gamma-ray bursts - university of wisconsin–madisonmorsony/150/lectures/lecture18_grbs.pdf ·...
TRANSCRIPT
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Gamma-Ray Bursts
• How do GRBs make gamma-rays?
• What are the impacts of GRBs on Earth?
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What isn’t known about GRBs?
" Where does the afterglow come from?
" How do they make gamma-rays? " What conditions are needed to make a
GRB?
" What powers them? (Neutron star or BH) 3
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How does a GRB make gamma-rays?
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GRB Observations • 080916C 080916C
Zhang et al. 2010 arxiv:1009.3338
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Old Model: Internal Shocks
" Energy injected by central engine at 107 – 108 cm " As jet propagates,
relativistic material accelerates and cools " At photosphere (1012 – 1013
cm), little internal energy, not much emission " Internal variation in flow
leads to shocks at 1014-1015 cm " Gamma-rays from
synchroton radiation
" Kinetic energy converted to internal energy, synchrotron radiation produces prompt burst
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Synchrotron radiation
" Non-thermal radiation from particles in a magnetic field
" Power-law slope at low and high frequencies
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Problems with Internal Shock Model
" Not very efficient, <10% of energy converted to gamma-rays " Also problems reproducing observed spectrum " Problem reproducing the Amati relation and other GRB correlations
" New Models: Photospheric emission or magnetically dominated jets
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GRB Simulation
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Jet Collimation
" In simulations, stellar envelope collimates the jet with oblique shocks " Opening angle at a few
stellar radii only 3-5°, not 10° " Lorentz factor ~20, not
~200
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Jet Collimation " Collimation keeps
the jet hot " Internal energy ~
rest mass energy " Lorentz factor ~200,
not saturated at 400
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Photospheric Emisson " Light curves " LBol up to 1052
erg/s " Near the jet axis,
there is a bright initial phase
" Off axis, dim then slowly brightens
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Photospheric Efficiency " Very efficient,
20 % to 40%
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Amati Relation
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Amati Relation
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Problems with Photospheres " GRBs don’t
look like a black body " Need power law both below and above peak " Black body is too steep at both ends " Peak energy too low for Amati
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GRB Observations 090902B 080916C
Zhang et al. 2010 arxiv:1009.3338
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Magnetically Dominated Jets
" Hide photospheric emission by putting energy into magnetic field " Instabilities lead to reconnection " Convert magnetic field into particle energy " Emits synchrotron radiation
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Magnetically Dominated Jets
" Needs fast reconnection
" Not certain how particles are accelerated
" Not certain if observed spectrum can be reproduced
" Should not produce GeV neutrinos
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GRB Summary
• Long GRBs made from jets inside a dying star
• We are looking down the jet, see very relativistic material making gamma-rays
• Still a lot that we don’t know about them
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GRB Record Holders
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GRB Records • Most distant GRB
measured at z = 9.4, Universe only 520 million years old
• Closest GRB at z 0.0085, 110 Mly away
• Brightest GRB was Eiso = 8.8 × 1054 erg
• Brightest afterglow reached mag. 5.8 in the optical, from z=0.937 (7.5 Gly)
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How close would brightest GRB need to be to be brighter than the
Sun? • Brightest afterglow reached mag. 5.8 in the
optical. • From z=0.937 (7.5 Gly) • Sun is mag. -26.74
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How close would brightest GRB need to be to be brighter than the
Sun? • Brightest afterglow reached mag. 5.8 in the
optical. • From z=0.937 (7.5 Gly) • Sun is mag. -26.74
• Would need to be at 2,328 light years
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How far away does a typical GRB need to have more energy hit the Earth than the Sun does?
• Sun luminosity at Earth = 1300 W/s/m2 = 1.3 106 erg/s/cm2
• Typical GRB = 1051 erg/s
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How far away does a typical GRB need to have more energy hit the Earth than the Sun does?
• Sun luminosity at Earth = 1300 W/s/m2 = 1.3 106 erg/s/cm2
• Typical GRB = 1051 erg/s
• Answer: about 7,800 light years
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GRB Impacting Earth • GRB 030329 had a measurable effect on the
ionization of the upper atmosphere, from z=0.168 (2.1 Gly)