Probing the Low-radio-frequency Emission in PG Quasars with the uGMRT. II

Abstract We present results from uGMRT 685 MHz observations of 87 QSOs belonging to the Palomar–Green (PG) quasar sample with z &lt; 0.5. Radio emission is detected in all sources except for three radio-quiet (RQ) sources, namely PG 0043+039, PG 1121+422, and PG 1552+085. The radio-loud (RL)–RQ dichotomy persists at 685 MHz with only one source, PG 1216+069, changing its classification from RQ to RL. Approximately one-third of the detected RQ quasars display active galactic nucleus–dominated radio emission while the rest may show additional contributions from stellar-related processes. Consistent with this, the RL and RQ quasars occupy distinct tracks on the “fundamental plane” of black hole activity. We find that RL quasars have <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>log</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>685</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">MHz</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">W</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">Hz</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&gt;</mml:mo> <mml:mn>25.5</mml:mn> </mml:math> , while RQ quasars have <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>log</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>685</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">MHz</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">W</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">Hz</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>23.5</mml:mn> </mml:math> . Furthermore, the radio sizes display the RQ−RL divide as well with RQ sources typically having sizes ≲30 kpc, with only 16 (∼22%) of RQ sources having sizes between 30 and 100 kpc, where there is an overlap with RL quasar sizes. A strong correlation exists between 685 MHz radio luminosity and black hole mass that is tightened when accretion rate is considered, highlighting the important role played by the accretion rate and accretion disk structure in jet production. We found no difference in the minimum-energy magnetic field strengths of the radio cores of RL and RQ quasars; however, different assumptions of source volume and volume-filling factors may apply. High-resolution X-ray observations and radio-X-ray flux comparisons are needed to independently test the “magnetic flux paradigm.”