Time Series Analysis of Fermi-LAT Blazars

Abstract

Active Galactic Nuclei (AGN) are extremely energetic sources with an intense luminescence emanating from the center of galaxies, caused by matter accretion on the SMBH. These luminous objects are essential for measuring cosmological distances and epochs, but their study is expanding swiftly in astrophysics due to the high rates and small scales at which they release energy. AGNs are divided into two primary categories: radio-loud and radio-quiet. Researchers have searched for Quasi Periodic Oscillations (QPOs) in AGNs for years, but the Fermi-LAT has provided ample data for long-term studies. Some studies have observed QPOs in gamma wavelength, with jets being the primary emission source. The binary black hole model is better explained for decade-long periodicities, while jets are better explained for year-time periodicities. Multiple physical models have been formulated to explain the existence of QPOs in AGNs. Variability in the AGNs are typically divided into three time scales, Intra Day Variability (IDV), Short Term Variability (STV), Long Term Variability (LTV). IDV spans on the timescale of a few minutes to less than a day, STV spans the timescale of a day to a few weeks or months and LTV spans over a few months to years. The search for QPOs in gamma-ray Light Curve (LC) involves treating the Light Curve as a time series and applying time series analysis methods. Common methods include Lomb Scargle Periodogram (LSP), Weighted Wavelet Z Transform (WWZT), and Power Spectral Density (PSD). In this study, LSP and WWZT are used simultaneously to search for QPOs. LSP is a version of the Discrete Fourier Transform (DFT) and is used to find periodicities in unevenly spaced temporal signals. WWZT, on the other hand, is a widely used wavelet transform method that fits sinusoidal signals varying frequency and location in the time series, allowing for identifying QPOs in specific parts of the time series. Multiple nations have collaborated on the Fermi Gamma-ray Space Telescope to investigate the universe at Gamma wavelength. In 1991, the Energetic Gamma Ray Experiment Telescope (EGRET) conducted the first comprehensive sky survey in the high High Energy Gamma Ray wavelength. Fermi features two instruments: an enhanced version of the gamma-ray imaging spectrometer onboard EGRET and another instrument to investigate gamma-ray bursts (GRB). The primary instrument, the Large Area Telescope (LAT), has a higher sensitivity, allowing for a more comprehensive coverage of transient phenomena. The Gamma Burst Monitor (GBM) is a secondary instrument designed to observe GRBs. This study has shown that there is a very good possibility of QPOs in gamma wavelength that were previously not noticed due to a lack of long-term data. Further, it is shown in the study that the QPO observed has no correlation between the Mass of SMBH at the centre of the galaxy and observed periodicity. Similarly, no correlation was observed between the Redshift of the galaxy and observed periodicity.