Minimal spanning tree algorithm for gamma-ray source detection in sparse photon images: cluster parameters and selection strategies

Abstract

The minimal spanning tree (MST) algorithm is a graph-theoretical cluster-finding method. We previously applied it to gamma-ray bidimensional images, showing that it is quite sensitive in finding faint sources. Possible sources are associated with the regions where the photon arrival directions clusterize. MST selects clusters starting from a particular "tree" connecting all the point of the image and performing a cut based on the angular distance between photons, with a number of events higher than a given threshold. In this paper, we show how a further filtering, based on some parameters linked to the cluster properties, can be applied to reduce spurious detections. We find that the most efficient parameter for this secondary selection is the magnitude M of a cluster, defined as the product of its number of events by its clustering degree. We test the sensitivity of the method by means of simulated and real Fermi-Large Area Telescope (LAT) fields. Our results show that sqrt(M) is strongly correlated with other statistical significance parameters, derived from a wavelet based algorithm and maximum likelihood (ML) analysis, and that it can be used as a good estimator of statistical significance of MST detections. We apply the method to a 2-year LAT image at energies higher than 3 GeV, and we show the presence of new clusters, likely associated with BL Lac objects.

16 pages, 7 figures. Accepted for publication in Astrophysics and Space Science

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• Application of the MST clustering to the high energy gamma-ray sky. II - Possible detection of gamma-ray emission from blazar candidates in the 1WHSP sample
• A MST catalogue of gamma-ray source candidates above 10 GeV and at Galactic latitudes higher than 20 degrees
• The Formation of Binary Star Clusters in the Milky Way and Large Magellanic Cloud
• Application of the MST clustering to the high energy gamma-ray sky. III - New detections of gamma-ray emission from blazars
• Visualizing the pulsar population using graph theory
• Minimum Spanning Tree cluster analysis of the LMC region above 10 GeV: detection of the SNRs N 49B and N 63A
• The Use of Minimal Spanning Trees in Particle Physics
• Two new high energy gamma-ray blazar candidates
• Application of the MST clustering to the high energy gamma-ray sky. IV - Blazar candidates found as possible counterparts of photon clusters
• Twelve-year update of the MST catalogue of gamma-ray source candidates above 10 GeV and at Galactic latitudes higher than 20 degrees
• High energy gamma-ray detection of supernova remnants in the Large Magellanic Cloud
• Search for $\gamma$-ray emission from SNRs in the Large Magellanic Cloud: a new cluster analysis at energies above 4 GeV
• A Fast Algorithm for Finding Point Sources in the Fermi Data Stream: FermiFAST
• New blazar candidates from the 9Y-MST catalogue detected at energies higher than 10 GeV