4. Ph. D. research

"Search for galactic diffuse gamma-ray emission

around 100 GeV with the CELESTE atmospheric Cherenkov detector"

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Laboratoire de Physique Théorique et Astroparticules (LPTA)

UMR 5207 CNRS/IN2P3 - Université Montpellier II

CC 070 - Bat. 13 - Place E. Bataillon

34095 Montpellier Cedex 05 - France

http://www.lpta.univ-montp2.fr/

French Ph.D. book and defense slides

Ph. D. Supervisor:

Dr. Agnieszka Jacholkowska

Directeur de Recherche CNRS

Ph. D. summary:

The study of the Galactic diffuse emission is an important topic of the GeV gamma-ray astronomy. It allows indirect knowledge of acceleration and propagation processes of charged cosmic rays in our Galaxy. This is possible since most of the diffuse gamma-ray emission from the Galactic plane is the result of charged cosmic rays interacting with gas of the interstellar medium and radiation fields such as CMB and infrared background. Detectors on satellites (EGRET) measured the Galactic diffuse gamma-ray fluxes at energies below few GeV and Milagro at TeV energies. Only upper limits were provided above 500 GeV (Whipple, HEGRA, TIBET). As there is a lack of measurements between 10 GeV and 500 GeV, I have searched for diffuse emission in the OFF-source data of the CELESTE experiment, first atmospheric Cherenkov detector with an energy threshold under 100 GeV.

A gamma-ray entering the atmosphere creates an electromagnetic shower which produces Cherenkov light in the blue-UV wavelengths which can be detected as a pool of several tens of meters on the ground. CELESTE was the first gamma-ray atmospheric Cherenkov telescope which detected signal from the Crab Nebula and the Mrk 421 and Mrk 501 blazars with an energy threshold below 100 GeV. Located in the French Pyrénées, 1650 m a.s.l., CELESTE operated from 1999 to 2004. It used between 40 and 53 of the 54-square meter mirrors of the former solar plant of Thémis and the 100 m-tower where secondary optics, photomulipliers and a 1 GHz fast acquisition electronics were installed. Because of the very strong hadronic isotropic background coming from the showers induced by cosmic protons and ions, data was taken in ON/OFF pairs, i.e. by pointing the telescope on the source direction (ON) and away from the source sky region (OFF). The gamma-ray signal is then extracted by subtracting ON and OFF source region data after analysis cuts on variables based on the geometric and timing properties of the Cherenkov wavefront, which allows to discriminate between signal and background events.

One first important part of my Ph. D. thesis was to perform and study a large set of Monte-Carlo (MC) simulations, after demonstrating the simulation/data compatibility for gamma rays by comparing the distributions of the discriminant variables. Trigger rate of the detector was reproduced by hadronic MC rates. Spectral models and diffuse gamma-ray simulations allowed estimations of the CELESTE sensitivity of diffuse gamma-ray radiation: from 0.2 to 1.5 gamma per minute.

In the view of detecting Galactic diffuse gamma-rays mostly located at low galactic latitudes, I performed, in a second important part of my Ph. D., the analysis of some CELESTE OFF source data, comparing data taken near the Galactic plane with data taken away from the Galactic plane, based on the same ON-OFF method, where the “ON” is now replaced by the OFF region nearest to the Galactic equator. Since models and extrapolations from fluxes measured at lower energies predict a rather low signal, I used an analysis method based on a composed discriminant variable. It aims to improve the sensitivity of the instrument, which is limited by the hadronic background. Data used in this study was selected with criteria based on atmospheric and acquisition stability. A first step in this analysis was to verify that there is no significant systematic effect in the OFF-OFF pair association of high Galactic latitudes data taken at different dates but at the sames local coordinates in the sky. A second step was to perform OFF-OFF analysis with other OFF acquisitions taken at different dates to search an event excess from the data acquisitions nearest to the Galactic equator.

With only one hour of data in the Galactic anti-centre direction, an upper limit with a 95 % C.L. was provided at 100 GeV, giving a limited integral flux of the Galactic gamma-ray diffuse emission:

F(E>100 GeV) < 8.3 x 10-3 m-2 s-1 sr-1.

In addition, I have actively participated in CELESTE data taking on the Thémis site till its closure in June 2004.