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Shiho Kobayashi Astrophysics Research Institute, Liverpool JMU
On behalf of the Liverpool Telescope Team I.A. Steele, C.G.Mundell, R.J.Smith, C.Guidorzi, R. Harrison
• GRB Jets: Baryonic vs Magnetized
• “Very Early” Afterglow – A few mins after GRBs
• Polarization measurements – Liverpool Robotic Telescope
• Implications to magnetic fields in GRB jets
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X
O
R
How Jets accelerated? Origin of mag fields?
Relativistic Outflow with gamma > 100 and E ~10^51 ergs
cm1014≈ cm1016>
Prompt emission: B~ 10^6 Gauss Afterglow: B ~ 1 Gauss
Synchrotron emission
How GRB jets formed? • Baryonic jets
– Fireball, Thermal pressure – Tangled magnetic fields generated
locally by instabilities in shock.
• Magnetized jets – Rotating BH, Magnetic pressure – Threaded with globally ordered B-fields
Tchekhovskoy’s talk Talks this afternoon
€
ν
• Difficult to resolve the image (z~1, R < sub pc)
GRBs
“Flying Pancakes” T. Piran
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R /Γ2
GRB ejecta is in a non steady state and its width is many orders of magnitude smaller than radius.
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Polarimetry: good option to study the composition
Optical Afterglow: GRB990510
P~1.7% (18.5hr after GRB)
(Covino et al. 1999; Wijers et al. 1999)
The smoking gun of synchrotron emission
However, 18.5hr is too late to study the Compostion of GRB ejecta
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ISM
Forward Shock (blast wave) Reverse Shock
ejecta
The deceleration of Pancake (GRB ejecta)
KS&Zhang2008
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tdec ≈ 200 E53
n1
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 3Γ
100⎛
⎝ ⎜
⎞
⎠ ⎟ −8 / 3
sec
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Rotating polarizer by 90 deg
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Π =I − I⊥I + I⊥
Polarization measurement for rapidly decaying sources
Afterglow luminosity changes very rapidly…
could produce false 5% polarization
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Δm = 0.1
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• Polaroid with wedge prism : rotated at 500rpm • A Source is imaged as a small ring.
Sluggish CCD and a solution
Point source
Polarization info recorded on RING
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GRB 060418 Afterglow polarization measurement
– 200 sec after GRB trigger (30 sec exposure) – At the onset of afterglow: 14mag – Polarization: 8% upper limit
Mundell et al. 2007
x IR/opt
Molinari et al. 2006
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GRB 090102
Steele et al. 2009; Gendre et al. 2010 (LT, TAROT, REM,GROND,Swift)
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Fν ~ t−1.5
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Fν ~ t−1
Polarization detection: P~10% at 160 sec (60 sec exposure) Steep decay phase (the signature of reverse shock emission)
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GRB 090102 Detection: 10% 1%
Two polarization measurements by RINGO
L
t GRB 060418 P < 8%
L
t
large scale field in fireball
Mundell et al. Science 2007 Steele et al. Nature 2009
t~200sec Just at the peak
t~160 sec In the steep decay phase reverse shock
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±
Flattening Light curve: magnetized? Fan et al. 2002; Zhang et al. 2003; Kumar & Panaitescu 2003; Gomboc et al. 2008
Off-axis jet: Ghisellini & Lazzati1999; Sari 1999
Gruzinov & Waxman 1999
Visible region contains many patches of coherent B-fields
Large scale, Coherent B-fields
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Due to relativistic beaming, only a small area around LoS can be observed.
A toroidal magnetic field produces large polarization degree
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GRB 090102 Detection: 10% 1%
Two polarization measurements by RINGO
L
t GRB 060418 P < 8%
t
1) No large scale field in jet 2) Strong B-field in jet
large scale field in fireball
Mundell et al. Science 2007 Steele et al. Nature 2009
t~200sec Just at the peak
t~160 sec In the steep decay phase reverse shock
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±
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The lack of Reverse Shock Emission
• We detect reverse shock emission for a small fraction of GRBs.
• Dust extinction? Magnetic suppression? it emits at lower freq?
Akerlof et al. 1999; Sari & Piran 1999; Meszaros & Rees 1997
GRB990123
Mimica et al. 2009
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Early Afterglow
L
t
The passage of FS typical freq
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ν€
Fν
RS FS
SK&Zhang2003; Zhang et al. 2003 Sari&Piran1999; SK 2000; SK&Sari2001
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Γ2€
E = Mejectac2Γ = MISMc
2Γ2
spectrum
L
Light curves t
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tdec
Low freq High freq
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Radio Flares • Reverse shocked ejecta:
– adiabatically cooled, radiates at lower and lower freq – The emission peaks in the radio about 1day after GRB – Many flare events observed in radio (private communication)
• Radio Polarimetry provides additional info – Optical Flash – Radio flare modeling
Radio light curve: GRB 990123
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• GRB Jets: Baryonic vs Magnetized • “Very Early” Afterglow: Reverse Shock Emission
– A few mins after the prompt emission – Detection P=10% and upper limit P<8%
• LT observation now always start with RINGO2 for 10 mins – Time evolution of polarization degree and angle in early afterglow
• Polarimetry in other bands: multi-band modeling – X-ray/gamma-ray: ; GEMS, NHXM, POET – Radio flares
• Liverpool GRB conference: June 2012 – Jet physics is one of the main topics
Coburn & Boggs2003; Gotz et al. 2009
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