Supervisor: Dr. Jasmina Lazendic-Gallowaypresented by Alex Li

X-ray Study of Supernova Remnant W28

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

The Project is about…

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)?

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

ejectaouter shell of ISM by forward shock

Types of SNRShell-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

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

Why Study SNR W28?

• -ray emission γ near the northeastern shell of W28 detected by H.E.S.S in 2008 and Fermi-LAT in 2010.

• -ray emission:γ Inverse Compton

scattering (electrons) Non-thermal

bremsstrahlung (electrons) π0 decay (protons)

Detection of non-thermal X-ray• helps determine if -ray emission γ by electron or proton acceleration.

Radio Image of W28

ROSAT X-ray Image of W28

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 subtractionSpectral analysis and fitting: XSPEC ver.12.5.1

Need reprocessing for event file?

Flaring exist?

YES

YES

NO

NO

Basic Procedure of Preparing Spectra

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. Temperature2. Abundances of elementsROSAT X-ray

Image of W28

Northeastern ShellOMg Si

S

MOS-CCD components

pn-CCD component

The CCD image of W28’s NE shell from XMM

OMg Si

S

The CCD image of W28’s NE shell from XMM

pn-CCD component

MOS-CCD components

Northeastern Shell

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.

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

Results from Chandra DataImage of the Chandra central chip with point sources moved

Results from Chandra DataThree-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

Results from Chandra DataChandra image with point sources moved

Power-law component was detected in R12.

Good news!!Γ

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

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⊙

Core-collapse Supernova

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⊙

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.

Summary1. 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.

THANK YOU

Results from Chandra DataO

Chandra image with point sources moved

Results from Chandra DataOxygen 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 movedChandra image with point sources moved

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

?

?

We use data from X-ray Multi-mirror Mission (XMM-Newton).

XMM-Newton Observatory

(above) XMM-Newton satellite and (left) its X-ray telescope’s basic structure.

CCDs of MOS1 (left) and MOS2 (right) cameras CCDs of pn camera

MOS1 camera pn camera

European Photon Imaging Camera (EPIC)(EPIC)

Chandra X-ray Observatory

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

Chandra’s Advanced CCD Imaging Spectrometer (ACIS)