1. An international research team led by Dr. Surajit Paul of Savitribai Phule Pune University (SPPU) who is a Visiting Associate of IUCAA, Pune has for the first time revealed the entire process of production of Cosmic rays in the Galaxy Clusters.
2. By creating many realizations of our Universe on IUCAA's High Performance Supercomputing system meant for the use of Indian Universities, this research closely studied the collisions of massive clusters of galaxies, which provided important insights into the long-standing puzzle of non-detection of Gamma rays from the clusters by understanding their brightest Cosmic ray injection phases. This research has recently been published in 'Monthly Notices of Royal Astronomical Society, London' (MNRAS, 2019,vol.488, no.1,page 1301-319; https://doi.org/10.1093/mnras/stz1785).
3. No scientific observation till now has conclusively identified galaxy clusters as the high energy Gamma-ray sources, even with the most sensitive Gamma-ray telescopes in the world such as FERMI-LAT, HESS, MAGIC, etc. The non-detection was either explained by low resolution and sensitivity of the available instruments or due to the inadequate understanding of the particle acceleration mechanisms. But, they found out that the problem lies with the wrong choice of targets for deeper studies.

Galaxy clusters are the largest known objects in the universe with a size of about 10²⁰ km and mass of 10⁴⁵ kg that consists of thousands of galaxies like milky-way. These structures form through the various energetic process such as gigantic mergers of clusters, the known most energetic process in the Universe expected to produce extremely high energy particles such as X-rays, Gamma-rays etc. Theoretically, thus they are largest particle accelerators in the universe and reservoir of Cosmic rays of highest energies of around 10¹⁹ eV.

This important research was possible because of the availability of the required huge computational facility of High-Performance Computing of IUCAA. Dr. Paul and his former Ph.D. student Dr. Reju Sam John, earlier from University of Pondicherry and now a postdoctoral research fellow at IUCAA said, “It means a lot when you produce cutting-edge science after extensive use of your own systems available in national research institutions like IUCAA. This experience strengthened our confidence to test and build the indigenous capabilities to meet global standards and go beyond.”

Dr. Somak Raychaudhury, Director, IUCAA said, “The Earth receives some very high energy particles called cosmic rays, which bombard the Earth's atmosphere, and the most energetic ones reach the surface of the Earth. We have been studying these for the past century, and we know that many come from the Sun. However, the source of the highest energy particles has always been a mystery, and it is clear that they originate in the most energetic phenomena in the Universe. I am glad that our computers, such as the ones at IUCAA Pune, are now powerful enough that simulations of the highest energy events in the Universe can be performed, to check whether these ultra-high-energy particles are coming from them.”

In the initial study of collisions of clusters, they found that the clusters that are clashing with each other during their formation, accelerates the highest energy Cosmic rays and becomes extremely luminous in contrast to the non-merging clusters. The particle acceleration in these sources happens because of shock formation during the mergers. The bulk velocity of the cluster medium in this situation exceeds the sound velocity in the same medium i.e. becomes super-sonic. Charged particles such as electrons and protons that are abundant in cluster medium, when passes through those shocks, takes up some energy from the shocks and get accelerated continuously until the particles gain enough energy to diffuse out of the medium. This method is known as Diffusive Shock Acceleration, first explained by Nobel laureate, Enrico Fermi.

Dr. Paul's team has implemented this theory in the computer codes to compute the emitted power of cosmic rays from the galaxy clusters that gave a new insight to the problem of non-detection of Gamma-rays from these sources. They said, “the problem lies with the wrong choice of targets for deeper studies made so far. We know very little about the energy evolution in these systems. So, studying the complete event of energy evolution during the cluster formation is absolutely necessary to pinpoint where to look into”. Therefore, the results from Dr. Paul's study intrigue further investigation on making proper selection strategies for a successful Gamma-ray detection from clusters and thus giving important insight into this long-standing puzzle of non-detection of Gamma rays in clusters.

Galaxy cluster formation process happens in cosmological time scales, usually a few times Giga (10⁹) years. This is an impossible task for the human being to follow such evolution process through observations. Both the time scales and the size of these systems also make it absolutely impossible to study these processes in laboratories. The only tool available is the theoretical simulations using super-computers. Computing algorithm for Cosmic Ray production from galaxy clusters was developed by this research team. The same was implemented over the basic Codes (ENZO & yt-tools) which was developed by the global scientific community for computing cosmological structures.

Research group led by Dr. Paul processed their codes in the IUCAA HPC facility and produced the highest ever resolved cosmological simulations in India. This type of simulation is also among the top few in the world in the context of study of formation of Large Scale structures existing in the universe. The highest resolution realizations used for this study took about 3 months with 1000 CPU's working in parallel. In other words, with a single high-end workstation it would have taken 250 years to compute a single such computer realization of a large volume of the universe.

Research contacts:

	Prof. Somak Raychaudhury Director, Inter University Center for Astronomy and Astrophysics, Tel.: +91-20-25604-231 (Off.) E-mail: somak_at_iucaa.in
	Dr. Surajit Paul Assistant Professor at Savitribai Phule Pune University (SPPU), Pune and Visiting Associates at Inter University Center for Astronomy and Astrophysics, Tel.: +91-9405510226 (Mob.) E-mail: surajit_at_associates.iucaa.in
	Dr. Reju Sam John Post Doctoral Fellows, Inter University Center for Astronomy and Astrophysics Tel.: +91-20-25604- 208 (Off.) & +91-7020103007 (Mob.) E-mail: reju_at_iucaa.in

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