The Fermi Large Area Telescope preliminary results from

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The Fermi Large Area Telescope: preliminary results from on-orbit high level performance verifications and

The Fermi Large Area Telescope: preliminary results from on-orbit high level performance verifications and monitoring S. Germani on behalf of the Fermi LAT Collaboration INFN and Università di Perugia germani@pg. infn. it 44 th Rencontres de Moriond La Thuile February 1 -8 2009 La Thuile 1 -8 Feb 2009 S. Germani

The Large Area Telescope Si Tracker pitch = 228 mm 18 planes g ACD

The Large Area Telescope Si Tracker pitch = 228 mm 18 planes g ACD segmented scintillator tiles The Observatory LAT: 4 x 4 modular array 3000 kg, 650 W 20 Me. V – 300 Ge. V e+ e. Cs. I Calorimeter hodoscopic arry La Thuile 1 -8 Feb 2009 S. Germani 2

Tracker Details Tracker Tower y x x y y x x y y x

Tracker Details Tracker Tower y x x y y x x y y x x y Tracker: angular resolution is determined by: multiple scattering (at low energies) => Many thin layers position resolution (at high energies) => fine pitch detectors Front Layers (2. 7%X 0 W converter) Back Layers (18%X 0 W converter) Blank Layers (No W converter) Si layer W coverter q Front (thin) conversion layers to have small multiple scattering errors at low energies q Back (thick) layers to increase conversion probability Cs. I Calorimeter See Atwood et al. 2007, Astropart. Phys. 28: 422 -434 La Thuile 1 -8 Feb 2009 S. Germani 3

LAT Gamma Candidate Events The green crosses show the detected positions of the charged

LAT Gamma Candidate Events The green crosses show the detected positions of the charged particles, the blue lines show the reconstructed track trajectories, and the yellow line shows the candidate gamma-ray estimated direction. The red crosses show the detected energy depositions in the calorimeter. La Thuile 1 -8 Feb 2009 S. Germani 4

LAT Performance - Field of View http: //www-glast. slac. stanford. edu/software/IS/glast_lat_performance. htm q The

LAT Performance - Field of View http: //www-glast. slac. stanford. edu/software/IS/glast_lat_performance. htm q The LAT has a huge field of view q The PSF is good up to high inclination angles La Thuile 1 -8 Feb 2009 S. Germani 5

Observatory performance Example of LAT position error box Point Spread Function (PSF) Effective area

Observatory performance Example of LAT position error box Point Spread Function (PSF) Effective area See Abdo et al. 2008, Science 322, 1218 La Thuile 1 -8 Feb 2009 S. Germani 6

First Fermi view of the Vela Pulsar 100 Me. V < E < 10

First Fermi view of the Vela Pulsar 100 Me. V < E < 10 Ge. V q Turns nearly completely off between the double pulses q Energy spectrum consistent with simple exponential cutoff (b=1): See M. Razzano Talk on: Early Fermi Lat observations of the Vela Pulsar See Abdo et al. 2009 Ap. J in press, ar. Xiv: 0812. 2960 La Thuile 1 -8 Feb 2009 S. Germani 7

PSF measurement with Vela 1 2 3 La Thuile 1 -8 Feb 2009 1.

PSF measurement with Vela 1 2 3 La Thuile 1 -8 Feb 2009 1. Select In-Pulse events 2. Use scaled Off-Pulse events to evaluate background 3. Subtract scaled Off-pulse events to In-Pulse events to get deviation from Vela position S. Germani 8

PSF scaling with Energy The cutoff in the Vela spectrum limits this method up

PSF scaling with Energy The cutoff in the Vela spectrum limits this method up to a few Ge. V La Thuile 1 -8 Feb 2009 S. Germani 9

Detailed PSF verification q PSF shows good agreement with expectations up to few Ge.

Detailed PSF verification q PSF shows good agreement with expectations up to few Ge. V q Also the PSF tails are well in agreement with the expectations Front+Back La Thuile 1 -8 Feb 2009 S. Germani 10

PSF vs Inclination angle q PSF shows good agreement with expectations q Very small

PSF vs Inclination angle q PSF shows good agreement with expectations q Very small variation with inclination angle q Very good angular resolution across all the field of view is confirmed La Thuile 1 -8 Feb 2009 S. Germani 11

PSF stability Azimuth angle Geomagnetic Latitude q PSF shows good stability with respect to

PSF stability Azimuth angle Geomagnetic Latitude q PSF shows good stability with respect to the azimuth angle q Background rates vary with the Geomagnetic Latitude but the PSF seems not to be affected La Thuile 1 -8 Feb 2009 S. Germani 12

PSF Monitoring with Vela q The Vela Pulsar is the most luminous steady point

PSF Monitoring with Vela q The Vela Pulsar is the most luminous steady point source in the high energy gamma-ray sky q Use Vela to perform one measurement per week q Very good stability La Thuile 1 -8 Feb 2009 S. Germani 13

Global Alignment and absolute pointing accuracy q The absolute pointing on Fermi is obtained

Global Alignment and absolute pointing accuracy q The absolute pointing on Fermi is obtained from 2 star trackers mounted on the spacecraft. q Can also determine the absolute pointing from an ensemble of known gammaray sources. q Alignment between star tracker and LAT has been calibrated on orbit and it is stable Misalignemnt representd by three axis rotation ( x , y , z) First 60 days of mission with different operation modes (pointed, survey) and temperature changes La Thuile 1 -8 Feb 2009 S. Germani 14

Conclusions q The PSF shows very nice agreement with expectations up to a few

Conclusions q The PSF shows very nice agreement with expectations up to a few Ge. V and up to very high inclination angles (80 deg) q The LAT performances are very stable q The LAT performances are not affected by background rate variations q High energy PSF verification is ongoing with alternative methods Thank You ! La Thuile 1 -8 Feb 2009 S. Germani 15