N. G. Kantharia
Abstract.
Giant Metrewave Radio Telescope (GMRT) which operates at
wavelengths longer than 20 cm (frequencies ≤ 1.4 GHz) has been used to search for
radio emission from Galactic novae systems since 2002.
Of the 11 Galactic novae observed with GMRT,
radio continuum emission has been detected in two of the systems
whereas atomic gas associated with two systems has been imaged and studied
in the 21 cm signal of Hi.
The two novae studied in the radio continuum with
the GMRT are the remnant of GK Persei, a classical nova which
had an outburst in 1901 and
RS Ophiuchi, a recurrent nova following its last outburst in 2006.
Combining the GMRT data on the classical nova GK Persei
with VLA data at earlier
epochs resulted in concluding that the nova remnant was undergoing
a secular decrease in its flux density and in its adiabatic
phase of evolution \citep{2005A&A...435..167A}. RS Ophiuchi was observed
at 1280, 610, 325 and 240 MHz with the GMRT days after its
outburst and detected at all the
observed frequencies. The near-simultaneous
monitoring of its flux density at the low GMRT frequencies, resulted in
the study of its spectral index which was indicative of synchrotron
emission at all epochs \citep{2007ApJ...667L.171K}. A supernova model
resulted in a reasonable fit to the observed light curves;
in particular the late appearance of emission at the lower
GMRT frequencies due to the foreground clumpy, ionized, thermal circumbinary
material. Comparison of these results with the previous outburst indicated
that the densities of this clumpy medium had
reduced making it optically thin to GMRT frequencies
in 2006 \citep{2007ApJ...667L.171K}.
It is important to complement the higher radio
frequency studies with observations at GMRT frequencies since these
study different regions and physics of the nova system.
Studies at GMRT frequencies can result in insights on the shock physics, distribution and
density of the circumbinary material or planetary nebula, magnetic field
generation and the spectral index evolution. Since the evolution of a nova system is
faster than a supernova and novae are more numerous; these can be studied
over shorter timescales. A sensitivity limit of 1 mJy
can detect radio emission at GMRT frequencies upto a distance
of 10 kpc, if the non-thermal luminosity of the novae system is
1013 W Hz-1.
Out of the total of about 33 novae detected in the radio bands, 9 have shown
the presence of non-thermal emission in their spectra and 4 of these
are recurrent in nature.
GMRT frequencies are ideal to observe the non-thermal emission
from the recurrent nova population
as the ejecta expands driving shocks into the dense circumbinary material
from the giant companion.
An important motivation for studying the non-thermal radio emission
from recurrent novae is to interrogate any evolutionary connection to
the lack of detectable radio emission from type 1a supernova systems which
recurrent novae are believed to evolve into and subsequently lend support to
this model.
Keywords: (stars:) novae, cataclysmic variables -- (stars:) white dwarfs -- radio continuum: stars
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