NEW FRONTIERS IN THE
STUDY OF GRB PHYSICS
In the era of the Liverpool Telescope
Drejc Kopac (LJMU),
on behalf of the Liverpool GRB group
RAS, London, 14 Nov 2014
Gamma-Ray Burst (GRB)
- The most powerful and luminous of stellar deaths
- Duration: 0.01s - 1ks
- Product of massive stellar collapse or NS-NS/NS-BH merger
- Cosmological distances
- Afterglow (X-ray, optical, radio); up to days – months
Autonomous GRB Follow-up (2004)
Swift


 LT and FTS/FTN (LCOGTN) in the Swift era
 Immediate automatic response (over-ride)
 No human intervention from receipt of alert:
 observations
 automatic object ID
 choice and execution of subsequent observations
(Guidorzi et al. 2006)
Optical afterglow light curves
“Rapid” optical follow-up soon after 1997 was about 10 hrs
GRB Fireball Model
Reverse
Shock
Gehrels et al. 2007
Forward
Shock
Fireball Magnetization
Internal shock region
 Standard synchrotron shock model



Baryon-dominated jet creates tangled B-field in shock layer
Prompt -ray variability (internal shocks)
Inefficient conversion of bulk:radiated energy
 Alternative: Poynting flow



Large-scale ordered magnetic fields advected outwards
Powerful acceleration and collimation
Dissipation through magnetic reconnection?
Magnetic Fields
Indirect diagnostics:
 Relative contributions of
RS and FS components
 Color evolution diagnostic
 Rapid response is key
Reverse+forward shock
GRBs 990123, 021211, 060111B, 060117,
061126, 080319B
(Gomboc et al. 2009)
See also: Zhang, Kobayashi & Meszaros, 2003, ApJ
Reverse Shock Sample
 Parent sample: 118 GRBs
10 reverse shocks with z
 Fainter than average FS
emission @ t>10ks
 High magnetization:

R = B,r/B,f ~ 2 – 104

Magnetized baryonic jets
Japelj, Gomboc, Kopac et al. 2014, ApJ
See also: Harrison & Kobayashi 2013, ApJ
Prompt optical emission sample
 Parent sample:
36 GRBs with
optical during 
18 peaks
 Steep rise/decay
 Variability

 Internal dissipation


Kopac, Kobayashi, Gomboc et al. 2013, ApJ
Polarization
Direct diagnostics of magnetic fields
●
-ray polarization intriguing:
• RHESSI:
GRB 021206
P~ 0 or 70-80%
• INTEGRAL:
GRB 041219A
P ~ 4%  43 ± 25%
• IKAROS/GAP:
GRB 100826A
GRB 110301A
GRB 110721A
P=27 ± 11%
P=70 ± 22%
P=84 ±1628%
(Coburn & Boggs 2003 vs Rutledge & Fox 2003/Wigger et al. 2003)
(Gotz et al. 2009; but also McGlynn+07, Kalemci+07)
(Yonetoku et al. 2011; Toma et al. 2012)
• Optical polarization:
• P ~ few % at late time (~ day)
• Reverse shock predicted to be polarized
• Rapidly fading signal
RINGO POLARIMETER
RINGO POLARIMETER
GRB 060418
 RINGO polarimetry of GRB 060418 at t = 203 s



Measurement coincided with deceleration of fireball

(~400; Rdec~ 1017 cm)
Strongly-constrained upper-limit: P<8%
Equal contribution from forward and reverse shocks
REM
XRT
RINGO
Steele et al. 2006, SPIE, 6269 ,179;
Mundell et al. 2007, Science, 315, 1822
GRB 090102
Reverse shock
RINGO
GRB 090102
Steele et al. 2009, Nature, 462, 767

60-s RINGO exposure began t = 160 s post-burst
Stars in field provide additional calibration
First detection of optically polarized GRB afterglow: P=10.2±1.3%

Ordered magnetic fields, low


 for bright RS
Temporal Coverage/Evolution
 Complex light curves
 Single-shot inadequate
 Time-resolved polarimetry
 In 2009-2010 recycled RINGO into RINGO2:
Fast read-out EMCCD
 Polaroid 8 rotations per sec
 125 millisecond exposures
 Images not rings!

GRB 120308A with RINGO2
SED modeled z ~ 2.22; RINGO2 at t=240 s
(restframe 74 s); ~4000 calibration stars
Mundell et al. 2013, Nature, 504, 119
GRB 120308A with RINGO2
 Time-resolved polarization
 High, declining %
 Stable position angle
 Forward + reverse shocks
Mundell et al. 2013 Nature, 504, 119
Highest measured optical polarization →
Long-lived, large-scale ordered magnetic field
Current State of the Art
Liverpool Telescope
1 min
VLT
1 hour
1 day
Fireball/jet physics
Large-scale geometry/ambient medium
Mundell+07, Science
Steele+09, Nature
Cucchiara+11, ApJ
Uehara+2012, ApJ
Mundell+13, Nature
Time in GRB restframe (sec)
+ 7 more poins in RINGO2 sample paper (Arnold+ in prep)
3 x RINGO2:
350 – 640 nm
650 – 750 nm
760 - 1000 nm
Spectral units
RINGO3
1
200 400 600 800 1000
Wavelength (nm)
Core programmes: GRB and AGN/blazars
Ultimately Tidal Disruption Events,
Fast Radio Bursts etc?
Each camera: 125-ms readout EMCCD.
Data rate ~ 1Gb/min
RINGO3 GRB (preliminary LC)
ray
X ray
Optical
Kopac et al. in prep
Conclusions
 Early time multi-wavelength data is essential


To characterize and understand GRB mechanism
To study jet composition and geometry

Also: to understand progenitors (fast spectroscopy - SPRAT)
 Future (especially in the context of LT2)
LT2: bigger, faster → GRB luminosity function (Melandri+ 08)
→ extending to faint-end high-z population
 → Study of prompt optical and RS (also with radio, Virgili+)
 Complicated LCs → various components, precise modeling
