Post on 13-Jan-2016
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 (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