Observations of particle acceleration in the blast waves of GRB - PowerPoint PPT Presentation

Observations of particle acceleration in the blast waves of GRB afterglows Peter A. Curran with Phil Evans, Alexander van der Horst, Rhaana Starling, Mat Page Laboratoire AIM, Irfu /Service d'Astrophysique CEA Saclay, France

GRB afterglows Electrons are accelerated by shock… emit synchrotron radiation External shocks: X-ray, optical, radio (Piran 2003)

GRB Synchrotron spectra Electrons are accelerated by shock… emit synchrotron radiation Cooling break freq. Optical X-ray (Slow cooling, adiabatic expansion) (Sari et al. 1998)

Fermi acceleration of electrons Electrons are accelerated by shock… to a certain distribution (value of p dependent on the underlying plasma physics) Electron energy distribution Number of electrons with Energy, E N(E) ∝ E -p p – electron energy distribution index Energy, E

Fermi acceleration of electrons Single, discrete value of p ? Or a distribution? What distribution? How does measured value compare to theory/ simulations? Universal value or variation between sources*? * GRB ( p~2 -3 ), Cosmic Rays ( p ≈ 2. 5-2.7), X-ray binaries ( p ≈ 2.6, σ ≈ 0.4), AGN ( p~3)

Derivation of p X-ray spectra ➝ p( β X , N H ) ⇒ multiple options ⇒ synchrotron cooling frequency above/below Compare predictions of p to light curves (optical & X-ray: small sample) to decide which value of p is correct

Distribution of p BeppoSAX & Swift (16 GRBs) Not consistent with single, discrete value (Starling et al. 2008; Curran et al. 2009)

Probability distributions 0.5 Cooling frequency: ( free parameter ) > X-ray < X-ray Transform p to β

X-ray spectral index, β X Swift X-ray Telescope (XRT) ( ~ 300 bursts) i) Assume underlying p distribution ii) Find most-likely parameters iii) Test hypothesis (via Monte Carlo) (data from Evans et al. 2009)

Monte Carlo hypothesis test ~10 5 synthetic data sets

Distribution of spectral index, β X Swift XRT ( ~ 300 bursts) Single discrete value of p ? synthetic data (Curran et al. 2010)

Distribution of spectral index, β X Swift XRT ( ~ 300 bursts) Not consistent with single, discrete value synthetic data (Curran et al. 2010)

Distribution of spectral index, β X Swift XRT ( ~ 300 bursts) Gaussian distribution of p ? (p =2.39, σ =0.6) synthetic data (Curran et al. 2010)

Distribution of spectral index, β X Swift XRT ( ~ 300 bursts) Consistent with a Gaussian distribution p =2.39, σ =0.6 (Curran et al. 2010)

Distribution of p via temporal indices X-ray spectra ➝ p( β X , N H ) ⇒ multiple options X-ray light curves ➝ p(indices, density structure, accretion) & accuracy of fit, model dependent ⇒ multiple options

Distribution of p via temporal indices Totally independent method, with different data! Preliminary p ≈ 2.36, σ≈ 0.36 ( ~ 180 bursts) (Evans et al. in preparation, using model of Van Eerten & Wijers 2009)

Fermi acceleration of electrons Q: Single value of p ? Distribution of p ? What distribution? Q: Single value of p ? Distribution of p ? What distribution? A: Gaussian distribution at p =2.39 and standard deviation, σ =0.6 A: Gaussian distribution at p=2.39 and standard deviation, σ =0.6 Electron energy distribution Number of electrons with Energy, E N(E) ∝ E -p Energy, E

Only 1 spectral peak?

Only 1 spectral peak? Swift XRT ( ~ 300 bursts) ?

Only 1 spectral peak? ≤ 6% with > 94% with cooling cooling ~94%: cooling frequency frequency frequency below > X-ray < X-ray Number of trials 3 σ limit 6% 94% Fraction of GRBs with ν c < X-ray (Curran et al. submitted)

Other explanations?  Bimodal distribution of p, with mode of p drawn from correlated with position of cooling break? Highly convoluted!  Wide distribution blurs out double peaks? Requires distribution far wider than observed  Not 2 β X ( p ) relationships?

Synchrotron spectra? Smoothly broken curves ⇒ continuous range of relationships Breaks evolve in time ⇒ spectral & temporal changes (inconsistent with observations) (Granot & Sari 2002)

Other explanations?  Bimodal distribution of p, with mode of p drawn from correlated with position of cooling break? Highly convoluted!  Wide distribution blurs out double peaks? Requires distribution far wider than observed  Not 2 β X ( p ) relationships? Inconsistent with observations No, majority of GRBs have cooling frequency below the X-rays - supported by preliminary Evans results

Conclusions  Fermi acceleration can be ‘observed’ in GRBs, and parameters constrained via statistical methods  Allows for comparison with other sources, as well as theory/simulations  not consistent with a single, discrete value of p  consistent with Gaussian of p ≈ 2.4, σ ≈ 0.6*  94% of GRBs have cooling frequency below the X-rays * Compared to Cosmic Rays ( p ≈ 2. 5-2.7), X-ray binaries ( p ≈ 2.6, σ ≈ 0.4), AGN ( p~3)