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NGC6791CXO - NGC 6791 Chandra X-Ray Point Source Catalog

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Overview

This table contains some of the results from the first X-ray study of NGC 6791, one of the oldest open clusters known (8 Gyr). This Chandra observation was aimed at uncovering the population of close interacting binaries down to an X-ray luminosity (LX) of ~1 x 1030 erg/s (0.3-7 keV). The authors detect 86 sources within 8 arcminutes of the cluster center, including 59 inside the half-mass radius of 4.42 arcminutes. centered on 19h 20m 53s, +37o 46' 18" (J2000.0). They identify 20 sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy, the authors confirm the nature of one CV. They also discover one new, X-ray variable candidate CV with Balmer and He II emission lines in its optical spectrum; this is the first X-ray-selected CV in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC 6791 are primordial. The authors compare the X-ray properties of NGC 6791 with those of a few old open clusters (NGC 6819, M67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the number of ABs brighter than 1 x 1030 erg/s normalized by cluster mass is lower in NGC 6791 than in M 67 by a factor ~3-7. CVs, ABs, and sub-subgiants brighter than 1 x 1030 erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e., X-ray-emitting) binaries.

The authors observed NGC 6791 with the Advanced CCD Imaging Spectrometer (ACIS) on Chandra from 2004 July 1 20:51 UTC until July 2 10:49 UTC for a total exposure time of 48.2ks (ObsID 4510). They obtained low-resolution spectra of candidate optical counterparts to guide the classification of the X-ray sources. A total of 16 candidate counterparts brighter than V ~18.3 were observed with the FAST long-slit spectrograph on the 1.5m Tillinghast telescope on Mt. Hopkins on nine nights between 2005 June 7 to September 2 (coverage from 3480 to 7400 Angstrom and a 3 Angstrom resolution). Candidate optical counterparts fainter than V ~17 were observed with the fiber-fed multi-object spectrograph Hectospec on the 6.5m Multi-Mirror Telescope. A total of 16 candidate counterparts were observed on the nights of 2005 May 13 and July 4-6 (spectra that cover 3700 to 9150 Angstrom with a 6-Angstrom resolution).

The authors performed source detections in broad (0.3-7.0 keV), soft (0.3-2.0 keV) and hard (2.0-7.0 keV) energy bands, also used in their Chandra study of M 67 (van den Berg et al. 2004, A&A, 418. 509), so as to facilitate comparison. The CIAO detection routine wavdetect was run for scales of 1.0 to 11.3 pixels, in steps increasing by a factor of sqrt(2), with the larger scales appropriate for large off-axis angles where the point-spread function (PSF) becomes significantly broader. The authors computed exposure maps for the response at 1 keV to account for spatial variations of the sensitivity. The wavdetect detection threshold was set to 10-6, from which the authors expect two spurious detections per detection scale (so 16 spurious detections in total) in the area that they consider here. Combination of the broad, soft, and hard-band source lists results in a master catalog of 86 distinct sources within 8 arcmin of the cluster center, of which 59 lie inside the half-mass radius rh. To investigate the validity of the sources, the authors also ran wavdetect with a threshold of 10-7 or an expected number of spurious sources of 1.6. The 14 sources not detected in this run are marked with a value of the source_flag parameter of 'T' in this table (replacing the '*' symbol used in the original table).


Catalog Bibcode

2013ApJ...770...98V

References

A Chandra X-ray study of the interacting binaries in the old open cluster
NGC 6791.
    van den Berg M., Verbunt F., Tagliaferri G., Belloni T., Bedin L.R.,
    Platais I.
   <Astrophys. J., 770, 98 (2013)>
   =2013ApJ...770...98V

Provenance

This table was created by the HEASARC in August 2015 based on the CDS Catalog J/ApJ/770/98 files table1.dat and table2.dat.

Parameters

Inside_HMR_Flag
This flag parameter is set to 'Y' if the source is inside the half-mass radius (4.42 arcminutes) of the cluster center (19h 20m 53s, +37o 46' 18" (RA, Dec J2000.0), or to 'N' if the source lies outside the half-mass radius but is still less than 8 arcmin away from the cluster center.

Source_Number
A running source number, in order of decreasing counts for those sources within the half-mass radius (1-59) and then for those outside the half-mass radius (60-86), which uniquely identifies the X-ray source. In the reference paper, the authors use the prefix CX together with the source number as a short-hand way of referring to the sources, e.g., 'CX 29'.

Source_Flag
This flag parameter is set to 'T' of the source detected by wavdetect using a threshold of 10-6, but was not detected by wavdetect for a threshold of 10-7.

Name
The J2000.0 position-based "unregistered" Chandra source designation as approved by the Dictionary of Nomenclature of Celestial Objects, e.g., 'CXOU J192013.8+374606'.

RA
The Right Ascension of the Chandra X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the original table, and is the position obtained after applying the boresight correction of DeltaRA = -0.06" +/- 0.06", DeltaDec = -0.21" +/- 0.04" (Chandra minus optical).

Dec
The Declination of the Chandra X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10-6 degrees in the original table, and is the position obtained after applying the boresight correction of DeltaRA = -0.06" +/- 0.06", DeltaDec = -0.21" +/- 0.04" (Chandra minus optical).

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

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

Error_Radius
The 95% uncertainty radius in the source position, r95, in arcseconds. To compute positional uncertainties, the authors adopted Equation (5) from Hong et al. (2005, ApJ, 635, 907). This formula relates the 95% confidence radius on the source position r95 to the wavdetect counts and the offset angle from the aimpoint, and is based on extensive simulated detections of artificial sources.

Radial_Distance
The angular offset of the Chandra source from the cluster center, in arcminutes.

Counts
The net 0.3-7 keV band source counts. The authors used the ACIS Extract package (version 3.107.2; Broos et al. 2010, ApJ, 714, 1582) to measure the net source counts. Events between 0.3 and 7.0 keV were extracted from a region corresponding to 90% of the PSF at 1.5 keV; for a few sources, this region was reduced to avoid contamination by a close neighbor. The background was determined from a source-free annulus centered on the source position.

Counts_Error
The uncertainty in the net 0.3-7 keV band source counts.

Flux_Flag
This flag parameter has the following possible non-blank values to indicate unreliable flux values:

      b = A source is detected but its proximity to the CCD edge could make the
          estimates for net counts and flux unreliable.
      c = Sources 67 and 71 are separated by 2.4". Counts were extracted from a
          region enclosing 39% of the PSF at 1.5 keV (compared to a PSF fraction
          of 90% for the other sources) to minimize contamination.
  

Flux
The unabsorbed (absorption-corrected) 0.3-7 keV band source flux, FX,u, in erg cm-2 s-1. The authors convert net count rates to fluxes within Sherpa with arf and rmf response files appropriate for the chip location and source-extraction area of each source. The fluxes FX,u are computed under the assumption that the underlying spectrum is an optically thin plasma (described by the xsmekal model) with kT = 2 keV. For source number 29, which lies near the aimpoint, 1 count s-1 corresponds to 6.8 x 10-12 erg cm-2 s-1. This temperature is appropriate for the most active ABs in the cluster (see, e.g., van den Berg et al. 2004, A&A, 418, 509), while it is too high for the least active ABs and too low for most CVs. For a 1 keV Mekal model, a power-law spectrum (powlaw1d) model with photon index of 2,and a 10 keV thermal-bremsstrahlung model (xsbremss), the conversion factor is 24% smaller, and 30% and 42% larger, respectively. In each case, the authors account for a column density equal to the cluster value (7.8 x 1020 cm-2) using the xsphabs model. For the adopted distance to NGC 6791 of 4.1 kpc, a 3-count detection limit corresponds to a limiting unabsorbed X-ray luminosity of (0.7-1.4) x 1030 erg s-1 (0.3-7 keV), where the range corresponds to the choice of models specified above.

Median_Energy
The median energy in the 0.3-7 keV band, in keV. This is given only for sources with more than 5 counts.

Median_Energy_Error
The uncertainty in the median energy in the 0.3-7 keV band, in keV.

Ctrpart_Flag
This flag parameter is set to 'Y' if the X-ray source has a detected candidate optical counterpart, else is set to 'N'. The authors looked for optical counterparts in the deep BVI photometric catalog of NGC 6791 compiled by Stetson et al. (2003, PASP, 115, 413), which covers the entire area studied here. The limiting magnitude is V ~ 24 for the central area but the sensitivity is lower for the outer regions. The authors looked for matches to X-ray sources with more than 10 net counts and a positional uncertainty r95 <= 1 arcsecond. For each source, the match radius is set to be the quadratic sum of the error in its X-ray position, and the typical 1-sigma error in the optical positions (0.27 arcseconds) scaled to a 95% error radius assuming a two=dimensional Gaussian error distribution.

Membership_Status
This parameter contains a code summarizing the cluster membership status of the source counterpart:

    pm = Proper-motion cluster member;
    ?  = Uncertain or no membership information;
    n? = Likely not associated with the cluster.
  

Second_Ctrpart_Flag
This flag parameter is set to 'a' to indicate that the X-ray source has another candidate optical counterpart listed in Table 3 of the reference paper, and reproduced below (where dOX is the offset between the X-ray source and second candidate optical counterpart positions, and pmu is the membership probability of the optical source based on its proper motion):

  Table 3 (alternative optical counterparts):

    Src   OID     dOX     Vmag    B-V     V-I     Comment
    Num         (arcsec)

    21    11098   0.78    18.48   0.93    0.97    pmu = 90
    27    11365   0.68    16.58   0.76    0.94    pmu = 0, F star
    34            0.68                            Extended
    57    7395    0.76    21.02   1.47    1.96    Non-member, K star
    78    569     3.12    16.50   0.95    1.03    pmu = 0
    82    13395   0.70    14.66   0.96    0.98
    84    15442   3.01    20.64           1.7
  

Stetson_Source_Number
The source number of the candidate optical counterpart in the S03 Catalog. S03 refers to the Stetson et al. (2003, PASP, 115, 413) source list: sources therein are referred to as 'Cl* NGC 6791 SBG NNNNN' in Simbad. Optical sources without a Stetson number were found in the HST/ACS images or (for source number 55 only) by visual inspection of the S03 cluster image.

CXO_Opt_Offset
The angular separation between the X-ray and optical source positions after the Chandra boresight correction had been applied, in arcseconds.

PM_Membership_Prob
The percentage probability of cluster membership for the optical counterpart based on the proper-motion studies of Platais et al. (2011, ApJ, 733, L1) (the case of source number 3 is described in more detail in Section 4.2 of the reference paper) and of K. Cudworth et al. (private communication).

HST_Membership_Prob
This parameter contains a code for the probable membership of the optical counterpart in the cluster based on the HST/ACS study described in Bedin et al. (2006, A&A, 460, L27), as follows:

        m = Member;
       nm = Non-Member.
  

Vmag
The V-band magnitude of the candidate optical counterpart to the Chandra X-ray source. The optical magnitudes and colors are from Stetson et al. (2003, PASP, 115, 413), except for the (unnumbered) HST counterparts where the F606W magnitudes and the F606W-F814W colors are listed, if available.

Vmag_Flag
This flag parameter has the following possible non-blank values to indicate the following qualifications to the listed V magnitude:

     : = The value is considered uncertain;
     b = The counterpart is not included in the S03 (Stetson et al. 2003,
         PASP, 115, 413) catalog. The listed value is the magnitude in the
         g' band (Platais et al. 2011, ApJ, 733, L1).
  

BV_Color
The B-V color index of the candidate optical counterpart to the Chandra X-ray source. The optical magnitudes and colors are from Stetson et al. (2003, PASP, 115, 413), except for the (unnumbered) HST counterparts where the F606W magnitudes and the F606W-F814W colors are listed, if available.

BV_Color_Flag
This flag parameter is set to ':' to indicate that the corresponding parameter value is considered by the authors to be uncertain.

VI_Color
The V-I color index of the candidate optical counterpart to the Chandra X-ray source. The optical magnitudes and colors are from Stetson et al. (2003, PASP, 115, 413), except for the (unnumbered) HST counterparts where the F606W magnitudes and the F606W-F814W colors are listed, if available.

VI_Color_Flag
This flag parameter is set to ':' to indicate that the corresponding parameter value is considered by the authors to be uncertain.

Lx
The unabsorbed (absorption-corrected) 0.3-7 keV band source luminosity, in erg s-1, calculated from the derived X-ray flux (q.v.) and an assumed source distance of 4.1 kpc corresponding to the distance of the cluster.

Log_Fx_Over_Fv_Limit
This limit flag parameter is set to '>' if the corresponding parameter value is a lower limit rather than an actual measured value.

Log_Fx_Over_Fv
The logarithm of the ratio of the X-ray (0.3-7 keV) to the V-band fluxes of the source after correcting for extinction.

Log_Fx_Over_Fv_Flag
This flag parameter is set to ':' to indicate that the corresponding parameter value is considered by the authors to be uncertain.

Log_Fx_Over_Fv_Error
The uncertainty in the logarithm of the extinction-corrected ratio of the X-ray (0.3-7 keV) to the V-band fluxes of the source.

Opt_Var_Name
The variable star designation of the associated optical counterpart, if applicable. Information on optical variability was gathered from the following references:

    * Kaluzny & Rucinski (1993, MNRAS, 265, 34);
    * Rucinski et al. (1996, MNRAS, 282, 705);
    * Mochejska et al. (2002, AJ, 123, 3460); (2003, AJ, 125, 3175);
      (2005, AJ, 129, 2856);
    * Bruntt et al. (2003, A&A, 410, 323);
    * Hartman et al. (2005, AJ, 130, 2241);
    * de Marchi et al. 2007, A&A, 471, 515);
    * Brogaard et al. (2011, A&A, 525, A2).
  

Var_Period
The period of the variability of the associated optical counterpart, in days.

Var_Period_Flag
This flag parameter is set to 's' to indicate that for this object (source number 23) there is another possible period value of 23.947 days.

Opt_Spectrum_Notes
The spectroscopic properties of the candidate optical counterpart to the Chandra X-ray source.

Source_Type
The authors' source classification, as follows:

      CV  = cataclysmic variable;
      CV? = candidate cataclysmic variable;
      AB  = likely active binary;
      AB? = candidate active binary;
      SSG = sub-subgiant;
      RG  = red giant;
      S   = likely foreground star not associated with the cluster without any
            indication of binarity;
      EG  = extra-galactic source;
      FB  = Faint object to the blue of the main sequence.
  

Comments
Additional comments on the object. Information on optical variability was gathered from the following references:

    * Kaluzny & Rucinski (1993, MNRAS, 265, 34);
    * Rucinski et al. (1996, MNRAS, 282, 705);
    * Mochejska et al. (2002, AJ, 123, 3460); (2003, AJ, 125, 3175);
      (2005, AJ, 129, 2856);
    * Bruntt et al. (2003, A&A, 410, 323);
    * Hartman et al. (2005, AJ, 130, 2241);
    * de Marchi et al. 2007, A&A, 471, 515);
    * Brogaard et al. (2011, A&A, 525, A2).
  

Class
The HEASARC Browse object classification based on the source_type value.


Contact Person

Questions regarding the NGC6791CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 9-Sep-2015