search for very high energy gamma ray emission from pulsars with h.e.s.s
DESCRIPTION
Search for Very High Energy Gamma Ray Emission from Pulsars with H.E.S.S. presented by Till Eifert Humboldt University Berlin. Research Seminar WS 2005/06, Experimental High Energy Physics. Outline. Pulsars H.E.S.S. Experiment Timing Analysis Results. Outline. Pulsars - PowerPoint PPT PresentationTRANSCRIPT
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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
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1. Pulsars
2. H.E.S.S. Experiment
3. Timing Analysis
4. Results
Outline
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1. Pulsars
2. H.E.S.S. Experiment
3. Timing Analysis
4. Results
Outline
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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
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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
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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
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Neutron Star Formation
Mass: ~ 8-10 MSolar
Radius: ~ 108 m
Rotational period: ~ 26 days
Star
time
Star
Gravitational collapse
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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
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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
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Overview Pulsars
Supernova Explosion => Neutron Stars
Fast Rotation (P = 2 ms..8 s)
Emitted radiation (magnetic dipole radiation)
Gradually slowing down (loss of energy)
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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
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(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
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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 !
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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
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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
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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
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1. Pulsars
2. H.E.S.S. Experiment
3. Timing Analysis
4. Results
Outline
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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
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several viewing angles for preciseevent-by-event source location!
Stereoscopic Observation Technique
source position
source image is on image axis
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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
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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!
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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
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closed lidLight catchers
and PMTs
960 pixels, ∅ 0.16
5 field of view
Camera
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1. Pulsars
2. H.E.S.S. Experiment
3. Timing Analysis
4. Results
Outline
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Simple beam pattern
Lightcurve and Phasogram
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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
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Pulse patterns up to ~ 10 GeV
Thompson (2000)
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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!
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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
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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)
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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!
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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
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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)
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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)
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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)
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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
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1. Pulsars
2. H.E.S.S. Experiment
3. Timing Analysis
4. Results
Outline
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Young Pulsar analysis results:
H.E.S.S.
(Conducted by Fabian Schmidt, HU Berlin 2004-2005)
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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
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PSR J0437-4715
Radio observation (Parkes)
Two phase cycles!
X-ray observations
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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
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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)
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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
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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
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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
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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)
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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
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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? )
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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
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Detection Area of a Cherenkov Telescope
about 50000 m2
good sensitivityup to highest energy( smallest fluxes )
~ 120 m
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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
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Camera Plane
Combination of Telescope Images
Source direction
θ 0 for signal
θ2 flat for background
θ 0 for signal
θ2 flat for backgroundθ
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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