Conclusions
In this thesis I have presented the work for my Ph.D. project, that focuses on the sim- ulation of γ-ray pulsar emissions for the GLAST mission, scheduled for launch at the end of 2007.
The GLAST Large Area Telescope (LAT) will be a powerful instrument for studying γ-ray pulsars, and we expect many important discoveries because of the optimal detec- tion performances of this new-generation γ-ray telescope.
GLAST will have sensitivity and angular and energy resolutions better than its pre- decessor EGRET and will study in more details the already-known classes of sources.
Additionally we expect that it will discover many new sources in the γ-ray sky.
The optimal detection performances of GLAST are a direct consequence of the new de- tection technology employed for designing the instrument subsystem, the Calorimeter, the Tracker and the AntiCoincidence Detector. A detailed description of the LAT has been given in Chapter 2. LAT is a pair conversion telescope, that allow the incoming γ-rays to convert in an electron-positron pair whose energy and direction can be mea- sured to find the energy and direction of the primary γ-ray.
In particular the silicon microstrip Tracker has many advantages compared to previous tracking detectors, e.g. high detection efficiency, high spatial resolution and no con- sumables. The segmentation of the Calorimeter will add the capability to image the electromagnetic shower produced by the electron-positron pair and will also improve the event recontruction and background rejection. The segmented Anticoincidence De- tector will help reduce the self-veto problem for high-energy photons. This will further increase efficiency and effective area at energies above 10 GeV.
The optimal detection performances of the LAT will provide higher statistics in order to study γ-ray pulsars and explore its emission at energies above 10 GeV.
Pulsars are not the only target of the LAT. From past missions we know that the γ- ray Universe is incredibly dynamic and rich in sources.
Since pulsars are source emitting in various energy window it is important to know how their appear at wavelengths other than γ-rays. In Chapter 3 I give a brief review of the basic properties of, and model for pulsars as Neutron Stars and their characteristic emission in radio, optical and X-rays.
Presently only seven γ-ray pulsars have been detected but we expect that LAT will sub- stantially increase this sample and our knowledge in this field, as it has been discussed in Chapter 4. The high timing accuracy will permit study of pulsar lightcurves with increased time resolution, in order to reveal temporal substructure in the lightcurve.
The study of pulsar specral cutoff of pulsars will be possible thanks to the wide spectral range and high energy resolution of the LAT at high energies, greater than few GeV. As
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discussed in more detail in Chapter 9 this will help to constrain the two main emission scenarios, the Polar Cap and Outer Gap models.
Another very exciting possibility of the LAT will be the discovery of many new γ-ray pul- sars. As discussed in Chapter 4 this number ranges from some tens to some hundreds of new pulsars.
Another interesting possibility is the discovery of new γ-ray pulsars, radio-loud or even radio-quiet like Geminga. Altough in this thesis the blind searches have been not de- scribed in detail, the LAT Collaboration has made also efforts in developing and im- proving such techniques.
In order to better understand the science capabilities of the LAT for pulsars it is impor- tant to have simulated data that are as realistic as possible.
The PulsarSpectrum simulator has been created and developed in this project in order to provide GLAST members with a complete tool for simulating detailed timing model and spectral distribution of γ-rays.
As discussed in Chapter 5 PulsarSpectrum is based on two main simulation models, a phenomenological model that produces a spectrum obtained from an analytical expres- sion, and a more flexible model that can simulate any arbitrary phase-energy photon distribution.
The photons that are extracted from the source model are then processed and corrected in order to consider several effects, mainly 1) barycentric effects due to GLAST motion and relativistic corrections; 2) period change with time; 3) timing noise and 4) orbital motion for simulated binary pulsars.
During this project I also improved them and I found that they permit to obtain high- detailed simulations that match very close the observed features of γ-ray pulsars ob- served by EGRET.
PulsarSpectrumhas been also used for testing the pulsar analysis tools of the LAT Stan- dard Analysis Environment, in order to find for possible bugs and to study the feasibility of possible improvements of the analysis tools. In order to show how PulsarSpectrum can be used to simulate γ-raypulsars and how LAT simulated data can be simulated I proposed some simple analysis cases in Chapter 6. This also helped show how Pul- sarSpectrum can be used for testing analysis tools, developing analysis techniques and gaining pratice with LAT data. Three of the EGRET pulsars - Vela, PSR B1706-44 and PSR B1951+32 - have been simulated and analyzed as examples.
The PulsarSpectrum simulator has been used by the GLAST LAT Collaboration for simulating the pulsars in the Data Challenge 2 (DC2), as explained in Chapter 7. Since the simulation of the whole pulsar population in DC2 has been under the responsibility of PulsarSpectrum simulator, I developed and extended PulsarSpectrum with a set of macros and programs of the Pulsar Simulation Suite , that made possible the simulation of hundreds of pulsars at the same time. These tools were very useful for generating the final catalog of simulated pulsars for the DC2 skymodel.
The DC2 offered a first possibity to have a realistic full sky simulation of the γ-ray sky, LAT scientists had the possibility to work on it and develop new analysis techniques and test LAT Analysis Tools. In particular, for pulsar analysis I developed a suite of scripts called pyPulsar for automated analysis of large samples of pulsars, discussed n Chapter 8. This program have been successfully applied to LAT DC2 data, demonstrating the feasibility of an automated, script-based analysis tool for γ-ray pulsars. This analysis
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highlighted some general LAT capabilities for studying pulsar population, when the the- oretical model is folded with a detailed description of the LAT response function as in the DC2.
One of the most interesting fields where LAT data are expected to be particularly useful is the study of high-energy spectral cutoffs in order to put constraints on the emission scenario. The Chapter 9 is devoted to such a study, where two sample simulations for Polar Cap and Outer Gap models for Vela pulsar are introduced and LAT ability to distinguish between them is presented. In this Chapter I show that LAT will be able to discriminate over a timescale of few of week of observation in scanning mode, but shorter observation times are required for pointed observation.
The PulsarSpectrum simulator that has been used successfully for Data Challenge 2 is also presently in use for the next full sky simulation runs called Service Challenges, that are also a testbench for studying the data production and calibration before and after launch.
The work presented in this thesis has been developed to provide a basic simulator for pulsars that has been expanded to reach a high degree of detail. This results in a wide use of PulsarSpectrum by the LAT Collaboration for several activities, in particular testing of analysis tools and development of new analysis techniques. In the period preceding launch it is being also used to obtain some general information on the LAT science capabilities for pulsar detection.
Most of the simulation and analysis work developed in this thesis have been proved useful to the LAT Collaboration for studying and developing detection and analysis strategy in order to better organize the high discovery potential of the LAT for γ-ray pulsar science.
Most of the instruments developed here can be easily adapted to study mode focused analysis strategies: this work is ongoing in collaboration with other members of the LAT Collaboration in order to insure high-detail simulations of pulsars for the period before the launch of the Gamma Ray Large Area Space Telescope.
