MSV-2035 Astronomy Document - Inside Design - FINAL - FINAL

Astronomy & Astrophysics 54 Newly discovered Galactic X-ray sources such as Cyg X-1, Cyg X-3, and Sco X-1 were observed using Indian balloon-borne instruments in the 1970s and 1980s, in which the variation of intensity and energy spectrumwere noted. Such observations contributed to the eventual identification of these sources as a binary system containing a neutron star or a black hole. In the 1990s, similar observations with newer telescopes continued. For example, the hard X-ray pulsar GX 1+4 was observed using the balloon-borne Xenon filled Multi-cell Proportional Counter (XMPC) telescope, which provided precise measurement of its pulse period. Using observations from the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3 rapid changes from a high-soft to a low-hard state of the black hole X-ray binary GRS 1915+105 were studied. Presence of an event horizon in this object was also proposed using the observed X-ray burst behaviour. Such observations contributed in characterizing the ongoing dynamical processes and emission mechanisms in the inflow of matter near compact objects. Very detailed theoretical models of the accretion flow around black holes and neutron stars were developed by theorists that were able to explain various observational details such as quasi-periodic oscillations in their emission, and the possible presence of outflow. Pulsars are rapidly spinning neutron stars in which the direction of the magnetic field is misaligned with the rotation axis. Indian astrophysicists established the limit on the accretion driven spin-up of a neutron star known as the 'spin-up line' and predicted the existence of a population of 'spun-up' pulsars that was later validated by the discovery of millisecond pulsars. Based on magnetospheric emission mechanisms it was predicted that millisecond pulsars may be strong emitters of ɤ-rays, which was later confirmed by the observations of Fermi ɤ-ray Space Telescope. Significant inroads have been made into the formation process of neutron stars in interacting binaries, the subsequent evolution of such systems, and stellar interactions leading to the formation of binary pulsars, millisecond pulsars and pulsars in globular clusters. Substantial progress has beenmade in our understanding of the decay of themagnetic fields of single radio pulsar as well as whether themagnetic field evolution in binary systems is driven by accretion. Observations of pulsars in the radio with a view to understand the emission geometry and the underlying physical mechanism has been one of the key science areas carried out with the GMRT. These observations have led to the detection of multiple components in the pulse profile of many pulsars. Relativistic models have been generated to study the pulsar emission and polarisation considering detailed geometry of the emission region, rotation, and modulation. Simulating a set of typical pulse profiles, the role of viewing geometry, rotation, and modulation in the pulsar polarisation profiles has been analysed. In the last decade, Indian astrophysicists have contributed to explaining the physical origin of the cyclotron resonance scattering feature (referred as the cyclotron line) in pulsars and the variation of its peak energy with time due to the growth of a mound on the polar cap of a neutron star due to accretion along the magnetic field lines and subsequent collapse due to instabilities. They showed that the disc flux to total flux ratio may be the parameter which determines the location of a neutron star binary in the hardness-intensity diagram. The spectra of such sources are sometimes dominated by the disc emissionwhile in other cases the Comptonization region obscures the disc. In one source, quasi- periodic oscillations with a frequency ~kHz (kHz QPOs) were observed at energy >10 keV, for the first time, with AstroSat. They addressed a long-standing question: is the pre-burst persistent emission spectrum in such binaries non- changing and may be subtracted as background to estimate the burst spectrum? Using the larger collecting area of AstroSat-LAXPC, sufficient time resolution could be obtained which enabled the finding that persistent emission also MEGA SCIENCE VISION-2035