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 XLFs and total L X inside and outside D 25 region of NGC253 Conclusions

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

X-ray vs. Star Formation Rate Grimm et al. (2003) tried calibrating SFR with total HMXB L X 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 M Sun /yr) No. of sources with 2-10 keV luminosity >2x10 38 erg/s NGC253: SFR = 4.0 Lx = 5x10 39

NGC 253 ~Edge on starbursting spiral galaxy in Sculptor group (~4 Mpc) ~25 x 7 arcmin 2 X-ray view shows what NGC 253 might look like if standard model were true

XMM observations of NGC253 XMM obs in 2000 & 2003 Combined source list 185 sources Chandra observed 140 sources, 3 confused Full image, Hist-eq scaledCentral region, linear scale keV keV 4-10 keV

Source analysis Made source extraction regions 12-40” radius (mostly 20”) Extracted 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

Fluxes from different methods APPROACH 1: Flux obtained from best fit model Freely modelled 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 keV flux equivalent to 1 count/s for standard model (power law with  = 1.7) with n H = 1.3x10 21 for an on-axis source region with 15” extraction radius (following the HEASARC tool WebPIMMS) We then compared resulting fluxes

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

Difference due to NH? Required absorption for SM to have same total L as best fits is ~ 7.1 x atom/cm 2 This is 25  higher than the mean measured absorption (2.0+/-0.2 x atom/cm 2 ) Discrepancy instead due to spectral differences… very luminous sources are have much softer spectra than standard model, and hence are poorly represented

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

Best fit and SM LFs (1) In D 25 (77) Out D 25 (92) Best fit SM

Best fit and SM LFs (2) Number density of NGC253 sources (red) 3.6 times higher than for sources outside D 25 (blue): sources outside likely bg. Total L X for best fits 2.8 times greater than SM inside D 25 … total L X for best fits 2.1 times greater outside D 25, hence difference is not systematic Empirical relation between N (L>2x10 38 ) and SFR (Grimm et al. 2003): SM gives ~1.7 M sun /yr, fits give ~4 M sun /yr Using a power law with mean  and N H yields fluxes 1.6x higher than SM… not perfect because high L sources systematically softer

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 L X to low N (L 2-10 >2x10 38 ) too low Better calibration requires close study of more nearby galaxies