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 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 Extended emission GRBs BAT Lightcurves Example: GRB T 90 = 103 s Redshift = 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 GRB be a new class (e.g. WD+NS, King et al. 2006)

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

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 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)

11

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 Metzger 2009; Metzger et al. 2011; etc)