J/MNRAS/388/849 NGC253 XMM observations (Barnard+, 2008)
A multi-coloured survey of NGC253 with XMM-Newton: testing the methods used for
creating luminosity functions from low-count data.
Barnard R., Greening L.S., Kolb U.
<Mon. Not. R. Astron. Soc., 388, 849-862 (2008)>
=2008MNRAS.388..849B 2008MNRAS.388..849B
ADC_Keywords: Galaxies, nearby ; X-ray sources
Keywords: galaxies: individual: NGC 253 - X-rays: binaries - X-rays: general
Abstract:
NGC 253 is a local, starbursting spiral galaxy with strong X-ray
emission from hot gas, as well as many point sources. We have
conducted a spectral survey of the X-ray population of NGC 253 using a
deep XMM-Newton observation. NGC 253 only accounts for ∼20 per cent of
the XMM-Newton EPIC field of view, allowing us to identify ∼100 X-ray
sources that are unlikely to be associated with NGC 253. Hence, we
were able to make a direct estimate of contamination from, for
example, foreground stars and background galaxies.
X-ray luminosity functions (XLFs) of galaxy populations are often used
to characterize their properties. There are several methods for
estimating the luminosities of X-ray sources with few photons. We have
obtained spectral fits for the brightest 140 sources in the 2003
XMM-Newton observation of NGC 253, and compare the best-fitting
luminosities of those 69 non-nuclear sources associated with NGC 253
with luminosities derived using other methods. We find the
luminosities obtained from these various methods to vary
systematically by a factor of up to 3 for the same data; this is
largely due to differences in absorption. We therefore conclude that
assuming Galactic absorption is probably unwise; rather, one should
measure the absorption for the population.
Description:
XMM-Newton observations are susceptible to periods of high
background levels, caused by increased flux of solar particles. We
screened the data from each of the EPIC cameras (MOS1, MOS2 and pn),
to remove flaring intervals. This process resulted in ∼46ks of good
time for the PN and ∼69ks for the MOS cameras.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tablea1.dat 44 185 Positions of X-ray sources in the XMM-Newton
observations of NGC 253, along with some
observed properties
tablea2.dat 105 185 Spectral properties for each source from
Observation 2
--------------------------------------------------------------------------------
See also:
J/ApJ/649/730 : ULX population in nearby galaxies from XMM (Winter+, 2006)
B/xmm : XMM-Newton Observation Log (XMM-Newton SOC, 2007)
Byte-by-byte Description of file: tablea1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- [BGK2008] Source sequential number (G1)
5- 6 I2 h RAh Right ascension (J2000)
8- 9 I2 min RAm Right ascension (J2000)
11- 15 F5.2 s RAs Right ascension (J2000)
17 A1 --- DE- Declination sign (J2000)
18- 19 I2 deg DEd Declination (J2000)
21- 22 I2 arcmin DEm Declination (J2000)
24- 28 F5.2 arcsec DEs Declination (J2000)
30 A1 --- inD25 [yn] indicates whether the source is within
the D25 isophotal region of NGC 253
32- 33 I2 arcsec Rs Source extraction radius
35- 39 F5.2 --- Ab/As Background-to-source area ratio
41- 44 F4.2 --- EEF Encircled energy fraction (1)
--------------------------------------------------------------------------------
Note (1): Sources with an EEF of zero were detected, but no suitable
spectra were obtained.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tablea2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- [BGK2008] Source sequential number (G1)
5- 9 I5 ct CtPN pn source counts
11- 15 I5 ct CtMOS MOS source counts
17- 26 A10 --- Mod Best fitting model (1)
27- 31 F5.3 10+22cm-2 NH ? Absorption
32- 36 F5.3 10+22cm-2 e_NH ? rms uncertainty on NH at
90% confidence level
37 A1 --- f_NH [f] f for fixed absorption
40- 44 F5.3 --- Param ? Parameter (spectral index or
temperature) (2)
46- 50 F5.3 --- e_Param ? rms uncertainty on Param at
90% confidence level
52- 55 F4.2 --- Param2 ? Second parameter value (spectral index or
temperature) (2)
56- 59 F4.2 --- e_Param2 ? rms uncertainty on Param2 at
90% confidence level
62- 66 F5.1 --- chi2 ? chi2 value
67 A1 --- --- [/]
68- 70 I3 --- DOF ? Degree Of Freedom (3)
72- 76 F5.3 --- gf ? Good fit probability
78- 86 F9.2 10+29W LBF ? Best-fitting luminosity
88- 96 F9.2 10+29W e_LBF ? rms uncertainty on LBF at
90% confidence level
99-105 F7.2 10+29W LSM ?=0. Standard model (Method I) luminosity
--------------------------------------------------------------------------------
Note (1): Best-fitting models for bright sources can be as follows:
PO = a power law
BB = blackbody
BR = bremsstrahlung
2C = a two-component model consisting of a BB plus power law
Faint sources are modelled using a best-fitting power law:
-----------------------------------------------------------------------------
Model NFnt NH/1020 Gamma chi2/dof [gf] Flux(pn thin) Flux (MOS med)
-----------------------------------------------------------------------------
IS 7 1.3 0.4 (.03) 71/54 [0.06] 12000(200) 36000(8000)
OS 36 1.3 1.23(.13) 55/44 [0.12] 4800(400) 19000(2000)
-----------------------------------------------------------------------------
Note (2): Parameter (spectral index or temperature), spectral index for
PO models, temperature for BB and BR models, spectral index and
temperature for 2C models.
Note (3): The quality of the spectrum may be deduced from the number of
degrees of freedom because the spectra are grouped to a minimum number
of counts per bin (brighter sources have more degrees of freedom).
--------------------------------------------------------------------------------
Global notes:
Notes (G1): Sources identified as [BGK2008] SNNN in Simbad.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 24-Dec-2008