What’s New with Gamma-Ray Bursts

Jay Norris *University of Denver,

Visiting Stanford/SLAC

GRB Bimodal Duration Distribution

Redshifts, Look-back times, Distances

Biggest News/Controversy on Long Bursts

Biggest News/Controversy on Short Bursts

Why Short Bursts are cool: tool for QG constraint

The odd, low-luminosity but (most) numerous GRBs

Summary: GRBs are still very hot, and getting hotter.

*Thanks to: Jeff Scargle, Neil Gehrels

GRB Bimodal Duration Distribution

Division roughly near 2 s. Present party line:Long bursts = “collapsars” — young, very low

metallicity hosts

Short bursts = coalesence events — no clear preference for host

Short GRBs: pulse FWHM ~ 5-30 ms spectral lags ~ 0-few ms few pulses

Long GRBs: pulse FWHM ~ 0.1-10 s spectral lags ~ 10-1000 ms many pulses

high energ

y

low energ

y

Selection Effect: Before Swift, ~ 1/2 of redshifts for long GRB came from emission lines of the host galaxies. Emission-line spectra tendto be obtained for z <~ 1 — these are under-represented in above plot.

Blue Histo: Short GRBsRed Histo: Long GRBs

Long Bursts only 46 absorption 16 emission

Swift GRBs with Redshifts: ~ 1/3 of BAT Sample

GRB Lookback Times

The Big Bang (13.7 Gyrs)

Trilobytes (500 Myrs)

Era of Long GRBs

Blue Histo: Short GRBsRed Histo: Long GRBs

Well-defined Era of Short GRBs?? Not so clear.

GRB Distances

QG timescale: ~ 8 ms / Gpc / GeV

z 0.8 :: T 6.8 Gyr “halfway” to Big Bang

z = 0.65

dcom = 2.4 Gpc

Tback = 6 Gyr

z = 3.5dcom = 7 Gpc

Tback = 12 Gyr

Rint ~ 1/2

Biggest News/Controversy for short GRBs: they’re not always short

<<< Short Bursts have Extended Emission ~ 25-30% of cases >>>

Rint ~ 1/600

Ratio of Intensities, initial pulse complex to extended emission, exhibits dynamic range of ~104.(Norris & Bonnell; Norris & Gerhels)

Theoretical problem (?): The timescale for coalescence is very short, ~ 1 second.

But, treatments of angular momentum flow in coalescing NS binaries do not consider inner zone, where accreting gas is optically thick to its own neutrino emission.(Narayan, Piran, & Kumar 2001)

Neutrino pressure may modulate coalescence (mass dependent).

“GRB 060614: Is it short, or is it long?”

Rint ~ 1/1

Initial short pulses complex ~ 0.1-3 s (6 s)

~ 5-10 s hiatus (5 s)

Extended Emission ~ 30 - 100 s (130 s)

150 s

*(z = 0.125 and no SN)

lag (ms) =

0.3 8.0

lag (ms) =

0.5 9.0

“New Gamma-Ray Burst Classification Scheme from GRB 060614” Gerhels et al. (Nature 2007) *

Biggest News/Controversy for long GRBs: Breaks not achromatic

<<< Jet breaks absent in most Swift/XRT Light Curves >>> This transition

to subrelativisticnear ~ 1 day usually missing in X-ray band

But “jet break” still there, and when expected, in optical(Ghirlanda et al. 2007):

Ep

eak

(keV

)

Egamma

(erg)

Explanation: reverse shock emission in slow shells dominates late X–ray flux (Uhm & Beloborodov); Grenet, Daigne & Mochkovitch), whereas optical flux reflects usual forward external shock mechanism. Or, late X–ray flux still dominated by central activity engine (Ghisellini et al. 2007).

GRB 930131 — “Superbowl Burst” aka Queen Beatrix Burst: Very bright short burst detected by Compton GRO

Time (seconds)

Co

un

ts /

64-m

s b

in

“Existence Proof” of very high-energy gammas in pulses of short GRBs. 1st six photons: 80, 30, 460, 170, 50, 165 MeV

GRB 051221a — Brightest burst detected by Swift/BAT (a once in ~ 2-year burst). “Most Salient Property” for our purpose: Negligible energy dependence of pulse peaks.

However, pulse widths do narrow at higher energies, but above 1 MeV this narrowing is essentially not mapped. So, model the pulse width energy dependence as w(E) E- with = 1/3, 1/4, 1/5 — this range covers reasonable possibilities — extrapolate to GLAST/LAT energies ...

... “detect” the burst with the LAT. Assuming z = 0.65, ~ 20 ms / GeV. Add this (hypthetical) QG-based energy-dependent dispersion. Explore optimal de-dispersion measures.

Most pulse narrowing

Least pulse narrowing

Most pulse narrowing + QG

(ms / GeV)

= 1/3

= 1/5

do trial transformations, ti' = ti

obs –

Eiobs

Ultra-low luminosity GRBs: Numerous, Nearby, with SNe

Supernovae Ib/c 9000 Gpc-3 yr-1 Underlum GRBs 700 Gpc-3 yr-1 “Classic” Long GRBs 70 Gpc-3 yr-1

GRB 060218 (BAT image trigger)

Luminosity ~ 10-6 most luminous GRBsDuration: ~ 2100 secondsSpectral Lag: ~ 100 s (BATSE chan 31)

z = 0.033 d = 145 MpcSN 2006aj (type Ib/c)Swift/BAT: Lag - Luminosity

Plot

GRBs are not just nascent black holes, with the most ultra-relativistic jets so far observed:

Short GRBs — with large dynamic range in intensity ratio, [initial pulse complex : extended emission] — may evidence interplay of neutrino opacity and viscosity, modulating angular momentum transfer via accretion flow near the collapsing object.

Long GRBs may be useful for exploring/constraining the Universe's expansion rate vs. time (although we are behind type Ia SNe by ~ few years in terms of calibration of systematics and sub-classes).

Very low luminosity GRBs, associated with type Ib/c SNe, usually undetected by Swift/BAT, are the most numerous sub-class — such objects may be worthy of much larger detectors, for in-depth study of black hole formation.

Summary: Latest in GRB Classes

QG Summary, Discussion, Caveats.

We explored several cost functions for QG-based energy-dependent dispersion recovery. The best appear to be Shannon and Renyi informations, which display relative insensitivity to pulse width (Scargle, Norris, & Bonnell 2007)

There are at least three sources of irreducible uncertainty in our treatment, in decreasing order of (probable) importance: finite pulse width, instrumental energy resolution, and pulse asymmetry.

[The first and third uncertainties would be much larger for long GRBs, than we expect for short GRBs — therefore short GRBs are the preferred tool.]

However, ignorance of pulse-width energy dependence above ~ 1 MeV presently represents the largest unknown factor.

While the brightest short burst in ~ 2 years would easily provide a stand-alone significant detection, attribution to QG would further require the demonstration of redshift dependence from several bursts.