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Recent Results and the
Future of Radio Afterglow
Observations
Alexander van der Horst
Astronomical Institute Anton Pannekoek
University of Amsterdam
Suite of radio follow-up observatories
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Very Large Array
Westerbork Synthesis Radio Telescope
Australian Telescope Compact Array
Giant Metrewave Radio Telescope
Ryle Telescope
Combined Array for Millimeter-wave Astronomy
New era in radio follow-up observations
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Very Large Array  Jansky Very Large Array
Westerbork Synthesis Radio Telescope  Apertif
Australian Telescope Compact Array
Giant Metrewave Radio Telescope
Ryle Telescope  Arcminute Microkelvin Imager
Combined Array for Millimeter-wave Astronomy
Atacama Large Millimeter Array
Low Frequency Array
Square
MeerKAT
Kilometer
Australian SKA Pathfinder
Array
Murchison Widefield Array
Long Wavelength Array
Broadband spectrum
Piran 2003
Wijers & Galama 1999
Radio observations:
• Peak flux & frequency
• Self-absorption frequency
• Non-relativistic evolution
• Scintillation & image size
Physical parameters
• Electron energy distribution index p
• Energy in electrons εe
• Fraction of emitting electrons ξ
• Energy in magnetic field εB
• Blast wave energy E
• Density of circumburst medium n
• Structure of circumburst medium k
• Jet opening angle θ0
• Observing angle θobs
GRB 030329
Berger, Kulkarni et al. 2003
Van der Horst, Kamble et al. 2008
Pihlstrom, Taylor et al. 2007
Current radio afterglow sample
• 1/3 of observed GRBs
detected in radio
• Narrow observed flux range
• Sensitivity limited
• No clear correlations with
other wave bands (optical?)
• Detected radio afterglows
brighter in prompt emission,
X rays & optical
• Biased because of strategy
Chandra & Frail 2012
Average light curves
Chandra & Frail 2012
• Forward & reverse shock
• Bright future for 8 GHz and
above (including ALMA)
• How about low radio
frequencies?
Radio predictions: p & εe
Radio predictions: εB & n
Radio predictions: E & z
Radio calorimetry
• Late-time evolution: no relativistic complications
• Blast wave spherical?  Progenitor constraints
• Very low frequencies and/or very late times
GRB 970508 & GRB 980703 (Berger et al.
2004)
GRB 030329 with LOFAR
(Van der Horst, Kamble et al. 2008)
LOFAR: afterglows & prompt emission
• (Very) late-time afterglows:
• Automatic monitoring on various timescales
• Complementing the radio afterglow sample
• Automated response:
• Triggers by high-energy satellites or LOFAR
• New beam formed pointing to GRB location
LOFAR: afterglows & prompt emission
• AARTFAAC: Amsterdam-ASTRON Radio
Transients Facility And Analysis Centre
• 24/7 all-sky monitor with 6 central stations
• Piggy-back mode in all LOFAR observations
• LBA: whole sky, HBA: 1000 deg2
• Transient Buffer Boards
• 5 second storage
• Dispersion delay  subband approach
• Coming soon: LOFAR UK-Chibolton responding
to Swift triggers (1 hour follow-up)
Reverse shock – radio flares
Kulkarni, Frail et al. 1999
Melandri, Kobayashi et al. 2010
• Optical flash  early radio flare
• Probe GRB ejecta & jet magnetization
AMI fast follow-up
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Large array at 15 GHz
~0.13 mJy rms in 10 minutes
5.5’ primary beam
~30’’ synthesised beam
First responses within 4-5 minutes!!
AMI fast follow-up
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Large array at 15 GHz
~0.13 mJy rms in 10 minutes
5.5’ primary beam
~30’’ synthesised beam
First responses within 4-5 minutes!!
AMI observations of GRB 120422A
Staley, Titterington et al., in prep.
Dark bursts
Radio
• Host galaxy
• High redshift
• Intrinsic?
X-ray
Rol, Van der Horst et al. 2008
GRB 051022
Van der Horst, Kouveliotou et al.
2009
Compact binary mergers
Piran, Nakar & Rosswog 2012
• Sub-relativistic dynamically ejected outflow
• Detectable up to ~300 Mpc  LIGO/VIRGO
• Short GRBs(?)  relativistic jet
Conclusions
• Dawn of a new radio era:
• Many SKA pathfinders
• Upgrades of new facilities
• Extensions of the frequency & time domains
• Radio observations crucial for pinning down
afterglow physics
• Current sample sensitivity limited
• Probing parameter space of “regular” afterglows
• Studies of early reverse shock emission and
possible coherent prompt emission