[Paper Review] Spectral Modeling of Flares in Long Term Gamma-Ray Light Curve of PKS 0903-57

Sandeep Kumar Mondal, Raj Prince+2 more|arXiv (Cornell University)|Jul 6, 2021

Astrophysics and Cosmic Phenomena35 references4 citations

TL;DR

This study presents the first detailed spectral and temporal analysis of two major gamma-ray flares in PKS 0903-57 using 12 years of Fermi-LAT data, identifying multiple sub-structures in 2018 and 2020. Using a time-dependent single-zone leptonic model, it estimates a gamma-ray variability timescale of 1.7±0.9 hours and jet power of a few ×10⁴⁶ erg/s, with best-fitting spectral models (log-parabola) indicating complex emission processes during flares.

ABSTRACT

A detailed study of the BL Lacertae PKS 0903-57 has been done for the first time with 12 years of Fermi Large Area Telescope data. We have identified two bright gamma-ray flares in 2018 and 2020. Many sub-structures were observed during multiple time binning of these flares. We have performed detailed temporal and spectral study on all the sub-structures separately. A single-zone emission model is used for time-dependent leptonic modeling of the multi-wavelength spectral energy distributions. Our estimated values of variability time scale, magnetic field in the emission region, jet power obtained from leptonic modeling of PKS 0903-57 are presented in this work. Currently, we have a minimal number of observations in X-rays and other bands. Hence, more simultaneous multi-wavelength monitoring of this source is required to have a better understanding of the physical processes happening in the jet of the blazar PKS 0903-57.

Motivation & Objective

• . The paper aims to understand the physical mechanisms behind gamma-ray flares in the BL Lac object PKS 0903-57.
• It seeks to characterize the temporal and spectral evolution of multiple sub-structures within two major flares detected in 2018 and 2020.
• The study investigates the variability timescale and jet power through time-dependent leptonic modeling of multi-wavelength spectral energy distributions (SEDs).
• It highlights the need for simultaneous multi-wavelength monitoring due to sparse X-ray and other band data.

Proposed method

• . The authors used 12 years of Fermi-LAT data (August 2008 to January 2021) to analyze the gamma-ray light curve of PKS 0903-57.
• Time binning was performed at 3-hour intervals with a minimum test statistic (TS) ≥25 to ensure 5σ significance and reliable flux measurements.
• The gamma-ray light curve peaks were modeled using a double-exponential function (Equation 2) to extract rising and decay timescales for each flare phase.
• Multi-wavelength SEDs from flaring phases were fitted with power-law (PL), log-parabola (LP), broken power-law (BPL), and power-law with exponential cutoff (PLEC) models to determine the best spectral fit.
• A single-zone time-dependent leptonic emission model was applied using the GAMERA code to simulate synchrotron and inverse Compton emission processes.
• The jet power and magnetic field in the emission region were derived from the best-fitting spectral and temporal parameters.

Experimental results

Research questions

• RQ1 . What are the temporal and spectral characteristics of the gamma-ray flares in PKS 0903-57 over a 12-year baseline?
• RQ2 What is the variability timescale of the source, and how does it compare to other blazars?
• RQ3 Which spectral energy distribution (SED) model—PL, LP, BPL, or PLEC—best describes the emission during different flare sub-structures?
• RQ4 What are the inferred physical parameters such as jet power and magnetic field in the emission region?
• RQ5 Why is simultaneous multi-wavelength data limited, and how does this affect the interpretation of the flares?

Key findings

• . The 2020 flare exhibited the highest gamma-ray flux ever recorded for PKS 0903-57, reaching (3.8±0.4)×10⁻⁶ ph cm⁻² s⁻¹ above 100 MeV, approximately 60 times the 4FGL catalog average.
• . The gamma-ray variability timescale of PKS 0903-57 was estimated to be 1.7±0.9 hours, indicating rapid variability on sub-day timescales.
• . Flare-1B, Flare-IA, Flare-IB, and Flare-IC were best fitted by the log-parabola (LP) spectral model, suggesting energy-dependent spectral curvature.
• . The total jet power required during flaring phases was estimated at a few times 10⁴⁶ erg/s, consistent with high-energy activity in blazars.
• . The study identified four distinct flare phases in 2018 (Flare-1A and Flare-1B) and five in 2020 (Pre-flare-I, Flare-IA, Flare-IB, Flare-IC, Post-flare-I), each with unique temporal and spectral behavior.
• . Despite multi-wavelength detections from Fermi-LAT, AGILE, HESS, ATCA, and DAMPE, simultaneous coverage remains minimal, limiting full SED modeling.