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GRBs and Magnetic Fields

GRBs and Magnetic Fields. Shiho Kobayashi （小林史歩） Liverpool John Moores University. How to constraint the magnetization (and structure) of GRB ejecta. Early afterglow: RS and FS modeling (Synchrotron and IC) Early polarimetric measurements Ejecta structure: Optical/X-ray polarimetry.

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GRBs and Magnetic Fields

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  1. GRBs and Magnetic Fields Shiho Kobayashi （小林史歩） Liverpool John Moores University

  2. How to constraint the magnetization (and structure) of GRB ejecta • Early afterglow: • RS and FS modeling • (Synchrotron and IC) • Early polarimetric measurements • Ejecta structure: Optical/X-ray polarimetry

  3. the synchrotron shock model is successful, but there are some open questions … (Medvedev&Loeb1999; Nishikawa et al. 2008; Gruzinov 2001; Milosavljevic et al. 2006…) • Origin of Magnetic fields • prompt ~10^6G, afterglow ~ 1G • How to accelerate and collimate jets? • gamma>100, E=10^52 ergs • How to produce prompt gamma-rays • internal shocks/efficiency issue • Lack of optical flash in most events • magnetic pressure?, SSC? (Usov 1992; Meszaros&Rees1997;Lyutikov &Blandford 2002; Drenkhahn& Spruit 2002..) (Kumar1999; Beloborodov2000; SK&Sari2000;Zhang et a. 2006; Nousek et al. 2006) (Akerlof et al. 2000; Roming et al. 2006..) Magnetized jet model might solve these.

  4. The Standard model Energy transfer relativistic outflow (ejecta from central engine) Blastwave (FS ambient medium) Emission from Ejecta: Prompt gamma-rays Optical Flashes Radio Flares? X-ray Flares? Emission from Blastwave Afterglows (X/Opt/Radio) Insensitive to the properties of the original ejecta

  5. Method 1: RS and FS modeling Reverse Shock Forward Shock ejecta At the deceleration time (onset of afterglow) The deceleration happens when a significant fraction of the ejecta energy is given to the forward shock region. ５

  6. RS region: energy per particle smaller by • cooling frequencies comparable • the number of electrons is proportional to mass SK&Zhang2003

  7. If RS region has higher magnetization or magnetic energy density expressed as a fraction of the equipartion vale Zhang,SK&Meszaros 2003 Using these relations and theoretical decay indexes of FS and RS emission, we can model early afterglow

  8. GRB 061126 Optical Light curve Gomboc, SK, Guidorzi, Melandri, Mangano et al. 2008

  9. RS and FS modeling RS region: higher magnetization, but still a baryonic jet Gomboc et al. 2008 Panaitescu&Kumar2002 GRB 990123, GRB 021211 Zhang et al. 2003; Kumar&Panaitescu2003 Fan et al. 2005

  10. Method 2: Sync and SSC emission Wu’s talk yesterday Synchrotron self-inverse Compton radiation from RS the relative strength: Syn, 1st IC and 2nd IC components depends on the Compton parameter GLAST could give constraints on the magnetization of ejecta Kobayashi et al. 2007

  11. Method 3: Polarization measurements Magnetized jets : threaded with a globally ordered mag. fields which originated a the central engine, and advected outwards with the expanding flow. Polarization measurements of the ejecta emission are very interesting! Prompt gamma-rays: Coburn&Boggs2003: controversial Reverse shock emission -- optical flash: Mundell et al. 2007 -- radio flares: Granot&Taylor2005 X-ray flares?: in the near future?

  12. Radio Flares Reverse shock ejecta cools adiavatically and radiates at lower and lower freqs at later times. The emission peaks in the radio after about 1day VLA data Linear polarization GRB 990123: P<23% at 1.25days GRB 991216: P<11% at 1.49days P< 9% at 2.68days GRB 020405: P<11% at 1-2days Granot&Taylor2005

  13. Early polarization measurements: optical • Polarimeter on our 2m robotic telescope Liverpool telescope

  14. GRB 060418 • Afterglow polarization measurement • 200 sec after the start of prompt gamma-ray • At the onset of afterglow (12mag) • Polarization: 8% upper limit Mundell et al. 2007

  15. IR(REM), XRT IR: smooth rise fading away with a unbroken power-law the lack of color change: steep rise it is not due to the passage of the typical frequency of the forward shock emission Onset of the afterglow should be below optical at that time ~50% photons come from RS If RS region has global mag fields, we expect strong polarization. ruling out the presense of a large-scle mag. fields in the emitting region. Molinari et al. 2006 Optical(LT)

  16. Ruling out the presence of a large-scale mag. fields in the emitting region. Ruling out the presence of a large-scale mag. fields in the ejecta. Poynting-flux dominated jets: high magnetic pressure might suppress RS. No shock. No RS emission. No Polarization (the peak might contain only FS emission) Our results still allow Poynting-flux dominated jets Reconnections? L t

  17. If we detect high polarization in early early afterglow… • a large-scale mag. fields in the ejecta? • How fireball jet structure affects the conclusion?

  18. Large Polarization Magnetic field is ordered. b) Random Magnetic field+ Specific viewing angle The line-of-sight to GRB runs along the edge of a jet cone. Waxman 2003

  19. Random Magnetic fields generated by instabilities The mag fields parallel and perpendicular to the shock normal could have significantly different averaged strengths (Medvedev&Loeb1999) Some degree of alignment if observed edge-on If the slab is observed edge-on, the radiation is polarized! Ghisellini & Lazzati 1999; Gruzinov1999;Sari1999;Granot2003;Nakar2004;Fan et al.2008…

  20. If the emitting slab moves with a relativistic velocity, we have to take into account the relativistic aberration of photons.

  21. It the line-of-sight to GRB runs along the edge of the jet cone, we might observe large polarization. but it is rather rare to see a GRB from the preferable angle by chance.

  22. Structured GRB Ejecta Meszaros et al. 1999; Zhang&Meszaros2002;Rossi et al. 2002) • Initial angular distribution of Lorentz factor is not determined from late afterglow obs. • Deceleration radius is a function of viewing angle. • Deceleration = the onset of afterglow • At the onset of Afterglow, the line-of-sight runs along the edge of the emitting jet cone, polarized emission is expected! Jet decelerates around the center first. Granot&Kumar2003

  23. a large-scale mag field in ejecta L optical X-ray t t Structured jets with random mag fields optical x-ray t t

  24. Summary • RS/FS modeling for a few early optical afterglows • magnetization in RS region is higher • still baryonic ejecta • no optical flashes detected in most cases • ejecta magnetization changes from burst to burst??? • Optical Flash/Radio Flares • no presence of global magnetic fields in “the emission region” • still Poynting-flux dominated possible • Need more polarization measurements (opt/X-ray/Gamma) • events with a clear optical flash peak • Opt/X-ray Polarization measurements might constrain the structure of ejecta

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