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Research Fellows Directory



Research Fellow


University of Southampton

Research summary

When a black hole attracts matter around it in the form of a disc (known as accretion disc), it emits X-ray light, and its intensity depends on the rate at which matters accumulate onto the black hole. At a low matter accumulation rate (known as mass accretion rate), X-rays are produced mostly due to the up-scattering of low-energy photons from the disc to the higher energy. Such phenomena are observed from stellar-mass black hole binary systems (BHXBs with the black hole mass from few to tens of solar mass) to the supermassive black holes at the centre of active galaxies known as AGN (a billion solar mass black hole systems). Recent observations found two BHXBs that show the existence of a critical accretion rate across which the nature of X-ray emission spectra changes drastically and indicate two different sources of low energy seed photons for Comptonization. My research would like to answer the challenging question: Does the same change occur in other BHXBs and AGN and, if so, is the critical accretion rate the same in AGN? Satisfactory answers to these questions will have a profound impact on our understanding of both systems. A successful attempt will take us one step further in developing a unification model across all black hole mass scales. Moreover, it will allow us to test for the first time the existence of a critical mass accretion rate in AGN that distinct spectrally harder-when-brighter behaviour at low accretion rate and spectrally softer-when-brighter behaviour at higher accretion rate. To answer these questions, I develop and employ advanced analysis techniques for measuring the spectro-temporal correlated variability, Fourier frequency-resolved rms spectra the time delay between two time-series at different wavelength using data from powerful, currently-operational, multi-wavelength observatories like AstroSat, XMM-Newton, and Swift.

Interests and expertise (Subject groups)

Grants awarded

A new seed-photon based X-ray spectral unification scheme for AGN and black hole X-ray binaries

Scheme: Newton International Fellowships

Dates: Feb 2018 - Jan 2020

Value: £99,000