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M81CXO - M 81 Chandra X-Ray Discrete Source Catalog


A Chandra X-Ray Observatory ACIS-S imaging observation is used to study the population of X-ray sources in the nearby (3.6 Mpc) Sab galaxy M 81 (NGC 3031). A total of 177 sources are detected, with 124 located within the D_25 isophote to a limiting X-ray luminosity of ~ 3 x 10^36 erg/s. Source positions, count rates, luminosities in the 0.3 - 8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported, including the low-luminosity active galactic nucleus, SN 1993J, and the Einstein-discovered ultraluminous X-ray source X6.

The primary X-ray data set is a 49926 s observation of M81 obtained on 2000 May 7 with the Chandra Advanced CCD Imaging Spectrometer (ACIS) spectroscopy array operating in imaging mode. The X-ray data were reprocessed by the Chandra X-ray Center (CXC) on 2001 January 4. These reprocessed data were used in this work. There are no significant differences between the reprocessed data and the originally distributed data analyzed by Tennant et al. (2001ApJ...549L..43T). The observation was taken in faint timed exposure mode at 3.241 s/frame at a focal plane temperature of -120 C. Standard CXC processing has applied aspect corrections and compensated for spacecraft dither.

The primary target, SN 1993J, was located near the nominal aimpoint on the back-illuminated (BI) device S3. The nucleus of M81 lies 2.79' from SN 1993J toward the center of S3 in this observation. Accurate positions of these two objects and two G0 stars located on device S2 were used to identify any offset and to determine absolute locations of the remaining Chandra sources as well as objects in other X-ray images and those obtained at other wavelengths. No offset correction was applied to the Chandra X-ray positions.

Catalog Bibcode



Chandra X-ray observations of the spiral galaxy M81.
    Swartz D.A., Ghosh K.K., McCollough M.L., Pannuti T.G., Tennant A.F., Wu K.
   <Astrophys. J. Suppl. Ser. 144, 213 (2003)>


This table was created by the HEASARC in March 2007 based on the CDS table J/ApJS/144/213, files table2.dat and table3.dat.


A running number for the X-ray source in order of increasing J2000.0 Right Ascension.

The name of the X-ray source using the '[SGM2003]' prefix followed by the X-ray source number, as recommended by the CDS Dictionary of Nomenclature of Celestial Objects.

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

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

The Galactic Longitude of the X-ray source.

The Galactic Latitude of the X-ray source.

The aperture-corrected X-ray source count rate in the ~0.2 to 8.0 keV band, in ct/s.

The uncertainty in the X-ray source count rate, in ct/s.

The signal-to-noise ratio of the X-ray source detection.

This parameter is set to '>' to indicate that a counterpart to the X-ray source was not detected in the V band.

The apparent visual magnitude of the counterpart to the X-ray source derived from either Hubble Space Telescope (HST) WFPC2 images or from the Perelmutter & Racine (1995, AJ, 109, 1055) catalog of bright objects. Objects within the 3-sigma uncertainty of the X-ray source positions were considered potential counterparts to the X-ray sources. The HST F555W, F606W, or F547M filter measurements and values have not been corrected for charge transfer efficiency (CTE) effects nor color-corrected to obtain true Johnson V magnitudes.

This flag parameter has the following possible values:

   H = located within HST field but without an optical candidate,
       implying fainter than the HST limiting magnitude of ~27 magnitudes.
   I = only F814W (I-band) image available

The Chandra ACIS CCD chip on which the X-ray source was detected.

The M 81 global environment of the X-ray source position, coded thus: "b" denotes bulge, "d" denotes disk, and "D_25_" denotes that the source is outside the D_25 isophote.

The unabsorbed luminosity L_X of the source in the 0.3 - 8.0 keV energy range, in erg/s. For 33 bright sources, this was obtained from the best-fit spectral models as described below. The luminosities for the remaining sources were estimated assuming an absorbed power-law spectrum with photon index Gamma = 1.5 and a hydrogen column density N_H = 11.0 x 10^20 cm^-2. These are the average values for the 31 of 33 bright sources whose spectra were best fitted with a power-law model. The source detection limit of 12 counts corresponds to an observed flux of F(0.3-8 keV) = 1.9 x 10^-15 ergs cm^-2 s^-1 for sources on the s3 chip (F(0.3-8 keV) = 2.5 x 10^-15 ergs cm^-2 s^-1 on the FI chips) or an unabsorbed 0.3 - 8 keV luminosity of L_X = 3.4(4.5) x 10^36 erg s^-1 for sources on the BI (FI) chips.

The uncertainty in the X-ray luminosity, in erg/s. The quoted errors are 90% confidence limits for a single interesting parameter based on the chi-squared fit statistic.

Additional comments on the source, such as corresponding X-ray detections from Einstein (denoted "X", Fabbiano, 1988ApJ...325..544F, sources named '[F88] X-NN' in Simbad) and ROSAT ("P" and "H" denoting PSPC- and HRI-identified sources, respectively, Immler & Wang, 2001, CDS Cat. <J/ApJ/554/202>, sources named '[IW2001] PNN' and '[IW2001] HNN' in Simbad), and potential source types based on spatial correlations with catalogued objects or on other information (SSS means in Swartz et al., 2002ApJ...574..382S). Explanations of the quantitative entries are given in the text of the reference paper.

The best-fit absorption equivalent Hydrogen column density, N_H, in H atoms cm^-2, for the X-ray source. The absorption column densities were constrained in the fitting procedure to be at least as large as the Galactic column density along the line of sight to M 81. This resulted in a best-fit value of N_H equal to this lower limit for eight sources. Therefore, the column densities are listed as NH = (4.0 +/- 0.0) x 10^20 cm^-2 for these sources.

The RMS uncertainty in the best-fit column density, in cm^-2. The quoted errors are 90% confidence limits for a single interesting parameter based on the chi-squared fit statistic.

The best-fit power-law photon index, Gamma, of the X-ray source spectrum, except for source numbers 160 and 161, where the listed spectral parameter is the blackbody temperature, kT, in keV. Statistically constrained model fits could be achieved for a total of 39 sources in the M 81 field. Detailed X-ray properties of SN 1993J, Einstein source X6, and the nucleus are presented in Section 4 of the reference paper and of the three brightest supersoft sources in Swartz et al. (2002, ApJ, 574, 382). The best-fit spectral model parameters for the remaining 33 bright sources are presented here. The power-law model provided a significantly better fit to all but the two X-ray sources listed above.

The RMS uncertainty in the value of the spectral parameter. The quoted errors are 90% confidence limits for a single interesting parameter based on the chi-squared fit statistic.

The chi-squared value of the best spectral fit to the X-ray source spectrum.

The number of degrees of Freedom of the spectral fit.

The Kolmogorov-Smirnov statistic, P_KS, used to test the X-ray sources (including background) for constant count rates over the duration of the primary observation, in percent. Sources with a low value of P_KS have a high probability of being variable.

Contact Person

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