Identifying TeV source candidates among Fermi-LAT unclassified blazars with Artificial Neural Network.

G.Chiaro,  M.Meyer,  M. Di Mauro,  D.Salvetti, G. La Mura,  D.J. Thompson

P R E L I M I N A RY 

Blazars and in particular the subclass of high synchrotron peaked objects are the main targets for the present generation of Imaging Atmospheric Cherenkov Telescopes (IACTs)and will remain of great importance for very high-energy gamma-ray science in light of the future Cherenkov Telescope Array (CTA).

The observation time of high energy sources by IACTs is limited by their small field of view, by the presence of many competing source populations to observe and science cases to study. Therefore, it is important to select the most promising targets in order to save observation time and consequently to increase the number of detections. 

The aim of this study is to search for unclassified blazars that are likely detectable with Cherenkov telescopes within reasonable observation times using Artificial Neural Network (ANN) algorithm.

The 3FGL catalogue contains two classes of source with uncertain classification that offer opportunities to identify HBLs and subsequently TeV candidates according with the TeV catalog census : (i) the 573 BCU and (ii) 1010 UCSs. Recently in https://arxiv.org/abs/1602.00385 the authors applied a number of machine-learning techniques to classify 3FGL UCSs as pulsars or AGN. The authors found 334 pulsars, 559 sources of AGN type (UCS$_agn$) and 117 remained uncertain.

In https://arxiv.org/abs/1705.09832 the 559 UCS_agn have been further analyzed with the ANN algorithm and 271 UCS_bll, 185 UCS_fsrq and 103 UCS_bcu have been identifyied . 

The resulting 573 BCU and 103 UCS_bcu  represent the first targets for this search and we apply an optimized version of the ANN described in https://arxiv.org/abs/1709.05727 in order to compute the likelihood distribution of HBL and non-HBL  sources

We perform an analysis of Fermi-LAT data in order to find the gamma-ray SED of our HBL candidates and confirm the nature of them.
We analyze 104 months of Pass 8 data, from 2008 August 4 to 2017 April 4, selecting gamma-ray events in the energy range E=[0.1,1000] GeV, passing standard data quality selection criteria.

We also compare the extrapolated fluxes of the candidates  against the sensitivity of present IACTs and the future CTA. We use the Fermi-LAT spectral shape of the sources obtained in the range between 0.1 and 300 GeV using the best-fit model parameters from the LAT 4-year 3FGL Catalogue and particularly we referred to the following relation derived from the spectral model that fits the data.

The study is in progress and paper wil come soon.

ref.contact  Graziano Chiaro       chiaro@lambrate.iasf.it

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Data

3FGL blazar subclasses  HBL and non-HBL distribution against gamma-ray flux

    Likelihood distribution of HBL and non-HBL  sources in 3FGL blazars by our applied ANN algorithm. 

   This result could show that the applied algorithm is not able to clearly identify HBLs but the likelihood distribution can still be acceptable for the aim of this study

Distribution of the ANN likelihood to be HBL candidates of 3FGL BCUs. (right) and UCS_bcu  (left). Vertical blue and steel blue lines indicate the applied classification thresholds  to identify sources as High Confidence candidates.

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Full list of BCU HBL candidates. In Cols. 9, 10, 11 the observabilty at the IACT sites. On the top of the list the candidates with the highest Likelihood  (L> 0.89) .

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Full list of BCU HBL candidates. In Cols. 8, 9, 10 the observabilty at the IACT sites. On the top of the list the candidates with the highest Likelihood  (L> 0.89) 

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The 3D plot shows the distribution of HBL candidates against  the 3FGL blazar subclasses HBL [blue], IBL [green] , LBL [ red ].

All the candidates lie in the clean HBL  area validating the ANN results that classified the target as HBL sources.

TeV  candidates

We compare the extrapolated fluxes of the candidates against the sensitivity of present IACTs and the future CTA. We used the Fermi-LAT spectral shape of the sources obtained in the range between 0.1 and 300 GeV using the best-fit model parameters from the LAT 4-year 3FGL Catalogue and particularly we referred to the spectral model that fits the data.

We compare the extrapolated fluxes with the CTA sensitivity for 50 hours (5hours) of observations as a solid (dashed) grey line.
Above a declination of 0 degrees we use the sensitivity of the northern array and the southern array otherwise.
The CTA sensitivity for 5 hours of observations is similar to that of currently operating IACTs for 50 hours of observations except a higher threshold energy of ~ 80 GeV.