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Explaining extended emission Gamma-Ray Bursts using accretion onto a magnetar

Paul O’Brien &

Ben GompertzUniversity of Leicester

(with thanks to Graham Wynn & Antonia Rowlinson et al.)

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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”

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Extended emission GRBs

BAT LightcurvesExample: GRB 060614T90 = 103 s Redshift = 0.125No 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)

•

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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

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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 exampleRelations between the initial spin period (P0), dipole field (Bp), plateau luminosity (L) and magnetar spin-down time (Tem):

L Bp2 / P0

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Tem P02 / Bp

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Magnetar spin-down component

Prompt decay PL

component

(Zhang & Mészáros 2001)

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Propellering and accretion

Schematic model: red circle = Alfvén radius (rm), green circle = co-rotation radius (rc). These depend on the magnetic dipole field (B) and spin period (P) respectively.

A) High accretion rate suppresses rm – magnetar is spun up and rc shrinks

B) As the accretion rate declines, rm expands

C) When rm > rc matter outside rm is propellered away (producing EM emission)

D) As accretion rate drops, rm expands, but rc also expands due to loss of ang. mom.

E) When disk depleted, rc slowly increases as spin lost to dipole emission

A B C D E

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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

DPoor fit at late times; maybe B varies?

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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 ~1015 G

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Filled symbol: use known z Open symbol: use average z

Spin break up period for a 1.4 Msolar NS

(Lattimer & Prakash 2004)

Magnetic field strength <1017 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

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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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Outcomes from NS-NS merger

Expect a relation between the pulsar initial spin period (P0), dipole field strength (Bp), luminosity (L) and the characteristic timescale (Tem) for spin-down:

L Bp2 / P0

4 and Tem P02 / Bp

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(Usov 1992; Duncan & Thompson 1992; Dai et al. 2006 Metzger 2009; Metzger et al. 2011; etc)