SK Minhajur Rahaman

Research Scholar
Email: rahaman [at] ncra.tifr.res.in
Phone: +91 - 20 - 25719446
Extn: 9446
Office: F254
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA


Main Research Areas: Pulsars; Pulsar Radio Emission Mechanisms

Supervisor: Dipanjan Mitra

Biography:

Minhajur Rahaman obtained his B.Sc degree in Physics from St. Xavier's College, Kolkata, in the year 2015. Following this, he joined NCRA-TIFR as an Integrated PhD student.  Having completed the requirements of the NCRA TIFR Graduate School in 2018, he is now working with Dipanjan Mitra for his doctoral research.

Research description:

Pulsars are highly magnetized (~10^6 to 10^9 Tesla), fast rotating (~ few millisecond to seconds) neutron stars which can emits beams of electromagnetic waves (from radio to gamma rays).  We observe a pulse of emission every time the beam sweeps across our line of sight. Pulsars lose their rotational kinetic energy as electromagnetic radiation and winds in a process called "spin down". Radio emission (observed from tens of MHz to tens of GHz) accounts for less than 1% of this energy loss.

However, the pulsar radio emission is characterized by very high brightness temperature (an equivalent blackbody temperature for a given intensity in the Rayleigh-Jeans limit) which is at least 18 orders of magnitude higher than the Compton limit ( maximum brightness temperature expected from a incoherent radio synchrotron source). This suggests both a non-thermal origin and a "coherent" emission mechanism, i.e the emission is not due to single particle processes but requires collective in-phase emission from charged particles.

For normal pulsars (having periods greater than 100 ms), a plethora of observational evidence suggests that the radio emission is excited by coherent curvature radiation from "charged" bunches ( i.e. a collection of charges emitting coherently at wavelengths larger than the size of the bunch) flowing relativistically along the open dipolar magnetic field lines. In the strong electric and magnetic fields near the magnetic pole, energetic gamma-ray photons are split into electron-positron pairs which are then accelerated to relativistic speeds. The growth of plasma instabilities in this relativistic electron-positron plasma is supposed to give rise to these "bunches". The origin and the stability of these "bunches" are not very well understood and a self-consistent theory is yet to be found. Even after 5 decades since their discovery, the pulsar radio emission mechanism remains an open problem.

In my doctoral research, I will be studying theoretical aspects of how charged bunches can be produced using the linear and especially non-linear plasma processes, as well as propagation effects in the relativistic plasma of the pulsar magnetosphere. The primary aim will be to identify physical mechanism for the charge bunch formation and how can they explain the radio observational data.

Selected publications:

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