Spectropolarimetry of NGC 1275 reveals a narrow-line radio galaxy with polarization parallel to its radio jet axis

TL;DR

This study presents optical spectropolarimetry of NGC 1275 obtained with NOT/ALFOSC during the IXPE Perseus campaign, revealing a type-2 AGN spectrum with narrow emission lines and a continuum polarization that is wavelength-dependent and relatively high (2–3%). Using four waveplate orientations, the authors derive $q$ and $u$, obtain a debiased polarization $P_d$, and analyze line properties including a reddening estimate of $E(B-V) \sim 0.34$ and electron density $n_e \sim 2\times10^3\ \mathrm{cm^{-3}}$. The polarized continuum shows $P_d \approx 3.23\%$ in the blue and $\approx 2.02\%$ in the red with polarization angles around $\theta \sim 164^{\circ}$ and $\theta \sim 155^{\circ}$, respectively, and the polarization in emission lines is lower, consistent with a mix of synchrotron emission and scattering. Importantly, the optical polarization angle aligns with the arcsecond-scale radio jet, implying a substantial jet-related synchrotron contribution superposed on scattering from the nucleus; variability and scale-dependent jet orientations motivate deeper, time-resolved spectropolarimetry to search for a hidden BLR and to quantify the scattered-to-synchrotron ratio.

Abstract

Concomitant with the Imaging X-ray Polarimetry Explorer (IXPE) observation of the Perseus cluster, we obtained optical spectropolarimetry of its central active galactic nucleus, NGC 1275, using the Alhambra Faint Object Spectrograph and Camera (ALFOSC) on the Nordic Optical Telescope (NOT). While the total-light spectrum confirms its edge-on, core obscured (type-2) classification, the polarized spectrum shows a polarization angle aligned with the arcsecond radio jet axis -- an exceptional behavior for type-2 objects. Our polarization analysis also reveals wavelength-dependent linear polarization at level 2-3% in the continuum, likely rising from a combination of variable syn

Figures (1)

• Figure 1: NGC 1275 was observed with NOT/ALFOSC on March 21, 2025. The top-left panel shows the total flux spectrum (corrected for instrumental response, in arbitrary units), and the top-right shows the polarized flux (total flux multiplied by the polarization degree). The bottom-left displays the linear polarization degree $P_{\rm d}$, while the bottom-right shows the polarization position angle $\theta$. All but the total flux spectrum were rebinned to 137 consecutive pixels ($\sim$237 Å), with observational errors indicated per spectral bin.