A Million Second Chandra View of Cassiopeia A Una Hwang
(NASA/GSFC, JHU) & J Martin Laming (NRL) Boston AAS 24 May
2011
Slide 2
Cas A First-light Chandra image (Hughes+ 2000) Red: Fe, Green:
Si Cassiopeia A Core-collapse SNR with the most prominent Fe ejecta
emission Si and Fe distributions are distinct (Hughes+ 2000, Hwang+
2000, Willingale+ 2002) Advanced evolutionary state: reverse shock
has heated a substantial portion of ejecta (Laming & Hwang
2003, Chevalier & Oishi 2003) Best studied SNR at all
wavelengths Explosion date: 1671 (to 1681; Thorstensen+ 2001, Fesen
2006) Distance: 3.4 kpc (Reed+ 1995) Shock velocities, radii
(Gotthelf+ 2001, DeLaney & Rudnick 2003; Helder & Vink
2008, Morse+ 2004)
Slide 3
Extensive progenitor mass loss: Aided by a binary companion
(Young+ 2006) SNR expansion into circumstellar wind matches
dynamics (Laming & Hwang 2003, Chevalier & Oishi 2003) CSM
modified by bubble (Hwang & Laming 2009) or dynamics modified
by particle acceleration (Patnaude & Fesen 2009) Shocked CSM
mass: ~10 M sun Likely mass at explosion: ~ 4 M sun (Willingale+
2003, Laming & Hwang 2003, Chevalier & Oishi 2003) Infrared
light echo spectrum: Cas A was Type IIb (core-collapse with partial
H envelope) Krause+2008
Slide 4
XMM-Newton spectral survey of Cas A Willingale+2002, 2003 15x15
grid two component fits Total mass: 2.2 M sun ejecta 7.9 M sun
CSM
Slide 5
Cas A X-ray Emitting Ejecta Census 1 million second VLP
observation with Chandra ACIS 2004 nine OBSIDs 2.8x10 8 photons
6202 extraction regions: 2.5, 5, or 10 along one side customized
spectral response off-source background scattered source spectrum
selected by azimuth Plane-parallel shock model with variable
abundances, elements O and heavier
Slide 6
Slide 7
Cas A Chandra Fitted Element Abundances
Slide 8
Slide 9
Classify each region by dominant spectral type Possible
contributions to each spectrum include: forward shocked thermal
emission from CSM nonthermal emission reverse shocked thermal
emission from ejecta Eliminate 1500 forward shock/nonthermal
dominated regions: plane-parallel shock with CSM-type abundances
optional power-law Consider >4000 remaining regions as ejecta:
plane-parallel shock with O as lightest element
Slide 10
Gallery of Spectral Types Normal CSM Nonthermal (not NS) Mixed
CSM nonthermal Fe dominated ejecta Normal composition O, Ne, Mg,
Si, etc Mixed ejecta Normal and Fe rich Two ejecta components:
normal + pure Fe (see also Hwang & Laming 2009)
Slide 11
Pure (very highly enriched) Fe Ejecta Chandra 50 ks (BG
subtracted) Hwang & Laming (2003) Fe/Si > 16 solar by #
Plausible site of -rich freeze out (products include Fe, 44 Ti,
Chandra Ms (BG modelled) Fe/Si ~ 20 solar by # src+bg src
Slide 12
Ejecta Mass Calculations Ejecta fits with (1) single vpshock or
(2) vpshock + NEI (Fe, Ni only): evaluate with f-test Use fitted
emission measure assume V=A 2/3 filling factor for 2.5 shell front
and back Total shocked ejecta mass = 2.8 M sun Mostly O (2.55 M sun
) Fe= 0.10 M sun (normal Si-burning) +0.04 M sun (pure, -rich
freezeout) (Chevalier & Oishi 2003) Narrow density peak at
contact discontinuity Total ejecta mass = 3.1 M sun Unshocked
ejecta mass = 0.3 M sun
Slide 13
Unshocked ejecta is probably Si Spitzer Observatory Smith+2009,
Rho+2008 Infrared observations show unshocked ejecta at remnant
center, primarily in [Si II] Little optical or infrared evidence
for Fe (Ennis+ 2006, Rho+ 2003, Isensee+ 2010, Hurford & Fesen
1996, Gerardy & Fesen 2001) Cool 35 K dust component consistent
with Si (Nozawa+ 2010; Sibthorpe+ 2010, Barlow+ 2010) Radioactive
heating of Fe ejecta by 56 Ni decay inhibits Fe dust condensation
Condensation less efficient in IIb events vs those without mass
loss
Slide 14
X-ray inferred mass of shocked Fe is 0.088 0.14 M sun depending
on assumptions consistent with expected mass of Fe 0.058-0.16 M sun
(Eriksen+ 2009) Fe associated with low or high Si about evenly
(consistent with Magkotsios+ 2010) All the Fe ejecta are found well
outside the center 44 Ti associated with pure Fe will also be
outside the center small LOS velocity (INTEGRAL; Martin & Vink
2008, Martin+ 2009) may be tested with NUSTAR Two other remnants
with 44 Ti are different from Cas A: SN 1987A : all the 44 Ti are
in the center (Kjaer + 2010) G1.9+0.3 : most of the 44 Ti are
outside (Borkowski+ 2010) Strong instabilities must operate to mix
the Fe far outwards
Slide 15
Neutron Star Kick Velocities of 1825 optical knots (Fesen+
2006) Inferred motion of NS (Thorstensen+ 2001) Neutron star speed
is inferred to be 330 km/s, roughly perpendicular to axis of ejecta
jets, fast optical knots Hydrodynamical simulations (3D,
non-rotating progenitor; Wongwathanarat+2010): Predict NS recoil
opposite maximum explosion strength (ie, opposite the Fe?)
Slide 16
Fe ejecta Due east Between NS motion and jet All ejecta East of
North 700 km/s 150 degrees from NS motion Remnant as a whole moves
opposite to NS: Suggests hydrodynamic origin for NS kick NS
motion
Slide 17
Three Dimensional Structure of Cas A Si/Mg ratio DeLaney+ 2010
3D structure from Doppler shifts: Infrared [Ar II] (Spitzer) High
[Ne II]/[Ar II] [Si II] X-ray Fe K (Chandra Ms) outer optical knots
(Fesen 2001, Fesen & Gunderson 1996) Si in center, in rings on
the surface Fe ejecta, high-velocity jets in outflows encircled by
outlying material
Slide 18
Summary 3 M sun ejecta is inferred from census of X-ray
emission and is also consistent with the observed remnant dynamics
Most of the Fe ejecta is already shocked, and sits well outside the
reverse shock; some of the Fe is pure 44 Ti is expected to have the
same distribution as pure Fe Long exposure crucial to find pure Fe
via Fe K emission Inferred momentum of Fe ejecta is perpendicular
to the jet axis, not opposite the NS; momentum of total ejecta
opposes NS Hydrodynamic mechanism for the kick looks likely Cas A
provides constraints on hydrodynamics of the explosion and is ripe
for targeted explosion models including progenitor rotation