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A multi-colour survey of NGC253 with XMM-Newton

A multi-colour survey of NGC253 with XMM-Newton. Robin Barnard, Lindsey Shaw Greening & Ulrich Kolb The Open University. Overview. X-ray populations of galaxies and what we think they mean NGC253, and our analysis of XMM data… Comparison of best fit luminosities vs. standard model

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A multi-colour survey of NGC253 with XMM-Newton

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  1. A multi-colour survey of NGC253 with XMM-Newton Robin Barnard, Lindsey Shaw Greening & Ulrich Kolb The Open University

  2. Overview • X-ray populations of galaxies and what we think they mean • NGC253, and our analysis of XMM data… • Comparison of best fit luminosities vs. standard model • XLFs and total LX inside and outside D25 region of NGC253 • Conclusions

  3. X-ray populations of galaxies • Point X-ray emission of external galaxies dominated by X-ray binaries (XBs) • LMXBs from old population, LMXB numbers related to galaxy mass • HMXBs young and short lived, hence numbers are dictated by ongoing star formation rate • Extragalactic X-ray binaries often too faint for us to model their X-ray spectra • Hence, fluxes are often estimated from count rates, assuming a particular emission model • Power law with spectral index 1.7, or 5 keV bremsstrahlung are commonly used • Alternatively, best fit to summed point source emission spectrum is sometimes applied

  4. X-ray vs. Star Formation Rate • Grimm et al. (2003) tried calibrating SFR with total HMXB LX for a sample of local galaxies with high SFR/mass so LMXBs can be ignored • They found that XLFs normalised by SFR to be ~same… “Universal XLF” • They also found relation between SFR and: • integrated X-ray luminosity of galaxy (linear at SFR > ~4 MSun/yr) • No. of sources with 2-10 keV luminosity >2x1038 erg/s NGC253: SFR = 4.0 Lx = 5x1039

  5. NGC 253 • ~Edge on starbursting spiral galaxy in Sculptor group (~4 Mpc) • ~25 x 7 arcmin2 • X-ray view shows what NGC 253 might look like if standard model were true

  6. XMM observations of NGC253 0.3-2.5 keV 2.5-4 keV • XMM obs in 2000 & 2003 • Combined source list 185 sources • Chandra observed 140 sources, 3 confused 4-10 keV Central region, linear scale Fullimage, Hist-eq scaled

  7. Source analysis • Made source extraction regions 12-40” radius (mostly 20”) • Extracted 0.3-10 keV pn and MOS spectra and lightcurves • For each spectrum, got corresponding RMF and ARF files • Selected corresponding background regions: • On the same CCD as the source region • At a similar off-axis angle • Source free • At similar distance from readout edges in pn • Area 1-36 x source area ( > 3 x for 75% of sources) • Obtained background lightcurves and spectra corresponding to source regions

  8. Fluxes from different methods • APPROACH 1: Flux obtained from best fit model • Freely modelled 0.3-10 keV spectra for 140 sources with >50 source counts in pn or combined MOS1+MOS2 spectra. • APPROACH 2: Flux obtained from intensity, assuming standard model • We corrected for vignetting and encircled energy fraction (extraction radius, off-axis angle, energy) • For conversion factor from intensity to flux, we obtained the 0.3-10 keV flux equivalent to 1 count/s for standard model (power law with G = 1.7) with nH = 1.3x1021 for an on-axis source region with 15” extraction radius (following the HEASARC tool WebPIMMS) • We then compared resulting fluxes

  9. Comparing SM with best fit fluxes • Here we compare SM fluxes (red dots) with 90% upper and lower limits from best fit models (lines) • SM OK for faint sources, underestimates brighter

  10. Difference due to NH? • Required absorption for SM to have same total L as best fits is ~ 7.1 x 1021 atom/cm2 • This is 25 s higher than the mean measured absorption (2.0+/-0.2 x 1021 atom/cm2) • Discrepancy instead due to spectral differences… very luminous sources are have much softer spectra than standard model, and hence are poorly represented

  11. Bright source fit vs. SM • Folded spectrum: best fit and SM have same area under curve, i.e. same count rate • However, SM appalling fit • Unfolded, unabsorbed spectrum reveals huge difference in flux • Hence SM drastically underestimates luminosities of bright sources

  12. In D25 (77) Out D25 (92) Best fit SM Best fit and SM LFs (1)

  13. Best fit and SM LFs (2) • Number density of NGC253 sources (red) 3.6 times higher than for sources outside D25 (blue): sources outside likely bg. • Total LX for best fits 2.8 times greater than SM inside D25… total LX for best fits 2.1 times greater outside D25, hence difference is not systematic • Empirical relation between N (L>2x1038) and SFR (Grimm et al. 2003): SM gives ~1.7 Msun/yr, fits give ~4 Msun/yr • Using a power law with mean G and NH yields fluxes 1.6x higher than SM… not perfect because high L sources systematically softer

  14. Conclusions • XLFs of local galaxies used to link X-rays with mass and SFR of distant galaxies. Such work often assumes a standard emission model for all sources • Our detailed study of NGC253 with XMM shows that the SM underestimates high L sources… • “Universal XLF” too steep • Total LX to low • N (L2-10 >2x1038) too low • Better calibration requires close study of more nearby galaxies

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