Core Collapse Supernovae and Neutron Star Kicks David Vollbach 2-6-08

What's the deal with Pulsars? Proper motions average ~200-500 km s-1 (Lai 2000) Some have velocities >1000 km s-1 (Lai 2000) Scale height = 330pc (Lorimer et. al. 2006) Many (~%20) will most likely escape the galaxy (Cordes & Chernoff 1998)

So how did that happen? Progenitor scale height only 0.13kpc. (Cordes & Chernoff 1998) Assumed progenitor velocities only ~10 km s-1(Lai 2000) Binary breakup only provides kick velocities of ~100 km s-1 (Lai 2000) There are many ways to get the necessary kick, but which ones fit the data?

Hydrodynamically driven kicks post-collapse instabilities insufficient with only ~100 km s-1(Janka & Muller 1994) cores must be globally asymmetric before collapse. (Lai 2000) Overstable oscillations of core w/respect to onion skin of nucleosynthesis seed kick. (Goldreich et.al. 1996) As core collapses and material expelled, low density regions are easier for material to escape through. --> asymmetric kick. Method does not require or imply spin/kick alignment. (Lai 2000)

Neutrino – Magnetic field driven kicks Three main types... 1. Parity violation: asymmetric dependence of neutrino opacity in nuclear media on neutrino momenta with respect to magnetic field. Nucleons in magnetic field scatter neutrinos asymmetrically. 2. Asymmetric field topology: absorption cross section of electron neutrinos depends on magnetic field strength, so asymmetric field = asymmetric absorption/emission. 3. Dynamical effect of magnetic field: Strong magnetic fields suppress convection creating dark spots with low neutrino flux. Trouble, all magnetic field methods require B~1015 G to provide effective kicks. Spin/kick alignment expected for P0 < a few seconds. (Lai 2000)

Electromagnetic Kicks (EM rocket) Magnetic dipole misaligned with spin axis induces EM radiation. As pulsar spins down, rotational Energy is transferred to EM radiation that provides a push. Method requires 1-2 ms initial period to transfer enough energy for kick. Also requires initial B~1015 G Kick would naturally be aligned with spin axis. (Lai 2000)

Observations No correlation found between B field strength and Pulsar velocity. (Cordes & Chernoff 1998) No correlation found between spin axis and kick direction (though correlation not ruled out). (Cordes & Chernoff 1998) Magnetic field strength generally ~1012 G (though could be stronger at birth). (Lai 2000)

The Crab

observational constraints v = 120 km s-1 (assuming a distance of 2 kpc) Best observations are consistent with spin axis/transverse velocity alignment, but not conclusive. (14° ± 2° ± 9°) Low velocity and uncertain distance measurement make Crab not the best candidate for constraining kick mechanisms. (Kaplan et. al. 2008)

What about pulsars elsewhere? Currently 138 pulsars in 25 Clobular Clusters ~60% are in binary systems Most are short period (millisecond) with P < 20ms (Ransom 2007)

Image credits http://wps.aw.com/wps/media/objects/500/512494/colorimages/source/17.html http://www4.ncsu.edu/~bjwilli2/snrintro.html http://astronomyonline.org/Stars/HighMassEvolution.asp http://www.etsu.edu http://www.astro.caltech.edu/palomar/exhibits/images/guitar.htm http://www.nature.com/nature/journal/v406/n6792/fig_tab/406139a0_F2.html NASA/CXC/PSU/G.Pavlov et al http://www.spacetelescope.org/images/html/opo0224a.html

References Lai, D. 2000, arXiv e-print, astro-ph 0012049v1 Lorimer, D.R. et.al. 2006, MNRAS 372 : 777-800 Cordes, J.M., Chernoff, D.F., 1998, ApJ 505 : 315-338 Janka, H.Th., Muller, E., 1994, A&A, 290, 496 Goldreich, P. Lai, D., Sahrling, M., 1996, in “Unsolved Problems in Astrophysics,” ed. J.N. Bahcall and J.P. Ostriker (Princeton Univ. Press) Kaplan, D.L., Chaterjee, S., Gaensler, B.M., Anderson, J. 2008, arXiv e- print, astro-ph 0801.1142v1 Ransom, S.M., 2007, arXiv e-print, astro-ph 0710.3626v1

Neutron Star Magnetic Fields and Kick Velocities David Vollbach 4-23-08 Image credit:http://www.nature.com/nature/journal/v406/n6792/fig_tab/406139a0_F2.html Image taken from www.science.psu.edu/alert/Fox8-2007.htm Credit: Casey Reed, courtesy of Penn State

Review of kicks (three types) Hydrodynamically driven kicks Magnetic-Neutrino driven kicks Electromagnetically driven kicks Asymmetries in progenitor lead to asymmetric SN explosion. Strong asymmetric magnetic field causes asymmetric neutrino emission. Off axis magnetic field leads to spin down of pulsar and EM “rocket.”

spin/kick alignment Kick velocity does show alignment with spin axis. (Wang, Lai, & Han 2007) (Lai, Wang, & Han 2006) Kick velocity does show alignment with spin axis. All kick mechanisms can produce this but hydrodynamically driven kicks fit observations and simulations best. (Wang, Lai, & Han 2007)

Correlation between B-field strength and Vt? Old data show little correlation. Any correlation is explained by selection effect with simulation. Recent data shows no change. (Itoh & Cotuda1994)

What about magnetars? Anomalous X-Ray pulsars (AXPs) and Soft Gamma Ray Repeaters identified as possible population of magnetars. Slower rotations than radio pulsars (5-12s) B field strengths ≥1015 G Image taken from http://www.atnf.csiro.au/news/press/magnetar_flare_site/magnetar_flare_images.html, credit: NASA

Observational constraints and associations with SNRs. Without a large population (11 known) can not constrain properties. Difficult to determine if associations with SNRs are real. Of “known” associations, Vt tend to be ≤500 km/s with ages <104 years. Likely that AXPs and SGRs are a young population of highly magnetized neutron stars. (Gaensler 2004)

References Gaensler, B. M., 2004, ASR 33, 645-653 Gaensler, B. M., et. al., 2001, ApJ 559:963-972 Gaensler, B.M., et. al., 2005, ApJ 620:L95-L98 Itoh, N., Kotuda, T., 1999 RNAAS conf. 305I, 305-314 Lai, D., Wang, C., Han, J.L., 2006 CJAA Suppl.2, 241-247 Vink, J., arXiv:0706.3179v1 Wang, C., Lai, D., Han, J.L., 2007 ApJ 656:399-407