Curious Case of CGRaBS J0211+1051: Observational Evidence of Lepto-Hadronic Origin of High-Energy Emission?

TL;DR

This work presents a decade-spanning multi-wavelength analysis of CGRaBS J0211+1051, focusing on two major GeV flares and extended optical activity to probe emission mechanisms. Employing a time-dependent one-zone lepto-hadronic framework (OneHaLe), the authors compare pure leptonic and hadro-leptonic SED models across quiescent and flaring epochs, finding that hadronic processes better explain the soft UV/X-ray turnover and high-energy spectra. The hadro-leptonic interpretation also predicts TeV emission and a detectable neutrino flux within IceCube Gen-2 sensitivity, positioning CGRaBS J0211+1051 as a potential multi-messenger laboratory for jet physics. The results motivate deeper X-ray observations and future very-high-energy and neutrino facilities (CTAO, IceCube Gen-2) to test the role of hadrons in blazar jets and refine particle acceleration scenarios.

Abstract

We present an extensive analysis of the multi-wavelength data of the low-synchrotron-peaked BL Lac object CGRaBS J0211+1051, which has been gathered over more than ten years with many observatories. Two major gamma-ray flares have been observed during the Fermi era: one in January 2011 and other in June 2019. During these events, CGRaBS J0211+1051 was also bright in other energy bands. On the other hand, there are also examples of optical activity that do not exhibit any comparable gamma-ray variability. Here, we study the temporal and spectral characteristics of the object in an attempt to understand the emission mechanisms operating in this source. A peculiar feature in its spectrum is the X-ray domain, which is unusually soft considering its object class. Interestingly, the relatively soft UV and optical spectrum does not extrapolate well to the X-rays. To mimic the observed SEDs during quiescent and flaring periods, we use both a purely leptonic and a hadro-leptonic modeling approach to reproduce four broadband SEDs from various epochs. When taking into account the steep optical-UV spectrum, we find that the hadro-leptonic scenarios better explains the SEDs compared to the purely leptonic model. The hadro-leptonic interpretation of the two gamma-ray flares suggests that CGRaBS J0211+1051 could be both a potential neutrino emitter and TeV-bright (E>10 TeV). Thus, it may offer a unique test bed to check for hadro-leptonic contributions to the multi-messenger emission in blazar jets.

Figures (4)

• Figure 1: Long-term optical lightcurve showing strong variability throughout. CRTS and ZTF in the legend stand for photometry from Catalina Real-Time Sky Survey and Zwicky Transient Factory. The magnitudes in ZTF filters are shifted by a constant value to fit in the frame. The vertical dashed lines represent different epochs used for SED extraction.
• Figure 2: (a): X-ray Spectra fitted with simple absorbed powerlaw model ( TBabs*powerlaw). (b): Same as (a) but with $\Gamma$ tied to the one from 2011 observations. (c): The contour plots for $\Gamma-$N distribution showing confidence intervals for three different X-ray spectra. (d): Simultaneous X-ray and UV spectral modeling. The black data points refer to optical - X-ray data taken from December 09 - 15, 2021, representing a fainter quiescence state of the source. The red and cyan data are unfolded data points in UV-X-rays from F2011 and F2109. S1 in the legends refers to the test scenario-1 (ref. § \ref{['sec:uvxspecfit']}): the spectral slope for UV and X-ray bands are the same, only norms are different, whereas S2 belongs to scenario-2: both norms and indices are kept free while modeling. The black X-ray data ('+') are PN spectra, whereas the orange X-ray data ('+') are from the MOS observations.
• Figure 3: (a): 10.5 year long multi-wavelength lightcurves of CGRABsJ0211+1051 during UTC 2010-10-31 to 2021-03-27. Top panel: photon flux in $\gamma$ rays [$\geq$100 MeV] with black points being 5-day averaged fluxes derived from our analysis and grey points being 7-day averaged fluxes obtained from the LAT Lightcurve repository. Middle panel: count rate in X-rays [0.3-10.0 keV]. Bottom panel: magnitudes in optical and UV bands [ Swift-UVOT, CRTS, Steward Observatory, and ZTF]. (b): A zoom-in on 950 days enclosing F2 and Q. (c): A zoom-in on F1. (d): A zoom-in on F2. The red and blue solid lines, repectively represent the $\gamma-$ray flares F1 and F2 modeled using Equation-\ref{['equ:flaremdl']}.
• Figure 4: (a): Multi-epoch SEDs (Q in magenta, F1 in red, and F2 in cyan) with the best-fit pure-leptonic model indicated by the correspondingly colored lines ( solid: Total emission w/o EBL absorption, and, densely dotted extension of the same: with EBL absorption of 2017A A...603A..34F). The shaded colored regions in $\gamma-$ray regime of the SEDs (E:$10^{22}-10^{26}$ Hz) of 12 year averaged (gray), F1 (red), and F2 (cyan), refer to the 1$\sigma$ uncertainty interval around the respective best fit LogParabola models. (b): Same as (a) but using a hadro-leptonic model. In both the SED plots, the dotted, dashed, dashed dotted, dashed double dotted, and densely dashed dotted magenta curves represent the BLR spectrum synchrotron, SSC, EC-BLR, and EC-DT components of Q, respectively. The inset shows the data and model for Q. These components are not plotted for F1 and F2 to keep the clarity in the figures. (c): The injection of leptons of the various origins plotted with $\gamma$ (i.e., energy) derived from the solutions of the Fokker Plank Equation. (d): The predicted neutrino ($\nu_\mu$,$\nu_e$,$\nu_\tau$) spectra corresponding to Q, F1, & F2 for the hadro-leptonic model used in (b). The IceCube-Gen2 sensitivity is shown by the dashed curve in olive color.