1. Supervisor: Dr. Jasmina Lazendic-Galloway
presented by Alex Li

2. Target: SNR W28 (G6.4-0.1)
 The Northeastern Shell (XMM-Newton data)
 The Central Region (Chandra data)
Aims:
 Detect powerlaw component (non-thermal X-
ray radiation) from W28’s NE shell for
broadband model
 Study plasma properties of SNR W28

3. Ejected stellar material (ejecta) from a supernova, mixed with interstellar
medium (ISM) and compressed by forward shock, forms a SNR.
What is supernova remnant (SNR)?

4. SNR Evolution
Free Expansion
Adiabatic or
Sedov phase
Radiative phase
 Mejecta >> Mswept-up
 Ejecta expands without deceleration
 Outer ISM shell by forward shock
 Mejecta ~ Mswept-up
 Significant deceleration
 Ejecta merges into forward shock
 Energy loss by adiabatic expansion
 Efficient particle acceleration by
strong forward shock
 Tejecta < 106K
 Efficient cooling by radiation
 Well-mixing between ejecta and ISM
 No efficient particle acceleration due to slow forward shock
Disappearance when velocity drops to the typical value of ISM
ejecta
outer shell of
ISM by forward
shock

5. Types of SNR
Shell-type (e.g. Cassiopeia A)
• non-thermal X-ray emitting
shells
Crab-like (e.g. Crab Nebula)
• non-thermal X-ray or radio
radiation from a pulsar at
the centre
• faint shells or no shell at all
Composite-type
• if appear as both shell-like
and Crab-like
Mixed-morphology (e.g. W44)
• radio shell emission
• thermal X-ray center-filled
emission
• uniform temperature profile
not common in shell-type
SNRs
Ejecta Cassiopeia A
Crab Nebula
Radio
shell
Radio image
Thermal X-ray
center-filled
emission
X-ray image
W44
Neutron star
(a little spot)Non-thermal X-
ray emitting shell
by forward shock
Pulsar

6. SNR W28
• Mixed-morphology SNR
• 35000-15000 year old
• 50’x45’ angular size
• ~ 2kpc distant away
• Possibly evolved into its
radiative phase
Previous X-ray observation
with ROSAT and ASCA (2002):
 Northeastern :
 single thermal component
(0.6 keV ),
 ISM abundance,
 Center:
 two thermal components
(1.8 keV and 0.6 keV)
Radio Image
of W28
ROSAT X-ray
Image of W28

7. Why Study SNR
W28?
• γ-ray emission near the
northeastern shell of W28
detected by H.E.S.S in 2008
and Fermi-LAT in 2010.
• γ-ray emission:
 Inverse Compton
scattering (electrons)
 Non-thermal
bremsstrahlung (electrons)
 π0 decay (protons)
Detection of non-thermal X-ray
• helps determine if γ-ray emission
by electron or proton acceleration.
Radio Image
of W28
ROSAT X-ray
Image of W28

8. Download raw data from data
archive HEASARC and check
them, e.g. version, mode, etc.
Create a new event file
Check light curve
Filter the event file
Select region and extract the region’s
spectrum from the event file
Background subtraction
Spectral analysis and
fitting: XSPEC ver.12.5.1
Need reprocessing
for event file?
Flaring exist?
YES
YES
NO
NO
Basic Procedure of
Preparing Spectra

9. The CCD image of W28’s
NE shell from XMM (10
regions)
The CCD image of W28
from Chandra (8 regions)
Parameters concerned in
spectral fitting:
1. Temperature
2. Abundances of elementsROSAT X-ray
Image of W28

10. Northeastern ShellO
Mg
Si
S
MOS-CCD
components
pn-CCD
component
The CCD image of W28’s NE
shell from XMM

11. O
Mg
Si
S
The CCD image of W28’s NE
shell from XMM
pn-CCD
component
MOS-CCD
components
Northeastern Shell

12. Results for Northeastern Shell
All regions’ spectra are best
fitted by two thermal
components, different to single
thermal component observed by
ROSAT and ASCA.
Low temperature components
correspond to interstellar
medium (ISM) and high
temperature components
correspond to SNR ejecta.
Power-law model was also used
for fitting spectra, but photon
indices Γ > 4.0 for all the
regions, except REG G that Γ =
3.8.

13. Enhanced abundances of O
were detected in all regions,
and most of them show
enhanced abundances of Mg
as well, confirming the
presence of ejecta in W28’s NE
shell.
Emission from ejecta is still
strong enough to be resolved by
XMM, not expected from old
SNRs like W28.
Results for Northeastern Shell

14. Results from Chandra Data
Image of the Chandra
central chip with point
sources moved

15. Results from Chandra Data
Three-thermal model best fits the central
spectra. The component of ~1.8keV was also
observed by ROSAT and ASCA (2002), but two
thermal only.
The third thermal component may correspond
to reflected shock by (molecular) cloud.
Further analysis is needed to determine
whether the reflected shock model fitted the
data of the central region.
Image of the Chandra
central chip with point
sources moved
Three
thermal
components
Z
also observed by
ROSAT & ASCA

16. Results from Chandra DataChandra image with
point sources moved
Power-law component was
detected in R12.
Γ

17. Constraint on Non-thermal X-ray Radiation in
Northeastern Shell
The multi-band spectra of the Fermi-LAT source at the
NE boundary of SNR W28 (taken from Abdo et al. 2010).
Fermi
H.E.S.S
EGRET

18. We also attempt to estimate the mass of the
progenitor star associated with W28 by:
1. calculating the ejecta masses of
overabundant elements
2. comparing our values to the well-known
results (Thieleman et al. 1996).
The progenitor star’s mass:
8 M⊙ < M < 13 M⊙
There should be a neutron star
near the remnant’s center.
Mass of Progenitor Star
associated with W28
Our values are 10
times smaller than
that for 13M⊙

19. Identifying Background Point Sources and
Potential candidates for Neutron star
The CCD image of W28
from Chandra
• Five possible candidates
were identified (blue
circles).
• But determination of
their flux is needed to
find out the strong
candidates.

20. Summary
1. New thermal components detected in both the NE shell
and the central region of W28.
2. Confirmation of high temperature plasma of ~1.8 keV at
W28’s center.
3. First detection of ejecta with enhanced abundances of O
and Mg in W28’s NE shell .
4. First detection of powerlaw components in both W28’s
NE and S regions.
5. Determination of lower limit of flux of non-thermal X-ray
for the NE shell’s broadband modeling.
6. First identification of possible candidates of neutron star
associated with W28.

22. Results from Chandra Data
O
Chandra image with
point sources moved

23. Results from Chandra Data
Oxygen line, not detected in other
Chandra’s regions we investigated, was
detected in both R1 and R2.
However I had trouble to determine the
best fits for R1 and R2 due to the
background spectrum dominant at
energies E > 3keV.
Considering consistency, the fits with solar
abundances are the preferred ones.
Chandra image with
point sources moved
Chandra image with
point sources moved

24. Ejecta
Injection of
ions to shock
Efficiency of
particle
acceleration
Amplification
of magnetic
field in shock
by CRs
Particle
acceleration or
re-acceleration
Collision
between
ejecta and
dense cloud
Reverse
shock
Modification
of shock by
CRs
?
Energy
spectrum
of CRs
SNR
Evolution
?
?

25. We use data from X-ray Multi-mirror Mission (XMM-Newton).
(above) XMM-Newton
satellite and (left) its X-ray
telescope’s basic structure.

26. CCDs of MOS1 (left) and MOS2 (right) cameras CCDs of pn camera
European Photon Imaging Camera (EPIC)(EPIC)

27. Chandra X-ray Observatory
(above) Chandra satellite and
(left) its X-ray telescope’s basic
structure.

28. Chandra’s Advanced CCD Imaging Spectrometer (ACIS)