A Million Second Chandra View of Cassiopeia A

Una Hwang (NASA/GSFC, JHU)&

J Martin Laming (NRL)

Boston AAS 24 May 2011

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 wavelengthsExplosion 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)

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 Msun

Likely mass at explosion: ~ 4 Msun

(Willingale+ 2003, Laming & Hwang 2003, Chevalier & Oishi 2003)

Infrared light echospectrum:Cas A was Type IIb(core-collapse withpartial H envelope)

Krause+2008

XMM-Newton spectral survey of Cas AWillingale+2002, 2003

15x15 gridtwo component fits

Total mass: 2.2 Msun ejecta 7.9 Msun CSM

Cas A X-ray Emitting Ejecta Census

1 million second VLP observation with Chandra ACIS 2004

nine OBSIDs2.8x108 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

Cas A Chandra Fitted Element Abundances

Classify each region by dominant spectral type

Possible contributions to each spectrum include:forward shocked thermal emission from CSMnonthermal emissionreverse 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

Gallery of Spectral Types

Normal CSM

Nonthermal (not NS)

Mixed CSMnonthermal

Fe dominatedejecta

“Normal” compositionO, Ne, Mg, Si, etc

Mixed ejecta“Normal” and Fe rich

Two ejecta components: “normal” + pure Fe

(see also Hwang & Laming 2009)

“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, 44Ti, ) Chandra Ms (BG modelled) Fe/Si ~ 20 solar by #

src+bgsrc

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=A2/3

filling factor for 2.5” shell front and back

Total shocked ejecta mass = 2.8 Msun Mostly O (2.55 Msun)Fe= 0.10 Msun (normal Si-burning)

+0.04 Msun (pure, -rich freezeout)

(Chevalier & Oishi 2003)Narrow density peak at contact discontinuity

Total ejecta mass = 3.1 Msun

Unshocked ejecta mass = 0.3 Msun

Unshocked ejecta is probably Si

Spitzer ObservatorySmith+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 56Ni decay inhibits Fe dust condensationCondensation less efficient in IIb events vs those without mass loss

X-ray inferred mass of shocked Fe is 0.088 – 0.14 Msun depending on assumptions

consistent with expected mass of Fe 0.058-0.16 Msun (Eriksen+ 2009)

Fe associated with low or high Si about evenly (consistent with Magkotsios+ 2010)

All the Fe ejecta are found well outside the center44Ti 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 44Ti are different from Cas A:SN 1987A : all the 44Ti are in the center (Kjaer + 2010)

G1.9+0.3 : most of the 44Ti are outside (Borkowski+ 2010)

Strong instabilities must operate to mix the Fe far outwards

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?)

Fe ejectaDue eastBetween NS motion and jet

All ejectaEast of North700 km/s150 degrees from NS motion

Remnant as a whole moves opposite to NS: Suggests hydrodynamic origin for NS kick

NS motion

Three Dimensional Structure of Cas A

Si/”Mg” ratioDeLaney+ 20103D 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 surfaceFe ejecta, high-velocity “jets” in outflows

encircled by outlying material

Summary

3 Msun 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”

44Ti 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

Abstract Deadline:31 May 2011