NGC1068CXO - NGC 1068 Chandra X-Ray Compact Source Catalog

Overview

This table contains some of the results from a study of the compact X-ray source population in the Seyfert 2 galaxy NGC 1068, imaged with a 50-ks Chandra observation. The authors find a total of 84 compact sources on the ACIS-S3 chip, of which 66 are located within the 25.0 B-mag/arcsec² isophote of the galactic disk of NGC 1068. In the reference paper, the spectra of the 21 X-ray sources with at least 50 counts were modeled with both multicolor disk blackbody and power-law models. The power-law model provides the better description of the spectrum for 18 of these sources. For fainter sources, the spectral index was estimated from the hardness ratio. Five sources have 0.4-8 keV intrinsic luminosities greater than 10³⁹ ergs s^-1, assuming that their emission is isotropic and that they are associated with NGC 1068. The authors refer to these sources as intermediate-luminosity X-ray objects (IXOs). If these five sources are X-ray binaries accreting with luminosities that are both sub-Eddington and isotropic, then the implied source masses are >~7 solar masses, and so they are inferred to be black holes. Most of the spectrally modeled sources have spectral shapes similar to Galactic black hole candidates. However, the brightest compact source in NGC 1068 has a spectrum that is much harder than that found in Galactic black hole candidates and other IXOs. The brightest source also shows large amplitude variability on both short-term and long-term timescales, with the count rate possibly decreasing by a factor of 2 in ~2 ks during this Chandra observation, and the source flux decreasing by a factor of 5 between this observation and the grating observations taken just over 9 months later. The ratio of the number of sources with luminosities greater than 2.1 x 10³⁸ ergs s^-1 in the 0.4-8 keV band to the rate of massive (>5 solar masses) star formation is the same, to within a factor of 2, for NGC 1068, the Antennae, NGC 5194 (the main galaxy in M51), and the Circinus galaxy. This suggests that the rate of production of X-ray binaries per massive star is approximately the same for galaxies with currently active star formation, including "starbursts."

The authors were concerned with the study of the discrete X-ray source population in NGC 1068, imaged within the 8.4 arcmin x 8.4 arcmin (35.3 kpc x 35.3 kpc) field of view of the ACIS S3 chip. Images were extracted from the reprocessed level 2 events file in soft (0.4-1.5 keV), hard (1.5-5.0 keV) and full (0.4-5.0 keV) energy bands. The authors used the CIAO program wavdetect to search the images in the three energy bands for discrete sources of X-ray emission. They analyzed the images using wavelet scales in the range from 1 pixel (0.492 arcsec) to 16 pixels (7.87 arcsec), separated by a factor of sqrt(2). The wavelet source detection threshold was set to 10^-6, which gives approximately one false source for the whole S3 chip. The total number of sources detected by wavdetect in the soft, hard, and full energy band images was 115, 67, and 138, respectively. Each of these sources was examined carefully by eye, and only those 84 sources that appear compact to the eye are included in this source list.

Catalog Bibcode

2003ApJ...591..138S

References

A Chandra X-ray study of NGC 1068. II. The luminous X-ray source population.
    Smith D.A., Wilson A.S.
   <Astrophys. J., 591, 138-153 (2003)>
   =2003ApJ...591..138S

Provenance

This table was created by the HEASARC in August 2015 based on the CDS Catalog J/ApJ/591/138 file table1.dat.

Parameters

Name
The standard J2000.0 position-based "unregistered" Chandra source designation, e.g., 'CXOU J024224.5+000008', as suggested by the authors in the reference paper. Note that the CDS created an alternative naming scheme for these sources which is described in the Dictionary of Nomenclature of Celestial Objects, viz., '[SW2003b] J024224.5+000008', where the prefix refers to the Smith and Wilson (2003) reference paper.

RA
The Right Ascension of the Chandra X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of 0.01 seconds of time in the original table.

Dec
The Declination of the Chandra X-ray source in the selected equinox. This was given in J2000.0 sexagesimal coordinates to a precision of 0.1 arcseconds in the original table.

LII
The Galactic Longitude of the Chandra X-ray source.

BII
The Galactic Latitude of the Chandra X-ray source.

FB_Counts_Limit
This limit flag parameter is set to '<' if the corresponding parameter value is a 90% upper limit rather than a detection. The counts upper limits are calculated for a confidence level of 90%, using either (a) equation (9) in Kraft, Burrows & Nousek (1991, ApJ, 374, 344) for instances when the total number of counts in the detection region is <=20, or (b) equation (3) in Gehrels (1986, ApJ, 303, 336), when the total number of counts is >20. The mean number of background counts was then subtracted from these calculated upper limits.

FB_Counts
The 0.4-5.0 keV background-subtracted source counts.

FB_Counts_Error
The error in the corresponding background-subtracted source counts. This error is calculated from the relation:

            [sigma_T² + sigma_B² * (A_T / A_B)²]^1/2

(e.g., Bevington & Robinson, 1992, Data Reduction and Error Analysis for the Physical Sciences (2d edition.; New York: McGraw-Hill) p. 41), where sigma_T and sigma_B are the 1-sigma errors on the total and background counts, respectively, derived from equation (7) in Gehrels (1986, ApJ, 303, 336G), and A_T and A_B are the areas of the source and background regions, respectively.

SB_Counts_Limit
This limit flag parameter is set to '<' if the corresponding parameter value is a 90% upper limit rather than a detection. The counts upper limits are calculated for a confidence level of 90%, using either (a) equation (9) in Kraft, Burrows & Nousek (1991, ApJ, 374, 344) for instances when the total number of counts in the detection region is <=20, or (b) equation (3) in Gehrels (1986, ApJ, 303, 336), when the total number of counts is >20. The mean number of background counts was then subtracted from these calculated upper limits.

SB_Counts
The 0.4-1.5 keV background-subtracted source counts.

SB_Counts_Error
The error in the corresponding background-subtracted source counts. This error is calculated from the relation:

            [sigma_T² + sigma_B² * (A_T / A_B)²]^1/2

HB_Counts_Limit
This limit flag parameter is set to '<' if the corresponding parameter value is a 90% upper limit rather than a detection. The counts upper limits are calculated for a confidence level of 90%, using either (a) equation (9) in Kraft, Burrows & Nousek (1991, ApJ, 374, 344) for instances when the total number of counts in the detection region is <=20, or (b) equation (3) in Gehrels (1986, ApJ, 303, 336), when the total number of counts is >20. The mean number of background counts was then subtracted from these calculated upper limits.

HB_Counts
The 1.5-5.0 keV background-subtracted source counts.

HB_Counts_Error
The error in the corresponding background-subtracted source counts. This error is calculated from the relation:

            [sigma_T² + sigma_B² * (A_T / A_B)²]^1/2

Hardness_Ratio_Limit
This limit flag parameter is set to '<' if the corresponding parameter value is an upper limit rather than a detection, or to '>' if the corresponding parameter value is a lower limit rather than a detection.

Hardness_Ratio
The "hardness ratio" of the Chandra source, defined as the ratio of the net counts H in the 1.5-5.0 keV band to the net counts S in the 0.4-1.5 keV band. The HEASARC notes that this parameter is more frequently referred to as the band ratio to distinguish it from the hardness ratio HR commonly defined as (H-S)/(H+S).

Hardness_Ratio_Error
The error in the band ("hardness") ratio of the source.

Log_Lx
The logarithm of the 0.4-5 keV luminosity of the source, in erg s^-1. This has been corrected for Galactic absorption only (3.53 x 10²⁰ cm^-2 according to Dickey & Lockman 1990, ARA&A, 28, 215), and assumes that (i) the source is in NGC 1068 (i.e., it has a distance of 14.4 Mpc) and (ii) the X-ray emission is isotropic.

Contact Person

Questions regarding the NGC1068CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: Friday, 28-Aug-2015 14:41:56 EDT