Investigating the evolution of the energetic budget during gamma-ray flares in blazars

Abstract

lactic and extragalactic sources, such as pulsars, X-ray binnaries, supernova remnants, relativistic jets in active galactic nuclei (AGN), γ-ray burst or starburst galaxies. Blazars dominate the extragalactic γ-ray sky, accounting for 94 % and 91 % of the known γ-ray emmiters in high and very high energy. Blazars are a type of AGN in which the relativistic jet is pointing towards the observer. Their Spectral Energy Distribution (SED) presents a double peak structure. According to their SED and the pressence of broad emission lines in their optical spectrum, blazars can be classified as Flat Spectrum Radio Quasars (FSRQ) and BL Lac objects (BLL). Depending on the energy of the SED peaks, BL Lacs are subdivided in Low Frequency Peaked (LBL), Intermediate Frequency Peaked (IBL) and Hard Frequency Peaked (HBL). Blazars show a great variability in flux in all wavelengths. Their flux could change in timescales from minutes to years. The periods in which the source increases its flux significantly are called flares. The flares could occur simultaneously at different energy bands, with a certain delay between bands, or in one single band. One of the highest flux variability is shown for γ-rays. These photons come from the relativistic jet, but the location and size of the emission region is still unkown. A rapid flux variability suggest that the emission comes from regions close to the black hole, and/or a small size emission region. However, some studies suggest that this emission comes from outer regions. Once they are produced, γ-rays do not travel uninterruptedly by the intergalactic medium. They interact via e −e + pair production with the ultraviolet, optical and infrarred photons emmited in galaxy evolution processes, star evolution and by heated dust. Such photons emitted during the whole Universe history are known as the Extragalactic Background Light (EBL). Since the Earth’s atmosphere is opaque to γ radiation, this spectral range cannot be observed directly from ground-based telescopes. For high energies (HE, E > 100 MeV), it is possible to detect γ-rays using space telescopes such as the Large Area Telescope onboard the Fermi satellite. However, as the number of γ-ray photons reaching the Earth decreases significantly with energy, detectors with large collection areas are needed to access the very high energy (VHE, E > 100 GeV) band. This makes unavailable the option of observing directly VHE photons from the space, therefore, indirect methods are needed. When γ-rays reach the atmosphere, they interact producing subatomic particle showers. As these particles travel faster than the light in the air, Cherenkov light flashes are produced. This phenomena can be used to indirectly detect γ-rays with ground Cherenkov telescopes such as MAGIC, HESS and VERITAS. The current generation of Cherenkov telescopes tipically work from energies above 100 GeV. The Cherenkov Telescope Array (CTA) will be the first VHE γ-ray open observatory. CTA is divided in two emplacements, CTA North is being constructed in La Palma, and it would work from energies above 30 GeV, reducing the gap between Earth and space detectors. Fermi-LAT monitorizes the entire sky each 3 hours while Cherenkov telescopes have a narrower field of view, using LAT data is crucial as trigger for Cherenkov observations. Within this context, the main objective of this work is to study the relationship between the times of arrival of the VHE photons and the lightcurve at high energy using Fermi-LAT available data, in order to optimize the observation strategy for CTA and current Cherenkov telescopes. To do that, we have studied a sample of all the 75 blazars from the TeVCat catalog. This catalog contains all sources detected in VHE using Cherenkov telescopes. To see if the features observed for the VHE detected blazars are common for the rest of the γ-ray blazars, we have compared the results obtained with a sample of the Fourth Fermi Gamma-ray LAT Catalog (4FGL). This comparison sample contains 1915 blazars. Another goal of this work is to study the homogeneity and the optical depth (τ) of the EBL. To do this study we have selected those sources whose redshift, z, is known. The sources selected are also at high galactic latitudes (|b| > 10º). The z data is obtained from the Fourth LAT AGN Catalog (4LAC). For these sources, we have computed the optical depth, τ, for their most energetic photons detected. The optical depth has been computed using the EBL models found in the bibliography, including Dominguez et al. 2011, using a python function. This python function gives the optical depth of a photon given the redshift and the energy. For the analysis of the Fermi-LAT data, we have used the package Fermitools developed by the Fermi Science Support Centre and the teams in charge of the instruments onboard Fermi. We have used photons with energies greater than 30 GeV (coincident withe the CTA North energy threshold) for a 3º ROI around each source. The timeframe studied consists of 13 years, between 04/08/2008 and 05/11/2021. We have obtained that for the 28 % of the FSRQ, 63 % of the LBL, 78 % of the IBL and the 92 % of the HBL LAT has detected photons of E > 30 GeV. In addition, BL Lac objects emit a greater number of photons at these energies than FSRQ. Also, the detected energies are greater. Within BL Lacs, the highest energies and the most of the photons are detected for HBL, followed by IBL. But the highest energy of the sample has been detected with LAT from an IBL with an energy of 1,97 TeV. This source is 4FGL J1558.9-6432, z=0,0796. The intrinsic spectra between 20 and 50 GeV of the BL Lacs follows a Power Law (PL) with spectral index, α = 1, 85. While FSRQ follow a PL with α = 1, 62. For higher energies, the number of intrinsic photons in FSRQ decreases faster with energy. For the sources with public lightcurves available, we have not observed flares with 5σ criteria but detected photons with E > 30 GeV for the 16 % of the FSRQ, 63 % of the LBL, 79 % of the IBL and the 90 % of the HBL. For the TeVCat sample, we have not found flares for 2 LBL, 4 IBL and 30 HBL. There is a general trend indicating higher energy photons are emitted when the relative flux, referred to the mean flux of the source, is smaller than for the less energetic ones. For FSRQ, we have seen higher relative fluxes when photons with E > 30 GeV are emitted than for the rest of the sample. In addition, for FSRQ, these photons are emitted at times closer to flares. We have also studied the clustering of emitted photons with E > 30 GeV for the TeVCat FSRQ, obtaining photons emitted with time differences shorter than 3 and 7 days. Finally, studying the EBL we have observed 3 sources at τ > 4: 4FGL J0035.2+1514 (τ = 4.68+0.18 −0.3 , z=1,09), 4FGL J1224.1+2239 (τ = 5.3+0.3 −0.5, z=0,48) and 4FGL J1522.1+3144 (τ = 8.23+0.03 −0.3 , z=1,49). However, a more detailed study would be needed fot the photon that produced the value τ = 8.23+0.03 −0.3 . Our results are also compatible with an homogeneous EBL as tested for γ-ray blazars located at |b| > 10º.