Poonam Chandra

Reader - F
Email: poonam [at] ncra.tifr.res.in
Phone: +91 - 20 - 25719290
Extn: 9290
Office: 204 (A)
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA

Main Research Areas: Circumstellar interaction in Supernovae; Gamma Ray Bursts Afterglows; Electromagnetic counterparts of gravitational wave sources; Radio magnetospheres of massive stars; Acceleration mechanisms in supernova remnants.


Poonam Chandra did her B.Sc. Hons. (Physics) from DayalBagh Educational Institute, Agra, and her M.Sc. (Physics) from Aligarh Muslim University. In 1999, she joined the Joint Astronomy Programme (JAP) run by the Indian Institute of Science, Bangalore, for her Ph.D.. In 2000, she moved to the Tata Institute of Fundamental Research, Mumbai, to pursue her JAP Ph.D. project on radio and X-ray studies of circumstellar interaction in supernovae. She obtained her Ph.D. in 2005 and then went to the University of Virginia, Charlottesville, as a Jansky post-doctoral fellow. She moved to Canada in 2008 as a research associate at the Royal Military College of Canada, Kingston. Poonam joined the National Centre for Radio Astrophysics in August 2012 as a Reader. She is currently on sabbatical, from August 2017 until May 2018, as a Guest Professor at Stockholm University.

Research description:

Circumstellar interaction in Supernovae:
Massive stars are generally expected to undergo a violent end once their nuclear fuel is exhausted. These violent explosions are termed as supernovae. In a supernova explosion, when the fast moving supernova ejecta collides with the circumstellar medium (CSM) created from the mass loss, a hot forward shock, moving with a velocity of ~10,000 km/s, and a reverse shock propagating into the stellar envelope are created. The heated shocked shells emit in X-rays, giving estimates of the densities and temperatures in the shocked ejecta and the CSM. The electrons undergo acceleration to relativistic energies in the forward shocked shell in the presence of the amplified magnetic field and give rise to synchrotron radio emission. One of my main research interests is to study these biggest explosions and understand their environments carrying the footprints of their progenitor. I am particularly interested in radio and X-ray aspects of these explosions. A special class of supernovae termed as Type IIn supernovae has been found to have progenitors with extremely high mass loss rates. Such mass loss rates are not predicted in standard stellar evolution theories. There are two main proposed channels of this high mass loss, luminous blue variables in their outbursts, or instability in the evolution of the star just before the explosion, enhancing the mass loss rate. The way to resolve this issue is to study the radio and X- ray emission created due to the ejecta interacting with the CSM, carrying unique footprints of the mass loss evolution of the progenitor star. Since the synchrotron peak moves to lower and lower frequencies with time, the best strategy to understand the mass loss evolution history of the SN progenitor star is to observe it at high frequencies at early epochs and at low frequencies at later epochs.
Radio afterglows of gamma ray bursts:
Gamma Ray Bursts (GRBs) are intense flashes of light for a short duration (a few msec to a few tens of seconds). Long duration GRBs are explained by the fireball internal-external shock model, in which a compact source gives off relativistic shells of variable energy and momentum which subsequently collide (internal shocks) giving rise to prompt gamma-rays. The hot, shocked matter acquires embedded magnetic fields and accelerated electrons, which then produce the radiation we see via synchrotron emission. While observations by the Swift and Fermi satellites have significantly advanced our physical understanding of GRBs, they have also raised fundamental questions about the origin of the prompt emission, the nature of the central engine(s), and the character of the relativistic outflow. The progenitors and environments of GRBs remain poorly understood. As the afterglow emission peaks at lower and lower frequencies at later and later times, radio observations offer the opportunity to follow the full evolution of the fireball emission through all of its different stages. The early time interstellar scintillation provides size constraints on the fireball, whereas the synchrotron self-absorption by the optically thick medium, which is mainly observed in the radio bands, provides density constraints. As the fireball transitions from the ultra-relativistic phase to the sub-relativistic phase and jet spreads sideways, afterglow emission visible only in radio bands provides strong constraints on the energetics independent of the jet geometry. I have been using the Very Large Array, the Giant Metrewave Radio Telescope, and various publicly available telescopes such as Swift, etc. to carry out these studies.
Electromagnetic counterparts of gravitational wave sources:
Gravitational waves (GW) were first directly observed in September 2015 by the Advanced Laser Interferometer GW Observatory (LIGO). While this event was from the merger of two black holes, it was followed by a broadband observing campaign at various electromagnetic (EM) bands. Even though no EM counterpart was detected, consistent with models of black-hole mergers, it paved the way for the beginning of multi-messenger astronomy. A major breakthrough came when, on August 17, 2017, a GW event (GW 170817) lasting for about 100s was detected by the Advanced LIGO and Advanced Virgo detectors. We started an extensive radio campaign with the GMRT, the VLA and the ATCA, following the source extensively. Our GMRT detection is the lowest frequency EM counterpart of this neutron star merger event. Our radio observations have revealed that the radio emission is consistent with a mildly relativistic outflow instead of a relativistic jet. We are continuing to follow this event at late epochs. In collaboration with various teams worldwide, we are preparing ourselves to follow future such events with the GMRT and other telescopes through time-critical observing proposals.
Radio magnetospheres of massive stars:
Hot, massive OB stars are the energetic and chemical engines of galaxies. Their high luminosity lights up and ionizes the nearby interstellar medium, and drives strong, high-speed stellar winds, resulting in mass outflows. This mass loss, combined with rapid, sometimes near-critical stellar rotation, can exert a strong, even dominant, influence on the formation and evolution of such massive stars, and on their demise as supernovae or gamma ray burst (GRB)-producing hypernovae. But recent advances in observations and theory indicate a third agent - magnetic fields - can also play a key role. The stellar wind properties of OB stars with magnetic fields are modified in important ways. Several of these magnetic stars have revealed non-thermal gyro-synchrotron emission in the radio bands. The relationship between the radio flux variation and the magnetic field encodes information about the physical origin of the emission and/or the geometry of the radio-emitting plasma. The variable radio emission of magnetic massive stars therefore provides a changing view of the radio magnetosphere as the star rotates. In addition to the widespread detection of gyro-sychrotron emission, highly directional coherent emission has also been detected in a small number of magnetic stars. Recently, coherent radio emission was discovered from the young magnetic B star HD 133880 at 610 MHz band with the GMRT by our team. The emission was highly directional and coincided with a null of the longitudinal magnetic field. We attributed this phenomenon to Electron Cyclotron MASER Emission (ECME). This is only the second hot magnetic star (after CU Vir) in which coherent radio emission has been detected. Characterization of ECME emission provides qualitatively new constraints on the properties of the magnetically-confined plasma surrounding the star. Moreover, because ECME produces pulsed emission, it can serve as a sensitive tracer of rotational period evolution. We are carrying out a systematic radio study with the GMRT and the VLA in order to understand complex magnetosphere properties of massive stars and their winds.
Acceleration mechanisms in supernova remnants:
Recent observations of young supernova remnants (SNRs) in the gamma-ray domains have raised several questions and triggered numerous theoretical investigations such as, when do the particle energies reach maximum, during the free-expansion phase or during the Sedov stage? How do cosmic rays escape from a SNR, what is the dynamics of escape, i.e. how does the maximum energy evolves with time? What is the primary particle population producing the gamma-ray emission? The first two questions are intimately connected with the intensity of the magnetic field, and hence with the maximum acceleration energies which are constrained by radiative losses and synchrotron radiation and hence by radio emission. The third one can be traced efficiently thanks to the detection of gamma rays in the high energy range with the Fermi-LAT or in the very-high-energy energy range with HESS. Multi-wavelength data, and especially radio and gamma-ray data, are thus crucial to understand the nature of these efficient particle accelerators in our Galaxy. We are investigating how young core-collapse supernova shocks accelerate Cosmic Rays (CR -- electrons or protons) to very high energies and how the acceleration efficiency evolve as the SN ages and moves to the supernova remnant stage.

Selected publications:

1: Circumstellar Interaction in Supernovae in Dense Environments—An Observational Perspective (P. Chandra, 2018, Space Science Reviews 214, 27)

2: Discovery of electron cyclotron MASER emission from the magnetic Bp star HD 133880 with the Giant Metrewave Radio Telescope (B. Das, P. Chandra, & G. A. Wade 2018, MNRAS Letters 474, L61)

3: 325 and 610 MHz radio counterparts of SNR G353.6-0.7 also known as HESS J1731-347 (Nayana, A. J., P. Chandra, S. Roy et al. 2017 MNRAS 467, 155)

4: Detection of 610-MHz radio emission from hot magnetic stars (P. Chandra, G. A. Wade, J. O.
Sundqvist et al. 2015, MNRAS 452, 1245)

5: Strong Evolution of X-Ray Absorption in the Type IIn Supernova SN 2010jl ( P. Chandra et al. 2012, ApJ Letters 750, L2)

6: A Radio-selected Sample of Gamma-Ray Burst Afterglows (P. Chandra & D. A. Frail, 2012, ApJ
746, 156)

7: Discovery of Radio Afterglow from the Most Distant Cosmic Explosion (P. Chandra, D. Frail, &
D. Fox et al. 2010 ApJ Letters 712, L31)

8: A Comprehensive Study of GRB 070125, A Most Energetic Gamma-Ray Burst (P. Chandra, B. Cenko, D. Frail et al. 2008, ApJ 683, 924)

9: Detection of Circumstellar Material in a Normal Type Ia Supernova (F. Patat, P. Chandra,  R. Chevalier et al. 2007, Science 317, 924)

10: Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806 - 20 (P. B. Cameron, P. Chandra, A. Ray  et al. 2005 Nature, 434, 1112)