Deep Xray Observation of the Galactic Plane with Chandra

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Deep X-ray Observation of the Galactic Plane with
Chandra

• Ken Ebisawa
• INTEGRAL Science Data Center and NASA/GSFC
• Ada Paizis
• INTEGRAL Science Data Center
• Y. Maeda, H. Kaneda, G. Sato
• ISAS
• S. Yamauchi
• Iwate University
• A. Bamba
• Kyoto University

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Introduction and Summary

• Galactic plane is a strong hard X-ray (2-10 keV)
  emitter

What is the origin of the Galactic plane hard
X-ray emission?
Is it really diffuse? Superposition of numerous
point sources?
Previous X-ray instruments did not have good
enough angular resolution (1 arcmin with ASCA)
Chandra finally could resolve point sources from
diffuse emission (0.5 arcsec resolution)
We found the Galactic plane hard X-ray is truly
diffuse emission
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Introduction and Summary
What is the origin of the hot diffuse plasma?
We do not know yet (this is the
question!) Thermal equilibrium plasma is
impossible Non-thermal mechanism is required
What is the nature of dimmest X-ray point sources
on the Galactic plane?
Two classes hard sources and soft sources
Hard sources are mostly AGN behind milky way Some
Galactic hard sources (quiescent novae?)
Soft sources are mostly X-ray active late type
stars
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Review
HEAO1 measurement of the Galactic Ridge
(Worrall et al. 1982)
Total luminosity 1.4x1038 erg s-1 Energy
spectrum softer than Cosmic X-ray BGD
EXOSAT Galactic scan (Warwick et al. 1985)
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Review

• Tenma observation
• Koyama et al. (1986)

6.7 keV iron line emission ? thin thermal plasma
emission Temperature of the plasma kT 6-10 keV
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Review

• Point sources or diffuse emission?
• (Warwick et al. 1985 Koyama et al. 1986)

Diffuse plasma?
107K plasma extended over the Galactic plane ?
P/k nT105 cm-3 K 100 times the neutral
interstellar gas! Thermal velocity exceeds the
escape velocity!
Point sources?
Very dim (Lxlt1033erg s-1) numerous (gt30,000 )
source required (cataclysmic variables?)
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Review

• Ginga scan observation
• (Yamauchi et al. 1993)

6.7 keV Fe-line mapping
6.7 keV line is a good tracer of thin thermal
plasma
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Review

• ASCA observation
• (Sugizaki et al. 2001)

10 resolved into point sources above 3x10-13
erg s-1cm-2 (2-10 keV)
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Observation

• Scutum region (l,b)(28.5,-0.1), where diffuse
  emission is strong
• Extensive ASCA observations (Kaneda et al. 1997)
• No point sources brighter than 2x10-13 erg
  s-1 cm-2 (2-10 keV)
• AO1, 2000 Feb 24 (100 ksec) AO2, 2001 May
  20 (100 ksec) Two fields partially
  overlapping
• Sensitivity 3x10-15 ergs-1 cm-2 (2-10 keV) and
  7x10-16 ergs-1 cm-2 (0.5 - 2keV)
  100 times more
  sensitive than ASCA

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AO1AO2 Superposed 0.5-3 keV Red 3-8 keV
Blue Strong diffuse Emission Many new point
sources
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Source Search

• wavdetect in CIAO package
• 0.5-3 keV, 3-8 keV and 0.5-8 keV seperately
• Select sources which exceed 4 s significance in
  either of the three bands
• Total 275 sources detected
• 183 sources detected in Soft band
• 79 sources detected in Hard band
• 26 sources detected both in Soft and Hard band

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275 sources detected (gt4 sigma either 0.5-3
keV, 3-8 keV or 0.5- 8keV)
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183 soft sources (gt4 sigma in 0.5 3 keV)
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79 hard sources (gt4 sigma in 3- 8 keV)
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Origin of the hard X-ray emission

• Total hard X-ray flux in the field of view
  1.1x10-10 ergs-1 cm-2 deg-2 (2-10 keV)
• Total point source flux 10-11 ergs-1 cm-2
  deg-2
• Point sources account for only 10 of the total
  hard X-ray flux
• Most of these sources are extragalactic (see
  below)
• Origin of the Galactic plane hard X-ray emission
  is truly diffuse (Ebisawa et al. 2001)

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log N-log S in the hard band

• Source density consistent with those at high
  galactic latitudes (assuming NH6x 1022cm-2
  Galactic absorption)
• Most of the hard X-ray point sources are
  extragalactic

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Hard X-ray diffuse feature

• Radio source G28.60-0.13 (Helfand et al.1989)
• Association of super nova remnants and Galactic
  plane diffuse emission?
• Ueno et al. (2002)

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Diffuse spectrum

• Numerous emission lines
• Emission from optically thin hot plasma

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Diffuse iron emission line

• Iron line central energy 6.56 ? 0.01 keV
• Significantly less than 6.67 keV (He-like iron)
• Two possibilities
• Contamination of 6.4 keV fluorescence line
• Non ionization equilibrium (ionization does not
  reach He-like iron)
• Presence of H-like line (6.97 keV) not clear

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Origin of the diffuse hard X-ray emission
Static and thermal equilibrium plasma is
impossible

• Heating and confinement of the plasma by
  interstellar magnetic fields
• Tanuma et al. (1999, 2001)
• Cosmic ray - interstellar matter interaction
• Cosmic ray electrons (Valinia et al. 2000 )
• Cosmic ray heavy irons (Tanaka 2002)

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Origin of the diffuse hard X-ray emission

• MHD simulation (Tanuma et al. 1999 2001)

Supernova triggers magnetic reconnection
(similar to solar flare)
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Origin of the diffuse hard X-ray emission
Valinia et al. (2000) Low Energy Cosmic Ray
electron model
ASCA GIS spectral fit
Fluorescent emission lines non-thermal
bremsstrahlung
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Galactic diffusehard X-ray and gamma-ray emission
Hard X-ray smoothly connects gamma-rays up to 1
GeV
Valinia et al. (2000)
Gehrels and Tueller (1993)
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Spectral study of point sources

• Only a dozen soft source sources had optical
  identification with R lt18
• No hard sources are optically identified
• Counts in 0.5-3, 3-8 and 0.5-8 keV bands obtained
  from source detection and spectra
• Normalized counts for 100 ksec (single
  observation) at the aim points
• Calculates Hardness Ratio (HR) as (H-S)/(HS)
• Calculate position dependent responses (RMF and
  ARF)
• Combine spectra and responses for sources with
  similar HR
• 91 soft sources with HR lt -0.8
• 44 hard sources with HR gt0.6

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Hardness ratio distribution

• Softest sources are most numerous
• Hardest sources are next numerous
• Two populations

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Intensity vs. HR

• No clear correlation
• Two populations are recognized.

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Soft source spectra (HR lt-0.8)
Thin thermal plasma model kT0.76/-0.04
keV NH(1.0/-0.1)x 1022cm-2 Small absorption ?
foreground sources Emission lines Presumably
active stars
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Hard source spectra (HR gt0.6)
Power-law p1.2/-0.4 NH(8.0/-2.4)x
1022cm-2 Absorption consistent with the
Galactic value Must be mostly AGN Weak iron
feature Either emission line or absorption
edge Spectral flatter than Cosmic X-ray BGD
(p1.4) Some Galactic sources (cataclysmic
variables?)
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Brightest source (HR0.74)
Power-law p0.7/-1.00.3 NH(6/-42)x
1022cm-2 Flat spectrum Iron feature Cataclysmic
variable (quiescent dwarf nova)
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Time variation
Among 275 sources, 17 sources gt99.9 significant
variations (K-S test) 13 sources had Hardness
ratio-1 Presumably foreground late type flare
stars
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Summary

• Hard X-ray emission from the Galactic plane is
  diffuse
• There are two populations of the point sources
  very soft ones and very hard ones
• Hard sources are mostly AGN and some bright
  quiescent novae
• Soft sources are probably nearby late types tars

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Next projects

• Ground telescope observation
• IR identification (with SOFI at ESO NTT)
• XMM
• Iron line study of hard point sources
• Discriminate AGN and galactic sources
• More precise iron line study of diffuse emission
• Plasma diagnostics (separate 6.40, 6.67 and 6.97
  keV lines)