Hunting for the GRB ProgenitorObservations of The GRB 030329 Radio Afterglow and Computer

Modeling of the Circumburst Medium

GTAC Conference August 29, 2011Bob Mesler (University of New Mexico)Chris Fryer (LANL)

Dan Whalen (LANL)Ylva Pihlström (University of New Mexico)

Progenitors-I

• Binary Star– Massive star in a binary system overflows its

Roche Lobe– Massive star loses its hydrogen envelope and

collapses

• Single, massive star– Massive star loses its hydrogen envelope

due to winds and rotation-induced mixing– Star collapses to form black hole

• He Merger– Primary star in a binary system collapses to form

black hole or neutron star, but system remains bound

– Secondary star overflows its Roche Lobe– Primary star spirals into secondary

Progenitors-II• He-He Merger

– Two massive stars in a binary system overflow their Roche Lobes nearly simultaneously

– Rapidly-rotating helium core is formed from merging stars

– Helium core collapses to black hole

• Magnetar– White dwarf collapses through accretion of

mass or 12-20 solar mass star collapses– Product is a magnetar rather than a black hole– Wind heated by neutrinos is collimated into

bipolar jet, becomes ultra-relativistic– Might explain ~100s X-ray emission seen

during 25-50% of short-period GRBs, which od ften rivals the initial burst in intensity

Gamma Ray Burst Emission

• Prompt Emission– Predominantly gamma rays– Short duration (a few minutes

or less)– Internal shocks

• The Afterglow– X-rays, UV, optical, IR, and

radio– Long duration (days to years)– External shocks (collisionless)– Synchrotron

Density Profiles

• Wind-like Medium– Stellar wind from evolved star blows bubble into

ISM– Density within bubble is characteristic of r-2 density

wind• Uniform Density Medium– GRB Progenitor loses material via episodes of mass-

loss within about 1000 years of the end of its life (dependent upon progenitor)

– GRB 020405

GRB 030329

• Brightest radio afterglow ever detected at radio frequencies (55 mJy at 43 GHz)

• Relatively close to Earth– z = 0.1685– d = 587 Mpc

• Only GRB to ever be resolved with VLBI• Detectable at 5 GHz by VLA at least through

2008

VLA/EVLA and WSRT Observations• Source observed at 5 GHz from 59 – 1828

days after the burst• Peak flux at 5 GHz was 11.6 mJy

– Brightest GRB ever recorded at this frequency

– Unusual intensity allowed radio afterglow to be detectable by EVLA through at least 2008

• Source decays with clear power law with α = -1.27 ± 0.03

VLBI Observations

• Direct measurements of linear size– Proximity to Earth make this

the only GRB to ever be resolved by VLBI

– Direct observations put much-needed constraints on the models

• Models of linear size at early times– Source unresolved until day 83– Estimate of angular size attained through model-dependent estimation of the

quenching of the scintillation– Large uncertainties due to reliance on imperfectly understood properties of the

ISM

Modeling with Zeus-MP

• Massively Parallel Computing• Physics with The Zeus-MP/2 Code

– MHD– gas hydrodynamics– flux-limited radiation diffusion– self gravity– multispecies advection

• Dynamic gridding makes high resolution possible over enormous range in size scales

• Allows accurate 3D modeling of the pre-burst environment

• Wind mass loss rate: 10-5 Msolar / yr• Duration of outburst: 10 years• Outburst time: 1000 years before GRB• Characteristic of a Binary System or He-Merger

The Future

• Currently running a suite of models that will shed light onto density profile of medium in proximity to progenitor at time of burst

• These models will help improve GRB emission models

• Better emission models applied to current data, and additional VLBI observations of GRB radio afterglows if possible, will place better constraints on the GRB progenitor