1. 1 Explaining extended emission Gamma-Ray Bursts using accretion onto a magnetar Paul O’Brien & Ben Gompertz University of Leicester (with thanks to Graham Wynn & Antonia Rowlinson et al.)

2. 2 GRB progenitors Long GRB: Collapsar Short GRB: Binary Merger LGRB: Collapsar model – occurs in region of massive (hence recent) star formation. Several examples known of associated super/hypernova signature SGRB: Merger model (e.g. NS-NS) – can occur in any type of galaxy, and also off of a galaxy due to natal dynamic kick and long merger time The “central engine” produced may be a either black hole or a “magnetar”

3. 3 Extended emission GRBs BAT Lightcurves Example: GRB 060614 T 90 = 103 s Redshift = 0.125 No supernova detected – short? Pluses: - Hard short episode followed by long softer hump - Short spectral "lag" (Norris & Bonnell) Minuses: - 5 s duration of hard episode - Brighter & more variable hump emission than others Could GRB060614 be a new class (e.g. WD+NS, King et al. 2006)

4. 4 Swift extended emission GRBs (Gompertz, O’Brien, Wynn, Rowlinson 2013) Similar luminosity extended “tail” Swift EE GRB sample: look for >30s of Extended emission (EE) (at 3  ) following a short (<few second) initial emission spike. The “Extended emission” looks similar in shape, duration and luminosity, suggesting a common physical process. Also see “late plateaus” (as in other SGRBs/LGRBs) Late plateau

5. 5 Example magnetar spin down fits (Rowlinson et al. 2013; Gompertz et al. 2013) SGRB examples Model can fit the “late-time” plateau in EE GRBs But what about the EE tail? EE GRB example Relations between the initial spin period (P 0 ), dipole field (B p ), plateau luminosity (L) and magnetar spin-down time (T em ): L  B p 2 / P 0 4 T em  P 0 2 / B p 2 Magnetar spin-down component Prompt decay PL component (Zhang & Mészáros 2001)

6. 6 Propellering and accretion Schematic model: red circle = Alfvén radius (r m ), green circle = co-rotation radius (r c ). These depend on the magnetic dipole field (B) and spin period (P) respectively. A) High accretion rate suppresses r m – magnetar is spun up and r c shrinks B) As the accretion rate declines, r m expands C) When r m > r c matter outside r m is propellered away (producing EM emission) D) As accretion rate drops, r m expands, but r c also expands due to loss of ang. mom. E) When disk depleted, r c slowly increases as spin lost to dipole emission A B C D E

7. 7 Example fits using propellering (P) plus dipole spin-down (D) Assumed 40% EM propeller efficiency; 5% for dipole; <0.9c ejection velocity; exponential fallback rate fits better than power-law (Fernández and Metzger 2013) P D Poor fit at late times; maybe B varies?

8. 8 EE GRB magnetar fit results Derived disk masses of 3x10 -3 to 3x10 -2 M  and outer radii of 400-1500 km (consistent with predictions for fallback disks, e.g. Lee et al. 2009). Initial spin period ~1ms; B field strength ~10 15 G

9. 9 Filled symbol: use known z Open symbol: use average z Spin break up period for a 1.4 M solar NS (Lattimer & Prakash 2004) Magnetic field strength <10 17 G (approx limit based on speed of sound on surface of NS) Not clear if such strong magnetar B fields or long lifetimes can occur Warning: points on this diagram from papers which assume different radiative efficencies Magnetar results (Gompertz et al. 2014; Rowlinson et al. 2013) EE GRBs

10. 10 Summary Generally get a good fit to the EE GRBs using a self-consistent combination of propellering and dipole spin-down emission for a magnetar+fallback disk model To work, propellering requires the efficient conversion (>10%) of K.E. into EM emission during the propeller phase Derived disk masses and sizes consistent with theoretical fallback discs Best fits require exponential rather than powerlaw accretion rates – as expected in presence of strong outflows (Fernández and Metzger 2013) Why do only some GRBs show an EE tail? Maybe these objects require a more unequal mass merger? May be able to test magnetar model using predicted radio emission (i.e. detect the energy injected), or use GW (extra signal if magnetar collapses)

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12. 12 Outcomes from NS-NS merger Expect a relation between the pulsar initial spin period (P 0 ), dipole field strength (B p ), luminosity (L) and the characteristic timescale (T em ) for spin-down: L  B p 2 / P 0 4 and T em  P 0 2 / B p 2 (Usov 1992; Duncan & Thompson 1992; Dai et al. 2006 Metzger 2009; Metzger et al. 2011; etc)