Rapid jet ejection from PKS 0215+015 coincident with a high-energy neutrino event

Abstract

Aims. We present a new neutrino-blazar multiwavelength flare coincidence observed in the blazar PKS 0215+015, which showed a strong multiwavelength outburst in coincidence with the IceCube neutrino track alert IC220225A, similar to the case of TXS 0506+056. We investigate the immediate response of the radio jet to the major flare. Methods. We performed target-of-opportunity observations with the Very Long Baseline Array (VLBA) at 15, 23, and 43 GHz in full polarization for six epochs with monthly cadence following the neutrino event. We combine the VLBA observations with monitoring data from the Effelsberg 100-m telescope, the Australia Telescope Compact Array, and Fermi/LAT. Results. Based on our VLBI kinematic analysis, we identified a new rapid jet component with an apparent speed of ~60-80c, which was ejected around the arrival of IC220225A. The fast component ejection is traced by a characteristic signature in polarization that suggests a shock-shock interaction with a quasi-stationary feature. By combining the VLBI results with radio variability data, we estimated a bulk Lorentz factor of $Γ= 105 \pm 56$ and a jet viewing angle of $\vartheta = (1.47 \pm 0.31)^\circ$. Conclusions. We note that the properties of the rapid component exceed previously reported maximum apparent jet speeds and Lorentz factors from continuous VLBI monitoring programs. This is likely only possible because we are observing an exceptional flaring event at high redshift (z=1.72) with higher observing cadence than in typical monitoring programs. We suggest that neutrino production in PKS 0215+015 can occur through pγ-interactions with protons possibly accelerated within the fast-moving feature. The target photon field could be external to the jet or explained by a multi-layered jet. The latter scenario is consistent with the presence of quasi-stationary features revealed in our analysis.

Figures (10)

• Figure 1: 90 % uncertainty regions of the neutrino IC220225A, as reported by the IceCube Collaboration via AMON (black dashed line) and GCN circular (blue ellipse and red dashed box). RFC sources within the GCN box region are displayed as black dots, gamma-ray detected sources from the 4FGL catalog are highlighted as green stars. PKS 0215+015 stands out as the only gamma-ray detected source within all three regions.
• Figure 2: Gamma-ray (upper panel) and radio (lower panel) light curves of PKS 0215+015. The source exhibited its historical high-state in both wavelengths coincident with the arrival time of IceCube neutrino event IC220224A (red dashed line). The new fast VLBI component 1 was ejected shortly before. Its possible ejection time range (1$\sigma$) is indicated by the gray shaded area.
• Figure 3: Stacked polarization images at 15 GHz (left), 23 GHz (center), and 43 GHz (right). Before stacking the individual images were convolved with the median beam (indicated by the gray ellipse) at each frequency separately. Contours show the total intensity, starting at four times the noise level and increasing by factors of two. The colormap displays linear polarization, and the sticks indicate the direction of the EVPA.
• Figure 4: Kinematic modelling of PKS 0215+015 at 15 GHz (left), 23 GHz (center) and 43 GHz (right). Component 1 is moving outwards, while components 2 and 3 are quasi-stationary. Fit parameters for all components at all frequencies are shown in Tab. \ref{['tab:kinematics']}.
• Figure 5: Top: Combined ATCA and TELAMON light curve with fitted flares. Center: TELAMON fractional polarization at 20 mm and 7 mm. The inclined lines indicate direction of the EVPA, errors are indicated by the shaded regions in EVPA and polarization. In addition to the TELAMON polarization values, we display the fractional polarization and EVPA of the VLBA data, for better visibility without uncertainties. Bottom: Spectral index evolution obtained from TELAMON observations between 20 mm and 7 mm. The red dashed line indicates the time of IC220225A.