DETERMINATION OF THE POINT-SPREAD FUNCTION FOR THE FERMI LARGE AREA TELESCOPE FROM ON-ORBIT DATA AND LIMITS ON PAIR HALOS OF ACTIVE GALACTIC NUCLEI

Ackermann, M.; Ajello, M.; Allafort, A.; Asano, K.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Bloom, E. D.; Bonamente, E.; Borgland, A. W.; Bottacini, E.; Brandt, T. J.; Bregeon, J.; Brigida, M.; Bruel, P.; Buehler, R.; Burnett, T. H.; Busetto, G.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Caraveo, P. A.; Casandjian, J. M.; Cecchi, C.; Charles, E.; Chaty, S.; Chekhtman, A.; Cheung, C. C.; Chiang, J.; Cillis, A. N.; Ciprini, S.; Claus, R.; Cohen Tanugi, J.; Colafrancesco, S.; Conrad, J.; Cutini, S.; D'Ammando, F.; de Palma, F.; Dermer, C. D.; do Couto, e. Silva E.; Drell, S.; Drlica Wagner, A.; Dubois, R.; Favuzzi, C.; Fegan, S. J.; Ferrara, E. C.; Focke, W. B.; Fortin, P.; Fukazawa, Y.; Funk, S.; Fusco, Piergiorgio; Gargano, F.; Gasparrini, D.; Gehrels, N.; Germani, S.; Giglietto, N.; Giordano, Francesco; Giroletti, M.; Glanzman, T.; Godfrey, G.; Grandi, P.; Grenier, I. A.; Grove, J. E.; Guiriec, S.; Hadasch, D.; Hayashida, M.; Hays, E.; Horan, D.; Hou, X.; Hughes, R. E.; Jackson, M. S.; Jogler, T.; Johannesson, G.; Johnson, R. P.; Johnson, A. S.; Kamae, T.; Kataoka, J.; Kerr, M.; Knoedlseder, J.; Kuss, M.; Lande, J.; Larsson, S.; Latronico, L.; Lavalley, C.; Lee, S. H.; Longo, F.; Loparco, Francesco; Lott, B.; Lovellette, M. N.; Lubrano, P.; Mazziotta, M. N.; Mcconville, W.; Mcenery, J. E.; Mehault, J.; Michelson, P. F.; Mignani, R. P.; Mitthumsiri, W.; Mizuno, T.; Moiseev, A. A.; Monte, C.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Naumann Godo, M.; Nemmen, R.; Nishino, S.; Norris, J. P.; Nuss, E.; Ohsugi, T.; Omodei, N.; Orienti, M.; Orlando, E.; Ormes, J. F.; Paneque, D.; Panetta, J. H.; Pelassa, V.; Perkins, J. S.; Pesce Rollins, M.; Pierbattista, M.; Piron, F.; Pivato, G.; Poon, H.; Porter, T. A.; Raino', Silvia; Rando, R.; Razzano, M.; Razzaque, S.; Reimer, A.; Reimer, O.; Reyes, L. C.; Ritz, S.; Rochester, L. S.; Romoli, C.; Roth, M.; Sanchez, D. A.; Saz, Parkinson P. M.; Scargle, J. D.; Sgro, C.; Siskind, E. J.; Snyder, A.; Spandre, G.; Spinelli, Paolo; Stephens, T. E.; Suson, D. J.; Tajima, H.; Takahashi, H.; Tanaka, T.; Thayer, J. G.; Thayer, J. B.; Thompson, D. J.; Tibaldo, L.; Tibolla, O.; Tinivella, M.; Tosti, G.; Troja, E.; Usher, T. L.; Vandenbroucke, J.; Vasileiou, V.; Vianello, G.; Vitale, V.; von Kienlin, A.; Waite, A. P.; Wallace, E.; Weltevrede, P.; Winer, B. L.; Wood, K. S.; Wood, M.; Yang, Z.; Zimmer, S.

doi:10.1088/0004-637X/765/1/54

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from approximate to 20 MeV to > 300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of gamma rays from pulsars and active galactic nuclei (AGNs). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of gamma-ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGNs. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low-and high-redshift BL Lac AGNs and the TeV blazars 1ES0229 + 200 and 1ES0347-121.