XMM-Newton study of SNR W28:

the thermal & non-thermal emission

Ping Zhou University of Manitoba, Nanjing University

Collaborators: Samar Safi-Harb University of Manitoba (supervisor) Yang Chen Nanjing University (supervisor) Xiao Zhang Nanjing University

The Cosmic Kaleidoscope 16 Aug, 2012

We acknowledge the funding support of CSC (China Scholarship Council) and NSERC (Natural Sciences and Engineering Research Council of Canada).

OUTLINE• Introduction

• previous observations of W28

• XMM study

• the SNR interior

• Spatially resolved spectroscopic analysis

• interpretation of the thermal emission

• the northeastern shell

• Spectroscopic analysis

• interpretation of the non-thermal emission

Introduction: a special SNR

ROSAT X-ray (0.5-2.4keV; Rho & Borkowski

2004)Contours: VLA1.4GHz

(Dubner et al 2000)

Mixed-Morphology SNR (MMSNR) with a

bright X-ray shell

shell

CO (2-1) image (Nicholas et al. 2010)

Black contours: H.E.S.S TeV γ-ray significance

Interacting with molecular clouds, hadronic-origin-

Cosmic-Ray(Aharonian et al. 2008)

shell

XMM observations• 4 archival XMM

observations with 3 different pointings.

• Red: 0.3-1.0 KeV

• Blue: 1.0-7.0 keV

• Contour: VLA 1.4GHz

Exposure PN (ks) MOS (ks)

Northeast 24 30

Center 5 4

North 1 1

solid red region: source

dashed red region: background

• A spatially resolved spectroscopic analysis

• Adequately select nearby background

Central gas: spectral results

a large gradient of NH, temperature and density

hothotdiffusdiffus

ee

coldcolddensdens

ee

vnei+ vmekal

abundance <1

VNEI (cold)+VMEKAL(hot)

RC

CN

C3

C2

C1

C0

Δχ^2 (d.o.f)

1.32 (619)

1.26 (453)

1.05 (475)

0.98 (437)

1.18 (495)

1.12 (391)

NH (1E22 cm^-2)

0,55

0,72

0,59

0,39

0,43

0,36

kT_c (keV)

0.36 (0.32--0.42)

0.28 (0.25--0.33)

0.31 (0.26--0.38)

0.57 (0.52--0.59)

0.54 (0.50--0.58)

0.64 (0.61--0.67)

from east to west:

NH generally

kT_c generally

kT_h generally

COLD

Tau_c (10E11 s cm^-3)

4.34 (2.61-7.27)

5.14 (2.17--17.2)

4.91 (>1.10)

282

1.54 (0.84--2.14)

7.28 (>1.48)

n_c (cm^-3)

1,48

1,79

2,01

0,73

1,17

0,91

HOT

kT_h (keV)

0.77 (0.75--0.79)

0.78 (0.74--0.84)

0.77 (0.74--0.81)

1.20 (0.95--1.41)

1.00 (0.94--1.07)

1.40 (1.05--1.59)

n_h (cm^-3)

0,69

0,64

0,81

0,35

0,63

0,42

• 12CO 3-2 (JCMT; Arikawa et al. 1999)• Hα emission (SuperCOSMOS Hα Survey)• X-ray 0.3-7.0 keV (XMM)• Contour: VLA 1.4GHz

• Interacting with dense material (CO and Hα filaments) in the east and inner radio ridge.

Central gas: interpretation• environment

• explains the gradient of

• foreground absorption

• density

• temperature

• filamentary Hα emission

• diffuse Hα emission

Popular scenarios:1. thermal conduction (Shelton et al 1999; Cox et al. 1999)2. cloudlet evaporation (White & Long 1991)

• thermal conduction -- Not efficient

• a large gradient of temperature and density from east to west.

• the low mass of X-ray-emitting gas (~30Msun) and the X-ray clumpiness are NOT consistent with the thermal conduction model (Rho & Borkowski 2002)

coldcolddensedense

hothotdiffusdiffus

ee

• thermal conduction Central gas: interpretation

• Advantages:

• clumpy X-ray structures.

• the colder component is denser and has a lower ionization timescale (model predicts the most recently evaporated material should be more under-ionized embeded in a hotter matrix)

• bright, diffuse Hα emission and low ratios of [SII]/Hα

• cloudlet evaporationCentral gas: interpretation

In addition to nonuniform environment, Cloudlet evaporation is an indispensable and important process to

influence the gas properties. Improved model is also required.

• Disadvantages:

• X-ray surface brightness is too sharply peaked at the center (Rho & Borkowski 2002).

• the gradient of temperature and density from west to east (rather than a symmetrical distribution).

• cloudlet evaporation

• VNEI+power law model

• soft component(``shell”):

• kT~0.3 keV

• abundance<1

non-thermal X-ray on the northeastern shell Γ~1.5 (1.3--1.9)

Γ~1.9 (1.4--2.2)

Γ~1.1 (0.2--1.4)Flat photon index (appears flatter in the south region ``S2”)No evidence of Fe Kα line at 6.4keV

shell

S1

S2

Possible origin of the non-thermal emission

2. Inverse Compton scattering?

not important in soft X-ray band, unless B <<10 μG or Urad >> UCMB

requires electrons accelerated to energy > tens of TeV. hard for old SNR as W28 (shock velocity~80km/s)X-ray spectrum is loss-steepened which results a typical photon index value 2--3.5 > observed value Γ~1.5

1. Synchrotron?

The hard non-thermal continuum (0.3--7.0 keV, in

region ``Shell”):

Possible origin of the non-thermal emission

a favorable explanation for the hard spectra in the MC-SNR interaction region. also need further calculation for the luminosity.

• 3. Non-thermal bremsstrahlung

• Bremsstrahlung radiation in X-ray can be produced by electrons with energy < tens of MeV.

• Coulomb loss dominates and flattens the spectrum.

•

• for Γ=1.1--1.9, α=2.1--2.9

• successfully explain the hard spectrum emitting clumps in γ Cygni (Uchiyama et al. 2002; Bykov et al. 2004).

Summary• W28 is an MMSNR with a deformed X-ray shell

and diffuse blobby X-ray emission in the interior.

• vnei (cold)+vmekal (hot) model well reproduces the spectra in the SNR interior, with a spatially varying spectral properties.

• thermal conduction is not efficient.

• the cloudlet evaporation model, together with a nonuniform environment, can explain many of the plasma properties.

• The shell gas is best characterized by a thermal plus a non-thermal model, with a hard photon index

• non-thermal bremsstrahlung is the most feasible origin.

Thank you!

4. non-thermal X-rays from Secondary leptons?

Possible origin of the non-thermal emission

The hadronic origin of γ-ray is widely accepted

The model of Gabici et al. (2009) is adopted

Red: synchrotrongreen: bremsstrahlungblack: pion decaybow tie: observed X-ray

ASCA, NEI model(Rho & Borkowski 2002)

An elevated hard tail compared to the

model

Energy (keV)

XMM PN 18ks obs , APEC+power law model (Ueno et al. 2003)

A flat photon index Γ~1.3 (0.4-1.9) is found. However, no

discussion about the origin of the non-thermal X-ray

Introduction: non-thermal X-ray on the northeastern shell?

• two XMM observations in NE

• PN and MOS data are used to joint fit the spectra

• VNEI+power law model

• compare the powerlaw index between the northern region ``S1”and southern region ``S2”.

• Energy cuts below 7keV

Compare to XMM study of Ueno et al (2003)

XMM hard X-ray (2.5--7.0keV)

Intro: debate on the central gas

Popular scenarios for central hot gas:1. cloudlet evaporation 2. thermal conductionothers: reflected shock, projection effects

ROSAT and ASCA studies:

(Rho & Borkowski)

W28 poses a challenge for existing models.X-ray emission at its center is a “fossil” radiation

ASCA study ASCA study (Kawasaki et al)(Kawasaki et al)

Search for Search for evidence of evidence of overionization, overionization, but none is foundbut none is found

SUZAKUSUZAKU(Sawada & Koyama)(Sawada & Koyama)

claim central claim central plasma is plasma is overionizedoverionized

Recombining plasma?

Einstein (Long et al. 1991)

Cloudlet evaporationYoung age of W28 (~2500yr)

γ-ray in W28

(Nicholas et al. 2010)

(Aharonian et al. 2010)

Fermi LAT 2-10 GeV count mapBlack contour: VLA 90cmAbdo et al. 2010

GeVTeV

Chandra and optical observation by Keohane et al.

2004

upper:ROSAT X-ray image

bottom:The ratio of [SII]/Hα

In W28 center:a)X-ray emitting gas is clumpy;b)Low ratio of[SII]/Hα

Spectral extraction region for ASCA, SUZAKU and XMM

•Black rectangle: ASCA

•Black circle: SUZAKU

Spectra for small-

scale regions in

W28 interior