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 ~ s spectral lags ~ ms many pulses high energy low energy

Selection Effect: Before Swift, ~ 1/2 of redshifts for long GRB came from emission lines of the host galaxies. Emission-line spectra tend to be obtained for z <~ 1 — these are under-represented in above plot. Blue Histo: Short GRBs Red 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 GRBs Red 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 d com = 2.4 Gpc T back = 6 Gyr z = 3.5 d com = 7 Gpc T back = 12 Gyr

R int ~ 1/2 Biggest News/Controversy for short GRBs: they’re not always short >> R int ~ 1/600 Ratio of Intensities, initial pulse complex to extended emission, exhibits dynamic range of ~10 4. (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 : Is it short, or is it long?” R int ~ 1/1 Initial short pulses complex ~ s (6 s) ~ 5-10 s hiatus (5 s) Extended Emission ~ s (130 s) 150 s *(z = and no SN)  lag (ms) = 0.3  8.0  lag (ms) = 0.5  9.0 “New Gamma-Ray Burst Classification Scheme from GRB ” Gerhels et al. (Nature 2007) *

Biggest News/Controversy for long GRBs: Breaks not achromatic >> This transition to subrelativistic near ~ 1 day usually missing in X-ray band But “jet break” still there, and when expected, in optical (Ghirlanda et al. 2007): E peak (keV) E gamma (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 — “Superbowl Burst” aka Queen Beatrix Burst: Very bright short burst detected by Compton GRO Time (seconds) Counts / 64-ms bin “Existence Proof” of very high-energy gammas in pulses of short GRBs. 1st six photons: 80, 30, 460, 170, 50, 165 MeV

GRB a — 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, t i ' = t i obs –  E i obs

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 (BAT image trigger) Luminosity ~  most luminous GRBs Duration: ~ 2100 seconds Spectral Lag: ~ 100 s (BATSE chan 3  1) z =  d = 145 Mpc SN 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.