The Milky Way

(Poonam Chandra, Jayaram N. Chengalur, Subhashis Roy, Nissim Kanekar, Former members: A. J. Nayana, Nimisha Kantharia, Swarna Kanti Ghosh, Narendra Nath Patra, Barnali Das, Ayan Biswas)

Understanding the physical conditions in the Milky Way, that stem from the interactions between the stars and the gas is an important area of research in astronomy and at NCRA-TIFR. Radio imaging and spectroscopy offers insights into conditions in the Milky Way that are not available at other wavelengths. An active area of research is the centre of the Milky Way (the ``Galactic Centre'' region), which is believed to be dominated by a compact, supermassive black hole with a mass of about two million solar masses. The Galactic Centre region also contains massive stars and strong star formation activity, as well as large molecular gas clouds and ionized regions, all of which can be studied at radio wavelengths to glean information on local physics.

Astronomers at NCRA-TIFR use deep radio continuum imaging studies to trace ionized gas structures arising from supernova remnants and ionized hydrogen regions in the Milky Way, and to derive densities, temperatures, and energetics therein. Low-frequency surveys for supernova remnants have been used to probe the star formation history of the Milky Way. Attempts are also under way to understand acceleration mechanisms in supernova remnants, via a combination of radio and gamma-ray studies. NCRA-TIFR astronomers also carry out research on Galactic objects like novae, which are bright explosions of compact stars, and on the magnetospheres of massive stars. There is also significant interest in black-hole X-ray binary systems, with radio monitoring studies used to probe conditions during the outburst states, when there is increased accretion onto the black hole.

Another important area of research at NCRA-TIFR is the interstellar medium (ISM) of the Milky Way and external galaxies, which consists of various gas phases, neutral atomic, ionized, and molecular gas, at different temperatures, pressures, and densities. The neutral atomic ISM is best probed with the hyperfine spectral line of neutral hydrogen at a rest wavelength of 21.11 cm. NCRA-TIFR astronomers have long used this spectral transition to study physical conditions in the neutral gas in the Milky Way. They have found evidence for a phase transition in the gas above a critical atomic hydrogen column density, as well as for a significant fraction of the neutral gas lying in the thermally unstable phase, and continue to work on the existence and nature of the equilibrium between the different gas phases in the Galaxy.

Recent Results
GMRT monitoring of the X-ray binary V404 Cygni during its June 2015 outburst
Chandra & Kanekar used the GMRT at 1280, 610, 325 and 235 MHz to monitor the black hole X-ray binary V404 Cygni during its 2015 June outburst, extending for a period of 2.5 weeks, and beginning on June 26.9 UT, a day after the strongest radio/X-ray outburst. They find the low-frequency radio emission of V404 Cygni to be extremely bright and fast-decaying in the outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate X-ray state. The radio emission settles to a weak, quiescent state roughly 11 days after the outburst, with a flat radio spectrum and a soft X-ray state. Combining the GMRT measurements with flux density estimates from the literature, the authors identify a spectral turnover in the radio spectrum at ~1.5 GHz on June 26.9 UT (see the attached image), indicating the presence of a synchrotron self-absorbed emitting region. They use the measured flux density at the turnover frequency with the assumption of equipartition of energy between the particles and the magnetic field to infer the jet radius, magnetic field, minimum total energy, and transient jet power. The relatively low value of the jet power, despite V404 Cygni’s high black hole spin parameter, suggests that the radio jet power does not correlate with the spin parameter.
GMRT imaging of a high-energy supernova remnant
Nayana et al. used the Giant Metrewave Radio Telescope (GMRT) to detect 325 and 610 MHz radio emission from HESS J1731-347, one of only five known very-high-energy (VHE; > 0.1 TeV) shell-type supernova remnants (SNRs). Multiple filaments of the SNR are clearly seen in the GMRT 610 and 325 MHz images, shown, respectively, in the left and right panels of the adjacent figure. However, the faintest feature in the GMRT bands corresponds to the peak in the VHE emission. This anti-correlation can be explained if the observed VHE gamma-ray emission has a leptonic origin. The individual filaments of the SNR (indicated by "1", "2", "3", and "4") have steep radio spectra, consistent with a non-thermal origin.