prof. Saikia titled “A Gamma-ray Emitting Collisional Ring Galaxy System in our Galactic Neighborhood” has just been published in the prestigious Astrophysical Journal Letters (https://doi.org/10.3847/2041-8213/ad4999).

The radio, optical, and ultraviolet observations of “Kathryn′s Wheel” have revealed intense star-forming activities surrounding the central spiral galaxy, possibly triggered by galaxy collision. The ‘bullet’ galaxy itself is undergoing rapid star formation.

The star-forming regions are the sites where the most energetic particles, also known as cosmic rays, are produced. The interaction of cosmic rays with the surrounding interstellar gas and radiation fields could produce the gamma-ray emission observed from such objects. Therefore, the gamma-ray radiation detected from “Kathryn′s Wheel” has been suggested to be produced by star-forming activities. The data taken by NASA′s Large Area Telescope instrument onboard the Fermi Gamma-ray Space Telescope was used to confirm the gamma-ray detection of this enigmatic galaxy collision. “This is the first-ever identification of a collisional ring galaxy system detected in the gamma-ray band and thus highlights the importance of high-resolution, multiwavelength observations needed to unravel the astrophysical treasures hidden in the cosmos”, says Prof. Dhruba J. Saikia.

Although located in our Galactic backyard, this object has been little studied because of the presence of a bright nearby star, HD 150915, and its location in a crowded, low-latitude area close to the Galactic plane, where identifying astrophysical sources is challenging due to extreme dust obscuration. “Kathryn′s Wheel” can be considered a test bed to explore the origin and transport of cosmic rays and their connection with star-forming activities given its proximity to the Milky Way”, says Prof. Vaidehi S. Paliya.

Research contacts:

	Prof. Vaidehi S. Paliya IUCAA, Pune E-mail: vaidehi.paliya_at_iucaa.in Phone: +91-20-2560 4220
	Prof. Dhruba J. Saikia IUCAA, Pune E-mail: dhrubasaikia_at_iucaa.in

* please change _at_ to @