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

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ν

How to diagnose the composition of GRB jets (ejecta)

M87, HST 5

•  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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•  2m robotic telescope at La Palma

Liverpool telescope (with RINGO polarimeter)

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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

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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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RINGO2: Starr Polarimeter

RINGO2 R<17mag

RINGO R<15 mag A few events/year

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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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www.astro.livjm.ac.uk/grb2012 Google “Liverpool GRB meeting”