Deep Chandra Observations of LMXB Populations in Normal Elliptical Galaxies

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Deep Chandra Observations of LMXB Populations in
Normal Elliptical Galaxies

• Nicola Brassington
• Harvard-Smithsonian CfA

The X-ray Universe, Granada 2008
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Overview

• Why study LMXB populations?
• Properties of target galaxies
• Summarise results
• Source Lx, colours, variability
• Difference between field and GC LMXBs
• X-ray Luminosity Function
• Diffuse emission
• Nuclei
• Conclusions

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LMXB Populations

• Origin and Evolution of LMXBs
• Chandra observations 20-70 of point sources
  form in globular clusters (Angelini et al. 2001)
• Evolution of primordial binaries in the stellar
  field
• Formation in globular clusters

• Evolution of native field binaries (see Verbunt
  van den Heuvel 1995)(Piro Bildsten 2002,
  Ivanova Kalogera 2006)
• Semi-detached binaries with large thermally
  unstable disks and giant donors
• Recurrent transients for gt75 binarys life
  (recurrence time gt100yr, outburst 1-100 yr)
• From multi-epoch Chandra observations should be
  able to identify these sources

• Formation in GCs (efficient two-body encounters
  Clark 1975, Fabian et al 1975)
• Ultra-compact NS-WD binaries (Bildsten Deloye
  2004)
• White dwarf orbiting NS
• 5-10 min orbit
• Short lifetime few 106 yr
• Transient at the LX 1037 erg s-1
• LX lt 2x1038 erg s-1
• High luminosity BH binaries(Kalogera, King
  Rasio 2004)
• Should be rare
• Possibly persistent (if from capture)
• Field LMXBs formed in GCs - then ejected or left
  alone in the field as GC was disrupted (Bildsten
  Deloye 2004)

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Galaxy Selection

• Elliptical galaxies provide clean sample
• NGC 3379 NGC 4278 chosen because
• Nearby (10.8 Mpc 16.1 Mpc)
• Contain little diffuse emission
• Optically similar (LB 1.3 x 1010 L? LB 1.5 x
  1010 L?)
• Difference in GC specific frequency (SGC 1.3
  12.3)

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NGC 3379

• Five separate Chandra observations
• All ACIS-S3
• One archival observation
• Four deeper pointings taken over 1 year

0.3-0.9 keV 0.9-2.5 keV 2.5-8.0 keV
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NGC 4278

• Larger distance than NGC 3379
• 16.1 Mpc (10.6 Mpc)
• Longer exposure to probe to similar Lx

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Source Luminosity Distributions

• Source detection with wavdetect for each
  observation, then cross-correlated
• Lx values calculated assuming canonical PL model
  for LMXB, with Galactic NH and ?1.7

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Variability

• Characterising variability of sources helps
  identify source populations through variation
  amplitudes and excess variance
• Sources defined as variable if reduced ?2 gt1.2
• NGC 3379 48 variable sources
• NGC 4278 44 variable sources
• This number increases with Lx cut - a consequence
  of improved statistics
• Compare these percentages to other systems

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Transient Candidates

• Monitoring campaign to search for transients
• Transient sources ratio gt 10 (or gt 5 possible
  transient candidates)
• Low Lx sources have large uncertainty therefore
  simple ratio not adequate
• Instead use BEHR (Bayesian Estimation of Hardness
  Ratio) to provide ratio and lower bound value

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Optical Correlations

• WFPC2 observations
• Cleaned for B/G and F/G objects
• Correlate GC-LMXB sources
• Search radius 0.6 /1.0

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• K-S test show that GC and field populations not
  from same population
• NGC 3379 99.8 confidence NGC 4279 81.4
  confidence

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X-ray Luminosity Function

• Important tool for constraining LMXB formation
• Well studied at high end
• Break at 5x1038 erg s-1 (Kim Fabbiano 2004
  Gilfanov 2004)
• Reflects two different binary systems NS/BH LMXBs
• Here can study low end of the XLF
• Voss Gilfanov (2006) claim break at 5x1037 erg
  s-1
• Include NGC 4697 in combined XLF

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Combined XLF

• Individual fits to each system very similar
• Single power-law slope 1.8-2.0
• However, combined XLF
• shows complexities

• Broken power-law - break 7x1037 erg s-1
• Slope 2.1 and 1.4
• See bump in residuals - fit power-law gauss
• Slope 1.8, bump peak 6x1037 erg s-1

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XLF Bump

• If bump feature real what is the contribution?
• Red giant donors

• All 3 systems age 9Gyr
• Red giant mass in narrow range 1M?
• Narrow range of Lx when they start mass transfer
  above the critical weight

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GC Vs Field XLF

• Compare XLF from field LMXBs and GC-LMXBs

• Relative lack of GC-LMXBs at lower Lx
• Bump still present 16 field LMXBs, 12
  GC-LMXBs

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Diffuse Emission

• We have detected hot ISM Lx3.5x1037 erg s-1
• Accounting for both unresolved LMXBS
• Contribution from stellar sources (e.g. CVs and
  active binaries)
• Specifically tailored model
  reproduces Lx, mass and
  profile well
• Trinchieri et al. (2008)
  - submitted
• NGC 4278 does not appear
  to contain hot ISM

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Properties of the Nuclei

• Nucleus of NGC 3379 varies Lx few 1038 erg s-1
• LINER 2/transition object
• Nucleus of NGC 4278 varies Lx few 1040 erg s-1
• LINER 1.9 - broad H? (Ho et al. 1997)

• Fit spectra with power-law plus APEC
• KT 0.5 keV
• ? varies massively

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Summary

• Probing LMXB population to 1036 erg s-1 reveals
  more details in population
• Relative deficit of GC-LMXBs at low Lx
• Not all LMXBs form in GCs
• Low end of XLF shows complexity
• Possible bump feature - from red giant donors
• Similar of field transients in both galaxies
• NGC 4278 contains BH-LMXB transient in GC
  (contrary to KKR04)
• NGC 3379 contains small (3.5x1037 erg s-1) hot
  ISM
• Variable LINER in NGC 4278, exhibiting hard-high
  behaviour
• Brassington et al (2007) astro-ph/0711.1289,
  Fabbiano et al (2007) astro-ph/0710.5126, Fragos
  et al. astro-ph/0801.1122
• Trinchieri et al (2008) - submitted, Kim et al
  (2008) - in prep, Brassington et al. (2008) - in
  prep