The Present and Future The Present and Future of GRB Cosmographyof GRB Cosmography

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

www.cosmicbooms.netwww.cosmicbooms.net

OutlineOutlineMotivations: What is GRB Cosmography?

Brief HistoryCurrent WorkFuture Prospects

MotivationsMotivationsGRBs – brightest explosions in universe Detectable out to high z (>5, ~10-20?)Gamma-rays penetrate dustAny evolution likely to be orthogonal to

that of Type Ia SNeSwift 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

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

MotivationsMotivations

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)

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

MotivationsMotivations

GRB Cosmography?GRB Cosmography?

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)

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

HistoryHistoryEiso distribution – spans several orders

of mag. – Eiso is a bad standard candle

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

Fluence in observed bandpass

Cosmological k-correction

Luminosity Distance (theory)

Hubble constant /70 kms-1Mpc-1

Redshift

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)

HistoryHistory

15 GRBs (fb), 17 GRBs (z) Frail et. al 2001

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

Updated comparison of Eiso and Eγ dist.

HistoryHistory

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

33 GRBs (fb), 39 GRBs (z)

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

Bloom et. al 2003

Frail et. al 2001

Ghirlanda et. al 2004

HistoryHistory

Friedman & Bloom 2004

1

23

4

Beaming inferred from steepening of afterglow light curve

Stanek et. al 1999

HistoryHistory

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

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

HistoryHistory