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M71CXO - M 71 Chandra X-Ray Point Source and Optical/Infrared Counterparts Catalog

HEASARC
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Overview

The authors observed the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources were found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources were found within the half-mass radius rh, reaching to a limiting luminosity of 1.5 x 1030 ergs/s (0.3-8 keV). From a radial distribution analysis, the authors find that 18 +/- 6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11 +/- 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range LX = 1030 - 1031 ergs/s than expected by extrapolating from other studied clusters using either mass or collision frequency. In their paper, the authors explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches.

The authors obtained a 52.4 ks Chandra observation (ObsID 5434) of M71 (nominal center of cluster at J2000.0 RA and Dec of 19 53 46.1 +18 46 42) on 2004 December 20-21 using the Advanced CCD Imaging Spectrometer (ACIS) in very faint (VF), timed-exposure mode with a 3.141s frame time. They searched for X-ray sources in the observed field by employing techniques described in Tennant (2006, AJ, 132, 1372) which use a circular Gaussian approximation to the point-spread function (PSF). Within twice the M71 half-mass radius (rh = 1.65 arcminutes), they set the signal-to-noise threshold (S/N) for detection to 2.0, but also required the number of source counts to be at least 5 times the statistical uncertainty in the local background estimate. The empirical relation derived by Tennant, Cmin = (S/N)2/0.81, then implies a point-source sensitivity limit of about 4.9 counts for rM71 <= 2rh and in the energy band 0.3-8.0 keV. Because of the increase in PSF size with off-axis distance and the associated increase in background within a detection cell, for RM71 > 2rh they set the S/N threshold for detection to 2.4 and again required the number of source counts to be at least 5 times the statistical uncertainty in the local background estimate. The point-source sensitivity limit thus rises to about seven counts.

This table contains 63 X-ray sources and their optical/infrared counterpart information, if any, for those sources with rM71 <= 2r_h which are listed in Table 1 of the reference paper: these sources have name prefixes of s01 to s63. It also contains 73 X-ray sources and their optical/infrared counterpart information, if any, for those sources with rM71 > 2r_h which are listed in Table 2 of the reference paper: these sources have name prefixes of ss01 to ss59 and is01 to is14. Each entry in this table corresponds to an X-ray source if there is no counterpart information or only a single identified counterpart or to a particular X-ray source and counterpart match if there are multiple counterpart identifications. There are thus 165 entries in this HEASARC table corresponding to 136 X-ray sources.


Catalog Bibcode

2008ApJ...687.1019E

References

Chandra X-ray observatory observations of the globular cluster M71.
    Elsner R.F., Heinke C.O., Cohn H.N., Lugger P.M., Maxwell J.E.,
    Stairs I.H., Ransom S.M., Hessels J.W.T., Becker W., Huang R.H.H.,
    Edmonds P.D., Grindlay J.E., Bogdanov S., Ghosh K., Weisskopf M.C.
   <Astrophys. J., 687, 1019-1034 (2008)>
   =2008ApJ...687.1019E

Provenance

This table was created by the HEASARC in March 2011 based on the electronic version of Tables 1 and 2 of the reference paper which were obtained from the CDS (their catalog J/ApJ/687/1019 files table1.dat and table2.dat).

Parameters

Location_Flag
This flag parameter has values which indicate the position of the X-ray source relative to the globular cluster half-mass radius rh, as follows:

    'rh'  = sources with rM71 <=rh (in part 1 of Table 1 of reference paper)
    '2rh' = sources with rh < rM71 <= 2rh
                                       (in part 2 of Table 1 of reference paper)
    't2'  = sources with 2rh <= rM71 (in Table 2 of reference paper)
  

Name
The name of the Chandra ACIS X-ray point source using the convention recommended by the CDS Dictionary of Nomenclature of Celestial Objects and including the source identification listed in Tables 1 and 2 of the reference paper, e.g., '[EHC2008] s01' for the first source listed in Table 1, where the prefix identifies the reference (Elsner, Heinke, Cohn 2008) and an 's' prefix indicates that the source lies within twice the half-mass radius (an 'ss' prefix indicates that the source lies outside twice the half-mass radius and was detected on an ACIS-S chip, and an 'is' prefix indicates that the source lies outside twice the half-mass radius and was detected on an ACIS-I chip).

RA
The Right Ascension of the Chandra ACIS X-ray point source in the selected equinox. This was given in J2000.0 equatorial coordinates to a precision of 0.001 seconds of time in the original reference.

Dec
The Declination of the Chandra ACIS X-ray point source in the selected equinox. This was given in J2000.0 equatorial coordinates to a precision of 0.01 arcseconds in the original reference.

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

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

Alt_Name
An alternative name of the Chandra ACIS X-ray point source (created by the HEASARC) using the standard convention recommended by the CDS Dictionary of Nomenclature of Celestial Objects for such sources, viz., 'CXOU JHHMMSS.s+DDMMSS'.

Detect_Cell_Radius
The radius of the detect cell used for collecting the X-ray counts for the source in question, in arcseconds.

Counts
The number of 0.3 - 8.0 keV source counts in the X-ray source which were collected in the area covered by the detect cell.

Source_Radius
The radius enclosing the true X-ray source position with 68% confidence, in arcseconds. The corresponding radii for inclusion with 95% or 99% confidence can be found by multiplying this value by 1.62 and 2.01, respectively.

CCD_Number
The CCD chip in which the X-ray source appear, 'I2', 'I3', 'S2' 'S3' or 'S4'. Due to dither, sources detected close to chip boundaries may have counts on two CCDs, as indicated by values such as 'S2-S3'.

Radial_Distance
The radial offset rM71 of the X-ray source from the nominal center of the globular cluster (J2000.0 RA and Dec of 19 53 46.1 +18 46 42), in arcminutes. This is only given for sources within 2 rh of the cluster center; thus, if left blank, rM71 > 2 rh.

OIR_CXO_Offset
The radial offset of the candidate optical or infrared counterpart from the X-ray source, in arcseconds. Candidate counterparts were found by searching the HEASARC 2MASS (see http://www.ipac.caltech.edu/2mass/, CDS Cat. II/246), USNO-B1.0 (Monet et al. 2003; CDS Cat. I/284), and Tycho-2 (Hog et al. 2000, CDS Cat. I/259) catalogs, and requiring that the candidate counterpart lie within the 99% confidence radius of the Chandra source. There are 6,414 and 3,376 2MASS sources, 15,324 and 9,196 USNO sources, and 7 and 9 Tycho sources appearing within the S2-S3-S4 and I2-I3 boundaries shown in Figures 3 and 4, respectively, of the reference paper. If this parameter is blank, then no candidate counterpart was found in any of these catalogs. (See Section 6.1 of the reference paper for further details).

Jmag
The J magnitude of the candidate counterpart found in the 2MASS Catalog.

R1_Mag
The R1 magnitude of the candidate counterpart found in the USNO-B1.0 Catalog.

R2_Mag
The R2 magnitude of the candidate counterpart found in the USNO-B1.0 Catalog.

VT_Mag
The VT magnitude of the candidate counterpart found in the Tycho-2 Catalog.

Chance_Probability
The percent probability of a chance coincidence, This is calculated from the relation Pcoinc = N(<m) (pi) r299/Asearch where N(<m) is the number of sources in the corresponding catalog inside the search area, Asearch, with a magnitude m smaller than that of the candidate counterpart, and r99 is the radius enclosing the X-ray source location with 99% confidence. (See Section 6.1 of the reference paper for further details).

Counterpart
This parameter contains information on the candidate counterpart to (e.g., '2MASS' if it is a 2MASS source, or the counterpart name if it is a Tycho or USNO source), or the location of the X-ray source if it is within the core radius of 0.63 arcminutes of the cluster center.


Contact Person

Questions regarding the M71CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 24-Mar-2011