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The Present and Future of GRB Cosmography. Andrew S. Friedman (Harvard-CfA) & Joshua S. Bloom (Harvard-CfA / UC Berkeley). Outline. Motivations: What is GRB Cosmography? Brief History Current Work Future Prospects. Motivations.

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The present and future of grb cosmography l.jpg
The Present and Future of GRB Cosmography

Andrew S. Friedman (Harvard-CfA) &

Joshua S. Bloom (Harvard-CfA / UC Berkeley)

Outline l.jpg

  • Motivations: What is GRB Cosmography?

  • Brief History

  • Current Work

  • Future Prospects

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  • GRBs – brightest explosions in universe

  • Detectable out to high z (>5, ~10-20?)

  • Gamma-rays penetrate dust

  • Any evolution likely to be orthogonal to that of Type Ia SNe

  • Swift satellite in space! Years before SNAP (2010?, zmax ~ 1.7)

A GRB standard candle could serve as an independent probe of the geometry & expansion history of the universe, complementary to SNe Ia

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  • But wait, high z is ΩM-dominated – boring!

  • Only low z regime is good for ΩΛ, w, right? And very few low-z GRBs (now), including peculiar bursts 980425, 031203, (+future)

  • However, survey in full range 0 < z < 2 is crucial to pin down dark energy systematics (Linder & Huterer 2003)

  • GRBs already comparable in number to SNe Ia in 1<z<2, (zmax ~ 1.7 now, SNAP)

Ultimately, independent techniques are always a good thing in science

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Turning the game around - Pick a cosmology - GRB standard candles can tell you about the progenitors of GRBs

  • Empirical standard candle ultimately derives from microphysics (e.g. SNe Ia, Chandra. Limit)

  • Scatter about standard candle can help quantify burst diversity (e.g. peculiar Ia’s)

  • Empirical corrections to scatter (e.g. MLCS, stretch – Ia’s), can yield new physical insight

  • Large deviations from the standard candle (outliers) may indicate different physical mechanisms for making GRBs – i.e. different progenitor systems (e.g. Type II SNe)

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GRB Cosmography?

What function of observables might serve as a useful GRB standard candle? Purely empirical approach.

  • It must be indep. of redshift (z), i.e. roughly constant from burst to burst

  • It can depend on properties of both the prompt emission and the afterglow, although the latter are more observationally expensive

  • Let’s review the search in the set of GRBs with known z (now ~40 events)

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  • Eiso distribution – spans several orders of mag. – Eiso is a bad standard candle

Fluence in observed bandpass

Cosmological k-correction

Luminosity Distance (theory)

Hubble constant /70 kms-1Mpc-1


Data from Friedman & Bloom 2004 (astro-ph/0408413), ApJ/subm

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  • But GRBs are beamed. Eγ distribution is much narrower than Eiso (Frail et. al 2001, Piran et. al 2001, Bloom et. al 2003), but not sufficient for cosmology (Bloom et. al 2003)

24 GRBs (fb), 28 GRBs (z) Bloom et. al 2003

15 GRBs (fb), 17 GRBs (z)

Frail et. al 2001

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  • Updated comparison of Eiso and Eγ dist.

Data from Friedman & Bloom 2004 (astro-ph/0408413), ApJ/subm

33 GRBs (fb), 39 GRBs (z)

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  • Eγ is getting worse as a standard candle with new data: e.g. 030329, 031203, XRF 020903,…

Frail et. al 2001




Bloom et. al 2003

Ghirlanda et. al 2004

Friedman & Bloom 2004

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  • Beaming inferred from steepening of afterglow light curve

Stanek et. al 1999

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  • Eg meaningful, modulo external density (usually unconstrained), gamma-ray efficiency, kcor bandpass, and jet model

This prescription of Egamma is model dependent! Top hat jet.