Top panel: GMRT and VLA data showing steepening in the
spectra of SN 1993J (Chandra et al. 2004). Lower panel: Th efits to only GMRT data and only VLA data. In absense of wide band spectrum, both measuremnts would have resulted in faulty
estimations of the synchrotron spectrum.
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With the radio spectrum of a young supernova, we can probe the
conditions in the magnetized plasma where this radiation originates
from relativistic electrons. These electrons are believed
to be accelerated in the interface region of the supernova blast
wave shock and the circumstellar medium.
The most critical parameter of the plasma that affects the synchrotron radiation spectrum is the strength of the magnetic field. This is often estimated indirectly under assumptions of equipartition of energy between the magnetic fields and that of relativistic particles or by fitting the radio flux density and turnover wavelength. In many classical radio sources, such as supernova remnants like the Crab or Cassiopeia A, or in luminous radio galaxies, the radio spectral index is found to steepen at high frequencies.
This is due to the so-called synchrotron aging of the source, as during the lifetime of the source, electrons with high-enough energies in a homogeneous magnetic field will be depleted because of efficient synchrotron radiation compared with the ones with lower energies. An observation of a synchrotron break can yield a measurement of the magnetic field independent of the equipartition argument if the age of the source is known.
Multifrequency radio studies of a supernova such as SN 1993J, which was bright enough offered
such a possibility.
The near-simultaneous spectrum of SN 1993J obtained by combining the Giant Metrewave Radio Telescope (GMRT) low-frequency data with the Very Large Array (VLA) high-frequency data around day 3200 since explosion showed signature of synchrotron aging, which enabled us to estimate the magnetic field in a young supernova independent of equipartition assumption, for the first time.
Our measurements yielded the magnetic field of the order of ~0.3 milli Gauss as opposed to the equiparition values of 0.034 milli Gauss. This indicated the the actual magnetic field to be order of magnetic higher
than the value one would have estimated using the equipartition assumption, and would have
led to wrong estimations of the physical properties of the supernova shock.
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