PART 2. S UMMARY Intro: VHE -rays Astrophysics Observation Technique Instruments and data taking...

It’s a kind of MAGIC!

COME I RAGGI GAMMA AD

ALTISSIMA ENERGIA

DIVENTANO SEGNALE

ELISA PRANDINIPadova University

MAPSES – LECCE 23 NOVEMBER 2011

PART 2

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SUMMARY Intro: VHE g-rays Astrophysics

Observation Technique

Instruments and data taking

From raw data to shower images

Background Rejection

Signal extraction

Results: Sky Maps Integral and differential fluxes

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Steps of the Analysis of CT data

Raw signal

Detection

Spectrum

Pixel signal extraction

Image cleaning

Image parameterization

Stereo- reconstruction

Background rejection

Background estimation

Source detection

Sky maps

Spectrum / light curve

Spectrum Unfolding

Event parameter reconst

Cleaned signal

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Characterization of the events

Once we have obtained the shower images, the next step is to obtain the CHARACTERISTICS OF THE PRIMARY PARTICLE which originated the shower

Nature of the particle (Background rejection)

Primary Direction

Primary Energy

Random ForestReloaded

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Random ForestsHow it works: The growing of a tree

this is a tree

ELEMENTS:- parameters (~10)- trees (100)- In each tree:

- nodes - POSSIBLE parameters in each

node (trials, ~3) RANDOM- BEST SEPARATOR and value

per node (via Gini index)- final node size (1-5)- each event is classified!

APPLICATION TO REAL DATA:- We apply each tree ( from training) to them- Each event is univocally classified (with a local hadronness) Final hadronness is an average among all the values!

Based on Monte Carlo Simulation

+ real data

TRAINING

Primary direction

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Primary direction

The ellipse major axis points to the center of the camera

With many telescopes:

Helpful to “detect the signal”

The intersection of the axes is related to the INCOMING DIRECTION

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The incoming direction

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Reconstruction of the incoming direction

Geometrical reconstruction: for more than 1 telescope

M2

M1Reconstructed direction

Δδ

M1

M2

In Plan ┴ direction

Reconstructed core impact

point

Impact

parameter

Efficient forδ 30 deg

Monte Carlo independent

Energy Reconstruction

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Energy reconstructionBasic fact: Energy ~ Image size

Based on Monte Carlo Simulation

Methods:

A parameterization:Energy = f(size, impact, zenith,…)

Look-up tables

Optimized decision trees(Random Forest)

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Energy reconstructionEnergy resolution:

20% at 100 GeV, down to 15% around 1 TeV

Big bias @ low energies.Solved with unfolding

ENERGY RESOLUTION:E(est) - E(true) / E(true)

ENERGY BIAS:E(est) - E(true) / E(est)

Estimated with Monte Carlo

(Gammas)

Signal Extraction

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What is the aim of our analysis?

Populate the VHE sky

Localize the emission

Characterize the emission

Energy

Time

DETECTION OF THE SIGNAL

SKY MAP

SPECTRUM

LIGHTCURVE

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o DATA ACQUISITION

o CALIBRATION

o IMAGE CLEANING AND HILLAS PARAMETERS CALCULATION

o HADRONNESS AND ENERGY RECONSTRUCTION

The analysis

FILE SET OF EVENTS

EVENT SET OF PARAMETERS CHARACTERIZING THE SHOWER

REMEMBER: EVENT IS A SHOWER (INDUCED BY A CR)

Former steps

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Characteristics of a g-like event

Therefore if we plot the parameter related to the reconstructed incoming direction, the gamma-like events will have it close to the telescopes pointing direction

To discriminate gamma-ray induced images from hadrons induced images, we use the square of the parameter Q, THE ANGLE BETWEEN THE POINTING DIRECTION (CAMERA CENTER) AND THE (RECONSTRUCTED) INCOMING DIRECTION.

AND WHAT ABOUT THE HADRONS?They are randomly distributed!

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Detection: the Theta2 plot!

Where is the signal???

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?We have a problem…

THE NUMBER OF BACKGROUND EVENTS IS ~ 104 TIMES THE NUMBER OF

GAMMA EVENTSWe have to reduce the number of bkg events:

HADRONNESS PARAMETER!

We apply a cut and reject the events that are likely hadrons

(MC data)

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The Detection Now we are ready to perform our detection plot (also

called theta-square plot)

VERITAS Collaboration, V. A. Acciari et al, ApJ 715 (2010) L49

Background data

Source data

“Signal”

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The Background We need a background in order to estimate the signal

Old technique: observe the source (ON data) and a region of the sky with the same characteristics but without a source (OFF data)PROBLEMS - time consuming!

- different observations conditions (weather, hardware)

SOLUTION: find an observing mode which allows to collect ON and OFF data simultaneously!

Wobble mode: the telescopes point to a region 0.4 deg offset from the source (and the background can be extracted)

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The Detection Some theta2 plots:

H.E.S.S. Collaboration, F. Aharonian et al. A&A 442 (2005) 177-183

VERITAS Collaboration, V. A. Acciari et al, ApJ 715 (2010) L49

Not always there is a detection of course…

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The Significance

SIGNIFICANCE is a measure of the likelihood that pure background fluctuations have produced the observed excess (i.e., assuming no signal)

The common rule is that a source is detected if the significance of the signal exceeds 5 sigma!

Li, T., and Ma, Y. 1983, ApJ, 272, 317

We need a tool to say if our observation is physically relevant

We use the SIGNIFICANCE of the signal

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Examples

A five sigma signal

PKS 1222+21MAGIC

PG 1553+113H.E.S.S.

Mkn 180MAGIC

A 10 sigma signal

A four sigma signal

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Standard Example: the Crab Nebula

MAGIC observations of the Crab Nebula:

WHICH IS THE DIFFERENCE?The energy range

considered!

E>300 GeV 60 GeV < E < 100 GeV

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Images and Energy

At low energies the characterization of a gamma-like is much more difficult!!!

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Therefore: The data analysis of IACTs telescopes is non trivial…

The first result to look for is a SIGNAL (is there a gamma ray source or not?):

The tool is the theta2 plot, a plot of the parameter theta2, that discriminates between hadrons (our background) and gamma-rays induced showers using the incoming direction

The signal is quantified through its SIGNIFICANCE:

< 5 SIGMA NO SIGNAL OR MORE STATISTICS NEEDED

> 5 SIGMA THERE IS A SIGNAL!THE ANALYSIS CAN

CONTINUE

Physical ResultsSky Maps

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Second step: the localization

Important: in general IACTs don’t operate in scan mode but in pointing mode!

Moreover our resolution is… ~ 0.1 DEGREES

Extended sources: are galactic and very large regions

Extragalactic objects, for the moment, are point-like!

In order to localize the emission, we perform the so-called SKY MAP, that is a bi-dimensional map of the reconstructed incoming directions of the primary gamma rays

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The sky map

For each event we reconstruct the INCOMING DIRECTION

Is the same parameter that we have used in the detection plot!

The background is modeled

Remember: our PSF is ~ 0.1 degree…

WE USE IT:

- As a check tool

- In few cases: extended emission or multiple sources in the field

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Example: the Crab Nebula

FIND THE DIFFERENCE!

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Example: the Crab Nebula

The angular resolution is strictly related to the ENERGY RANGE!

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Extended sources

Some galactic sources are extended enough to be mapped nicely at TeV energies

SNR HESS J1731-347

Very interesting studies on acceleration sites!

H.E.S.S. collaboration, A. Abramowski et al. A&A 531 (2011) A81

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NGC 1275 region

Clear signal from the head tail RG IC 310 at E > 400 GeV

And below this energy?E > 400 GeV

E > 150 GeV

If we go down to 150 GeV, a signal from the RG NGC 1275 becomes significant!

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The galactic scan

HESS Coll. ICRC 2009 Proceedings

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So, in the lucky case:

1. We have detected a significant signal

2. We have checked that the emission is coming from the direction that we expect (if not, go back to point 1, changing the true source position)

3. Now?

Spectrum

Light curve

Physical ResultsThe Spectrum

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The differential energy spectrum

The spectrum is essential in our study Allows the

characterization of the emission

A comparison is possible (also between different experiments and energy thresholds!)

What is it?

VERITAS Coll. APJ Letters, 709, L163-L167 (2010)

THE NUMBER OF VHE PHOTONS PER AREA AND TIME

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-ray flux: rate of -rays per unit area( to their direction)

units: [L-2] [T-1] (e.g. cm-2 s-1)

Needed ingredients: a number of -rays, a collection area and an observation time

Related concepts:

Differential energy spectrum: flux per interval in -ray energy (cm-2 s-1 TeV-1)

Integral flux: integrated in a given energy range (cm-2 s-1), e.g. :

Light curve: time evolution of integral flux: vs. t

Definitions

€

Φ =d2Nγ

dS dt

€

dΦdE

=d3N

dS dtdE

€

ΦE>200 GeV =dΦdE200 GeV

∞∫ dE

Courtesy of A. Moralejo

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Differential energy spectrum:

the observablesNUMBER OF DETECTED -RAYS: obtained from the observed excess

EFFECTIVE OBSERVATION TIME: not equal to the elapsed time!

EFFECTIVE COLLECTION AREA ESTIMATED ENERGY of the events

€

dΦ

dE=

d3N

dS dt dE

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Number of g-rays per energy

We have a “discretization” dN/dE becomes the number of excess per energy interval

How do we estimate this quantity?

THROUGH THE THETA2 PLOT (PER ENERGY INTERVAL)!

€

dΦ

dE=

d3N

dS dt dE

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Effective observation time The effective observation time is not equal to the elapsed

time between the beginning and end of the observations: there may be gaps in the data taking (e.g. between runs) there is a dead time after the recording of each event

Useful quantity: t, the time difference between the arrival time of an event and the next one

In a Poisson process, t follows an exponential

€

PPoiss(n,t) =(λt)ne−λt

n!probability of observing n events in time t, given event rate

probability that the next event comes after time t

€

P (tnext > t) =PPoiss(0,t) =e−λt

P (tnext > t) = dP(tnext =t)dt

dtt

∞∫ ⇒ dP(tnext =t)dt

=λe−λt

€

dΦ

dE=

d3N

dS dt dE

Courtesy of A. Moralejo

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Distribution of the time differences:

Calculation of effective observation time

In case of fixed dead time d, the distribution is still exponential with slope The true rate of events (i.e. before the detector) can be obtained from an exponential fit to the distribution

And teff = Nd,0 /

Courtesy of A. Moralejo

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Example effective time

And teff = Nd,0 /

Log scale!

l slopeN

intercept

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Effective Area Order of magnitude?

€

dΦ

dE=

d3N

dS dt dE

Aeff ~105 m2

Estimated from MC (gamma) data!

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Effective Area Order of magnitude Aeff ~105 m2

o It’s roughly the Cherenkov light pool

We estimate it with MC data

o Depends on the Zenith angle of the observations:

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Differential energy spectra

Numbers of bins is of course variable and set by the analyzer

Errors are larger at high energies… why?

Usually fitted with power laws

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The unfolding Before publishing our spectrum, there is still one thing

that we can do:

Use the MC data to calculate the errors that we perform and apply a correction to the data

- limited acceptance and finite resolution- systematic distortions-reconstructed energy is not true energy!

HOW? With a matrix (correlation matrix) correlating the true Energy (from simulations) to the reconstructed Energy (estimated through RF)

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Crab Nebula Spectrum

…and SED

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Other examples

Extremely variable objects

Extremely distant objects (z=0.536)

Physical ResultsThe lightcurve

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Integral Flux Differential (energy bins) integral (energy threshold)

If studied as a function of the time we make a LIGHT CURVE

How can we build a light curve?

Roughly speaking: we always have the same ingredients, but integral in energy: Theta2 plot above Eth

Integral effective area above Eth

The time… is the same!

€

ΦE>200 GeV =dΦdE200 GeV

∞∫ dE

Time evolutionstudies

H.E.S.S. collaboration, A. Abramowski et al. A&A. 520 (2010) A83

PKS 2155-304

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LC Examples

HESS PKS 2155-489

3C 279

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… we are only

one piece!

1ES 2344+514

1ES 2344+514

VERITAS Coll., Acciari et al., ApJ 738 (2011) 169

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By the way:

you can find the fits files of our final results at: http://magic.pic.es/pub/fits/

Last CommentEBL: The gamma ray horizon

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xx

x

VHE photons absorption by the Extragalactic Background Light

VHE photon + diffuse light electron-positron pairs production

VHEEBL e+e-

Absorption:

dF/dEOBS= (dF/dEEM) e-t

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EBL SED

VHE photon + diffuse light electron-positron pairs production

Hauser and Dwek (2001)

VHEEBL e+e-

VHE photons absorption by the Extragalactic Background Light

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VHE photon + diffuse light electron-positron pairs production

Large uncertainties!

VHE photons absorption by the Extragalactic Background Light

Dominguez et al. (2011)

VHEEBL e+e-

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z = 0.003

z = 0.01

z = 0.03

z = 0.1

z = 0.3z = 0.5z = 1

g-g opacity

Our range of observations

Absorption:

dF/dEOBS= (dF/dEEM) e-t

EBL ModelFranceschini et al. (2008)

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z = 0.003

z = 0.01

z = 0.03

z = 0.1

z = 0.3

z = 1

Strong suppression

z = 0.5

g-g opacityAbsorption:

dF/dEOBS= (dF/dEEM) e-t

EBL ModelFranceschini et al. (2008)

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EBL absorption effect

EBL model:Franceschini et al. (2008)

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Current “limit” The FSRQ 3C 279 at redshift 0.536

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Conclusions The analysis of Cherenkov data is non trivial…

…but it is worth!!!

Probably there are still large margins of improvements: New analysis techniques More powerful instruments (CTA)

The present and future is MULTI-INSTRUMENT!

THANK YOU!

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Exercise… how many Crab gammas in 1 hour?

dN/dE = (3.3±0.11)×10−11E−2.57±0.05cm−2s−1TeV−1

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Model analysis: A Global reconstruction method

An alternative to the use of image parameterization Analytic model (based on MC) gives the

expected signal in each pixels as a function of E, Direction & Impact

A fit of the MC templates on the real data reconstructs at same time the E, direction, and nature (gamma/hadron)

This method developed by CAT and then by HESS is time consuming but provides the best results (for telescope arrays).

MC template

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Reconstruction of the incoming direction

DISP method: Developed for single telescope data

centroid

major axis

DISP

DISP can be determined with:

- A parameterization:

- Optimized decision trees (Random Forest)

reconst. direction

All methods are based on Monte Carlo Simulation

Possible confusion withsymmetric directionImage asymmetry and timegradient help the distinction