kobayashi - ICCUBicc.ub.edu/congress/HEPROIII-2011/slides/Thursday_morning/Kobayashi... · Shiho Kobayashi Astrophysics Research Institute, Liverpool JMU On behalf of the Liverpool

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

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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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”