Supernovae and Gamma-Ray Bursts Summary of Post-Main-Sequence Evolution of Stars Supernova Fusion proceeds; formation of Fe core. M > 8 Msun Subsequent ignition of nuclear reactions involving heavier elements Fusion stops at formation of C,O core. M < 4 Msun Fusion of Heavier Elements 4 He → 16 O + g C + 6 2 8 16 O + 4 He → 20 Ne + g 8 2 10 12 16 16 O → 28 Si + 4 He O + 8 8 14 2 Onset of Si burning at T ~ 3x109 K → formation of S, Ar, …; → formation of 5426Fe and 5626Fe Final stages of fusion happen extremely rapidly: Si burning lasts only for ~ 2 days. → iron core The Life “Clock” of a Massive Star (> 8 Msun) Let’s compress a massive star’s life into one day… H → He 11 12 1 Life on the Main Sequence + Expansion to Red Giant: 22 h, 24 min. 2 10 9 3 4 8 H burning 7 6 5 H → He He → C, O 11 12 1 2 10 He burning: (Horizontal Branch) 1 h, 35 min, 53 s 9 3 4 8 7 6 5 H → He He → C, O C → Ne, Na, Mg, O 10 11 12 1 2 3 9 C burning: 6.99 s 4 8 7 6 5 C → Ne, Na, Mg, O H → He He → C, O Ne → O, Mg Ne burning: 6 ms 23:59:59.996 H → He He → C, O C → Ne, Na, Mg, O Ne → O, Mg O → Si, S, P O burning: 3.97 ms H → He He → C, O 23:59:59.99997 C → Ne, Na, Mg, O Ne → O, Mg O → Si, S, P Si → Fe, Co, Ni Si burning: 0.03 ms The final 0.03 msec!! Observations of Supernovae Total energy output: DEne ~ 3x1053 erg (~ 100 L0 tlife,0) DEkin ~ 1051 erg DEph ~ 1049 erg Lpk ~ 1043 erg/s ~ 109 L0 Supernovae can easily be seen in distant galaxies. ~ Lgalaxy! SN 2006X in M 100 Observed with the MDM 1.3 m telescope Type I and II Supernovae Core collapse of a massive star: Type II Supernova Light curve shapes dominated by delayed energy Type II P input due to radioactive decay of 5628Ni Collapse of an accreting White Dwarf exceeding the Chandrasekhar mass limit Type II L → Type Ia Supernova. Type I: No hydrogen lines in the spectrum Type Ib: He-rich Type II: Hydrogen lines in the spectrum Type Ic: He-poor The Famous Supernova of 1987: SN 1987A Before At maximum Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987 Progenitor: Blue supergiant (denser than normal SN II progenitor) 20 M0; lost ~ 1.4 – 1.6 M0 prior to SN Evolved from red to blue ~ 40,000 yr prior to SN The Remnant of SN 1987A Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays. vej ~ 0.1 c Neutrinos from SN1987 have been observed by Kamiokande (Japan) Escape before shock becomes opaque to neutrinos → before peak of light curve provided firm upper limit on ne mass: mne < 16 eV Remnant of SN1978A in X-rays Color contours: Chandra X-ray image White contours: HST optical image Supernova Remnants X-rays The Crab Nebula: Remnant of a supernova observed in a.d. 1054 Optical The Cygnus Loop The VeilANebula Cassiopeia Synchrotron Emission and Cosmic-Ray Acceleration The shocks of supernova remnants accelerate protons and electrons to extremely high, relativistic energies. →“Cosmic Rays” In magnetic fields, these relativistic electrons emit Synchrotron Radiation. Synchrotron Radiation Power-law distribution of relativistic electrons: Ne(g) ~ g-p In Opt. thick n5/2 jn ~ n-a a = (p-1)/2 kn ~ n-b b = (p+4)/2 Opt. thin n-(p-1)/2 n Synchrotron Spectra of SNR shocks (I) Electrons are accelerated at the shock front of the supernova remnant: Ne = Ne(g, t) . ) + Q(g,t) ∂Ne/∂t = -(∂/∂g)(gN e Ne (g,t) Q(g,t) = Q0 g-q g-q Uncooled Cooled g-(q+1) gc g Synchrotron Spectra of SNR shocks (II) Resulting synchrotron spectrum: Opt. thin, uncooled In Opt. thick n-(q-1)/2 n5/2 Opt. thin, cooled n-q/2 nsy,c = nsy (gc) Find the age of the remnant from . t = (gc/g[gc]) n Gamma-Ray Bursts (GRBs) Short (sub-second to minutes) flashes of gamma-rays GRB Light Curves Long GRBs (duration > 2 s) Short GRBs (duration < 1 s) Possibly two different types of GRBs: Long and short bursts General Properties • Random distribution in the sky • Approx. 1 GRB per day observed • No repeating GRB sources Afterglows of GRBs On the day of the GRB 3 days after the GRB X-ray afterglow of GRB 970228 (GRBs are named by their date: Feb. 28, 1997) Most GRBs have gradually decaying afterglows in X-rays, some also in optical and radio. 1 day after GRB 2 days after GRB Optical afterglow of GRB 990510 (May 10, 1999) Optical afterglows of GRBs are extremely difficult to localize: Very faint (~ 18 – 20 mag.); decaying within a few days. Optical Afterglows of GRBs Host Galaxy Optical Afterglow Optical afterglow of GRB 990123, observed with Hubble Space Telescope (HST/STIS) Long GRBs are often found in the spiral arms (star forming regions!) of very faint host galaxies Energy Output of GRBs Observed brightness combined with large distance implies huge energy output of GRBs, if they are emitting isotropically: E ~ 1054 erg L ~ 1051 erg/s … another one, observed Energy equivalent to the entire mass by us with the MDM 1.3 m of the sun (E = mc2), converted into gamma-rays in just a few seconds! telescope on Kitt Peak! Beaming Evidence that GRBs are not emitting isotropically (i.e. with the same intensity in all directions), but they are beamed: E.g., achromatic breaks in afterglow light curves. GRB 990510 Models of GRBs (I) There’s no consensus about what causes GRBs. Several models have been suggested, e.g.: Hypernova: Supernova explosion of a very massive (> 25 Msun) star Iron core collapse forming a black hole; Material from the outer shells accreting onto the black hole Accretion disk => Jets => GRB! Models of GRBs (II) Black-hole – neutron-star merger: Black hole and neutron star (or 2 neutron stars) orbiting each other in a binary system Neutron star will be destroyed by tidal effects; neutron star matter accretes onto black hole => Accretion disk => Jets => GRB! Model works probably only for short GRBs.