Julie McEnery

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GLAST Science Simulations
Julie McEnery NASA/GSFC
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Flow of Simulation
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Reconstruction Overview

• Goal of the reconstruction
• Determine the incident gamma direction and energy
• Provide tools for suppressing background
• Follow a two pass approach for Cal and Tracker
• Best energy from Calorimeter needs tracking
  information
• Best tracks from Tracker needs best energy
  reconstruction
• Solve by breaking into two passes
• Background suppression
• ACD Recon
• MIP Finder
• Analysis step at end of reconstruction
• Create output ntuple for detailed performance
  studies
• Event Classification determine the particle
  type and quality
• Create FT1 output for analysis (convert to RA/DEC
  etc)

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Background Model

• The background model consists of several
  components
• Gamma-ray albedo from the Earth
• Electron, positron and proton albedo (splash and
  reentrant)
• Primary cosmic-ray (Electons, positrons, protons
  and heavy ions)
• Background spectra and flux model includes orbit
  position and direction dependencies.

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Astrophysical sources

• The sources are defined by creating input files
  which specify location, flux, spectrum, spatial
  extent etc
• Blazar Populations (provide flux, spectra and
  redshift)
• Modified version of Chiang and Mukerjee
• Stecker
• Giommi (astro-ph/0508034) this also provides
  optical - X-ray fluxes
• Variability models
• Simple red noise model
• Inverted PSDs (Timmer and Konig 1995)
• Shot noise model (Terrel and Olsen 1972)
• Non-linearity (Uttley et al 2005)
• Spectral models (for variabilty studies have been
  fairly simple power-laws, broken power laws or
  log-parabola

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Astrophysical Sources

• Extragalactic background light for source
  classes that have been specified with a redshift
  we can apply a choice of EBL models to attenuate
  the spectra.
• Kneiske best fit model (Kneiske et al 2004)
• Kneiske high-UV (Kneiske et al 2004)
• Primack99 (Primack et al 1999)
• Primack05 (Primack et al 2005)
• Salamon and Stecker 1998
• Stecker 2005
• It is relatively straightforward to mix and match
  population, variability and EBL models.

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Astrophysical Sources

• Pulsar populations generated using code developed
  by Alice Harding and Peter Gonthier. Given a
  pulsar emission model it provides flux, spectra,
  timing properties and location for populations of
  radio-loud and radio quiet pulsars.
• The simulated pulsar spectra are parameterised as
• The lightcurves are either randomly generated
  (useful for making a population of pulsars) or
  can be explicitly specified.

Nel and De Jager, 1995
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Astrophysical Sources

• GRB
• Phenomological Specify alpha, beta, flux
  duration and the package will generate a GRB.
• Physical model more complete physical simulation
• Extended sources
• Individual sources modeled as 2 or 3 d maps
  (Galactic diffuse, SNR, dark matter etc)

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Data Challenges

• A series of end-to-end studies
• Produce 1 day one precession period (55 days)
  release this to the collaboration for science
  analysis.
• Drives instrument performance studies, exercises
  detector simulation software.
• Science input include in the simulation
  science/astrophysical features we expect to see
  in the GLAST gamma-ray sky, verify that the
  analysis tools can find them.
• Tests data servers, user level documentation and
  more
• DC1. Modest goals. 1 simulated day.
• DC2, start March 06. More ambitious goals.
  Encourage further development, based on lessons
  from DC1. 55 day simulation.

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Data Challenge 2 (March May 2006)

• 55 days of simulated GLAST data
• Brings together many elements Including
• Full (Geant4) detector simulation
• Realistic detector description (electronic
  deadtime, some small imperfections)
• Realistic orbit and attitude profile
• Updated particle background model (including
  orbit variations)
• Updated event reconstruction and event
  classification analysis
• Updated instrument response functions
• Data formats for input to high level tools
• Plausible model of the gamma-ray sky (This is
  kept a secret until the close of DC2)
• Generation of datasets (Level 1 pipeline)
• Populate and exercise dataservers at GSSC and LAT
• Alpha test high level science tools and
  documentation
• The challenge is for the DC2 participants to
  figure out the properties of the simulated
  gamma-ray sky.

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The DC2 Sky

• DC2 sky in galactic coordinates

Plot by Seth Digel
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DC2 Catalog
Catalog analysis pipeline developed by Jean
Ballet and collaborators, runs a source detection
algorithm and then runs more detailed analysis on
each source to produce a table of the basic
gamma-ray properties of each source. Released at
the beginning of DC2, provides a starting point
for more detailed analyses.
380 sources
Verifies the automated pipeline processing from
low-level data to source characterization.
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More on Catalogs
Cross reference the catalog sources against other
catalogs to produce identifications. Displays the
catalog data in an interactive way and links in
data from other wavelengths.
Produced by ASDC group Giommi and collaborators
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Lightcurves and Spectra
From the catalog at ASDC, there are links from
each source to DC2 data products
Quicklook lightcurves have been produced by
Benoit Lott for every source in the catalog.
Spectral fits for each source were produced by
Luis Reyes. These are simple power-law fits, but
provide a plot which can guide users to different
models if needed.
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More on lightcurves
The LAT PSF varies rapidly as a function of
energy. A simple binned analysis will can only be
an approximation because the fractional
background level will be a function of source
spectrum, and there may be confusion from other
nearby sources Jennifer Carson and Jim Chiang
have been looking at systematic effects
associated with using simple binned analysis
rather than likelihood to make lightcurves.
Source flux
Extragalactic diffuse flux
Spectral index
Comparing flux estimates from likelihood and
simple binned analysis
Lightcurve produced with Likelihood analysis
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Spectral studies of Blazars Intergalactic
Radiaton fields.
The DC2 data support more sophisticated spectral
analyses. Luis Reyes has searched for evidence of
attenuation of the gamma-ray flux due to the EBL
background. He looks for high energy cutoffs in
blazar spectra which vary in a systematic way
with redshift. He uses the results to set
constraints on the DC2 EBL model.
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GRB simulations

• In addition to LAT data. Simulated GBM data was
  generated for a population of bursts.
• This included all the burst data products and
  also included a GCN-like notice to provide
  summary GBM information.
• This allows us to more fully explore the
  possibilities of GRB studies in the GLAST era.
  Where we will have data from 10 keV to 300 GeV
  for many GRB.

64 GBM bursts
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GRB Analyses
A search for high energy gamma-ray afterglows
discovered a hard bright component occuring
several hours after the prompt emission in one
of the simulated bursts (Nicola Omodei)
Joint spectral fits This illustrates the very
broad energy range (10 keV to gt300 GeV that GLAST
will measure for the prompt phase of GRB (Nukri
Komin)
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GRB Analyses
A search for bursts in the LAT data found a
short, very hard burst which produced a
significant signal in the LAT (gt5 GeV) but did
not trigger the GBM. (Nukri Komin)
David Band has compared the difference between
the LAT determined and GBM determined locations
and compared that with the quoted statistical
uncertainties to look for systematic offsets.
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Pulsars
One of the DC2 data products was pulsar
ephemerides to describe the subset of known
pulsars in the DC2 sky. Many people have run the
GLAST timing tools on the DC2 data and
discovered a bug in the simulation (now
fixed). Damien Parent wrote a set of scripts to
automatically process all the pulsars in the DC2
ephemeris and produce phase plots.
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More Pulsars

• Markus Ziegler performed a blind search for
  periodicity in candidate pulsars in the DC2 data
  (i.e. not using an ephemeris). He found
  significant periodicity in several of the sources
  listed in the ephemeris (which is reassuring),
  and also identified a few radio-quiet pulsars.
• Composite sources Riccardo Rando found a
  source that appears to consist of two components
  a steady power-law component and a hard pulsed
  component

Refit with a composite source consisting of a
power-law and a log normal component
Power-law point source background model is a
very poor fit to the data
Phase vs energy plot shows that the pulsed
emission dominates above 1 GeV
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Comparing the simulated data with the Model
Analysis by Andy Strong
Excess at 60 GeV!
Blue line is the sum of the modeled galactic
diffuse and extragalactic diffuse emission. The
red points are the DC2 data. Differences between
them could be due to contributions from point
sources, residual particle background or
unmodeled diffuse components (from eg dark
matter).
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Connecting to VHE energies

• Possibility of extrapolating the source spectra
  to higher energies to get an estimate of the
  number of sources detectable by both GLAST and
  ground-based instruments for given
  luminosity/redshift distributions and EBL model.
• For some source classes, the extension of the
  spectrum to higher energies may need to be more
  careful than a simple extrapolation.
• To get an estimate of how many would actually be
  detected by both, we would need to simulate an
  observing schedule.
• All of the astrophysical sources in the GLAST
  simulation can also be used in a fast
  simulation
• Skip the full detector simulation and instead
  produce high level results by convolving source
  descriptions with parameterisations of effective
  area, energy and angular resolution. Could this
  be extended to include VHE instruments?

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Summary

• All of the results shown here are simulated
  figments of our imagination. However, they
  illustrate phenomena that GLAST might and could
  see after launch.
• Describing astrophysical source models in a
  consistent way is extremely useful.
• An excellent test of our processing pipelines and
  data handling infrastructure
• 200,000 cpu hours/400 cpus
• 5 TB of data
• Provided an opportunity for an early, realistic
  test of science tools usability.
• Improve the analysis software and documentation
  based on user feedback.
• DC2 closeout will be May 31 June 2 at GSFC.
• Participants can present their analyses of the
  DC2 data.
• Organisers will reveal the true contents of the
  simulation.
• The GLAST users committee will beta test the
  science tools using DC2 data in November 2006.