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 LX inside and outside D25 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 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

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

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

0.3-2.5 keV2.5-4 keV4-10 keV

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

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 = 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

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 1021 atom/cm2

This is 25 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

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 D25 (77)Out D25 (92)

Best fit SM

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 systematicEmpirical 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 and NH 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 LX to low

N (L2-10 >2x1038) too low

Better calibration requires close study of more nearby galaxies