arXiv subjects

Dharam Vir Lal

Publications and source records attributed to Dharam Vir Lal.

26 records · Page 2Linked to original sources

arXiv2007

Using Fast-Switching Data to Characterize Atmospheric Phase Fluctuations at the Submillimeter Array

For the submillimeter band observations, we have been routinely adopting the calibration cycle time of 20-30 minutes, which is the same as any typical centimeter and millimeter band observations. This cycle time, largely corrects only the instrumental phase fluctuations and there exists residual phase fluctuations, which are attributed to temporal and spatial atmospheric phase fluctuations. Hence, the classical calibration cycle needs closer attention for any future submillimeter band observations. We have therefore obtained fast-switching test data, cycling between three nearby calibrators, using the submillimeter array (SMA) with a cycle time of $\sim$90 sec, in order to understand and optimize the calibration cycle suitably, thereby to achieve the projected sensitivity, angular resolution and dynamic range for the SMA. Here, we present the preliminary results from this study.

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arXiv2006

GMRT observations of X-shaped radio sources

We present results from a study of X-shaped sources based on observations using the Giant Metrewave Radio Telescope (GMRT). These observations were motivated by our low frequency study of 3C 223.1 (Lal & Rao 2005), an X-shaped radio source, which showed that the wings (or low-surface-brightness jets) have flatter spectral indices than the active lobes (or high-surface-brightness jets), a result not easily explained by most models. We have now obtained GMRT data at 240 and 610 MHz for almost all the known X-shaped radio sources and have studied the distribution of the spectral index across the sources. While the radio morphologies of all the sources at 240 and 610 MHz show the characteristic X-shape, the spectral characteristics of the X-shaped radio sources, seem to fall into three categories, namely, sources in which (A) the wings have flatter spectral indices than the active lobes, (B) the wings and the active lobes have comparable spectral indices, and (C) the wings have steeper spectral indices than the active lobes. We discuss the implications of the new observational results on the various formation models that have been proposed for X-shaped sources.

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arXiv2004

Spectral structure of X-shaped radio sources

Analysis of Giant Metrewave Radio Telescope low frequency data for an X-shaped source, 3C 223.1 has revealed an unusual result as discussed earlier by Lal & Rao (2004, MNRAS in press). The radio morphologies of it at 240 and 610 MHz show a well-defined X-shaped structure with a pair of active jets along the north-south axis and a pair of wings along the east-west axis, that pass symmetrically through the undetected radio core. The wings (or low surface brightness jet) seem to have flatter spectral index with respect to the high surface brightness jet. This clearly shows the value of mapping the sample of X-shaped sources at low frequencies. Here we present our preliminary results for two more such sources.

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arXiv2004

3C 223.1: A source with unusual spectral properties

Analysis of Giant Metrewave Radio Telescope low frequency data for an X-shaped source, 3C 223.1 has revealed an unusual result. The radio morphologies of it at 240 and 610 MHz show well defined X-shape with a pair of active jets along the north-south axis and a pair of wings along the east-west axis, that pass symmetrically through the undetected radio core. The wings (or low surface brightness jets) have flatter spectral indices with respect to the high surface brightness jets, which confirms the earlier marginal result obtained at high frequency by Dennett-Thorpe et al. (2002). Although unusual, it is a valuable result which puts stringent constraints on the formation models and nature of these sources. This result clearly shows the value of mapping the sample of X-shaped sources at low frequencies.

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arXiv2004

Seyfert galaxies: Nuclear Radio Structure and Unification

We have observed a sample of Seyfert (Sy) galaxies with the global Very Long Baseline Interferometry (VLBI) to test the predictions of the unification scheme for the Sy galaxies which hypothesises that Sys of Type 1 and of Type 2 constitute a single population of objects. If Sy 1 and Sy 2 galaxies differ only in the orientation of the axisymmmetric active nucleus with respect to the observer, then, the parsec scale radio structures of the two types should be similar. The overall goal of the thesis is to rigorously test the predictions of the unified scheme hypothesis for the Sy galaxies. We chose a sample of 10 Sy 1s and 10 Sy 2s such that the two subsamples were intrinsically similar within the framework of the unified scheme. This was done by matching them as far as possible in the orientation-independent parameters {\it viz.}, host galaxy properties and measures of intrinsic Active Galactic Nuclei (AGN) power. We here present the results obtained from these observations and their implications on the unification scheme. We also discuss the results obtained from the measurements at all IRAS and 2-10 keV X-ray wavebands for our Sy galaxy sample using data from the literature.

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arXiv2004

Low Frequency Observations of a Head-Tail Radio Source

We have mapped the head-tail radio galaxy, 3C 129 at 240 and 610 MHz using the GMRT and studied the detailed morphology and spectral index variations in this object. This is the first attempt to observe a sample of head-tail sources at low frequencies. We find weak spectral steepening as we go away from the head along the jet. The Crosspiece has a spectral index of 0.9 (S$_ν \propto ν^{-α}$) and is flatter than the spectral index estimated by Lane et al. (2002). We also see a low surface brightness, diffuse feature at 240 MHz and could be a possible radio relic candidate (see Figure).

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arXiv2004

Milliarcsec-scale radio structure of a matched sample of Seyfert 1 and Seyfert 2 galaxies

We have obtained mas-scale resolution very long baseline interferometry (VLBI) images of a sample of Seyfert 1 and Seyfert 2 galaxies at 5 GHz (wavelength, $λ$ = 6 cm). The Seyferts of the two types were chosen to be matched in several orientation-independent properties, primarily in order to rigorously test predictions of the unified scheme. We detected all the 15 objects that we observed. In this paper we describe the observations and data reduction procedures, and present the VLBI radio images as well as simultaneous Very Large Array images that we obtained for these 15 Seyferts.

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arXiv2004

3C 129 : The GMRT observations

We present radio maps of the head-tail radio galaxy, 3C 129 at 240 and 610 MHz using the Giant Metrewave Radio Telescope. We have studied in detail the morphology of the source and the distribution of spectral index over the object. We find large-scale spectral steepening along the jet away from the core. Using synchrotron spectral ageing theory, we infer the age (time since particle acceleration) of electrons at various locations along the jet and ascribe an age of $\sim$200 Myr to 3C 129. We find that the "Crosspiece" has a spectral index of ${\sim}-1$, similar to the jet, which is not consistent with the suggestion by Lane et al. (2002) that it could be a radio relic. We have also studied 3C 129.1 and the other sources in the field.

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