Low frequency carbon recombination lines with wavelengths up to twenty five meters became important means of low density interstellar plasma diagnostic. Over the past twenty years impressive amount of information was obtained for Cassiopeia A and other galactic directions.

A complete survey of radio recombination lines (RRLs) near 327 MHz from the galactic plane (l = 330° − 0°-89°, b = 0°) was carried out using a section of the Ooty Radio Telescope (ORT) with an angular resolution of 2° × 2°. A subset of regions in the same area was observed using the whole telescope which has a beam of 2° × 6'. Hydrogen RRLs were detected in most of the positions that were observed. The lv diagram and radial distribution computed from the observed spectra and their comparison with other species in the galactic plane indicate that the low density gas detected in the survey is distributed similar to the star forming regions. For an assumed temperature of 7000 K, we estimate that the densities and sizes of the regions are in the range 1 — 10 cm−3 and 20 — 200 pc respectively. Our data suggests that the low density ionized gas is in the form of outer envelopes of normal HII regions.

To address its temperature and density we made observations of the galactic plane in CO (2-1) line with VST1 and VST2. Using a kinetic model we estimate the volume emissivities and the line intensity ratio as a function of galactocentric radius. Using data in the first and 4th quadrants separately we got shallow gradient in the both quadrants; at the smaller radius is the higher in ratio. The gradient is contributed by less bright pixels in the l-v diagram, suggesting molecular gas in interarm is different along the galactocentric radius but in arm is not.

We have made HI 21cm-line absorption measurements using the GMRT towards 15 directions in the Galaxy which are known to have high random velocity clouds as seen in the optical absorption lines of CaII and NaI. For the first time, in 6 out of these 15 directions we detect HI absorption features corresponding to the high random velocity optical absorption lines. The mean optical depth of these detections is ∼ 0.08.

A pilot project to observe recombination lines of hydrogen and carbon from Galactic HII regions near 20cm (n = 168) and 49cm (n = 220) using the 30-station correlator at the Giant Metre-wave Radio Telescope was undertaken. The preliminary results from observations of the HII regions W3 and S106 are presented here.

A survey of radio recombination lines (RRLs) in the Galactic plane (l = 332° − 0° − 89°) near 327 MHz made using the Ooty Radio Telescope (ORT) has detected carbon RRLs from all the positions in the longitude range 0° < l < 20° and from a few positions at other longitudes. The carbon lines detected in the survey are, most likely, emission counterparts of the absorption lines observed at frequencies below 150 MHz. Observations towards l = 13°.9, b = 0°.0 indicate that the broader (∼ 38 km s−1) carbon line detected in the lower resolution observation consists of multiple narrow components (∼ 10 km s−1) with different central velocities. The implications of the presence of such narrow components for the modeling of line emission is discussed.

We report the detection of carbon radio recombination lines (CRRLs) near 34.5 MHz (n ∼ 575) and near 328 MHz (n ∼ 270) from 6 positions with l < 17°, b = 0°. Modelling the data shows that the observed CRRLs can arise under a range of physical conditions which are found in atomic HI, molecular and photo-dissociation regions (PDRs) in the galactic disk.

Investigations of carbon low frequency radio recombination lines measured with the world biggest decameter wavelength array UTR-2 are described.

The problem of non-LTE populations has been considered in terms of the departure coefficients ∂bn/∂n as functions of the kinetic temperature Te, the electron density Ne, the continuum radiation flow Ic and the ratios of IHnα, IHnβ, IHnδ and IHnε (the line radiation flows). The ratio of IHnα/IHnβ are sensitive to the thermal radiation from HII regions. Characterized by the relation of ∂2bn/∂n2 > 0, the populations are shown to be inhabited radiatively.

Pulsars are among the most exciting and interesting objects in the Universe. They typically have very steep radio spectra, so they are most easily observed at relatively low radio frequencies. In this review, I summarise some recent important observational results and describe recent searches for pulsars, including the very successful Parkes multibeam survey.

Radio signals from pulsars are significantly affected by propagation effects such as dispersion, faraday rotation and scintillations in the interstellar medium (ISM). In this paper, I review some aspects of our understanding about pulsars and interstellar scintillations (ISS). The study of pulsar scintillation has dual benefits in that it allows us to learn about the properties of the ISM using pulsars as probes, as well as to infer some properties about pulsars, using the ISM as a tool. Both these aspects are addressed in this paper. The main emphasis is on recent developments in the following topics : (i) the shape of the spectrum of electron density fluctuations in the interstellar medium (ii) the distribution of scattering plasma in the local ISM and (iii) resolving pulsar emission regions using ISS.

One of the unique capabilities of the GMRT is the facility for simultaneous multi-frequency observations of pulsars. We have initiated such observations with the aims of (i) studying frequency dependence of pulsar emission, (ii) accurate estimates of pulsar dispersion measure and its variations and (iii) multi-frequency interstellar scintillation studies. Here we present some results from the ongoing dual frequency observations of pulsars.

To study the structure of emission beam, we have analysed the single pulse data of PSR B0329+54 at 325 and 606 MHz. In order to unambiguously detect the weak emission components, we have developed a new data analysis technique, which we term “window—thresholding”. By applying this technique to the data, we have detected three new emission components, and also confirmed the presence of a component which was proposed earlier. Hence our analysis indicates that PSR B0329+54 has nine components, which is among the highest of all the known pulsars. The symmetric distribution of pulse components about the pulse centre, indicates that the emission beam is conal.

We have analyzed the single pulse data of PSR B0301 + 19 and B0355+54 at 1.41 GHz taken from 100-m Effelsberg radio telescope of MPIfR on 22 and 30 August, 1996. The gray-scale maps are made for the polarization quantities to study the distribution of polarization modes at different pulse longitudes. Circular polarization flips the sign at the pulse center in the case PSR B0355+54 but in the case of PSR B0301 + 19 it is mostly positive throughout the pulse. We find the evidence for depolarization that most of the pulsar radiation depolarizes due to the superposition of orthogonal polarization modes.

The systematic increase of component separation and profile widths with decreasing frequency in pulsars suggests that the radiation at different frequencies is emitted from different altitudes above the polar cap. This concept is known as the radius-to-frequency mapping. According to RFM the observed low-frequency radiation should be generated farther from the neutron star than higher-frequency radiation. We discuss radial locations of the pulsar emission regions using a geometrical method for the estimation of emission altitudes. It is argued that the emission altitude at a given frequency is slightly different for young and old pulsars.

Integrated profiles of the Crab pulsar at frequencies 102, 111, 127, 147, and 196 MHz compensated for the interstellar scattering are presented. Observed profiles were compensated for interstellar scattering and intrinsic pulsar profiles were restored. The component structure and their frequency evolution was analyzed. The main pulse is two-component and a so-called precursor is its first component. The interpulse have two components too.

We present observational results from long term monitoring of giant pulses from two pulsars. One of the pulsars (PSR 0950+08) has been observed simultaneously at three different observatories, at different radio frequencies. There is evidence to suggest that the giant pulse phenomenon is intrinsic to the pulsar.

The Westerbork Telescope has good low-frequency capability, with receivers covering much of the frequency range from 250 to 1800 MHz. For pulsar observations the array elements can be coherently summed to provide the equivalent of a 93 m aperture. To exploit this potential, a new backend, PuMa, has recently been commissioned. It can be configured in various ways, with up to 4096 frequency channels per band, a total bandwidth of 80 MHz, and time resolution of 50 nanoseconds. PuMa can be used in search mode, recording just two polarization channels, or for observations of known pulsars, determining all four Stokes parameters and correcting for dispersion and Faraday rotation. In many of the low-frequency bands (especially 300 — 380 and 800 — 900 MHz) interference-free observing is possible much of the time, enabling wide-band studies of ISS, refraction and the Faraday effect.

Based on our examination of the characteristics of about 280 known pulsars and 40 millisecond pulsars (MSPs), we have derived selection criteria for MSP candidates: steep spectrum, highly linearly polarized point sources. The first sample of 14 candidates from the NVSS at 1.4 GHz and the WENSS at 325 MHz has been observed by the WSRT using PuMa at both 92 and 21 cm in May 1999. We have finished processing all data and find no evidence for MSPs for a dispersion measure range of 0 —- 200 cm−3 pc in this first sample.

We present results of the first low frequency measurements of integrated profiles and flux densities of a large set of millisecond pulsars at 102 MHz. Combining our observations with data at higher frequencies, borrowed from literature we performed the comparative analysis of the frequency dependence of profile width and spectra of millisecond and normal pulsars, searching for similarities and differences between their properties. Millisecond pulsars are differ to “normal” ones in much weaker frequency dependence of the width of integrated profile and the absence of the low- frequency turn-over in pulsar spectra.