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

2

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

3

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

4

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

6

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

8

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

9

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

10

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

11

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

12

Crab Consistency Check

0

5

10

15

20

25

30

35

40

45

DataSkyMap

m30h05m40h0520.5

21

21.5

22

22.5

23

23.5

24

24.5

0

5

10

15

20

25

30

TestSkyMap

m30h05m40h0520.5

21

21.5

22

22.5

23

23.5

24

24.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

NullSkyMap

m30h05m40h0520.5

21

21.5

22

22.5

23

23.5

24

24.5

0

5

10

15

20

25

30

35

40

Sign

ifica

nce

NullSkyMap Li & Ma (0.1 deg radius)

m00h00m00h00m00h00m00h00m00h00m00h00

21

21.5

22

22.5

23

23.5

24

null_resid_sig_copy

Entries 15213

Mean 0.4382− Std Dev 1.089

6− 4− 2− 0 2 4 6 8 10

1

10

210

310null_resid_sig_copy

Entries 15213

Mean 0.4382− Std Dev 1.089

0.8 0.9 1 1.1 1.2 1.3Mean Scaled Width

2−1−012

data

erro

rda

ta -

mod

el

0.8 0.9 1 1.1 1.2 1.3

Mean Scaled Width

200

400

600Cou

nts

Graph

0.8 0.9 1 1.1 1.2 1.3Mean Scaled Width

0.5−0

0.511.5

mod

elda

ta -

mod

el

3−

2−

1−

0

1

2

3

Sign

ifica

nce

TestSkyMap Li & Ma (0.1 deg radius)

m00h00m00h00m00h00m00h00m00h00m00h00

21

21.5

22

22.5

23

23.5

24

null_resid_sig_copy

Entries 15213

Mean 0.03153− Std Dev 0.9542

6− 4− 2− 0 2 4 6 8 10

1

10

210

310null_resid_sig_copy

Entries 15213

Mean 0.03153− Std Dev 0.9542

Event  Counts Source  +  BG Background

Residual:  Data  – Full  Model   Residual:  Data  – Background  Model  

13

Crab Consistency Check

0.8 0.9 1 1.1 1.2 1.3Mean Scaled Width

2−

02

data

erro

rda

ta -

mod

el

0.8 0.9 1 1.1 1.2 1.3

Mean Scaled Width

0

20

40

60

80

Cou

nts

Graph

0.8 0.9 1 1.1 1.2 1.3Mean Scaled Width

1−

012

mod

elda

ta -

mod

el

14

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)

30

40

50

60

70

Smoothed Acceptance Plot

15

Results

16

Residual:  Data  – Background  Model Souce +  Background  Model

17

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

18

= 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

19

1                    2                  3                  4                    5

5

4

3

2

1

degrees

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

21

Residual:  Data  -­‐ Background Residual:  Data  – Full  Model

Residual:  Data  -­‐ Background Residual:  Data  – Full  Model

Approximate source location

Pointing Strategy

22

Results• 25 pointings with new diffusion model

• Pointings spaced 1 degree apart

• 7.5 hours/pointing

• Significance = ~11 sigma

23

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

25

Acknowledgements

• National Science Foundation

• John Parsons, Amy Garwood, and Georgia Karagiorgi

• VERITAS group

• Brian Humensky and Qi Feng

26

Backup slides

27

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

28

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

29

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

31

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.

33

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.

34

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

35