The Large Area Telescope on-board of the Fermi Gamma-Ray Space Telescope Mission

Transcription

1 The Large Area Telescope on-board of the Fermi Gamma-Ray Space Telescope Mission 1

2 Outline Mainly from 2009 ApJ The Pair Conversion Telescope The Large Area Telescope Charged Background and Events Classification Energy Resolution Containment and Source Location Effective Area and Sensitivities Exposure A summary of the LAT Parameters 2

3 The Pair Conversion Telescope Photon cross-section in Pb. PDG compilation Photo-electric effect dominant below 300 kev Compton scattering dominant between 300 kev and few MeV Pair-production dominant above few MeV 3

4 The Pair Conversion Telescope Gamma Direction and Energy measurement, backgrounds rejection Gamma conversion in a electron-positron pair within the tracker No magnetic field, straight lines tracks, but Multiple Coulomb Scattering Resulting two tracks point back to incident Gamma incoming direction Measurement total energy of electron-positron pair in the Calorimeter Surrounding Anti-Coincidence Detector (ACD) for vetoing charged particles γ Charged particle anti-coincidence shield Conversion foils (W) tracking detector Calorimeter (energy measurement) e+ e- Scheme of pair conversion telescope 4

5 The Large Area Telescope 4X4 towers modular structure No consumables Precision Si-strip tracker : Si-strip detector, W converter foils, 80 m2 of Si active area, 228 micrometers pitch 1.5 radiation lengths on-axis Front = 12X0.03 rad.lenght Back = 4X0.18 rad. lenght Hodoscopic CsI calorimeter : array of 1536 CsI(Tl) crystals in 8 layers. 8.6 radiation lengths on-axis. Segmented Anti-Coincidence Detector : 89 plastic scintillator tiles and 8 ribbons. charged particles veto ( average detection efficiency). 5

6 The Particles Background Orbit averaged background fluxes of the various components protons (green filled triangles up), He (purple filled triangles up), electrons (filled red squares), positrons (light blue squares), Earth albedo neutrons (black squares) Earth albedo gamma-rays (dark blue filled triangles down). The geomagnetic cutoff is seen at 3 GeV for protons and electrons, and at higher energy for helium nuclei. At low energies the curves show the sum of re-entrant and splash albedo for electrons and positrons. 6

7 Events Classification Track reconstruction (Calorimeter Seeded Pattern Recog, Blind Search Pattern Recog., Kalman Filter) Energy reconstruction (Parametric Correction, Shower Profile, Likelihood) Classification with Classification Trees exploiting a large number of experimental observables (event topology, ADC, energy) Transient Class (2 Hz, maximum effective area) SourceClass (0.4Hz, bck comparable to EGB) Diffuse Class (0.2Hz., bck comparable to irriducible) Possibility to define new extra-cuts and classes The integral gamma-raydiffuse flux considered is ph cm 2 s 1 sr 1 above 100 MeV. 7

8 Energy Resolution Upper figure: Energy resolution versus energy for normal incidence (solid curve) and at 60 off-axis (dashed curve). Lower figure:energy resolution as measured with the LAT calibration unit in CERN electron beam tests. the reconstructed energy, with the LK method is the solid peak example with beam at 10GeV The beams entered the calibration unit at an angle of 45 to the detector vertical axis. 8

9 Containment Angle and Source Location Above: 68% containment radius versus energy at normal incidence (solid curve) and at 60 off-axis (dashed curve) for conversions in the thin section of the tracker. Below: LAT 68% confidence radii localizations for a source with integral flux (above 100 MeV) of 10 7 ph cm 2 s 1 versus source spectral index for a source detected in the one-year sky survey. Variations of viewing angle are considered. In effect, the source viewing angle is averaged over in sky-survey mode. 9

10 Effective Area Below: Effective area versus energy at normal incidence for Diffuse (dashed curve), Source (solid curve), and Transient (dotted curve) analysis classes. Above: Effective area versus energy at normal incidence (solid curve) and at 60 off-axis (dashed curve) for Source analysis class. 10

11 Differential Sensitivity Above: Differential source sensitivity ( 5 sigmas) for 1 year sky survey exposure. The assumed source photon index is 2.0 and background integral flux (above 100 MeV) of ph cm 2 s 1 sr 1 (dotted curve) with photon index 2.1 (similar to diffuse extra-galactic emission). The backgrounds 10 and 100 times higher for the dashed and solid curves. 11

12 Integral Sensitivity Below: Integral source sensitivity ( 5 sigmas) for 1 year sky survey exposure. Same assumption as for the differential sensitivity plot. 12

13 Sensitivity vs Exposure LAT source sensitivity for exposures on various timescales. Each map is an Aitoff projection in galactic coordinates. In standard sky-survey mode, nearly uniform exposure is achieved every 2 orbits, with every region viewed for 30 min every 3 hours. 13

14 The Lat Parameters 14

15 Scientific Results: stats LAT: ~2Hz gamma-ray rate LAT collaboration ~ 240 members 65 articles published by the collaboration (March 2010) 1 in in in the first months of accepted for pubbl. 7 articles on Science 1 article on NATURE + 1 accepted Additionally tens of articles have been published by collaborators smaller groups First article on Science by external authors with LAT data 15

16 Scientific Results: Catalogs Fermi Large Area Telescope First Source Catalog arxiv: , 2010 ApJS accepted. (1FGL)contains 1451 sources detected and characterized in the 100 MeV to 100 GeV, first 11 months data. The First Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope arxiv: , includes 671 gamma-ray sources at high Galactic latitudes ( b > 10 deg), with TS> 25 and associated statistically with AGNs. The First Fermi Large Area Telescope Catalog of Gamma-ray Pulsars 2010ApJS A. Contains 46 high-confidence pulsed detections using the first six months of data 16

17 An Example of Very Bright Source: Vela The Vela pulsar is the brightest persistent source in the GeV sky and thus is the traditional first target for new γ-ray observatories. We report here on initial Fermi Large Area Telescope observations during verification phase pointed exposure and early sky survey scanning. We have used the Vela signal to verify Fermi timing and angular resolution. The high quality pulse profile, with some 32,400 pulsed photons at E 0.03GeV, shows new features, including pulse structure as fine as 0.3ms and a distinct third peak, which shifts in phase with energy. We examine the high energy behavior of the pulsed emission; initial spectra suggest a phas averaged power law index of = with an exponential cut-off at Ec = 2.9 ±0.1GeV. Spectral fits with generalized cut-offs of the form e (E/Ec)b require b 1, which is inconsistent with magnetic pair attenuation, and thus favor outer magnetosphere emission models. Finally, we report on upper limits to any unpulsed component, as might be associated with a surrounding synchrotron wind nebula (PWN). 17

18 Another Example: an Extended Source Discovery of Extended Gamma-ray Emission in the Direction of Supernova Remnant W51C The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant ( 104 yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the GeV band exhibits steepening toward high energies. The luminosity is greater than erg s 1 given the distance constraint of D > 5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shockcloud interactions. The decay of neutral -mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to nhwp (D/6 kpc)2 erg cm 3. Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays. 2009ApJ...706L...1A 18

19 Another Example: an Extended Source 2009ApJ...706L...1A 19