The April 2011 gamma-ray flare: A new astrophysical puzzle R. Buehler for the Fermi-LAT collaboration and A. Tennant, P. Caraveo, E. Costa, D. Horns, C. Ferrigno, R. Mignani, A. Lobanov, A. Luca, M. Tavani, M. Weisskopf Berkeley Astronomy Department Colloquium 13 October 2011

2 Outline ● Introduction to pulsars and their nebulae ● Introduction to the Crab Nebula ● Observations of Crab Nebula gamma-ray flares ● Flare implications and open questions ● Summary and conclusion

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

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

5 Pulsar Wind & Magnetic Field Equatorial plane Near Field (~2R L = c / 2πP) Far Field (>>2R L ) toroidal Force Free MHD Spitkovsky 2006, → B equator ~ G “Striped” Pulsar Wind Г ≈ 10 6 N e+e- ≈ s -1 Electrostatic potential at pulsar surface due to Lorentz force separating charges: ~

6 Pulsar Wind Nebula Pulsar Wind interacts → 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 Examples of 54 Chandra observed PWN by Kargaltsev et al.

7 Radio VLA Optical HST X-rays Chandra The Crab Nebula SN 1054, M1 10''~0.1pc

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9 Inner Nebula dynamics Monthly images by Chandra and Hubble telescopes Velocities ~0.5c observed

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 ~1m 2 ) ● Angular resolution increases with energy between ~5°–0.3° Crab Nebula (~0.05°) is a point source

11 The Crab with Fermi Pulsar and nebula are bright gamma-ray sources Preliminar y 33 months April flare

12 Average spectrum Preliminar y 33 months Nebula between synchrotron and Inverse Compton component Pulsar spectrum falls harder than exponential. VHE detection to ~400 GeV makes curvature radiation unlikely

13 Flare discovery in all-sky search Flares with significance greater 3 sigma in weekly bins

14 Flare discovery in all-sky search

15 Flare discovery in all-sky search Crab nebula

16 Simultaneously discovered by AGILE, both published in Science Tavani et al 2011, Abdo et el. 2011

17 Long term light curve to be continued.... pulsar + IC nebula pulsar + IC + synch. nebula February 2009 Preliminar y

18 Long term light curve Persistent variability from yearly to daily time scales April 2011 September 2010 Preliminar y

19 The April 2011 flare ● Flare ~five previous flares, synch. nebula increase of ~30 ● Flux doubling in < 8 hours ● No changes in pulsar flux and phase ~9 min time bins (adaptive binning cause of Earth occultation) Preliminar y

20 Spectral evolution Preliminar y “Pulsar like” erg/s ~1% E pulsar ● γ = 1.26 ± 0.11 E peak = 361 ± 26 MeV

21 Power vs Cutoff Preliminar y Luminosity (P 100 ) scales with the power of 3.4 ± 0.8 the cutoff energy (E C )

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

23 Preliminary Chandra during the 2011 flare Optical

24 How about the pulsar region ? Knot 1 Pulsar PI A. Melatos (Hai Fu, Roger Romani, J. Graham, C. Max) Changes of <20%, also no correlations detected in radio (Teddy Cheung, Greg Taylor)

25 How about the pulsar region ? Knot 1 Pulsar PI A. Melatos (Hai Fu, Roger Romani, J. Graham, C. Max) Changes of <20%, also no correlations detected in radio (Teddy Cheung, Greg Taylor)

26 Assembling the puzzle Absence of MW counterpart and flare spectrum reveal hard electron spectrum: Causality implies small emission region: At peak 1% of pulsar spin down power emitted → how is it focused? Flare energy is carried by the highest energy electrons

27 Emission is very likely synchrotron, peak frequency of ~360 MeV is difficult to explain: Is possible e.g. in magnetic reconnection, or two zone models. Alternative Doppler beaming (also relaxes energy focusing problem). Assembling the puzzle E < B <, Uzdensky et al. 2011, Cerruti et al 2011

28 Assembling the puzzle Spectral evolution is compatible with fixed spectrum varying in frequency and luminosity with power ~3.4 → Compatible with boosting variations Brightness + High frequency + Spectral evolution Emission region is likely moving relativistically towards us and variations in motion are producing flux variations

29 Where is the emission region ? At the wind termination, knot 1 or inner ring ? At the pulsar, at the wind launch ? ● Lack of pulsations imply R > R L ↔ energy focusing implies R small In the jet ? ● Natural focus of energy, but nature not clear yet knot 1 pulsar Knots on inner ring jet Chandr a HST Atel nd October Komissarov et al. 2011, Luytikov et al 2011

30 How are particles accelerated? Diffusive Shock acceleration does not work ● Theory says difficult at wind termination ● Synchrotron peak → too fast cooling ● Spectrum typically have δ < -1 Magnetic reconnection in striped wind? ● At wind termination electron energies probably too low ● Before the wind termination In the electrostatic field of the pulsar? Alternatives: Gallant et al. 1992, Arons 2004, Sironi et el Lyubarsky et al. 2003, Kirk et el Abdo et el Sironi et el. 2011

31 Summary and Outlook Flares from the Crab are a surprising discovery, which gives us look into the “unknown” pulsar wind region. Some basic implications can already be drawn: ● Changes in relativistic motion produce the flux variations ● Flare energy is carried by the highest energy electrons Theoretical interpretation just started: ● Where does the emission come from? ● How are the particles accelerated? Monitoring and ToO programs at optical and x-rays put in place. Fermi is waiting for the next flare.

32 Backup slides

33 Chandra after the 2010 flare 6 days after September 2010 flare, followed by ~monthly images 28 th Sept ~mCra b

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