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Register a new userProbing the origin of low-frequency radio emission in PG quasars with the uGMRT -- I
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Silpa S.
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We present the results from 685 MHz observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) of 22 quasars belonging to the Palomar-Green (PG) quasar sample. Only four sources reveal extended radio structures on $sim$10-30 kpc scales, while the rest are largely a combination of a radio core unresolved at the uGMRT resolution of $sim$3-5 arcsec, surrounded by diffuse emission on few kpc to $sim$10 kpc scales. A few sources reveal signatures of barely resolved jets and lobes in their spectral index images that are created using the uGMRT 685 MHz data and similar resolution GHz-frequency data from the Very Large Array. On the basis of their position on the radio-IR correlation as well as the spectral index images, we find that the radio emission in the two radio-loud (RL) quasars and nearly one-third of the radio-quiet (RQ) quasars is active galactic nucleus (AGN) dominated whereas the remaining sources appear to have significant contributions from stellar-related processes along with the AGN. While the two RL sources exhibit inverted spectral index in their cores, the RQ sources exhibit a range of spectral indices varying from flat to steep ($-0.1gtrsimalpha_{R}gtrsim-1.1$) indicating the presence of unresolved jets/lobes or winds. Except for a significant correlation between the 685~MHz radio luminosity and the Eddington ratio, we do not find strong correlations between other 685 MHz radio properties and black hole (BH) properties in the RQ PG sources. This lack of correlations could be explained by the contribution of stellar-related emission, or radio emission from previous AGN activity episodes which may not be related to the current BH activity state.
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The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter ($mathcal{R} = frac{L_{mathrm{144-MHz}}}{L_{mathrm{i,band}}}$). The examination of the radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of $mathcal{R}$). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of $mathcal{R}$ on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).
Low-mass ($M_{rm{500}}<5times10^{14}{rm{M_odot}}$) galaxy clusters have been largely unexplored in radio observations, due to the inadequate sensitivity of existing telescopes. However, the upgraded GMRT (uGMRT) and the Low Frequency ARray (LoFAR), with unprecedented sensitivity at low frequencies, have paved the way to closely study less massive clusters than before. We have started the first large-scale programme to systematically search for diffuse radio emission from low-mass galaxy clusters, chosen from the Planck Sunyaev-Zeldovich cluster catalogue. We report here the detection of diffuse radio emission from four of the 12 objects in our sample, shortlisted from the inspection of the LoFAR Two Meter Sky Survey (LoTSS-I), followed up by uGMRT Band 3 deep observations. The clusters PSZ2~G089 (Abell~1904) and PSZ2~G111 (Abell~1697) are detected with relic-like emission, while PSZ2~G106 is found to have an intermediate radio halo and PSZ2~G080 (Abell~2018) seems to be a halo-relic system. PSZ2~G089 and PSZ2~G080 are among the lowest-mass clusters discovered with a radio relic and a halo-relic system, respectively. A high ($sim30%$) detection rate, with powerful radio emission ($P_{1.4 {rm GHz}}sim10^{23}~{rm{W~Hz^{-1}}}$) found in most of these objects, opens up prospects of studying radio emission in galaxy clusters over a wider mass range, to much lower-mass systems.
The origin of the radio emission in radio-quiet quasars (RQQs) remains unclear. Radio photons may be produced by a scaled-down version of the relativistic jets observed in radio-loud (RL) AGN, an AGN-driven wind, the accretion disc corona, AGN photon-ionisation of ambient gas (free-free emission), or star formation (SF). Here, we report a pilot study, part of a radio survey (`PG-RQS) aiming at exploring the spectral distributions of the 71 Palomar-Green (PG) RQQs: high angular resolution observations ($sim$50 mas) at 45~GHz (7 mm) with the Jansky Very Large Array of 15 sources. Sub-mJy radio cores are detected in 13 sources on a typical scale of $sim$100 pc, which excludes significant contribution from galaxy-scale SF. For 9 sources the 45-GHz luminosity, $ u L_{45~{rm GHz}}$, is above the lower frequency ($sim$1--10 GHz) spectral extrapolation, indicating the emergence of an additional flatter-spectrum compact component at high frequencies. The X-ray luminosity and black hole (BH) mass, correlate more tightly with the 45-GHz luminosity than the 5-GHz. The 45GHz-based radio-loudness increases with decreasing Eddington ratio and increasing BH mass. These results suggest that the 45-GHz emission from PG RQQs nuclei originates from the innermost region of the core, probably from the accretion disc corona. Increasing contributions to 45-GHz emission from a jet at higher BH masses and lower Eddington ratios and from a disc wind at large Eddington ratios are still consistent with our results. Future full radio spectral coverage of the sample will help us investigating the different physical mechanisms in place in RQQ cores.
We discuss a probe of the contribution of wind-related shocks to the radio emission in otherwise radio-quiet quasars. Given 1) the non-linear correlation between UV and X-ray luminosity in quasars, 2) that such correlation leads to higher likelihood of radiation-line-driven winds in more luminous quasars, and 3) that luminous quasars are more abundant at high redshift, deep radio observations of high-redshift quasars are needed to probe potential contributions from accretion disk winds. We target a sample of 50 $zsimeq 1.65$ color-selected quasars that span the range of expected accretion disk wind properties as traced by broad CIV emission. 3-GHz observations with the Very Large Array to an rms of $approx10mu$Jy beam$^{-1}$ probe to star formation rates of $approx400,M_{rm Sun},{rm yr}^{-1}$, leading to 22 detections. Supplementing these pointed observations are survey data of 388 sources from the LOFAR Two-metre Sky Survey Data Release 1 that reach comparable depth (for a typical radio spectral index), where 123 sources are detected. These combined observations reveal a radio detection fraction that is a non-linear function of civ emission-line properties and suggest that the data may require multiple origins of radio emission in radio-quiet quasars. We find evidence for radio emission from weak jets or coronae in radio-quiet quasars with low Eddingtion ratios, with either (or both) star formation and accretion disk winds playing an important role in optically luminous quasars and correlated with increasing Eddington ratio. Additional pointed radio observations are needed to fully establish the nature of radio emission in radio-quiet quasars.
We report radio imaging and monitoring observations in the frequency range 0.235 - 2.7 GHz during the flaring mode of PKS 2155-304, one of the brightest BL Lac objects. The high sensitivity GMRT observations not only reveal extended kpc-scale jet and FRI type lobe morphology in this erstwhile `extended-core blazar but also delineate the morphological details, thanks to its arcsec scale resolution. The radio light curve during the end phase of the outburst measured in 2008 shows high variability (8.5%) in the jet emission in the GHz range, compared to the lower core variability (3.2%) seen at the lowest frequencies. The excess of flux density with a very steep spectral index in the MHz range supports the presence of extra diffuse emission at low frequencies. The analysis of multi wavelength (radio/ optical/ gamma-ray) light curves at different radio frequencies confirms the variability of the core region and agrees with the scenario of high energy emission in gamma-rays due to inverse Compton emission from a collimated relativistic plasma jet followed by synchrotron emission in radio. Clearly, these results give an interesting insight of the jet emission mechanisms in blazars and highlight the importance of studying such objects with low frequency radio interferometers like LOFAR and the SKA and its precursor instruments.
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