What’s New with Gamma-Ray Bursts Jay Norris * University of Denver, Visiting Stanford/SLAC

1. 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

2. low energy high energy GRB Bimodal Duration Distribution 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 • 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

3. Swift GRBs with Redshifts: ~ 1/3 of BAT Sample Blue Histo: Short GRBs Red Histo: Long GRBs Long Bursts only 46 absorption 16 emission 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.

4. Era of Long GRBs The Big Bang (13.7 Gyrs) Trilobytes (500 Myrs) GRB Lookback Times Blue Histo: Short GRBs Red Histo: Long GRBs Well-defined Era of Short GRBs?? Not so clear.

5. z = 3.5 dcom = 7 Gpc Tback = 12 Gyr z = 0.65 dcom = 2.4 Gpc Tback = 6 Gyr GRB Distances QG timescale: ~ 8 ms / Gpc / GeV z  0.8 :: T  6.8 Gyr “halfway” to Big Bang

6. Rint ~ 1/2 Rint ~ 1/600 Biggest News/Controversy for short GRBs: they’re not always short <<< Short Bursts have Extended Emission ~ 25-30% of cases >>> 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).

7. 150 s “GRB 060614: Is it short, or is it long?” “New Gamma-Ray Burst Classification Scheme from GRB 060614” Gerhels et al. (Nature 2007) * Rint ~ 1/1 lag (ms) = 0.3  8.0 lag (ms) = 0.5  9.0 Initial short pulses complex ~ 0.1-3 s (6 s) ~ 5-10 s hiatus (5 s) Extended Emission ~ 30 - 100 s (130 s) *(z = 0.125 and no SN)

8. But “jet break” still there, and when expected, in optical (Ghirlanda et al. 2007): This transition to subrelativistic near ~ 1 day usually missing in X-ray band Epeak (keV) Egamma (erg) Biggest News/Controversy for long GRBs: Breaks not achromatic <<< Jet breaks absent in most Swift/XRT Light Curves >>> 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).

9. Counts / 64-ms bin Time (seconds) GRB 930131 — “Superbowl Burst” aka Queen Beatrix Burst: Very bright short burst detected by Compton GRO “Existence Proof” of very high-energy gammas in pulses of short GRBs. 1st six photons: 80, 30, 460, 170, 50, 165 MeV

10. 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 ...

11. Most pulse narrowing Most pulse narrowing + QG do trial transformations, ti' = tiobs– Eiobs  = 1/3 Least pulse narrowing  = 1/5 ... “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.  (ms / GeV)

12. Ultra-low luminosity GRBs: Numerous, Nearby, with SNe GRB 060218 (BAT image trigger) Luminosity ~ 10-6 most luminous GRBs Duration: ~ 2100 seconds Spectral Lag: ~ 100 s (BATSE chan 31) z = 0.033  d = 145 Mpc SN 2006aj (type Ib/c) Swift/BAT: Lag - Luminosity Plot Supernovae Ib/c 9000 Gpc-3 yr-1 Underlum GRBs 700 Gpc-3 yr-1 “Classic” Long GRBs 70 Gpc-3 yr-1

13. Summary: Latest in GRB Classes 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.

14. 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.