High Energy Astrophysics

IUCAA has a very diverse and active research group in the area of High energy astrophysics where faculty members work on multiple aspects related to the hot and energetic universe. The group is actively involved in several ongoing and next-generation X-ray space missions. An overview of the different topics is given below:

1. AGN: Active Galactic Nuclei (AGNs) are the most luminous sources in the Universe, which emit across the entire electromagnetic wavelength from radio to gamma-ray. The bolometric luminosity of an AGN ranges from 10^42 to 10^47 erg s-1. A considerable fraction of this bolometric luminosity arises from the accretion disk in the optical/UV band and hot corona in the X-ray band. It is believed that the accretion disk emits thermal photons via accretion of matter from the host galaxy onto a central supermassive black hole. A fraction of these thermal photons then interact with the high-energy electrons of the corona and are up-scattered to the X-rays. The X-ray emission from the corona illuminates the accretion disk and interacts with the disk matter via photo-electric absorption, Compton scattering, fluorescent line emission, and bremsstrahlung processes, and gives rise to the emission lines and reflection spectrum. The interplay between the disk and corona emission provokes a complex spectral and temporal behavior of AGNs. At IUCAA, researchers are continuously working to understand the nature of the disk/corona emission and their causal connections by performing spectral and timing analyses using the multi-wavelength data acquired by various space-observatories such as AstroSat, XMM-Newton, Suzaku, Chandra, Swift, and NuSTAR.
2. Blazars: Blazars are sources where the powerful jet of an AGN is oriented such that it is pointing towards us. Blazar spectra are dominated by the jet emission in all wave bands and  are highly variable.  The emission mechanism is due to synchrotron, synchrotron self-Compton and external photon Comptonization of non-thermal particles. The origin of the jet, its energy source and the physical processes by which the non-thermal particles are produced are largely unknown. At IUCAA, researchers are studying the broad band energy spectra as well as the variability of these sources, to understand these physical processes. This involves detailed data analysis of different wave-bands, developing theoretical models that can explain both the spectra and the variability, and statistical techniques to test them against data.
3. Compact Objects: X-ray binaries are systems where a black hole or a neutron star accretes matter from a companion star. The matter forms an accretion disc around the compact object which in the inner regions produces X-rays. Since the X-rays are produced near the black hole/neutron star they provide important clues to the behavior of matter under the influence of strong gravity and the system becomes a laboratory to test the predictions of General Theory of Relativity.  These sources are also highly variable on time-scales ranging from decades to milli-seconds. The rapid variability implies emission close to the compact object and understanding its origin would enable us to map the extreme space time geometry. Scientists at IUCAA study in detail the X-ray spectra of these sources and the rapid variability, by developing and implementing sophisticated and efficient analysis techniques, as well as constructing theoretical models to explain the extreme phenomena.
4. Population studies of compact objects: Population studies of compact objects like white dwarfs, neutron stars and black holes provide a bridge between stellar physics, galactic astronomy, and cosmology. In the Milky Way, they give detailed local insight into stellar evolution and Galactic history. In external galaxies, they provide statistical samples to understand how environment and galaxy type influence compact object formation and evolution. The group at IUCAA is involved with large X-ray surveys to study population of compact objects with eROSITA, XMM-Newton, Chandra , Swift and NuSTAR observations. Additionally, the group at IUCAA is involved in multi-messenger astronomy and studying the possible association of gravitational wave transients with AGNs and a combined population study.
5. Novae and masses of white dwarfs: Nova is a sudden, bright burst of electromagnetic radiation from a binary system where a white dwarf accretes matter from a companion star, leading to a thermonuclear runaway reaction on its surface. While often discovered in the optical band, their X-ray emission provides the most direct insight into the nuclear burning on the white dwarf’s surface. These X-ray observations allow to probe fundamental physics: accretion processes, surface temperatures, and the timescales of nuclear burning. Further, the duration of the "supersoft X-ray phase" of the nova is a robust indicator to the white dwarfs mass. If the white dwarf is growing in mass toward the Chandrasekhar limit, it could eventually trigger a Type Ia supernova one of the most violent explosions in the universe.
6. eROSITA: In particular, eROSITA X-ray telescope aboard the SRG mission has created the first map of the hot, energetic universe which contains over one million X-ray sources. This has doubled the number of sources discovered in the entire 60-year history of X-ray astronomy. The high energy astrophysics group at IUCAA is actively involved in the compact objects working group under the eROSITA-DE consortium to understand the compact X-ray source population in the Milky Way and nearby galaxies.