[Paper Review] Searching for $γ$-ray signature in WHSP blazars: Fermi-LAT detection of 150 excess signal in the 0.3-500 GeV band

B. Arsioli, Yu‐Ling Chang|arXiv (Cornell University)|Sep 27, 2016

Astrophysics and Cosmic Phenomena61 references16 citations

TL;DR

This study uses targeted likelihood analysis on 400 high-synchrotron-peak (HSP) blazars from the 2WHSP catalog—previously undetected in Fermi-LAT catalogs—to search for $γ$-ray emission. It detects 150 $γ$-ray excess signals (85 with TS > 25), resolving a significant fraction of the extragalactic isotropic $γ$-ray background (IGRB), particularly at high energies (100–200 GeV), and introduces the 1BIGB catalog for faint $γ$-ray blazars.

ABSTRACT

A direct search of $γ$-ray emission centered on multi-frequency selected candidates is a valuable complementary approach to the standard search adopted in current $γ$-ray Fermi-LAT catalogs. Our candidates are part of the 2WHSP sample, that was assembled with the aim of providing targets for Imaging Atmospheric Cherenkov Telescopes (IACT), and is currently the largest set of high synchrotron peaked (HSP) blazars. We perform a likelihood analysis with the Fermi Science Tools using positions from 400 2WHSP blazars as seeds of tentative $γ$-ray sources. This enabled us to detect 150 $γ$-ray excess signals that have not yet been reported in previous $γ$-ray catalogs (1FGL, 2FGL, and 3FGL). By identifying new sources, we solve a fraction of the extragalactic isotropic $γ$-ray background (IGRB) composition, improving the description of the $γ$-ray sky. Our analysis considers the 0.3-500 GeV energy band, integrating over 7.2 yrs of Fermi-LAT observation and making use of the Pass 8 data release. Within the 150 2WHSPs that showed excess $γ$-ray signature: 85 are high-significance detections with test statistic (TS)$>$25, and 65 are lower-significance detections with TS between 10 to 25. We study the $γ$-ray photon spectral index distribution, the likelihood of detection according to the synchrotron peak brightness, and plot the measured $γ$-ray LogN-LogS of HSP blazars, also discussing the portion of the IGRB that has been resolved by this work. We report on four cases where we could resolve source confusion and find counterparts for unassociated 3FGL sources with the help of high-energy TS maps together with multi-frequency data. The 150 new $γ$-ray sources are named with the acronym 1BIGB for the first version of the Brazil ICRANet Gamma-ray Blazar catalog, in reference to the cooperation agreement supporting this work.

Motivation & Objective

• To detect previously undetected $γ$-ray emission from high-synchrotron-peak (HSP) blazars not present in 1FGL, 2FGL, or 3FGL catalogs.
• To improve the characterization of the extragalactic isotropic $γ$-ray background (IGRB) by resolving faint $γ$-ray sources.
• To validate a targeted search method using multi-frequency positions as seeds for likelihood analysis, enhancing detection sensitivity beyond blind surveys.
• To resolve source confusion and identify counterparts for unassociated 3FGL $γ$-ray sources using high-energy TS maps and multi-frequency data.

Proposed method

• Conduct likelihood analysis using Fermi Science Tools with 400 2WHSP blazar positions as seed sources, focusing on the 0.3–500 GeV energy band.
• Apply Pass 8 data from 7.2 years of Fermi-LAT observations to maximize sensitivity and reduce background contamination.
• Use a power-law spectral model in the 0.3–500 GeV band to characterize detected sources and compute test statistics (TS) for significance.
• Generate energy-dependent test statistic (TS) maps to improve source localization, especially for hard-spectrum sources with large PSF effects.
• Cross-reference high-energy TS maps with multi-frequency data (radio, IR, X-ray) to resolve source confusion and identify counterparts.
• Apply a figure-of-merit parameter based on synchrotron peak brightness to assess detection likelihood and spectral consistency.

Experimental results

Research questions

• RQ1 Can targeted $γ$-ray source detection using known HSP blazar positions improve sensitivity beyond blind Fermi-LAT catalogs?
• RQ2 What fraction of the extragalactic isotropic $γ$-ray background (IGRB) is resolved by detecting faint $γ$-ray sources in the HSP blazar population?
• RQ3 How effective are high-energy TS maps and multi-frequency data in resolving source confusion and improving source localization?
• RQ4 To what extent do HSP blazars with bright synchrotron peaks contribute to the high-energy IGRB (e.g., 100–200 GeV)?
• RQ5 Can previously unassociated 3FGL $γ$-ray sources be linked to HSP blazars using targeted analysis and multi-wavelength data?

Key findings

• A total of 150 $γ$-ray excess signals were detected from 400 2WHSP blazar seed positions, with 85 sources having test statistic (TS) > 25 and 65 with TS between 10 and 25.
• The 150 new $γ$-ray sources are cataloged as the 1BIGB (first version of the Brazil ICRANet Gamma-ray Blazar catalog), providing a new resource for $γ$-ray astronomy.
• The study resolves 6–8% of the extragalactic isotropic $γ$-ray background (IGRB) in the 100–200 GeV energy band, primarily from faint HSP blazars.
• High-energy TS maps (E > 3 GeV) improved source localization by up to 6.9′, demonstrating the value of energy-dependent analysis for resolving positional uncertainties.
• Four cases of unassociated 3FGL sources were resolved by identifying counterparts using high-energy TS maps and multi-frequency data, confirming the method’s utility in source confusion mitigation.
• The spectral index distribution of detected HSP blazars and their flux distribution indicate that the method effectively probes the faint-end of the $γ$-ray source function, down to TS ≈ 10.