Minijets and Broken Stationarity in a Blazar : Novel Insights into the Origin of $γ$-ray Variability in CTA 102

TL;DR

This paper investigates the origin of GeV variability in CTA 102 using 18 years of Fermi-LAT data. It finds that the GeV flux distribution is not strictly log-normal before or after the 2017 flare; a significant skewness reduction suggests magnetic relaxation and a transition to a steadier, lower-energy state. The authors propose a modified minijets-in-jet model within an external Compton framework, with many minijets aligning toward the BLR and the line of sight, producing large flares; The flux distributions are well described by a Modified Log-Normal Power-Law distribution (MLP), with α differing between pre- and post-flare states, consistent with changes in reconnection rate. They validate the model against simulated light curves and discuss implications for jet magnetization, reconnection, and external photon fields.

Abstract

High-energy blazar light curves, in X-rays and beyond, have historically preferred a log-normal flux distribution, signifying multiplicative processes either in the jet itself or due to connection(s) with accretion. Here we present 18 year archival Fermi-LAT light curves (0.1-100 GeV) of the flat spectrum radio quasar (FSRQ) CTA 102 from August 2008 to November 2025, which underwent a huge flare in 2017, with a $\sim$ factor of 100 jump in $γ$-ray flux, along with similar flaring in X-rays. Our statistical analyses confirm that neither the pre nor the post-flare total GeV light curves follow a strictly log-normal distribution. Instead, we observe a statistically significant reduction in skewness from the pre to the post-flare light curves, which implies the blazar transitioned from an energetic state with frequent flaring to a more plateaued state with occasional flaring. We further find that this state transition can be explained through magnetic relaxation, where many reconnection events caused the 2017 flare, after which the magnetic field was ordered and its energy reached a minimum. To explain this further, we use a Monte Carlo simulation of a modified minijets-in-a-jet model where GeV flares are produced only when a maximum number of minijets move toward the broad line region and towards the line of sight, in the context of an external Compton model. The flux distributions (both observed and simulated) could be fit by a modified log-normal power-law distribution, implying our minijets model can reproduce the GeV flares in CTA 102 as well as their flux distributions.

Figures (8)

• Figure 1: The 18 year Fermi-LAT light curve of CTA 102, with the gray region showing the massive flaring period.
• Figure 2: Figure shows the histograms of the logarithmic fluxes for the pre-flare and post-flare light curves, in blue and red, in upper and bottom panels respectively. The "flux transfer" from the tail to the centre is evident.
• Figure 3: Left panel (a) shows the "per-1 year" bin skewness for both the pre-flare and post-flare light curves, in addition to the cumulative skewness taking more and more points into account until the flare, from either side of the light curve. Right panel (b) shows the rolling skewness of a window of size 1 year, advanced by one point. A beginning of a transition from a high to a low skewness is visible in the post-flare state.
• Figure 4: Figure shows the Fermi photon index histograms for the pre-flare and post-flare states.
• Figure 5: Sketch shows a basic schematic of the minijet model as observed from the lab frame. Red and blue arrows emanating from the minijets denote minijets moving toward us and away from us, i.e., in and against the direction of bulk jet flow.