Jet reorientation revealed by intermittent jet activity in radio galaxy 0954+556

TL;DR

The study investigates intermittent jet activity and rapid jet reorientation in the young radio galaxy 0954+556 (z=0.903) by combining extensive VLA and VLBA imaging across 1.4–43 GHz with spectral aging and polarization analyses. It identifies a pc-scale core region in component N, detects faint pc- and kpc-scale counter-jets, and reveals a diffuse bridge linking pc and kpc forward jets, all pointing to at least two distinct activity episodes. Spectral-index and radiative-age modeling indicate separate jet episodes with different orientations, supported by a spine-sheath polarization pattern that implies a helical magnetic field. The results favor a CSO classification with intermittent activity and jet reorientation on timescales near 1 Myr, likely driven by accretion-disk instabilities, and demonstrate rapid realignment between restarting episodes with significant pc–kpc misalignment.

Abstract

Intermittent jet activity of AGNs is a common phenomenon, whereas significant jet reorientation during episodic jet activity in relatively young radio galaxies are rarely reported. The quasar 0954+556 at redshift of 0.903 is an intriguing source exhibiting an unusual radio jet structure with significantly different jet directions at kpc and pc scales. At kpc scales, images from the VLA exhibit a bright core, a linear jet extending 24 kpc to the northwest, and a discrete jet component 16 kpc to the northeast. At pc scales, images from the VLBA show a two-component structure with a projected separation of 360 pc in the north-south direction. The peculiar structure of 0954+556 might result from jet reorientation. Here, our aim was to investigate the possible mechanism via multiscale and multifrequency deep radio images. We performed VLA and VLBA observations of 0954+556. Together with some existing data in the NRAO data archive, we made multiple VLA images at 1.4-22 GHz and VLBA images at 1.7-43 GHz for various image analyses of the jet structure. We identified the location of the radio core at pc scales, detected the faint counter-jets at both pc and kpc scales for the first time, and revealed a diffuse emission region connecting pc and kpc scale forward jets. Our spectral index distribution and spectral aging analysis indicate that 0954+556 might undergo at least two episodes of jet activity during the current AGN phase. Moreover, pc scale polarization maps display a well-resolved spine-sheath polarization structure. It seems that the jet direction of 0954+556 changed significantly during intermittent jet activity. This may explain the different jet orientations and spectral ages observed from kpc to pc scales. The research provides a strong case that AGN jet direction might change rapidly on timescales of one million years.

Figures (10)

• Figure 1: VLA flux density images of 0954+556 at kpc scale. The contours are drawn at -1, 1, 2, 4, 8, ..., of the first contour level (3$\sigma_\mathrm{rms}$). The synthesized beams are plotted in the bottom left corner of each image. In the top left panel, the red circles overlapping the contours present the results of modelfit. In the top middle panel, the 5 GHz image shows a weak counter-jet. To better reveal the weak counter-jet, the 5 GHz image was restored with a circular beam of 900 mas, as shown in the top right panel.
• Figure 2: VLBA flux density images of 0954+556 at pc scale. The contours are drawn at -1, 1, 2, 4, 8, ..., of the first contour level (3$\sigma_\mathrm{rms}$). The synthesized beams are also plotted in the bottom left corner of each image. The image in the top right corner is the tapered image at 1.7 GHz (UVTAPER 0.5, 10).
• Figure 3: VLBA flux density images of 0954+556 at 5 GHz. Left panel: Reconstructed image of the visibilities from a combination of data on 20 July 2002 and 22 September 2002. Middle panel: Reconstructed image of the visibilities from a combination of data on 14 June 2004 and 3 November 2004. Right panel: Reconstructed image of the visibilities from a combination of data on 4 April 2005 and 18 August 2005. The contours are drawn at -1, 1, 2, 4, 8, ..., of the first contour level (3$\sigma_\mathrm{rms}$). The synthesized beams are plotted in the bottom left corner of each image.
• Figure 4: Light curve of 0954+556. The top panel shows a Fermi-LAT $\gamma$ -ray light curve of 0954+556 from 8 August 2008 to 6 December 2024, divided into seven-day bins. All points are plotted along with their statistical errors. The bottom panel is the radio light curve obtained from OVRO at 15 GHz, from 19 April 2009 to 30 December 2011, with an interval of four days. The gray area in the top panel corresponds to the bottom panel.
• Figure 5: Spectral index distribution maps of 0954+556. From left to right, the panels show the distribution of spectral index between 1.4 and 5 GHz at kpc scales, between 5 and 8.4 GHz at kpc scales, between 1.7 and 5 GHz at pc scales, and between 5 and 8.4 GHz at pc scales. The overlapped contour images are a VLA image at 1.4 GHz with a $uv$ range $5.1-168.7 \mathrm{K}\lambda$, a VLA image at 5 GHz with a $uv$ range $11.3-561.2 \mathrm{K}\lambda$, a VLBA image at 1.7 GHz with a $uv$ range $2.3-48.0 \mathrm{M}\lambda$, and a VLBA image at 5 GHz with a $uv$ range $5.4-39.5 \mathrm{M}\lambda$, respectively. All counters start at 3$\sigma_\mathrm{rms}$ and increase by a factor of 2. The color represents the value of spectral index $\alpha$. Pixels with flux density below 5.4$\sigma_\mathrm{rms}$, alongside those with spectral index uncertainty exceeding 40%, are blanked.