First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC 4235

summary

The gist

The gist: This study presents the first-ever analysis of polarization across two frequency bands of the uGMRT and provides constraints on rotation measure (RM) at lower frequencies for a weak radio

In short

This study analyzed polarization across two frequency bands of the uGMRT to constrain rotation measure (RM) in NGC 4235, a radio-quiet galaxy. By combining data from 663 MHz and 1.3 GHz, the researchers found a core RM of $198.4 ext{ rad m}^{-2}$. This suggests a thin, low-density magnetized sheath or wind surrounding the AGN's outflow, providing constraints on the magnetic field structure in this environment.

Key concepts

Rotation Measure (RM): RM measures how much the polarization angle of radio waves rotates as they travel through a magnetized plasma. It is calculated using electron density and magnetic field strength along the line of sight. This value helps diagnose both the environment near an AGN and intervening media, like our own galaxy.
uGMRT Broadband Polarimetry: The uGMRT was used to observe NGC 4235 at two different frequencies (663 MHz and 1.3 GHz) using different feed configurations. This allowed the researchers to analyze polarization across a wide range of wavelengths, which is crucial for resolving ambiguities in RM measurements.
Equipartition Assumption: This is a standard assumption used to estimate the magnetic field strength (B-field) in radio-emitting sources like AGN cores. It assumes that the energy density of the magnetic field is comparable to the energy density of relativistic particles. This assumption allows researchers to estimate a characteristic B-field strength, which is then used alongside electron density estimates.
Faraday Rotation: Faraday rotation describes how linearly polarized light rotates when passing through a magnetized plasma. The observed rotation depends on the wavelength squared and the integrated product of electron density and magnetic field along the line of sight. In this study, it was used to determine the properties of the magnetized medium surrounding NGC 4235.

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This episode discusses

The paper

First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC 4235 · Read on arXiv

Salmoli Ghosh, Preeti Kharb, Janhavi Baghel

National Centre for Radio Astrophysics - Tata Institute of Fundamental Research (NCRA-TIFR) · Department of Physics, University of Pretoria · Inter-University Institute for Data Intensive Astronomy

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC 4235".

Vera: The gist:

Jocelyn: First, who's behind it and why it matters.

Paper summary: Vera: So we're looking at this paper now, "First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC four thousand two hundred thirty-five". It presents the very first analysis of polarization across two different frequency bands from the uGMRT.

Jocelyn: The authors are Salmoli Ghosh, Preeti Kharb, and Janhavi Baghel from NCRA-TIFR and the University of Pretoria. They used their own uGMRT-based polarization data reduction pipelines in CASA to get Stokes I, Q, U, and V images.

Subrahmanyan: The main point they're making is that the modest rotation measure value for NGC four thousand two hundred thirty-five suggests there isn't strong depolarization happening right there in the core.

Vera: And they also found consistency between the VLA-scale ordered magnetic fields and these uGMRT-scale rotation measure values. This means the kpc-scale radio emission in NGC four thousand two hundred thirty-five is being rotated by magnetized plasma in the host galaxy’s interstellar medium.

Jocelyn: The paper uses this multi-frequency approach to provide more constraints on polarimetric modeling and better information about AGN jets and their environments.

Subrahmanyan: It suggests that the rotation predominantly originates in an external, low-density magneto-ionized medium rather than significant internal depolarization within the core.

Vera: So, this work using the uGMRT to constrain rotation measure in NGC four thousand two hundred thirty-five shows how important multi-frequency polarimetry is for probing magnetized environments around AGN.

Conclusion: Vera: This paper, titled "First uGMRT Constraints on Rotation Measure in the Radio-Quiet Seyfert Galaxy NGC four thousand two hundred thirty-five", uses two different radio frequencies from the uGMRT to measure rotation measure at a galaxy called NGC four thousand two hundred thirty-five.

Jocelyn: It's been a big effort by Salmoli Ghosh, Preeti Kharb, and Janhavi Baghel to get these constraints on rotation measure for this specific AGN.

Subrahmanyan: They observed the source at one point three GHz and six hundred sixty-three MHz using different feed configurations on the uGMRT. It’s about probing the magnetic environment near this galaxy.

Vera: Exactly, they used those two bands to get a first look at polarization across a wide range of frequencies, which is what they needed for that rotation measure calculation.

Jocelyn: What’s really interesting is how they managed to get these constraints on the lower frequency data and what that tells us about the magnetic fields in this galaxy.

Subrahmanyan: They found a modest rotation measure value of one hundred ninety-eight point four rad m-two for the core, which suggests there isn't strong depolarization happening right there in the center.

Vera: So, what does that modest number actually mean for the physical environment around NGC four thousand two hundred thirty-five?

Jocelyn: It points toward a thin and uniform Faraday-rotating medium, which is pretty significant because it’s constraining how much magnetized plasma is present.

Subrahmanyan: And they linked this to VLA measurements of magnetic fields, showing consistency between those scales. It implies the kpc-scale radio emission is indeed being rotated by the host galaxy's interstellar medium.

Vera: So, in simple terms, this work uses uGMRT polarization to show that the galaxy’s surrounding plasma is rotating the light from its radio jets.

Jocelyn: It’s a good example of how multi-frequency polarimetry helps us map out these magnetic fields in complex environments like AGN.

Subrahmanyan: Their next step, which I think is crucial, will be to see if they can use these constraints to model the structure of those winds or outflows you mentioned earlier.

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