Browse
this table...

NGC253XMM - NGC 253 XMM-Newton X-Ray Point Source Catalog

HEASARC
Archive

Overview

This table contains the NGC 253 XMM-Newton x-Ray point source catalog. NGC 253 is a local, starbursting spiral galaxy with strong X-ray emission from hot gas, as well as many point sources. The authors 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 them to identify ~ 100 X-ray sources that are unlikely to be associated with NGC 253. Hence, they 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. The authors 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. They 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. The authors therefore conclude that assuming Galactic absorption is probably unwise; rather, one should measure the absorption for the population. In addition, they find that standard estimations of the background contribution to the X-ray sources in the field are insufficient, and that the background active galactic nuclei (AGN) may be systematically more luminous than previously expected. However, the excess in their measured AGN XLF with respect to the expected XLF may be due to an as yet unrecognized population associated with NGC253.

XMM-Newton observations are susceptible to periods of high background levels, caused by increased flux of solar particles. The authors screened the data from each of the EPIC cameras (MOS1, MOS2 and pn), to remove flaring intervals. This process resulted in ~ 46 ks of good time for the pn and ~ 69 ks for the MOS cameras.

The authors combined the cleaned MOS and pn data, and ran the source detection algorithm provided with the XMM-Newton data analysis suite SAS version 7.0. They accepted sources with maximum-likelihood detections > 10 (equivalent to 4 sigma).


Catalog Bibcode

2008MNRAS.388..849B

References

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    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in April 2009 based on the electronic version of Tables A1 and A2 from the paper which were obtained from the CDS (their catalog J/MNRAS/388/849 files tablea1.dat and tablea2.dat).

Parameters

Source_Number
A unique identification number for each XMM-Newton X-ray source in this table in order of increasing J2000.0 Right Ascension.

Name
The NGC 253 XMM-Newton X-ray source designation, as recommended by the CDS Dictionary of Nomenclature of Celestial Objects, using the '[BGK2008] S' prefix (for Barnard, Greening, Kolb (2008), source) and the source number.

RA
The Right Ascension of the XMM-Newton X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of up to 0.01 seconds of time in the original table. These coordinates are not astrometrically corrected, but were taken directly from the source detection routine.

Dec
The Declination of the XMM-Newton X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of up to 0.01 arcseconds in the original table. These coordinates are not astrometrically corrected, but were taken directly from the source detection routine.

LII
The Galactic Longitude of the XMM-Newton X-ray source.

BII
The Galactic Latitude of the XMM-Newton X-ray source.

In_D25_Flag
This flag parameter indicates whether the X-ray source is within ('Y') or outside ('N') of the V-band D_25 isophote of NGC 253 (shown in Figure 1 of the reference paper). Sources within the D_25 isophote are labelled as IS ('inner sources') in the reference paper, while those outside are labelled as OS ('outer sources').

Extraction_Radius
The XMM-Newton source extraction radius, in arcseconds. For every source, the authors obtained an extraction region with radius in the range from 10-40 arcseconds. In general, they used a radius of 20 arcsec, except for sources with large point spread functions due to high off-axis angles, where a 40 arcsec radius was used, or in very crowded regions, where a radius of 12-15 arcsec was used. The central region of NGC 253 is fairly crowded, and the authors ignored sources with another source < 10 arcsec away. This resulted in the loss of only a few faint sources.

Bck_Src_Area_Ratio
The background-to-source extraction area ratio for the XMM-Newton source. In addition to the source extraction region (see the extraction_radius parameter), the authors also created a corresponding background region for every source. They required that the background be on the same CCD as the source for all three EPIC cameras, that there be no point sources in the background, and that its intensity per unit area be smaller than for the source region. The resulting background regions had areas one to 35 times greater than their corresponding source regions; for 75 per cent of sources, the background area was more than three times larger than the source area.

Eef
The encircled energy fraction (EEF) for the X-ray source. EEF is a function of the source region radius R and the off-axis angle theta. The procedure by which it was calculated is discussed in Section 3.3.2 of the reference paper. Sources with an EEF of zero were detected, but no suitable spectra were obtained.

PN_Counts
The net source counts in the pn detector in the energy range from 0.3 to 10 keV.

Mos_Counts
The net source counts in the MOS detectors in the energy range from 0.3 to 10 keV.

Spectral_Model
The best-fitting model to the X-ray source spectrum for sources with > 50 source counts in either their pn or their MOS spectrum, or, for faint sources, the location of the source relative to the D_25 isophote of NGC 253, coded as follows:

       PO = a power law
       BB = blackbody
       BR = bremsstrahlung
       2C = a two-component model consisting of a BB plus power law
       OS = faint outer source
       IS = faint inner source
  

Faint sources are modeled using a best-fitting power law listed in Table 1 of the reference paper, and as listed below:

    Table 1: Best-fitting parameters for power-law models applied to the summed
               spectra of the IS and OS faint source populations

   Model NFnt  NH  Gamma    chi2/dof [gf]   Flux(pn thin) Flux (MOS med)

    IS      7   1.3E20   0.4 (.03)   71/54  [0.06]  12000(200)    36000(8000)
    OS     36   1.3E20   1.23(.13)   55/44  [0.12]   4800(400)    19000(2000)
  

The authors only accept models where the good fit probability is > 0.05. Those sources with no acceptable models have a value for this parameter of 'Cannot fit'. Some models have the minimum number of counts per bin quoted in parentheses following their spectral model type, for example, PO(30). This indicates that the spectrum had non-standard grouping. Such grouping was done if the standard grouping for a spectrum resulted in a data point that was unusually high or low as an artefact of grouping; grouping the spectrum to, for example, a minimum of 30 counts per bin rather than 50 counts per bin removes the artefact, showing that this outlier is not intrinsic to the source.

NH
The best-fit line-of-sight absorption column density n_H to the X-ray source, in H atoms cm-2, if an emission model was fitted. If n_H falls below the Galactic absorption in this direction (as measured by Stark et al. 1992, ApJS, 79, 77) of 1.3 x 1020 H atoms cm-2, then the absorption is fixed at this value, indicated by an nh_flag value of 'f'.

NH_Error
The 90% confidence level uncertainty of n_H, in H atoms cm-2.

NH_Flag
This flag parameter is set to 'f' to indicate that the absorption was fixed. Faint sources are modeled using a best-fitting power law listed in Table 1 of the reference paper, also listed below:

    Table 1: Best-fitting parameters for power-law models applied to the summed
               spectra of the IS and OS faint source populations

   Model NFnt  NH  Gamma    chi2/dof [gf]   Flux(pn thin) Flux (MOS med)

    IS      7   1.3E20   0.4 (.03)   71/54  [0.06]  12000(200)    36000(8000)
    OS     36   1.3E20   1.23(.13)   55/44  [0.12]   4800(400)    19000(2000)
  

Spect_Param_1
The best-fit spectral parameter value (spectral index or temperature) of the first component, which is the power-law spectral index for PO and 2C spectral models, or the temperature kT in keV for BB and BR spectral models.

Spect_Param_1_Error
The 90% confidence level uncertainty in the best-fit spectral parameter of the first component.

Spect_Param_2
The best-fit spectral parameter value (spectral index or temperature) of the second component, which is the temperature kT in keV for 2C spectral models. The HEASARC notes that source number 37 has a spectral model of 'PO' given in this table, but has 2 spectral components listed. The type of the second parameter in this case is thus uncertain.

Spect_Param_2_Error
The 90% confidence level uncertainty in the best-fit spectral parameter of the second component.

Chi_Squared
The best-fitting Chi-squared value for the fit to the X-ray source spectrum.

DoF
The number of degrees of freedom of the spectral fit. 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).

Good_Fit_Prob
The good fit probability of the spectral fit. The authors only accept models whicj have a good fit probability of > 0.05.

Lx_Best
The best-fitting luminosity of the X-ray source, in erg s-1, based on the assumed or inferred best-fit spectral model and an adopted distance to NGC 253 of 4 Mpc. Obviously, if the source is actually a foreground or background object instead, this luminosity will not be correct.

Lx_Best_Error
The 90% confidence level uncertainty of the best-fitting X-ray luminosity, in erg s-1.

Lx_Standard
The best-fitting luminosity of the X-ray source, in erg s-1, based on the 'Standard Model' (Method I) spectral assumptions, as described in Section 3.3.1 of the reference paper, i.e., an absorbed power-law spectral model with the absorption set to the Galactic line-of-sight absorption value and a power-law spectral index of 1.7.


Contact Person

Questions regarding the NGC253XMM database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 3-Apr-2009