# Presentation: 394459

## X-ray Studies of Supernova Remnants

- Una Hwang (NASA/GSFC, JHU)
- X-ray Astronomy School 2007
- George Washington University

## Supernova Explosions

- Ia Thermonuclear Runaway
-  Accreting C-O white dwarf reaches Chandrasekhar mass limit, undergoes thermonuclear runaway
-  Results in total disruption of progenitor_x0010_
-  Explosive synthesis of Fe-group plus some intermediate mass elements (e.g., Si)
-  Uncertain mechanism and progenitor: probably a delayed detonation (flame transitions from subsonic to supersonic speed)
- II/Ib/Ic Core-Collapse of Massive Progenitor
-  Massive progenitor core forms neutron star or black hole
-  Explosive nucleosynthesis products near core (Si and Fe) plus hydrostatically formed outer layers (O, Ne) are expelled
-  Most of the explosion energy is carried away by neutrinos
-  Uncertain mechanism details involve neutrinos, probably large-scale shock instabilities, rotation, possibly magnetic fields

- Supernovae and their progenitors provide most of the heavy elements in the Universe and deposit kinetic energy (1051 ergs each) into the interstellar medium

## Supernova Remnant Cartoon

- Shocked ISM

- Unshocked Ejecta

- Forward Shock
- into Interstellar or Circumstellar Medium

- Contact
- Discontinuity

- Reverse Shock
- into Ejecta

- Shocked ISM

- Forward shock moves supersonically into interstellar/circumstellar medium
- Reverse shock propagates into ejecta, starting from outside

- Shocked Ejecta

## Remnant Evolution

- Free Expansion
- Ejecta expand without deceleration r~t
- Adiabatic (Sedov-Taylor, or “atomic bomb”)
- Ejecta are decelerated by a roughly equal mass of ISM r~t2/5
- Energy is conserved
- Evolution of density, pressure is self-similar
- Temperature increases inward, pressure decreases to zero
- Radiative
- Dissipation of remnant energy into ISM
- Remnant forms a thin, very dense shell which cools rapidly
- Interior may remain hot

## Shocks compress and heat gas

- FS

- RS

- Mass, momentum, energy conservation give relations
- (for =5/3)
- 
- V = 3/4 *v*shock
- m/mH (*v/1000 km/s) **2 *keV
- X-rays are the characteristic emission
- These relations change if significant energy is diverted to accelerating cosmic rays

- The shock is “collisionless” because its size scale is much smaller than the mean-free-path for collisions (heating at the shock occurs by plasma processes)
- Collisions do mediate ionizations and excitations in the shocked gas

## Electron Heating at SNR Shocks

- Rakowski et al. 2003

- Compare Te to Tp

- Temperatures behind shock are proportional to mass
- Electrons and ions will equilibrate their temperatures by Couloumb collisions, but possibly more quickly by complicated collisionless plasma processes
- The efficiency of heating depends on the Mach number (shock velocity): faster electron heating in slower shocks

## Instabilities

- Kifonidis et al. 2000

- irregular shock boundariesmixing between ejecta layersmixing between ejecta and ISM

- Fe bubbles Blondin et al. 2001

## Time-Dependent Ionization

- Equilibrium

- Impulsive Shock Heating

- Fully stripped

- H-like ion

- He-like ion

- Oxygen heated to 0.3 keV
- (Hughes & Helfand 1985)

- Ionization is effected by electron-ion collisions, which are relatively rare in the ~1 cm-3 densities of SNRs
-      Ionization is time-dependent
- Ionization timescale = net
- electron density x time since impulsively heated by shock
-       Ionization equilibrium attained at
-       net ~ 104 cm-3 yr
- Ionizing gas can have many more H- and He- like ions, which then enhances the X-ray line emission
- Inferred  element abundances will be too high if ionization equilibrium is inappropriately assumed for an ionizing gas

## Supernovae and Supernova Remnants

- Supernovae
-    powered mostly by radioactive decay: 56Ni  56Co  56Fe
-    T~ 5000 K
-    characteristic emission is optical and infrared
-    timescale ~ year
- Supernova remnants
-    powered by expansion energy of supernova ejecta,
-       dissipated as the debris collides with interstellar material
-       generating shocks
-    T ~ 106-7 K
-    characteristic thermal emission is X-rays
-    timescale ~100-1000 years

## SNR 1987A in Large Magellanic Cloud

- HST, Burrows et al. 1995

- Park et al. 2002 , Burrows et al. 2001

- X-ray emission is approaching inner circumstellar ring
- X-rays correlate well with radio

## X-ray Emission from Supernova Remnants

- Thermal Emission
-  characterized by electron temperature, ionization timescale, element abundances
-  primarily bremsstrahlung continuum
-  collisionally excited emission lines_x0010_
- Nonthermal Emission
-  blackbody or power law from pulsar/neutron star if present (across electromagnetic spectrum)
-  synchrotron emission from electrons accelerated at the shock (usually radio, sometimes up to X-rays)

## The model is multiplied by the detector effective area and folded through a matrix that distributes photons of a given energy over the appropriate range of pulse height values.

## CCD

- Capella
- Chandra HETG
- Canizares et al. 2000

- Brickhouse et al. 2000

- Si

- Mg

- O

- Transmission Grating

- Spectral Resolution
- CCD devices have limited spectral resolution:
- “line” features in CCD spectra are generally blends of multiple emission lines

## Broadband X-ray Spectral Fitting

- Data are accumulated by the detector as counts per pulse height bin.  The detector “gain” gives the average relation between pulse height bin and photon energy.

- Pulse Height

- Energy

## Why Study Supernova Remnants?

- Supernova explosion:
- How is mass and energy distributed in the ejecta?
- What was the mechanism of the supernova explosion?
- What elements were formed in the explosion, and how?
- What are the characteristics of the compact stellar remnant?
- Shock physics:
- How is energy distributed between electrons, ions, and cosmic rays in the shock?
- How do electrons and ions share energy behind the shock?
- Interstellar medium:
- What is the structure of the interstellar medium, and how does the shock interact with that structure?

## Cassiopeia A: Explosive Nucleosynthesis

- Featureless

- Incomplete Si

- .. plus Fe

- O-burning

- (Hughes et al. 2000  ApJ, 518, L109)

## Explosive Nucleosynthesis

- Nuclear processing as the supernova shock wave propagates through the star (e.g., see Arnett 1996)

- C burning produces O, Ne, Mg, etc               T ~ 2 x 109 K
- Ne burning produces O, Mg, etc                  T ~ 2.3 x 109 K
- O burning produces Si, S, Ar, Ca, etc          T ~ 3.5 x 109 K
- Si burning produces Fe, Si, S, Ca, etc           T ~ 5 x 109 K

## Nucleosynthesis Products in SNRs

- Tycho’s SNR
- Type Ia
- White dwarf + companion
- Si, S, Ar, Ca
- Expect lots of Fe: most not yet shocked
- E0102-72
- Core-collapse
- ~25 Msun
- mostly O, Ne, Mg

## Identifying Ejecta in Middle-Aged SNRs

- Hughes et al. (2003  ApJ, 582, L95)

- DEM L71 in LMC
- Outer shock identified; follows optical H emission (Ghavamian et al. 2003, Rakowski et al. 2003)
- Central X-ray emission is dominant in Fe L emission (n=2 transitions, ~1 keV)
- Enhanced Fe abundances in center
- Position of the CD relative to shock gives shocked ejecta mass ~1.5 solar mass  probably Type Ia

## Energy Selected X-ray Imaging

- Cassiopeia A Chandra ACIS spectrum

- Continuum 4-6 keV Si and Fe
- Cassiopeia A observed with Chandra

## Si K (red)  S K (green)  Fe K (blue)

- Doppler shifts
- Line of sight velocity

- DeLaney & Rudnick 2003
- DeLaney et al. 2003

- Difference Image for two epochs
- Transverse velocity

- Si K (red)  S K (green)  Fe K (blue)

- Willingale et al. 2001

## Bipolar “jets” in Cassiopeia A

- Si HeMg Heratio image

- compact source

**Notes:**
What role did the jets play during the explosion?

## What causes the bipolar morphology in Cas A?

- (Blondin et al. 1996)

-  (Khokhlov et al. 1999)

- Circumstellar cavity formed by progenitor

- Jets of ejecta

## Interpreting Thermal SNR Spectra

- Time evolution of density, pressure, temperature in the remnant
-    expansion of the gas (P=nkT)
-    cooling of the gas by line emission
-    heating of the gas by collisions, radiation
-    loss of energy to particle acceleration
- Atomic processes in the gas
-    collisional ionization
-    excitation of ions and emission of radiation
- Things to keep in mind:
-    emission is weighted by n2V: denser gas is brighter
-    line emission is proportional to ion abundance:
-       no lines in fully ionized gas, or gas that is too cold to be ionized
-   nonthermal continua may be present (e.g., synchrotron)

## Tycho’s SNR (SN 1572)

- Mostly thermal emission from ejecta (Si and Fe ejecta)
- Rim emission is thermal + nonthermal
- (Hwang et al. 2002)
- Proximity of ejecta to FS is inconsistent with age, observations for models for adiabatic remnants
- Implies efficient acceleration of cosmic rays
- (Warren et al. 2005)
- Knowing physical scale, width of synchrotron filaments can be used to infer B field
- B~100G

## RXJ1713-3946

- Like SN1006 is dominated in X-rays by synchrotron emission
- PL spectrum with 
- Uchiyama et al. 2002, Cassam-Chenai et al. 2004)

## Neutron Stars in SNRs

- Pulsars are highly magnetized NS: beacons of light emitted along axis are detected as pulsations
- Infer energy loss rate, B field from pulsation characteristics

- Crab pulsar and nebula
- Weisskopf et al. 2000

- NS in Cas A
- Tananbaum et al. 1999

- 3C58
- Slane et al. 2004

- Relativistic wind is seen as a nebula around pulsar
- Thermal (blackbody) emission is also emitted from the surface of the NS

- G292.0+1.8
- Park et al. 2002, Hughes et al. 2001

- G21.5-0.9
- Matheson & Safi-Harb 2004

- Composite Remnants: Shell + PWN

## Examples of	 Interstellar/Circumstellar Interactions

- Examples of	 Interstellar/Circumstellar Interactions

## Cygnus Loop SNR

- Evolving into a circumstellar cavity
- Transmitted and reflected shocks at cavity wall
- Infer density contrast, interaction timescale

- Levenson et al. 2002

## Some Pitfalls

- Sufficient angular and spectral resolution are both important
- Broadband spectral fits are seldom unique and unambiguous
- Only a limited number of spectral parameters can be independently constrained.
- elements abundances generally should not be fitted if you cannot see the corresponding lines
- continua can often be fitted in numerous ways