Gamma-ray Burst Progenitors Ross Church Department of Astronomy and Theoretical Physics, Lund With Melvyn B. Davies (Lund), Andrew Levan (Warwick), Chunglee Kim (West Virginia) Wednesday, February 8, 2012
Binary Stars as Long Gamma-ray Burst Progenitors Ross Church Department of Astronomy and Theoretical Physics, Lund With Melvyn B. Davies (Lund), Andrew Levan (Warwick), Chunglee Kim (West Virginia) Wednesday, February 8, 2012
Gamma-ray Bursts Flashes of gamma-rays from immensely energetic extra- Galactic explosions Detected from space (Swift, Fermi) Afterglow fades and reddens through X-ray, optical & IR to radio Follow-up via ground & space based photometry and spectroscopy Two classes based on duration: long and short Wednesday, February 8, 2012
Gamma-ray Bursts Flashes of gamma-rays from immensely energetic extra- Galactic explosions Detected from space (Swift, Fermi) Afterglow fades and reddens through X-ray, optical & IR to radio Follow-up via ground & space based photometry and spectroscopy Two classes based on duration: long and short Wednesday, February 8, 2012
Long Gamma-ray Bursts - Observations Longer-lasting emission, softer spectrum, higher fluence Found in star-forming regions out to very high redshift Many bursts show co-incident Type Ib/Ic supernova • Type Ibc supernovae show neither H nor Si lines • Thought to be the outcome of core collapse of massive stars (>40ish solar masses) • Winds during the stars’ lifetimes remove the hydrogen envelopes (and He in the case of Ic) See Hjorth & Bloom, arXiv 1104.2274 for a recent review Wednesday, February 8, 2012
Long Gamma-ray Bursts - Model Core collapse of rapidly-rotating massive star Rotation leads to high specific angular momentum Some material falls back into a disc around a newly-formed black hole Accretion of the disc produces relativistic jets at the poles Gamma-ray burst observed if the jet points towards us Woosley (1993) ApJ 405 273 Wednesday, February 8, 2012
Problem Strong winds carry off angular momentum ⇒ spins star down Wednesday, February 8, 2012
Problem Strong winds carry off angular momentum ⇒ spins star down Potential solution Can a binary companion prevent spin-down? Wednesday, February 8, 2012
Close binaries In a close binary, one star will raise tidal motions on the surface of the other. Orbital angular Spin angular X frequency Ω frequency ω Wednesday, February 8, 2012
Close binaries In a close binary, one star will raise tidal motions on the surface of the other. Orbital angular Spin angular X frequency Ω frequency ω If ω < Ω Wednesday, February 8, 2012
Close binaries In a close binary, one star will raise tidal motions on the surface of the other. Orbital angular Spin angular X frequency Ω frequency ω If ω < Ω X Wednesday, February 8, 2012
Close binaries In a close binary, one star will raise tidal motions on the surface of the other. Orbital angular Spin angular X frequency Ω frequency ω If ω < Ω X Tidal torque transfers angular momentum and can spin the star up Wednesday, February 8, 2012
Tide-powered spin-up To form an accretion disc after supernova, we require that the material at the edge of the core of mass M c have specific angular momentum √ 6 GM c j < j lso = c Levan, Davies & King (2006), MNRAS 372 1351 Wednesday, February 8, 2012
Tide-powered spin-up To form an accretion disc after supernova, we require that the material at the edge of the core of mass M c have specific angular momentum √ 6 GM c j < j lso = c Assume that the orbit is tidally locked at its closest point, so √ 6 GM c Ω = R c � 0 . 2 R � R 2 c c Levan, Davies & King (2006), MNRAS 372 1351 Wednesday, February 8, 2012
Tide-powered spin-up To form an accretion disc after supernova, we require that the material at the edge of the core of mass M c have specific angular momentum √ 6 GM c j < j lso = c Assume that the orbit is tidally locked at its closest point, so √ 6 GM c Ω = R c � 0 . 2 R � R 2 c c Taking a typical iron core radius for 25-50 solar mass stars � − 2 / 3 � M tot � 1 / 3 � M c a crit = 7 . 36 R � 1 . 7 M � 20 M � Levan, Davies & King (2006), MNRAS 372 1351 Wednesday, February 8, 2012
Tide-powered spin-up Critical separation for tide-powered spin-up is � − 2 / 3 � M tot � 1 / 3 � M c a crit = 7 . 36 R � 1 . 7 M � 20 M � The binary must both be close and rather massive ⇒ A black-hole companion after binary interaction has brought the two stars close together Perform binary population synthesis to see whether we expect to form such binaries. Wednesday, February 8, 2012
Population at time of supernova 5 10 20 Companion mass M 1 / M � 5 10 20 Relevant binaries occupy a well- 5 Supernova mass loss ∆ M 2 / M � defined phase space: ∆ M 2 / M � 4 4 a � 3 − 5 R � 3 3 M 2 , pre � 7 − 8 M � 3 4 Companion mass M 1 / M � 5 10 20 M BH � 5 − 15 M � 6 6 Pre-supernova separation a/ R � Pre-supernova separation a/ R � 5 a/ R � 5 5 Distribution reflects 4 realistic initial 4 4 conditions 3 3 3 5 10 20 3 4 5 Companion mass M 1 / M � Supernova mass loss ∆ M 2 / M � M 1 / M � ∆ M 2 / M � Wednesday, February 8, 2012
Recap Long gamma-ray bursts occur during type Ib/c supernovae of massive, rapidly rotating stars. A binary companion can spin some stars up sufficiently to make a gamma-ray burst. To spin the star fast enough the companion must be a black hole of roughly ten solar masses. Such binaries should exist but will be rare. Wednesday, February 8, 2012
Wednesday, February 8, 2012
Black-hole formation by accretion Some black holes are thought to form by fallback (accretion onto a newly-formed massive neutron star). Figure from MacFadyen, et al. (2001) Wednesday, February 8, 2012
Black-hole formation by accretion Some black holes are thought to form by fallback (accretion onto a newly-formed massive neutron star). Material from the stellar core is ejected radially to distances up to 10 12 cm (10 7 km) Figure from MacFadyen, et al. (2001) Wednesday, February 8, 2012
Black-hole formation by accretion Some black holes are thought to form by fallback (accretion onto a newly-formed massive neutron star). Material from the stellar core is ejected radially to distances up to 10 12 cm (10 7 km) The reverse shock stalls it It falls back and is accreted onto the black hole Figure from MacFadyen, et al. (2001) Wednesday, February 8, 2012
Black-hole formation by accretion Some black holes are thought to form by fallback (accretion onto a newly-formed massive neutron star). Material from the stellar core is ejected radially to distances up to 10 12 cm (10 7 km) The reverse shock stalls it It falls back and is accreted onto the black hole Figure from MacFadyen, et al. (2001) How does a binary companion affect the fallback? Wednesday, February 8, 2012
Modelling the effect of the companion Eject particles radially, with velocities distributed to re-produce the fallback history from hydrodynamical simulations. Treat the particles as “dust” and follow their trajectories in a binary system. Deflected material will form a disc in the orbital plane; follow its accretion onto the newly-formed black hole. Wednesday, February 8, 2012
Example trajectories Location of supernova z/ 10 6 km 0.5 Particle 0 trajectories -0.5 2 1 y/ 10 6 km 0 -1 -2 Orbit of newly-formed -3 -2 -1 0 1 2 0 x/ 10 6 km z/ 10 6 km black hole Wednesday, February 8, 2012
The view from above 0 rot � M 1 = 15 . 61 M � , M 2 = 5 . 694 M � , r 0 = 1601 km , a = 3 . 359 R � , v rot = 6145 km , a = 3 . 359 R � , v 3 Long fall-back M 1 = 15 . 61 M � , M 2 = 5 . 694 M � , ∆ M 2 = 2 . 709 M � M = 5 694 time material 3 Material falling lands outside back into disc 2 either star’s around new Roche Lobe 1 y/ 10 6 km black hole (as in single star 0 case) Some gas -1 sufficiently deflected to -2 Newly-formed form a disc black hole -3 around the -3 -2 -1 0 1 2 3 x/ 10 6 km companion black hole Wednesday, February 8, 2012
First accretion curve 10000 Accretion rate (scaled units) 1000 100 10 1 0 . 1 0 . 01 100 1000 10000 t/ s Wednesday, February 8, 2012
First accretion curve Early times the same as 10000 single star Accretion rate (scaled units) 1000 case 100 10 1 0 . 1 0 . 01 100 1000 10000 t/ s Wednesday, February 8, 2012
First accretion curve Early times Sharp cut-off the same as from Roche 10000 single star Lobe Accretion rate (scaled units) 1000 case truncation 100 10 1 0 . 1 0 . 01 100 1000 10000 t/ s Wednesday, February 8, 2012
First accretion curve Early times Sharp cut-off the same as from Roche 10000 single star Lobe Accretion rate (scaled units) 1000 case truncation 100 10 Possible 1 flaring 0 . 1 activity? 0 . 01 100 1000 10000 t/ s Wednesday, February 8, 2012
First accretion curve Early times Sharp cut-off the same as from Roche 10000 single star Lobe Accretion rate (scaled units) 1000 case truncation 100 10 Possible 1 flaring 0 . 1 activity? 0 . 01 100 1000 10000 t/ s Next steps: model M 1 disc, look at other binaries Wednesday, February 8, 2012
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