presented by

Till EifertHumboldt University Berlin

Search for Very HighEnergy Gamma Ray Emission

from Pulsars with H.E.S.S.

Research Seminar WS 2005/06, Experimental High Energy Physics

1. Pulsars

2. H.E.S.S. Experiment

3. Timing Analysis

4. Results

Outline

1. Pulsars

2. H.E.S.S. Experiment

3. Timing Analysis

4. Results

Outline

What is a Pulsar?

rapidly spinning Neutron Star (NS)

Why is it pulsing?

because it’s rotating

What is emitted?

spectrum goes from radio waves to visible light to gamma rays

~ cosmic light house

What is a Pulsar?

rapidly spinning Neutron Star (NS)

Why is it pulsing?

because it’s rotating

What is emitted?

spectrum goes from radio waves to visible light to gamma rays

Beam aligned

Beam misaligned

Crab Pulsar, recorded in X-Rays

Motivation

Pulsar discovery: 1967 by Jocelyn Bell & Anthony Hewish (radio waves)

Today .. visible light, X-rays up to low gamma rays …

But Pulsed VHE emission not detected (yet) !?! Unique opportunity to learn:

How do pulsars work?

What pulsar model is correct?

First observation of pulsars

Neutron Star Formation

Mass: ~ 8-10 MSolar

Radius: ~ 108 m

Rotational period: ~ 26 days

Star

time

Star

Gravitational collapse

Neutron Star Formation

time

Star Grav. collapse

Supernova explosion

Supernova

Supernova remnant

Supernova remnant

Neutron starMass: ~1.4 MSolar

Radius: ~10 km

Rotational Period: 2ms..8s

Neutron star

Supernova Explosion

time

Star Grav. collapse Supernova Supernova remnant Neutron star

m107 8R

26 daysP BS = O(10-2T) (surface field)

2 RBS2RT 2 8P ms s

km10R

BS = O(108T)field lines frozen

into stellar plasma

part of angularmomentum carried

away by shell

Neutron star

Overview Pulsars

Supernova Explosion => Neutron Stars

Fast Rotation (P = 2 ms..8 s)

Emitted radiation (magnetic dipole radiation)

Gradually slowing down (loss of energy)

Eind surface forces 1012 times stronger than

gravity (Crab)

Charged particles (e-..) pulled out of surface and

accelerated to large energies

→ Magnetosphere electrically charged

(Too) Simple Electrodynamics

SindS BEB

v rotating

(P. Goldreich, W.H.Julian: Astrophys. J. 157 (1969) 839.)

|| :tionsimplifica mp

B

Magnetosphere charge density

02el B Rotating charge density:

Neutral cone at:

Herewith, two models:

Polar Cap

Outer Gap

Polar Cap Model

Polar Cap, r ~ 800 m e- accelerated at polar

caps gammas via

Inverse Compton Curvature+Synchrotron Radiation

(Sturrock (1971); Ruderman & Sutherland (1975); Harding (1981))

but limited by

pair production in huge B

Observer

Open field lines

Magnetosphere

model predicts super exponential cutoff in the high energy Gamma-ray spectra !

Outer Gap Model

Vacuum gap in outer magnetosphere (B=0)

Same interactions: ICS, Synchrotron, Curvature radiation

But: B field lower

(outer gap farther)

(Cheng, Ho & Ruderman (1986); Romani (1996))

model allows for IC peak around O(100) GeV !

Observer

Open field lines

Magnetosphere

Num

ber

log( T / s )

”Normal“ PulsarsT > 20 ms

Crab: T = 33 ms Vela: T = 89 ms

2 Pulsar Groups

810 TSB Millisecond Pulsars1 ms < T < 20 ms

510 TSB O

Thompson (2000)

Millisecond pulsars

Normal pulsars

Sample of Radio Pulsarsmore than 1500 radio pulsars

~50 X-ray pulsars

7 gamma-ray pulsars ~ 10 GeV

+3 candidates

low B ~ 104 -106 T Mostly in binary systems Very precise & more complex

timing corrections necessary

for analysis

0 ( )B sqrt P P

1. Pulsars

2. H.E.S.S. Experiment

3. Timing Analysis

4. Results

Outline

At 100 GeV

~ 10 Photons/m2

(300 – 600 nm)~ 120 m

Focal Plane

~ 10 km ParticleShower

Image Shape Primary Particle

Intensity Shower Energy

Image Orientation Shower Direction

5 nsec

Detection of Cosmic Rays and Gamma Rays

Cherenkov Light

120 m

Detection of Gamma Rays via Cherenkov Light of Air ShowersGamma

Ray

several viewing angles for preciseevent-by-event source location!

Stereoscopic Observation Technique

source position

source image is on image axis

4 telescopes operational since December 2003 Energy threshold: 100 GeV (at zenith) Single shower resolution: 0.1 Pointing accuracy: ≲ 20 Energy resolution: ≲ 15%

June 2002 September 2003 February 2003 December 2003

High Energy Stereoscopic System

Stereoscopic Imaging Atmospheric Cherenkov Array

Zenith

Energy threshold ~ Zenith Angle

Earth

~ 1

0 k

m

40 deg.

At Zenith: Mirror dish collects a faire amount of the Cherenkov light

At large ZA: Mirror dish collects only a small fraction of the Cherenkov light

→ low energy events (faint Cherenkov light) are seen at low ZA only!

Altitude rail

Azimuth rail

13m dish, mirror area 107 m2

382 spherical mirrors, f =15mPoint spread 0.03°-0.06°

960 pixel PMT cameraPixel size: 0.16°

On-board electronicsWeight: 800 kg

closed lidLight catchers

and PMTs

960 pixels, ∅ 0.16

5 field of view

Camera

1. Pulsars

2. H.E.S.S. Experiment

3. Timing Analysis

4. Results

Outline

Simple beam pattern

Lightcurve and Phasogram

Lightcurve and Phasogram

Lightcurve

Inte

nsity

Time

Fold into 1 rotational phasePhasogram

Inte

nsity

Rotational Phase [P]

Averaging periodic signal Radio: ~2 min smooth phase VHE: no intensity but single

gamma events

long averaging essential

Pulse patterns up to ~ 10 GeV

Thompson (2000)

How to get the phasogram?

Simply fold event times into phasogram …

But: observatory is not inertial to pulsar !!!

telescopes on rotating Earth

Earth orbiting Sun

Pulsar accelerating (if binary)

Solution: transfer times into

Solar System Barycenter (center of mass) and Binary Barycenter

as best approx. to inertial frames available!

Analysis of pulsar timing dataGiven: GPS event time stamp from CentralTrigger

intrinsic accuracy of GPS 10 μs

Phase of a pulsar waveform depends on: Spin-down (→ Radio observatories) Motion of Earth within the solar system (→ barycenter correction) Orbital motion of the pulsar (→ binary correction)

20

0

1, ,

2( ) 0

ff t t t T T

tT

t = time of arrival in UTC

tb = SSB corrected arrival time

Barycenter correction

∆tSSB transfer to SSB (Roemer time

delay)

∆tE “Einstein delay” (gravitational redshift

& time dilation due to motions of the Earth = TDB correction)

∆tS “Shapiro delay” (caused by

propagation of the pulsar signal through curved spacetime)

Binary modelsPulsar in binary system → significant acceleration

Blandford-Teukolsky (BT) model: Keplerian ellipse Newtonian dynamics Einstein delay patched into model

afterwards additional effects are accommodated

by nonzero time derivatives

Damour-Deruelle (DD) model: more general & precise Roemer time delay Orbital Einstein and Shapiro delay Aberration caused by rotation

Position and velocity need to be predicted by binary model!

Statistical TestsSearch for peaks in the phasogram

2 test flat distribution

good for narrow and high peaks

weak for wide and small profiles

Z2m probe sin/cos modes

powerful for sinusoidal profiles Kuiper-Test

search max deviation from

uniform distribution

sensitive for most peak structures

Test of timing corrections

Old H.E.S.S. timing corrections:

Deviation with respect to radio

astronomers tool (TEMPO):

∆t ~ 2 ms O(ms pulsar period)

OK for young pulsars

Not applicable for analysis over

long observation period of close

ms pulsars

No binary corrections available2004

∆tDeviation (H.E.S.S. – Tempo)

Test of (new) timing corrections

New H.E.S.S. timing corrections:

good agreement (<μs) with

radio astronomers tool

Including binary corrections!

2004

∆tDeviation (H.E.S.S. – Tempo)

Test of (new) timing corrections

New H.E.S.S. timing corrections:

good agreement (<μs) with

radio astronomers tool

Including binary corrections!

2004

∆tDeviation (H.E.S.S. – Tempo)

Test of timing analysis using Optical Crab Data Recorded with one H.E.S.S.

telescope in Nov. 2003

~ 2 min data analyzed and

corrected with (new) H.E.S.S.

software

Phasogram clearly shows typical

two-peak structure

Frequency Scan confirms correct

(radio) pulsar frequencyRadio frequency

1. Pulsars

2. H.E.S.S. Experiment

3. Timing Analysis

4. Results

Outline

Young Pulsar analysis results:

H.E.S.S.

(Conducted by Fabian Schmidt, HU Berlin 2004-2005)

PSR J0437-4715 Distance ~ 140 pc

P ~ 5.75 ms, dP/dt ~ 10-20

Low B ~ 108 -1010G

Binary orbit ~ 5.74 days

Low mass companion ~ 0.2 MSolar

No optical brightness variation

Pulsed emission visible in radio,

X-rays

GeV emission unknownHarding, A.K., Usov, V. V., Muslimov, A. G., 2005, ApJ, 622, 531

Polar Cap model prediction

PSR J0437-4715

Radio observation (Parkes)

Two phase cycles!

X-ray observations

Data analysis ~ 9 hours taken in October 2004 Zenith angle range: 23.9 – 30 deg Standard analysis to select

gamma ray events Standard background estimation

using 7 background regions→ Energy threshold ~ 200 GeV

Statistical tests for phasogram:

Z2m, Kuiper, Chi2

Timing analysisOn region

Z21 = 5.6 (Prob. 0.06)

Z22 = 5.7 (Prob. 0.23)

Kuiper = 0.05 (Prob. 0.10)Chi2 = 8.1 (Prob. 0.51)

All energies, DC: 0.4 σ

OFF regions (summed)

~ flat907 events

Z21 = 0.7 (Prob. 0.70)

Z22 = 0.8 (Prob. 0.94)

Kuiper = 0.01 (Prob. 0.94)Chi2 = 7.9 (Prob. 0.54)

Timing analysis, energy bins

On region

Z21 = 6.4 (Prob. 0.04)

Z22 = 6.7 (Prob. 0.15)

Kuiper = 0.06 (Prob. 0.09)Chi2 = 7.8 (Prob. 0.54)

Energies < 0.5 TeV, DC: 0.5 σ

OFF regions flat

Energies > 0.5 TeV, DC: -0.2 σ

On region

Z21 = 0.2 (Prob. 0.92)

Z22 = 2.2 (Prob. 0.70)

Kuiper = 0.07 (Prob. 0.93)Chi2 = 4.9 (Prob. 0.84)

751 events 156 events

Zenith angle

DC SignificanceEnergy < 0.5 TeV

Maximize signal/noise ratio for low energy by using

very small zenith angles only

DC SignificanceEnergy < 0.5 TeV

Final ResultsAll energies < 0.5 TeV, zenith angle < 25 deg

On region

DC: 2.0 σ 5.6 h livetime

Z21 = 9.4 (Prob. 0.009)

Z22 = 11.3 (Prob. 0.02)

Kuiper = 0.1 (Prob. 0.005)Chi2 = 15.1 (Prob. 0.09)

OFF regions flat 414 events

Summary

Pulsars – extreme physics inside

VHE pulsed emission detection still missing!

Timing corrections working in H.E.S.S.

(Ready for Pulsar detections)

J0437 … no clear evidence (more data is

needed)

H.E.S.S.High Energy Stereoscopic System

MPI für Kernphysik, Heidelberg

Humboldt-Universität zu Berlin

Ruhr-Universität Bochum

Universität Hamburg

Universität Kiel

Ecole Polytechnique, Palaiseau

College de France, Paris

Universite Paris VI-VII

LEA Saclay

CESR Toulouse

GAM Montpellier

LAOG Grenoble

Paris Observatory

Durham University

Dublin Inst. for Advanced Studies

Charles University Prag

Yerewan Physics Institute

North-West University, Potchefstroom

University of Namibia, Windhoek

The Future: H.E.S.S. Phase II

Build a large telescope Improve sensitivity: 4 small 1 large better

than 8 small Reduce threshold to O( 20 GeV ) Implement robotic operation ( future high

altitude site? )

H.E.S.S. Site

Clear sky Galactic centre culminates

in zenith Mild climate Easy access Good local support

23o16’ S, 16o30’ E, 1800 m asl

Farm Göllschau, Khomas Hochland, 100 km from Windhoek

Detection Area of a Cherenkov Telescope

about 50000 m2

good sensitivityup to highest energy( smallest fluxes )

~ 120 m

Camera Plane

Single Telescope Image Analysis

Source direction

Expected orientation of principal axis for signal events

“Tail cuts” on camera pixels Image cleaning

Cuts on “scaled” Width, Length,... Typically 99.9% of background removed

“Tail cuts” on camera pixels Image cleaning

Cuts on “scaled” Width, Length,... Typically 99.9% of background removed

Dist

ance

Hillas ParametersHillas Parameters

Camera Plane

Combination of Telescope Images

Source direction

θ 0 for signal

θ2 flat for background

θ 0 for signal

θ2 flat for backgroundθ

Leap seconds in UTC|UT1-UTC| < 0.9 seconds → leap seconds

UT1: time scale based on the Earth’s rotation (irregular fluctuations, general slowing down)

UTC: TAI (International Atomic Time) + leap seconds

Taken from Earth Orientation Center

UT1UTC