Application of the MST clustering to the high energy γ $\gamma $ -ray sky. IV—Blazar candidates found as possible counterparts of photon clusters

Abstract

We present the results of a cluster search in the Fermi -LAT Pass 8 γ-ray sky by means of the Minimum Spanning Tree algorithm, at energies higher than 10 GeV and at Galactic latitudes higher than 25• . The selected clusters have a minimum number of photons higher than or equal to 5, a high degree of concentration, and are without a clear corresponding counterpart in blazar catalogues. A sample of 30 possible γ-ray sources was obtained. These objects were verified by applying the standard Maximum Likelihood analysis on the Fermi -LAT data. A search for possible radio counterparts in a circle having a radius of 6 was performed, finding several interesting objects, the majority of them without optical spectroscopical data. These can be considered as new blazar candidates. Some of them were already noticed as possible blazars or Active Galactic Nuclei in previous surveys, but never associated with high energy emission. These possible counterparts are reported and their properties are discussed.

New blazar candidates from the 9Y-MST catalogue detected at energies higher than 10 GeV

“…It is interesting that about all objects are BL Lac candidates and the only one with a FSRQ bordeline position has a featureless SDSS spectrum. This finding agrees with the results of a previous blazar search (Campana et al 2016c) in which only three objects in a sample of 30 exhibited mid-IR colours in the FSRQ region. A possible explanation is that our cluster selection at energies higher than 10 GeV is biased to extract BL Lacs, and particularly HBL sources, rather than FSRQs.…”

“…We searched for possible counterparts and obtained a sample of 24 spatial clusters of photons whose centroids have an angular separation from radio sources comparable to those found for clusters already associated with optical/IR objects exhibiting properties typical of blazars. In a series of papers (Bernieri et al 2013;Campana et al 2015Campana et al , 2016aCampana et al ,b,c, 2017 we described the results of similar searches of possible new blazar counterparts of γ-ray clusters found by means of MST, which were, with a very few exceptions, confirmed by subsequent analyses.…”

Minimum Spanning Tree cluster analysis of the LMC region above 10 GeV: detection of the SNRs N 49B and N 63A

“…In a series of papers (Bernieri et al 2013;Campana et al 2015Campana et al , 2016cCampana et al ,b,a, 2017 we applied successfully the Minimum Spanning Tree (hereafter MST, Campana et al 2008Campana et al , 2013 source-detection method for searching new γ-ray sources closely associated with known BL Lac objects and blazar candidates, and illustrated how MST works in finding clusters having a small number of photons, but likely related to point-like sources.…”

Application of the MST clustering to the high energy γ $\gamma$ -ray sky. II—Possible detection of γ $\gamma$ -ray emission from blazar candidates in the 1WHSP sample

“…In a companion paper (Campana et al 2016) we will instead focus on the analysis of clusters matching sources already classified as blazars and reported in the Roma BZCAT catalogue, extending the results of Paper I.…”