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VERITAS Observation of GemingaPulsar using the 3D Maximum
Likelihood Method Columbia University, Nevis Laboratories REU 2018
Emily HarrisUniversity of Pittsburgh
Outline
• Introduction o VHE Gamma-ray Astronomyo VERITASo Pulsars and Pulsar Wind Nebulaeo Project Motivation - Positron Excess
• Methods and Studieso 3-D Maximum Likelihood Methodo Diffusion modeling for Geminga Pulsar
• Results • Conclusion
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VHE Gamma-Ray Astronomy
• Why study very high energy gamma-rays ?
o Understand particle acceleration in extreme astrophysical environments
• Over 200 VHE sources detected:
o Blazars
o Supernova Remnants
o Pulsars
o Pulsar Wind Nebulae
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VERITAS
• Gamma-ray observatory
o located at the Fred Lawrence Whipple Observatory in Arizona
• 4 imaging atmospheric Cherenkov telescopes
o Energy Range: 85 GeV to 30 TeV
o 3.5 degree FoV
o 350 mirrors
o Camera with 499 PMTs
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VERITAS detection of gamma-rays• VHE gamma-ray reaches Earth’s atmosphere
o Interacts with an atmospheric nucleus
o pair produces
• e ± interact with more nucleio Undergo Bremsstrahlung – secondary gamma-rays
o More pair production
• e ± moving faster than the speed of light in airo Polarize atoms they pass
o Shock wave of Cherenkov radiation
o Detected by IACTs 5
VERITAS Technique
• Mirrors reflect Cherenkov light onto PMTso Amplify, digitize, and record image of the shower
• Gamma-ray showers appear as long ellipses o MSW can be measured
• Helps discriminate against background
• Stereoscopic observation with array systemo Shower reconstruction
o Long axis – direction of gamma-ray
o Intersection of axes – position of source
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Sources: Pulsars• Formed when a star reaches the end of its life
o All of the fuel is exhausted in its core
o leads to supernova explosion
• Supernova leaves behind a remnanto rapidly rotating neutron star
o Rotation period: ms to s
• Strong magnetic fields producedo ~10^12 Gauss
• Radiation emitted along its magnetic poles o Beam sweeps around as star rotates
o If beam falls within line of sight of Earth – pulsed signal detected7
Sources: Pulsar Wind Nebulae
• Pulsar creates high energy particle wind from its spin-down power
• Initially surrounded by expanding nebula remnant from supernova
• Pulsar winds expand into nebulao Confined by the termination shock
• Leptons accelerated to relativistic speedso Emit inverse Compton radiation in gamma
wavelengths – detectable by VERITAS
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Project Motivation
• Positron fraction in Earth’s atmosphereo Secondary particles from cosmic-ray interactions
• In 2008, PAMELA observed an excess of positrons above 10 GeV
• Possible explanations for positron excesso Annihilation of dark matter particles
o Nearby pulsars or PWNe (Geminga)
• Geminga is a likely sourceo Due to its close proximity and age
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Project Motivation
• HAWC began to study Geminga’s emission region
• Recently detected extended emission in the TeV range
o Emission region several degrees across
• Due to leptons diffusing away – inverse Compton
scattering
o Diffusion constant 100x smaller than typical
o Suggests that contribution from Geminga is small
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Geminga’s emission region
Project Motivation
• VERITAS wants to extend the Geminga data from HAWC to lower energieso Better angular resolution of 0.1°
o Search for substructure in gamma-ray emission
o Possibly reinterpret HAWC data
• But Geminga is a very extended source• Relatively weak: 30% of the Crab flux
• Standard VERITAS analysis techniques poorly suited for very extended sources (> 0.5 degrees)• ~50 hours of existing data on Geminga but no source has been detected
o New method needed to study Geminga
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3D Maximum Likelihood
• 3D Maximum Likelihood Method created by a group at Iowa State
• New technique to study extended emission in VERITAS data
o 2 spatial dimensions
o 1 mean scaled width dimension
• This method generates and analyzes Monte Carlo simulations of
observations of a source
o Use to study the emission region around Geminga
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Crab Consistency Check
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DataSkyMap
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NullSkyMap Li & Ma (0.1 deg radius)
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TestSkyMap Li & Ma (0.1 deg radius)
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Event Counts Source + BG Background
Residual: Data – Full Model Residual: Data – Background Model
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Crab Consistency Check
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Crab Consistency Check
• Generated files from 3D MLM compatible for VEGAS analysiso Compare to normal Crab data
o See how well 3D MLM performs
0 0.5 1 1.5 2 2.5Squared Angle from Center of FoV (deg^2)
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Smoothed Acceptance Plot
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Results
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Residual: Data – Background Model Souce + Background Model
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Residual: Data – Background Model Souce + Background Model
Source Model
• Next step is to create a new model with a more complicated, diffuse
emission around Geminga
o Equations derived from HAWC used to model the diffusion of leptons
around Geminga
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= diffusion angle of leptons
= angular distance from pulsar
= Flux normalization = 8e-9 photons/m^2/s/Tev
= Spectral index = -2.23
= Normalization Energy = 2 TeV
Diffusion Modeling
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1 2 3 4 5
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Results• Simulating diffusion model for a 15hr observation in single pointing
centered on Geminga
• Want to model existing VERITAS data and see how 3D MLM results compare
• Left: significance map for the 3D MLM simulation
• Right: significance map from analysis of VERITAS data from Andy Flinders
20Likelihood analysis Significance = ~4 sigma
Results
• Geminga 0.5 deg south offset• MLM Significance = ~4 sigma
• Geminga 1.0 deg south offset• MLM Significance = ~4 sigma
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Residual: Data -‐ Background Residual: Data – Full Model
Residual: Data -‐ Background Residual: Data – Full Model
Approximate source location
Pointing Strategy
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Results• 25 pointings with new diffusion model
• Pointings spaced 1 degree apart
• 7.5 hours/pointing
• Significance = ~11 sigma
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Residual: Data – Full Model Residual: Data – Background Model
Results• Repeated this 25 pointing configuration 3 more times
o Check for consistency in results
24Significance = ~11 Significance = ~11.7 Significance = ~10
All 3 Residuals: Data - Background
Summary of Results
• A good observation strategy for VERITAS is 25 pointings with 7.5 hours at
each pointing
o Total time needed: ~187 hours
• Using this pointing strategy, VERITAS will be able to detect the Geminga
source with high significance
• Future studies: extensions could be added to improve 3D MLM code which
would allow us to search for substructure
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Acknowledgements
• National Science Foundation
• John Parsons, Amy Garwood, and Georgia Karagiorgi
• VERITAS group
• Brian Humensky and Qi Feng
26
Backup slides
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Diffusion Modeling
• Comparison of new model and 4-deg disk model
o 4-deg disk model is a flat extended emission
o New model is more diffuse and peaks in the center
o See how the diffuse emission around Geminga effects 3D MLM simulations
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Pointing Strategy Test
• 25 pointing skymap created using a 4-degree disk model
o Meant to simulate a simple, flat extended source
o No detection seen
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data – full model data – background model
PWNe Emission
• Synchrotron emission• Radiation emitted when a particle moves in helical
path around magnetic field line
• Emitted inside the pulsar wind nebula but is scattered due to random arrangement of magnetic field lines
• Inverse Compton emission• High energy electron transfer energy to a lower
energy photon
• Usually upscatter synchrotron photons - creates the gamma-rays detectable by VERITAS 30
VERITAS Analysis
• First rejects events that look like air showers from cosmic rays based on
MSW parameter
• Then estimate remaining background from parts of the FoV that are
located away from the source (OFF regions)
• Once you know what this background looks like, you can estimate level of
background contaminating region around the source (ON region)
• Hard to determine background for extended sources
• thus the estimate of the source flux will have large uncertainty
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significance• How is significance calculated?
• The total emission in the vicinity of the source is estimated by integrating all events within some distance of the source position.
• This region is referred to as the ON region.
• The level of background flux in the ON region is estimated by summing all of the counts in a region in the field of view which is expected to contain no gamma-ray emission.
• Referred to as the OFF region
• Li and Ma equation is typically used to estimate the significance of the counts in the ON region as compared to the prediction of counts from the OFF region.
• Essentially a ratio of ON and OFF regions
• Higher significance means greater counts in the ON region32
Residual maps
• Residual maps help to compare the results of the model fit to the data
• How the map is computed: NOFF is estimated by integrating the models
within 0.1◦ of a given bin position, either including or excluding the source
model. NON is estimated by integrating the data within the same region.
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Spectral maps
• The spectral energy distribution represents the best fit values of the
parameters for a given spectrum.
• Once the best fit values of these parameters are stored, a plot is generated which
represents this distribution.
• A spectral butterfly plot is a way of showing the error on the differential
flux at a given energy.
• It is computed by taking the errors on the various spectral parameters and
propogating those errors through the spectral formula.
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Geminga Diffusion
• Cooling time of electrons:
• Life of electrons after they emit synchrotron/inverse compton radiation
• When the cooling time of electrons is less than the age of the pulsar, the diffusion
radius of the electrons increases with energy
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