On the Origin and Evolution of Curvature of the Spectral Energy Distribution of Fermi Bright Blazars

Abstract: The origin and evolution of spectral curvature in blazar spectral energy distribution (SED) is still unclear. Since the observed SED curvature is related to an intrinsic curvature in emitting electron energy distribution (EED), we study this question by employing a log-parabolic EED with a curvature parameter and peak energy to model the quasi-simultaneous broadband SEDs of selected blazars in Fermi-LAT Bright AGN Sample (LBAS) using synchrotron and inverse Compton (IC) processes. We find that log-parabolic IC model can successfully explain the emission in all blazars in our sample. On average, FSRQs have higher magnetic field, Doppler factor, and curvature than BL Lac objects. The BL Lac objects show an anticorrelation between the curvature parameter of the EED and its peak energy, which is a signature of stochastic acceleration. FSRQs do not manifest such correlation and rather show a mild positive relationship between these parameters. This suggests that the evolution of spectral curvature in the BL Lac objects is dominated by a strong stochastic acceleration component, whereas the curvature in FSRQs evolves in a cooling dominated regime due to an additional external Compton (EC) component. The strong cooling in FSRQs not only restricts the electron peak energy but also adds extra curvature to the high energy tail of emitting EED. Since the curvature decreases from FSRQs toward high peak BL Lac objects (HBLs), opposite to peak energy, the curvature parameter can be considered a third parameter of the blazar sequence in addition to peak frequency and luminosity.

• The paper analyzes the origin and evolution of intrinsic curvature in the spectral energy distribution (SED) of Fermi-bright blazars using a log-parabolic electron energy distribution model.
• FSRQs and BL Lacs show distinct characteristics, with FSRQs having higher magnetic fields, Doppler factors, and curvature, suggesting stronger cooling effects in these sources.
• Significant differences in the anticorrelation (BL Lacs) and mild positive correlation (FSRQs) between electron energy distribution curvature and peak energy imply curvature could be a third parameter in the blazar sequence.

An Analysis of the Curvature Evolution in the Spectral Energy Distributions of Bright Fermi Blazars

The study presented in the paper by Anjum et al. focuses on understanding the intrinsic curvature observed in the spectral energy distribution (SED) of blazars, particularly those Fermi-bright in nature. This investigation explores the origin and evolution of such curvature, leveraging a log-parabolic energy distribution model for emitting electrons (EED) against the backdrop of synchrotron and inverse Compton (IC) processes.

Methodology and Key Findings

The researchers utilized a sample from the Fermi-LAT Bright AGN Sample (LBAS), encompassing both flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs), and employed a log-parabolic model with curvature and peak energy parameters to fit the observed broadband SEDs. This approach was instrumental in deriving conclusions about the relationship between intrinsic EED curvature, Doppler factor, and magnetic field in different blazar types.

Among the primary findings:

1. Model Validation: The log-parabolic model was shown to effectively capture the characteristics of the SEDs in the sampled blazars, aligning well with data across a wide range of frequencies.
2. Distinct Characteristics: FSRQs were observed to have higher average magnetic fields, Doppler factors, and curvature parameters compared to BL Lacs, suggesting a more potent influence of radiative cooling processes facilitated by an external Compton (EC) component.
3. Curvature and Peak Energy Correlation:
   • A significant anticorrelation between EED curvature and peak electron energy was found for BL Lac objects, indicative of a strong stochastic acceleration influence.
   • Conversely, FSRQs exhibited a mild positive correlation, pointing to a cooling-dominated evolution in these sources.
4. Blazar Sequence Implications: The differences in curvature and peak energy evolution suggest that curvature can act as a third parameter in the blazar sequence, in addition to peak frequency and luminosity, providing deeper insights into jet dynamics and particle acceleration processes.

Broader Implications and Future Directions

The work enhances our comprehension of blazar physics, notably the interplay between acceleration and cooling in dictating spectral characteristics, advancing the theoretical underpinnings related to particle acceleration dynamics in relativistic jets.

Moreover, the analysis opens up several avenues for future research:

• Expanding the sample size with simultaneous data from current and next-generation gamma-ray observatories could refine these findings.
• Detailed exploration of the emission region's location concerning the jet structure in FSRQs could clarify variations in curvature evolution.
• Further studies could also involve leveraging machine learning approaches to improve the predictive power regarding spectral characteristics based on observed SED data.

This research underscores the complexity of blazar emissions and the critical role curvature plays in understanding these fascinating cosmic objects, contributing to a more nuanced blazar classification framework. While the conclusions draw from models rooted in specific assumptions, they provide a foundation for both observational and theoretical work aimed at probing the multifaceted nature of AGN jets.