[Paper Review] Is there anything special about GRB 080319B?

S. Dado, Arnon Dar+1 more|ArXiv.org|Apr 4, 2008

Gamma-ray bursts and supernovae14 references3 citations

TL;DR

This paper argues that GRB 080319B, despite its extreme luminosity and visibility to the naked eye, was not uniquely special but an ordinary long gamma-ray burst viewed nearly on-axis via the cannonball model. The model explains its prompt emission via inverse Compton scattering of supernova glory photons and the afterglow via synchrotron radiation, reproducing all observed spectral and temporal properties with standard parameters.

ABSTRACT

We show that the properties of gamma ray burst 080319B and its afterglow are well reproduced by the cannonball model of long gamma ray bursts (GRBs). It was an ordinary GRB, produced by a jet of highly relativistic plasmoids (CBs), ejected in a core-collapse supernova (SN) and viewed, as some others before, particularly close to the CB-emission axis. It still remains to be seen whether GRB 080319B was associated with an SN akin to SN1998bw, the SN type ordinarily associated with GRBs, or with a much more luminous SN.

Motivation & Objective

• To determine whether GRB 080319B's extreme luminosity and visibility to the naked eye indicate a fundamentally different physical mechanism than standard long GRBs.
• To test whether the cannonball model can reproduce the full set of observational properties of GRB 080319B, including its prompt emission and afterglow light curves and spectra.
• To assess whether the burst was associated with a standard supernova (like SN1998bw) or a more luminous one (like SN2006gy), based on afterglow and potential SN signatures.
• To evaluate the consistency of the cannonball model with established correlations between $E_{ ext{iso}}$, $L_p$, and $E_p$ in GRBs.

Proposed method

• Modeling the prompt $oldsymbol{ u}$-ray and X-ray emission as inverse Compton scattering (ICS) of photons from the 'glory' (early supernova light) by relativistic electrons in cannonballs (CBs), using the relativistic beaming and Doppler factor $oldsymbol{ ilde{ u}}_0 = 2oldsymbol{ ilde{ u}}_0 / (1 + oldsymbol{ ilde{ u}}_0^2 heta^2)$.
• Applying the ICS spectral model with a power-law tail and exponential cutoff, matching the observed spectral index $oldsymbol{ ilde{ u}} = 1.01 \pm 0.02$ in the 15–350 keV band and the Band function fit from Konus-Wind.
• Using the synchrotron radiation (SR) model for the afterglow, with flux density $F_{\nu} \propto n^{(1+\beta)/2} R^2 \gamma^{3\beta-1} \delta^{3+\beta} \nu^{-\beta}$, where $\beta$ is the spectral index and $\delta$ the Doppler factor.
• Fitting the optical light curve with a pulse shape function $F_{\bar{\nu}}(t) \propto \left[ e^{-(T/(t-t_0))^{1+\beta}} \right] \left[ 1 - e^{-(T/(t-t_0))^{1+\beta}} \right]$, capturing rise and decay of individual pulses.
• Estimating the CB bulk Lorentz factor $\gamma_0 \approx 1250$ from the peak energy relation $E_p \approx 0.23 \gamma_0 \delta_0 \epsilon_\gamma / (1+z)$ with $\epsilon_\gamma \sim 2$ eV.
• Assessing the break time $t_b \sim 70$ s when the CB sweeps up a mass equal to its rest mass, marking the transition from ICS to SR dominance in the afterglow.

Experimental results

Research questions

• RQ1 Was GRB 080319B's extreme brightness due to unique physical conditions or simply viewing it nearly on-axis in the cannonball model?
• RQ2 Can the cannonball model reproduce the observed $[(1+z)E_p, E_{\text{iso}}]$ and $[(1+z)E_p, (1+z)^2 L_p]$ correlations seen in GRB 080319B?
• RQ3 Does the optical afterglow light curve and spectral evolution of GRB 080319B match the predictions of the cannonball model's synchrotron radiation framework?
• RQ4 Is the observed spectral softening at $t \sim 5000$ s consistent with a density enhancement causing a 'bend frequency' crossing the optical band?
• RQ5 Could GRB 080319B have been triggered by a supernova more luminous than SN1998bw, such as SN2006gy, and how would this affect detectability?

Key findings

• The cannonball model successfully reproduces the observed $E_{\text{iso}} \approx 1.32 \times 10^{54}$ erg and $L_p \approx 9.67 \times 10^{52}$ erg s$^{-1}$ for GRB 080319B, consistent with its $z = 0.937$ redshift and luminosity distance of ~6 Gpc.
• The observed spectral index $oldsymbol{ ilde{ u}} = 1.01 \pm 0.02$ in the 15–350 keV band matches the cannonball model's predicted ICS spectrum with a cutoff at $E_p \approx 1260$ keV.
• The optical afterglow's temporal decay index $oldsymbol{ ilde{ u}}_{\text{opt}} = 1.40 \pm 0.04$ implies a late-time spectral index $eta_{\text{opt}} \approx 0.90$, consistent with the model's prediction of $eta_{\text{opt}} \approx \beta_X$ after the bend frequency crosses the optical band.
• The observed spectral softening at $t \sim 5000$ s, with $eta_X = 0.70 \pm 0.05$, is consistent with a predicted $eta_{\text{opt}} \approx \beta_X - 0.5 = 0.2 \pm 0.05$ due to a density enhancement causing a backward crossing of the bend frequency.
• The cannonball model predicts that a SN1998bw-like supernova would have a peak J-band flux density of ~4.4 $\mu$Jy by late July 2008, comparable to the fading afterglow, making it potentially detectable via color changes.
• The model shows that GRB 080319B's properties are not unique: similar values of $E_{\text{iso}}$, $L_p$, and $E_p$ were seen in GRBs 990123, 050717, and 061007, indicating it was an ordinary GRB viewed nearly on-axis.