Poonam Chandra

My research interests lie in understanding the explosive transient phenomenon such as supernovae and gamma ray bursts (GRBs), and the massive stars which give rise to these events.


Magnetic Massive Stars:

The hot and massive stars ($\ge 8 M_\odot$) are the energetic and nucleosynthetic engines of galaxies. All massive stars evolve towards an unavoidable explosive end, representing one of the most exotic and energetic events in the universe, known as supernovae and GRBs. In recent years with three-dimensional general-relativistic magneto-hydrodynamic simulations,  it has become clear that magnetic fields, rotation and asymmetry play a crucial role towards the end evolution of massive stars. Observational and theoretical studies have revealed that the stellar wind properties of magnetic stars with are modified in important ways as compared to the winds of non-magnetic stars, introducing significant changes across the entire electromagnetic spectrum. While radio emission from non-magnetic hot stars is expected to be thermal free-free emission from the ionized wind in the circumstellar environment, however, in the presence of magnetic fields electrons can be accelerated to sub-relativistic energies giving rise to gyrosynchrotron radio emission. Gyrosynchrotron radio emission at different frequencies come from different depths, with emission at low frequencies coming from further away from the star. Thus studying the radio emission at different frequencies reveal the 3-D magnetic field topology of the star, which could be a crucial input in the simulations of massive star explosions.

During the last decade, theoretical and observational resources have been leveraged to yield significant progress in understanding the basic physics of massive-star magnetospheres. We are carrying out a systematic survey of studying all magnetic massive stars at low frequencies with the GMRT and high frequencies with the VLA, to characterize the radio emission properties of their magnetospheres at very low frequencies, and understand magnetospheric properties.



In a supernova explosion, much of a star's material (ejecta) is expelled with a velocity up to 30,000 km/s, driving a strong shock wave into the surrounding circumstellar medium (CSM) created by the winds from the progenitor star moving with 10--100 km/s. Since the shock velocities are typically 100--000 times the speed of the progenitor wind, the shock wave probes the past history of the star by sampling regions of the wind lost many thousands of years ago. The shock interaction with the CSM gives rise to radio and X-ray emission. Both radio and X-ray data trace the supernova ejecta and the surrounding density environments created by the progenitor star during its evolution. We are carrying out the low-frequency GMRT study of core collapse supernovae to trace the density environments and progenitor history of these supernovae.

We have also been working towards understanding a class of supernovae exploding in extremely
dense environments (named Type IIn supernovae or SNe IIn). Unlike other class of core-collapse supernovae, this class of is the most puzzling subclass of core-collapse supernovae where extreme external environments play the crucial role in energetics and evolution of these SNe IIn. However, neither their evolutionary status nor the origin of their extremely dense environments caused by tremendous mass loss of their pre-supernovae progenitors are known. Luminous blue variables (LBVs), pair-instability, common envelope etc. are some of the models which are invoked to account for the overwhelming density in SNe IIn. Radio studies can constrain the circumstellar density, and X-ray studies determine the shock temperature and also provide unique constraints on the ejecta structure. Optical observations illustrate the temporal evolution and, when combined with other observations, can reveal the chemical composition of the ejecta, and the nature of the progenitors. Hence, understanding the SNe IIn phenomenon requires a complex observational study of their optical, infrared, X-ray and radio emission, and modeling of the emission characteristics in terms of physical models that include ejecta and the surrounding medium properties (density, extension, clumpiness, explosion asymmetry). We are using GMRT, VLA, Chandra, XMM-Newton and Swift telescopes to carry out the above studies.

Gamma Ray Bursts:

It is still a debate as to under what circumstances the stellar collapse of a massive star will power a relativistic (Lorentz factor $\Gamma \sim$ few hundred) collimated explosion, i.e. gamma ray burst (GRB). Though a typical GRB explosion lasts from milliseconds to several minutes, the afterglow emission can be seen for few days, few weeks and few months in X-ray, optical and radio bands, respectively. The relativistic ejecta interacting with the surrounding gives rise to a forward shock moving into the ambient circumburst medium and powering afterglow emission, and a reverse shock going back into the ejecta. While the forward shock constrains the properties of the circumburst medium (medium surrounding the GRB), the reverse shock emission uniquely probes the composition and properties of the ejecta, such as thickness and Lorentz factor.

We are mainly studying the GRB afterglows in radio bands. A major advantage of radio afterglow emission is that due to slow evolution it peaks at much later time and lasts longer, for months or even years. Thus unlike short-lived optical or X-ray afterglows, radio observations present the possibility of following the full evolution of the fireball emission from the very beginning till the non-relativistic phase, when the geometry of collimation becomes insignificant and energetics can be determined more accurately. Therefore, the radio regime plays an important role in understanding the full broadband spectrum. This constrains both the macrophysics of the jet, that is, the energetics and the circumburst medium density, as well as the microphysics, such as energy imparted in electrons and magnetic fields necessary for synchrotron emission. Some of the phenomena routinely addressed through radio observations are interstellar scintillation, synchrotron self-absorption, forward shocks, reverse shocks, jet breaks, non-relativistic transitions, and obscured star formation.


TeV Supernova Remnants:

When a supernova ejecta accumulates enough circumstellar medium and starts to interact with the interstellar medium, it makes a transition to supernova remnant (SNR). SNRs are considered to be instrumental in Galactic cosmic rays production. Galactic cosmic rays are energetic particles believed to be mostly composed of hadrons with energies ranging from $\sim$ 0.1 to $10^{8-9}$ GeV. How are these particles accelerated to these high energies remains a fundamental question. SNR shocks are considered as one of the probable candidates of these particle acceleration sites. Association of spatially resolved shell-like gamma -ray sources with SNRs (which will mainly be seen in radio bands) is the signature of particle acceleration at supernova shocks. Studying these objects provides a unique opportunity to probe the cosmic ray particles that emit
high-energy gamma -rays.

However, so far, there are only five spatially resolved shell-like VHE sources firmly identified as SNRs. We are systematically studying the TeV SNRs at the GMRT frequencies tounderstand the connection between TeV and radio emission. Under this project, we have recently discovered a shell type TeV supernova remnant SNR G353.6-0.7 in the GMRT 325 MHz and 610 MHz band. Our radio observations suggest inverse-Compton origin of gamma rays.



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