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Search for Very High Energy Gamma Ray Emission from Pulsars with H.E.S.S.

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

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Search for Very High Energy Gamma Ray Emission from Pulsars with H.E.S.S.

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  1. Search for Very HighEnergy Gamma Ray Emission from Pulsars withH.E.S.S. presented by Till Eifert Humboldt University Berlin Research Seminar WS 2005/06, Experimental High Energy Physics

  2. Outline • Pulsars • H.E.S.S. Experiment • Timing Analysis • Results

  3. Outline • Pulsars • H.E.S.S. Experiment • Timing Analysis • Results

  4. What is a Pulsar? ~ cosmic light house • 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

  5. What is a Pulsar? Crab Pulsar, recorded in X-Rays • 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

  6. First observation of pulsars 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?

  7. time Star Mass: ~ 8-10 MSolar Radius: ~ 108 m Rotational period: ~ 26 days Gravitational collapse Star Neutron Star Formation

  8. time Supernova explosion Supernova remnant Neutron star Mass: ~1.4 MSolar Radius: ~10 km Rotational Period: 2ms..8s Star Grav. collapse Supernova Supernova remnant Neutron star Neutron Star Formation

  9. time part of angular momentum carried away by shell BS = O(108T) Star Grav. collapse Supernova Supernova remnant Neutron star Neutron star field lines frozen into stellar plasma Supernova Explosion BS = O(10-2T) (surface field)

  10. Overview Pulsars • Supernova Explosion => Neutron Stars • Fast Rotation (P = 2 ms..8 s) • Emitted radiation (magnetic dipole radiation) • Gradually slowing down (loss of energy)

  11. (Too) Simple Electrodynamics • Eind surface forces 1012 times stronger than gravity (Crab) • Charged particles (e-..) pulled out of surface and accelerated to large energies → Magnetosphere electrically charged

  12. Magnetosphere charge density (P. Goldreich, W.H.Julian: Astrophys. J.157 (1969) 839.) Rotating charge density: Neutral cone at: Herewith, two models: Polar Cap Outer Gap

  13. Observer Magnetosphere Open field lines Polar Cap Model (Sturrock (1971); Ruderman & Sutherland (1975); Harding (1981)) • Polar Cap, r ~ 800 m • e- accelerated at polar caps • gammas via Inverse Compton Curvature+Synchrotron Radiation • but limited by pair production in huge B model predicts super exponential cutoff in the high energy Gamma-ray spectra !

  14. Observer Magnetosphere Open field lines Outer Gap Model (Cheng, Ho & Ruderman (1986); Romani (1996)) • Vacuum gap in outer magnetosphere (B=0) • Same interactions: ICS, Synchrotron, Curvature radiation • But: B field lower (outer gap farther) model allows for IC peak around O(100) GeV !

  15. 2 Pulsar Groups Number ”Normal“ Pulsars T > 20 ms Millisecond Pulsars 1 ms < T < 20 ms Crab: T = 33 ms Vela: T = 89 ms log( T / s )

  16. low B ~ 104 -106 T • Mostly in binary systems • Very precise & more complex timing corrections necessary for analysis Sample of Radio Pulsars more than 1500 radio pulsars ~50 X-ray pulsars 7 gamma-ray pulsars ~ 10 GeV +3 candidates Normal pulsars Millisecond pulsars Thompson (2000)

  17. Outline • Pulsars • H.E.S.S. Experiment • Timing Analysis • Results

  18. Detection of Gamma Rays via Cherenkov Light of Air Showers Gamma Ray Particle Shower ~ 10 km 5 nsec Intensity  Shower Energy At 100 GeV ~ 10 Photons/m2 (300 – 600 nm) Image Orientation  Shower Direction ~ 120 m Image Shape  Primary Particle Detection of Cosmic Rays and Gamma Rays Focal Plane Cherenkov Light 120 m

  19. Stereoscopic Observation Technique source position source image is on image axis  several viewing angles for precise event-by-event source location!

  20. High Energy Stereoscopic System Stereoscopic Imaging Atmospheric Cherenkov Array • 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

  21. Energy threshold ~ Zenith Angle Zenith 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! ~ 10 km Earth

  22. 960 pixel PMT camera Pixel size: 0.16° On-board electronics Weight: 800 kg Altitude rail 13m dish, mirror area 107 m2 382 spherical mirrors, f =15m Point spread 0.03°-0.06° Azimuth rail

  23. Camera Light catchers and PMTs closed lid 960 pixels, ∅  0.16 5 field of view

  24. Outline • Pulsars • H.E.S.S. Experiment • Timing Analysis • Results

  25. Lightcurve and Phasogram Simple beam pattern

  26. Lightcurve and Phasogram Lightcurve Intensity Time Phasogram Fold into 1 rotational phase Intensity Rotational Phase [P] • Averaging periodic signal • Radio: ~2 min smooth phase • VHE: no intensity but single gamma events long averaging essential

  27. Pulse patterns up to ~ 10 GeV Thompson (2000)

  28. How to get the phasogram? • But: observatory is not inertial to pulsar !!! telescopes on rotating Earth Earth orbiting Sun Pulsar accelerating (if binary) • Simply fold event times into phasogram … • Solution: transfer times into Solar System Barycenter (center of mass) and Binary Barycenter as best approx. to inertial frames available!

  29. Analysis of pulsar timing data Given: 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)

  30. Barycenter correction t = time of arrival in UTC tb = SSB corrected arrival time ∆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)

  31. 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 Binary models  Position and velocity need to be predicted by binary model! Pulsar in binary system → significant acceleration

  32. Statistical Tests • 2 test flat distribution good for narrow and high peaks weak for wide and small profiles Search for peaks in the phasogram • Z2mprobe sin/cos modes powerful for sinusoidal profiles • Kuiper-Test search max deviation from uniform distribution sensitive for most peak structures

  33. Test of timing corrections ∆tDeviation (H.E.S.S. – Tempo) 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 available 2004

  34. Test of (new) timing corrections ∆tDeviation (H.E.S.S. – Tempo) New H.E.S.S. timing corrections: • good agreement (<μs) with radio astronomers tool • Including binary corrections! 2004

  35. Test of (new) timing corrections ∆tDeviation (H.E.S.S. – Tempo) New H.E.S.S. timing corrections: • good agreement (<μs) with radio astronomers tool • Including binary corrections! 2004

  36. 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 frequency Radio frequency

  37. Outline • Pulsars • H.E.S.S. Experiment • Timing Analysis • Results

  38. Young Pulsar analysis results: (Conducted by Fabian Schmidt, HU Berlin 2004-2005) H.E.S.S.

  39. Polar Cap model prediction Harding, A.K., Usov, V. V., Muslimov, A. G., 2005, ApJ, 622, 531 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 unknown

  40. PSR J0437-4715 Two phase cycles! X-ray observations Radio observation (Parkes)

  41. Standard analysis to select gamma ray events • Standard background estimation using 7 background regions → Energy threshold ~ 200 GeV Data analysis • ~ 9 hours taken in October 2004 • Zenith angle range: 23.9 – 30 deg • Statistical tests for phasogram: Z2m, Kuiper, Chi2

  42. Timing analysis All energies, DC: 0.4 σ On region OFF regions (summed) ~ flat 907 events Z21 = 5.6 (Prob. 0.06) Z22 = 5.7 (Prob. 0.23) Kuiper = 0.05 (Prob. 0.10) Chi2 = 8.1 (Prob. 0.51) Z21 = 0.7 (Prob. 0.70) Z22 = 0.8 (Prob. 0.94) Kuiper = 0.01 (Prob. 0.94) Chi2 = 7.9 (Prob. 0.54)

  43. Timing analysis, energy bins Energies < 0.5 TeV, DC: 0.5 σ Energies > 0.5 TeV, DC: -0.2 σ On region On region 156 events 751 events Z21 = 6.4 (Prob. 0.04) Z22 = 6.7 (Prob. 0.15) Kuiper = 0.06 (Prob. 0.09) Chi2 = 7.8 (Prob. 0.54) Z21 = 0.2 (Prob. 0.92) Z22 = 2.2 (Prob. 0.70) Kuiper = 0.07 (Prob. 0.93) Chi2 = 4.9 (Prob. 0.84) OFF regions flat

  44. Zenith angle Maximize signal/noise ratio for low energy by using very small zenith angles only DC Significance Energy < 0.5 TeV DC Significance Energy < 0.5 TeV

  45. Final Results On region All energies < 0.5 TeV, zenith angle < 25 deg 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

  46. 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)

  47. 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

  48. 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? )

  49. Farm Göllschau, Khomas Hochland, 100 km from Windhoek H.E.S.S. Site 23o16’ S, 16o30’ E, 1800 m asl • Clear sky • Galactic centre culminates in zenith • Mild climate • Easy access • Good local support

  50. ~ 120 m Detection Areaof a Cherenkov Telescope about 50000 m2 • good sensitivity up to highest energy ( smallest fluxes )

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