Particle acceleration during the gamma-ray flares of the Crab Nebular

Transcription

1 Particle acceleration during the gamma-ray flares of the Crab Nebular SLAC Accelerator seminar SLAC 15 June 2011 R. Buehler for the LAT collaboration and A. Tennant, E. Costa, D. Horns, C. Ferrigno, A. Lobanov, M. Weisskopf

2 Outline Introduction to pulsars and their nebulae Introduction to the Crab Nebula Observations of Crab Nebula gamma-ray flares Flare implications on particle acceleration 2

3 Neutron stars Emerge after core collapse Super Novae: Neutron Fermi pressure balances immense gravitation (Density ~atomic nucleus, escape velocity ~0.3c, radius of ~12km) Carries most of progenitor stars angular momentum 3

4 Pulsars First discovered 1967 About 2000 discovered so far Periods between ms 10 s Ω α μ Precise cosmic clocks, have e.g. been used for first evidence for gravitational waves Strong magnetic fields Energy outflow electromagnetically dominated 4

5 Pulsar Wind & Magnetic Field Near Field (~2RL= c / 2πP) Far Field (>>2RL) toroidal Striped Pulsar Wind Г ~~ 106 Ne+e-~~ 1039 s-1 Force Free MHD Spitkovsky 2006 Equatorial plane, Bequator ~ G Electrostatic potential at pulsar surface due to Lorentz force separating charges: ~ 5

6 Pulsar Wind Nebula Examples of 54 Chandra observed PWN by Kargaltsev et al. Pulsar Wind interacts with surrounding material Pulsar Wind Nebula. About 100 known today. They generally have: Intense synchrotron radiation from radio to x-ray Particle cooling observed by shrinking size Inverse Compton component with TeV emission 6

7 The Crab Nebula (M1) Radio VLT Optical HST X-rays Chandra 7

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9 The Crab Nebula x-rays Chandra ~0.3 ly synchrotron IC 10 arcsec ~0.1 pc / ly Hubble 2nd October Atel nd October 2010 Remnant from 1054 AD Super Novae at 2 kpc (~6 kly) Standard reference in X-rays and VHE Overall emission expected to be stable within few percent 9

10 Large Area Telescope on Fermi Pair creation detector with tracker and calorimeter in space: Detects gamma-rays between ~20 MeV to 1 TeV Unprecedented sensitivity (Collection area of ~1 m2) Angular resolution increases with energy between ~5 0.3 Crab Nebula (~0.05 ) is a point source 10

11 Introduction to Crab with Fermi 31 month average Preliminary Preliminary Pulsar Inverse Compton Nebula Off-peak Synchrotron Nebula Pulsar and nebula on top of each other, use phase to separate 11

12 Three day Crab synchrotron curve Abdo et al 2011, Tavani et al 2011, Balbo et al 2011 Synchrotron Nebula 3 day binning Preliminary April 2011 flare 2007 flare by AGILE Feb flare Sept flare Three flares as extremes of persistent variability Flux increase by ~5 during 2009 and 2010 flares. 12

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14 2009 and 2010 flares spectrum 2010 flare 2009 flare Photon index: 2.7 ± ± ± 0.11 Second flare has hard synchrotron spectrum and extends >1 GeV 14

15 Short term variability during 2010 flare Pulsar + Nebula >100 MeV Balbo et al. September 2010 flare composed of 3 rapid (~12h) flares 15

16 Pulsed emission preliminary by P. Ray Nothing unusual during the flares in the timing residual 16

17 The 2011 outburst August April April 2011 Crab Geminga > 100 MeV 1º smoothed During the flare, the Crab was the brightest source in the gamma-ray sky 17

18 2011 flare in ~20 min. binning Synchrotron Nebula Preliminary ~20 min time bins average Synchrotron nebula increased by factor ~30 during Fast variability ( ~1h) 18

19 Nebula 2011 flare spectrum 1% of spin down power Preliminary Preliminary April 2011 flare Pulsar Average nebula New component with pulsar like spectrum but no sign of pulsation in flare photons 19

20 Multi-wavelength correlations? After 2009 and 2010 flares simple idea: Chandra 10 arcsec Find correlation between gamma-ray flares and X-ray, Optical or Radio Angular resolution allows to pin down gamma-ray emission site Monitoring Programs Inner ring (Chandra program led by Martin Weisskopf) The following results produce by Allyn Tennant.. 20

21 Chandra after the 2010 flare 28th Sept ~mcrab 6 days after September 2010 flare, followed by ~monthly images 21

22 Chandra during the 2011 flare Preliminary Optical Two more observations on 21th 28th April

23 Multi-wavelength correlations? No correlation found with the nebula, how about the pulsar? Keck observations of Hai Fu analyzed by Roger Romani (Thanks J. Graham, C. Max) Pulsar Knot 1 PI A. Melatos Also no correlations, measured in optical and radio 23

24 And here, poor fool! with all my lore I stand, no wiser than before..

25 Not true.. Basic conclusions: New pulsar like spectral component with an energy ~1% of the pulsar spin-down power released at ~500 MeV Very hard spectrum before the spectral break electron energy distribution close to mono-energetic For magnetic field strength of Bperp <105 G electron energies E >TeV, accelerated on hour time scales Rapid variability implies synchrotron emission from a compact emission region < ly No pulsations Flare originates from outside the light cylinder Emission likely from the Pulsar Wind region between the light cylinder and the wind termination 25

26 How are particles accelerated? Standard astronomers recipe of Diffusive Shock Acceleration? Thought to be at work at termination of pulsar wind. Severely challenged by the flares. Synchrotron emission above 1 GeV larmor radius E B E 2 B cooling length E B peak E 2 B 200 MeV larmor radius peak cooling length 4 26

27 Alternatives? Acceleration directly related to electromagnetic field from the pulsar: Magnetic reconnection of the Striped Wind? (At the termination of the pulsar Wind? Close the light cylinder?) Acceleration in the Electrostatic Field of the pulsar? Just beginning to explore these possibilities, they will likely revise our understanding of pulsars and their connection to pulsar winds and their particle acceleration.. 27

28 Alternatives? Acceleration directly related to electromagnetic field from the pulsar: Magnetic reconnection of the Striped Wind? (At the termination of the pulsar Wind? Close the light cylinder?) Acceleration in the Electrostatic Field of the pulsar? Just beginning to explore these possibilities, they will likely revise our understanding of pulsars and their connection to pulsar winds and their particle acceleration....lets ask the experts? 28

29 Backup slides

30 x-rays Chandra ~0.3 ly 10 arcsec ~0.1 pc 2010 / 0.3 ly Hubble 2nd October