The Crab pulsar and its nebula: Surprises in gamma-rays

Transcription

1 The Crab pulsar and its nebula: Surprises in gamma-rays Based on Bühler & Blandford ROPP 2014 accept. arxiv At 2 kpc 10''~0.1pc Rolf Bühler, DESY Zeuthen Physics Colloquium, Delaware, 7th May 2014

2 Outline I Introduction II Observational overview - The Crab nebula - Crab pulsar III Theory of Pulsar Wind Nebulae - The pulsar magnetosphere - The cold pulsar wind - The synchrotron nebula IV Pulsation at Very High Energy V The flaring nebula VI Summary & Outlook 2

3 SN 1054 AD.. the appearance of a guest star, above which some yellow-colored light was faintly seen there is a person of great wisdom and virtue in the country. Chaco Canyon Yang Welt, August 25, 1054 A.D. One of the most studied sources over centuries, was e.g. key in the to establish connection between Super Novae, Neutron Stars and cosmic-rays Duncan 1921, Hubble 1928, Baade & Zwicky 1934, Minkowski 1942, Pacini 1967, Gold 1968 Today one of the prime targets to understand the non-thermal universe I 3

4 Radio VLA Optical HST X-rays Chandra 10''~0.1pc 4

5 Radio VLA Optical HST X-rays Chandra 10''~0.1pc 5

6 II 6

7 Spectral morphology Radio Infrared Arendt et al Temim et al X-rays Mori et al Veron-Cetty & Woltjer 1993 α: 0.24 to ' α: 0.30 to 0.8 4' α: 1 to 2 50'' Spectrum softens as electrons are transported outwards. Torus and jet have similar spectra in IR and X-rays II 7

8 Nebula polarization Moran et al II Polarization measurements show azimuthal magnetic field. Structures inside the nebula are linearly polarized up to ~60%. Intensity varies, polarization degree/angle does not 8

9 Nebula polarization Moran et al II Polarization measurements show azimuthal magnetic field. Structures inside the nebula are linearly polarized up to ~60%. Intensity varies, polarization degree/angle does not 9

10 Nebula polarization Moran et al II Polarization measurements show azimuthal magnetic field. Structures inside the nebula are linearly polarized up to ~60%. Intensity varies, polarization degree/angle does not 10

11 The Crab pulsar Double peaked at P~33.6 ms injecting P~ ergs s-1 Peaks in phase up to small shifts of φ~0.01 Oosterbroek et al. 2008, Abdo et al Degree and angle of polarization change with phase differently in radio and optical. Lyne and Manchester 1988, Slowikowska et al Phasogram very noisy, giant radio pulses in radio/optical, glitches ~once per year Popov & Stappers 2007, Strader et al. 2013, Wang et al II 11

12 The Crab pulsar Pulsar Nebula Three spectral components and ~1% of rotational-energy loss Increased flux in second peak within each component 12

13 Relativistic magnetized outflows Active Galaxies M87 Gamma-Ray Bursts Pulsar Wind Nebulae Vela GRB991216, not resolved 700 pc 0.15 pc All images from Chandra Are ubiquitous in the Universe, typically emitted by compact sources, form jets and rapidly accelerate particles III Pulsar wind nebulae are best resolved in space and time and we can measure the injected power

14 Pulsar Wind Nebula α~45 ζ~60 Oblique rotator releases most of its energy in a relativistic magnetized wind of (predominantly) electrons / positrons, which illuminate the nebula Rees & Gunn 1974, Kennel & Coroniti 1984 III 14

15 The pulsar magnetosphere Surface magnetic field B~ G induces potential drop U~ V. Increasingly realistic simulations of the magnetosphere Charge density R² Goldreich & Julian 1969, Komissarov 2002, Spitkovsky 2006, Li et al. 2012, Kalapotharakos et al. 2012, Tchekhovskoy et al. 2013, Philippov & Spitkovsky 2014 Particle acceleration in gaps or via magnetic reconnection Sturrock 1971, Cheng et al III Bai & Spitkovsky

16 The pulsar magnetosphere Surface magnetic field B~ G induces potential drop U~ V. Increasingly realistic simulations of the magnetosphere Charge density R² Goldreich & Julian 1969, Komissarov 2002, Spitkovsky 2006, Li et al. 2012, Kalapotharakos et al. 2012, Tchekhovskoy et al. 2013, Philippov & Spitkovsky 2014 Particle acceleration in gaps or via magnetic reconnection Sturrock 1971, Cheng et al III Bai & Spitkovsky

17 The cold pulsar wind Split monopole approximation: undulating current sheet and azimuthal magnetic field reversing at crossing. Simulations confirm this out to ~10 RLC Bogovalov 1999, Kalapotharakos et al. 2012, Tchekhovskoy et al Magnetization, particle density, Lorentz factor unknown, expected to be Г ~ 106, Ne+e-~ 1039 s-1 Energy density concentrated towards the equator Current sheet f (r, θ) sin(θ)/r 2 III 17

18 The synchrotron nebula Sample magnetic field lines Porth et al Velocity [cm s-1] MHD simulations reproduces torus and wisp dynamics. 3D simulations show significant magnetic dissipation solving the σ-problem Komissarov & Lyubarsky 2004, Bucciantini et al Begelman 1998, Mizuno et al

19 The synchrotron nebula optical X-rays Porth el al Tracing maximum energy particles for synchrotron maps. Qualitative agreement but jet and inner ring too faint III Del Zanna et al. 2006, Volpi et al. 2008, Volpi et al. 2009, Camus et al

20 The VHE pulsations Gamma-ray emission up to ~400 GeV detected by MAGIC and VERITAS telescopes Aliu et al. 2008, Aliu et al. 2011, Aleksic et al. 2011, Aleksic et al Makes curvature radiation as emission mechanism unlikely. Inverse Compton emission viable alternative Lyutikov et al Maybe from outside the light cylinder Bogovalov & Aharonian 2000, Kirk et al. 2002, Aharonian et al and Petri 2012 IV 20

21 The VHE pulsations Gamma-ray emission up to ~400 GeV detected by MAGIC and VERITAS telescopes Aliu et al. 2008, Aliu et al. 2011, Aleksic et al. 2011, Aleksic et al Makes curvature radiation as emission mechanism unlikely. Inverse Compton emission viable alternative Lyutikov et al Maybe from outside the light cylinder Bogovalov & Aharonian 2000, Kirk et al. 2002, Aharonian et al and Petri 2012 IV 21

22 The VHE pulsations Aleksic et al 2014 Counts Bridge emission Phase VHE pulsation where not expected, they should be explained self-consistently in pulsar emission models, giving a new constraint on the processes in the magnetosphere IV 22

23 The flaring Nebula When searching for flaring sources in the whole sky.. V A catalog of flaring LAT sources in Ackermann et al

24 The flaring Nebula The Crab shows up! Galactic coordinates, Aitoff projection Fermi and AGILE detected 7 flares so far ~1 per year V Abdo et al. 2011, Tavani et al. 2011, Balbo et al. 2011, Striani et al 2011/3 24

25 Photon flux >100 MeV [ 105 cm-2 s-1] The flaring nebula Daily binning April 2011 Feb Sept March October March 2014 MET [ 108 s] At HE gamma-rays the nebula is variable on all time scales which can be resolved and shows strong flares of approximately week duration V 25

26 The April 2011 flare Deepest view so far during Fermi ToO. Nebula flux increase of ~30 during, doubling in <8 hours. Buehler et al 2012 ~20 min bins Chandra Keck VLA V 26

27 Spectral evolution April 2011 Cutoff 375MeV ~1% Ppulsar L~ Ecut3.4 ± 0.8 Photon index of 1.26 ± 0.11 V 27

28 The March 2013 flare Flux increase by factor ~20 during Fermi ToO, bursts of ~6h duration Mayer et al ~40 min bins Chandra Keck HST V (Also H.E.S.S., VERITAS, NuStar) 28

29 Spectral evolution March 2013 Spectrum softer than in April 2011 again cuts off at ~0.5 GeV V 29

30 Spectra at flare maximum V 30

31 Origin within the nebula? Correlated emission at lower frequency would allow to pinpoint the origin of the flares in the nebula dense monitoring and ToO campaigns (>15 accepted proposals with ~1 Ms of observations) V 31

32 Counterpart at lower frequency? Chandra Keck VLA April 2011 campaign Weisskopf et al No counterpart found to date electrons spectrum unusually hard, in April 2011 close to mono-energetic V 32

33 What we have learned Compact region (<10-3 pc) with significant energy (~1% P) Synchrotron emission >160 MeV Emission anisotropic Doppler boosted Bednarek et al. 2011, Clausen-Brown et al 2012, Lyutikov et al 2012 Shock acceleration does not work, magnetic reconnection does Likely part of magnetic dissipation Uzdensky et al. 2011, Sironi et el. 2011,Cerutti et al. 2013/14, Begelman 1998, Komissarov 2012, Porth et al V Synchrotron emission from magnetic reconnection in PIC simulations from Cerutti et al

34 What we would like to know Where do the flares come from, the base of the jet? Lyubarsky 2012 Or from the inner knot? is this the arch shock of the pulsar wind? Komissarov and Lyutikov 2011 Simulated emission at 100 MeV Inner Knot V 34

35 Summary & Outlook We have come a long way in understanding the guest star of 1054 AD. Today it is one of the prime laboratories to study the behavior of relativistic magnetized plasma Large theoretical advances in the past decade largely due to PIC and MHD simulations The VHE pulsations put new constraints of the processes in the magnetosphere Gamma-ray flares likely caused by magnetic reconnection, giving us a view into the magnetic dissipation in the nebula VI 35

36 Backups 36

37 Pulsar during flare? 33 months April flare No changes in pulse phase and magnitude and no pulsations in the flare emission. No change found in radio and x-ray pulsations Abdo et al. 2011, Morii et al. 2011, Buehler et al

38 Typical time scale? 38

39 Typical time scale? April 2011 flare Low activity All time Index

40 Pulse polarization Radio Optical Moffet & Hankins GHz PD PA 1.4 GHz Slowikowska et al Phase Degree and angle of polarization change with phase (with-)in radio and optical II 40