NUGALCEN - NuSTAR Hard X-Ray Survey of the Galactic Center

Overview

This table contains some of the the first survey results of hard X-ray point sources in the Galactic Center (GC) region obtained by NuSTAR. The authors have discovered 70 hard (3-79 keV) X-ray point sources in a 0.6 deg² region around Sgr A* with a total exposure of 1.7 Ms, and 7 sources in the Sgr B2 field with 300 ks. They identify clear Chandra counterparts for 58 NuSTAR sources and assign candidate counterparts for the remaining 19. The NuSTAR survey reaches X-ray luminosities of ~4 x 10³² and ~8 x 10³² erg/s at the GC (8 kpc) in the 3-10 and 10-40 keV bands, respectively. The source list includes three persistent luminous X-ray binaries (XBs) and the likely run-away pulsar called the Cannonball. New source-detection significance maps reveal a cluster of hard (>10 keV) X-ray sources near the Sgr A diffuse complex with no clear soft X-ray counterparts. The severe extinction observed in the Chandra spectra indicates that all the NuSTAR sources are in the central bulge or are of extragalactic origin. Spectral analysis of relatively bright NuSTAR sources suggests that magnetic cataclysmic variables constitute a large fraction (>40% - 60%). Both spectral analysis and logN - logS distributions of the NuSTAR sources indicate that the X-ray spectra of the NuSTAR sources should have kT > 20 keV on average for a single-temperature thermal plasma model or an average photon index of Gamma = 1.5 - 2 for a power-law model. These findings suggest that the GC X-ray source population may contain a larger fraction of XBs with high plasma temperatures than the field population.

The observations of the GC region with NuSTAR began in 2012 July, shortly after its launch. The original survey strategy for the GC region was to match the central 2 degree x 0.7 degree region covered by the Chandra X-ray Observatory (Wang et al. 2002, Nature, 415, 148; Muno et al. 2009, ApJS, 181, 110). The field of views (FOVs) of neighboring NuSTAR observations in the survey were designed to overlap with each other by ~40%. Multiple observations of the same region with relatively large FOV offsets tend to average out the vignetting effects of each observation, enabling a more uniform coverage of the region. Multiple observations are also suitable for monitoring long term X-ray variability of sources in the region. Even when observing a single target, the NuSTAR observation is often broken up into two or more segments with relatively large pointing offsets to allow an efficient subtraction of a detector coordinate-dependent background component (e.g., Mori et al. 2013, ApJ, 770, L23).

Catalog Bibcode

2016ApJ...825..132H

References

NuSTAR hard X-ray survey of the Galactic Center region.
II. X-ray point sources.
    Hong J., Mori K., Hailey C.J., Nynka M., Zhang S., Gotthelf E.,
    Fornasini F.M., Krivonos R., Bauer F., Perez K., Tomsick J.A., Bodaghee A.,
    Chiu J.-L., Clavel M., Stern D., Grindlay J.E., Alexander D.M., Aramaki T.,
    Baganoff F.K., Barret D., Barriere N., Boggs S.E., Canipe A.M.,
    Christensen F.E., Craig W.W., Desai M.A., Forster K., Giommi P.,
    Grefenstette B.W., Harrison F.A., Hong D., Hornstrup A., Kitaguchi T.,
    Koglin J.E., Madsen K.K., Mao P.H., Miyasaka H., Perri M., Pivovaroff M.J.,
    Puccetti S., Rana V., Westergaard N.J., Zhang W.W., Zoglauer A.
   <Astrophys. J., 825, 132-132 (2016)>
   =2016ApJ...825..132H    (SIMBAD/NED BibCode)

Provenance

This table was created by the HEASARC in March 2018, based on CDS Catalog J/ApJ/825/132 files table2.dat, table3.dat, table4.dat and table5.dat.

Parameters

Field_Name
This HEASARC-created parameter Specifies the field in which the NuSTAR source was detected. For sources in the Galactic Center Region it is set to 'GC', whereas for sources in the Sgr B2 region it is set to 'B2'.

Association_Flag
This parameter, referred to as the group number in the reference paper, specifies the confidence level of the association of a Chandra source with the NuSTAR source, as follows:

  Value Significance of Association

  1 = Clear association. The sources in group 1 have a relatively clear Chandra
      counterpart which is usually the only bright
      (F_X > 3 x 10^-6 ph/cm²/s¹) Chandra source around the NuSTAR
      detection;
  2 = Unclear association. The sources in group 2 have solid NuSTAR detections
      (except for NGP 65, which is a bit marginal), but their association with
      the Chandra sources is not as clear either because multiple Chandra
      sources of similar fluxes are found within the uncertainty of the NuSTAR
      positions (e.g., NGPs 55 and 56) or because a diffuse origin of the hard
      X-ray emission cannot be ruled out (e.g., NGPs 53 and 59,
      see Section 6.1 of the reference paper).

Source_Number
The NuSTAR Galactic Center Point (NGP) source identification number for each source in the catalog. In the reference paper, sources are identified as, for example, NGP 55 for source number 55.

Name
This HEASARC-created parameter gives the source designation in the style recommended by the Dictionary of Nomenclature of Celestial Objects, using the prefix '[HMH2016]' for Hong, Mori, Hailey 2016 and the source number value, e.g., '[HMH2016] 55'.

RA
The Right Ascension of the NuSTAR X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10^-5 degrees in the original table. The local peak location of the NuSTAR trial map within 30" of the Chandra position. They are weighted average values among the trial maps with detections. For sources with bright neighbors, the authors limit the search radius to 10" or 15" depending on the proximity. The peak position is determined by a 2D Gaussian fit on the trial map.

Dec
The Declination of the NuSTAR X-ray source in the selected equinox. This was given in J2000.0 decimal degrees to a precision of 10^-5 degrees in the original table. The local peak location of the NuSTAR trial map within 30" of the Chandra position. They are weighted average values among the trial maps with detections. For sources with bright neighbors, the authors limit the search radius to 10" or 15" depending on the proximity. The peak position is determined by a 2D Gaussian fit on the trial map.

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

BII
The Galactic Latitude of the NuSTAR X-ray source. chandra_source_number =. The candidate Chandra counterpart source number given in Muno et al. (2009, ApJS, 181, 110, available at the HEASARC as the GALCENCXO table).

Chandra_Source_Number
Chandra Counterpart Identifier by Muno et al. 2009 (J/ApJS/181/110)

Chandra_Name
The candidate Chandra counterpart designation using the style recommended by the Dictionary of Nomenclature of Celestial Objects, i.e., the prefix 'CXOGC' and the J2000.0 Right Ascension and Declination truncated to 0.1 seconds of time in RA and to 1 arcsecond in Declination, e.g., 'CXOGC JHHMMSS.s+DDMMSS'.

Chandra_Photon_Flux
The observed (i.e., absorbed) 2-8 keV flux of the Chandra counterpart (from Muno et al. 2009, ApJS, 181, 110, available at the HEASARC as the GALCENCXO table), in photon cm^-2 s^-1. X-ray photon fluxes and luminosities quoted in this paper are all absorbed quantities using the assumed or estimated N_H values unless otherwise noted.

Offset
The angular offset between the NuSTAR source and the Chandra counterpart positions, in arcseconds.

Exposure
The combined exposure of the two NuSTAR FPMs at the Chandra source position, in seconds (converted by the HEASARC from the ks units used in the original table).

Detect_Bands
This parameter indicates whether the source was detected in the soft (< 10 keV) band only if set to 'S', the hard (> 10 keV) band only if set to 'H', or both bands if set to 'SH'.

Trial_Map_Value
The trial map value for the NuSTAR detection at the Chandra source position: see Section 3 of the reference paper for more details.

Trial_Map_Band_Lower
The lower limit of the energy band of the trial map with the local peak value, in keV.

Trial_Map_Band_Upper
The upper limit of the energy band of the trial map with the local peak value, in keV.

Trial_Map_PSF_Fraction
The source cell size of the trial map with the local peak value, in terms of the percentage of the NuSTAR PSF enclosed in the cell.

Num_Trial_Map_Detects
The number of trial maps above their respective thresholds at the Chandra position.

Comments
This comment field can contain a previously known name for the source, the name of a nearby Chandra source, and/or the presence of any notable diffuse feature that is nearby.

Counts
The net counts in NuSTAR source in the 3-40 keV band.

Counts_Error
The uncertainty in the net counts in NuSTAR source in the 3-40 keV band. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

Hardness_Ratio
The mode of the posterior distribution of the Bayesian Enhanced X-Ray Hardness Ratio (BEHR: Park et al. 2006, ApJ, 652, 610, see Section 4.3), (H - S)/(H + S) where H and S are the net counts in 3-10 and 10-40 keV energy bands, respectively.

Hardness_Ratio_Error
The uncertainty in the mode of the posterior distribution of the Bayesian Enhanced X-Ray Hardness Ratio (BEHR). The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

Hardness_Ratio_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

Median_Energy
The median energy of the counts comprising the NuSTAR source in the 3-40 keV energy band, in keV.

Median_Energy_Error
The uncertainty in the median energy of the counts comprising the NuSTAR source in the 3-40 keV energy band, in keV. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

Median_Energy_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

Quartile_Ratio
The quartile ratio. This is the relative ratio of the 25% and 75% quartiles, 3(E₂₅ - 3 keV)/(E₇₅ - 3 keV), for the NuSTAR source, equivalent to the y-axis value in the NuSTAR quantile diagram (Hong et al. 2004, ApJ, 614, 508, see Section 4.3).

Quartile_Ratio_Error
The uncertainty in the quartile ratio. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

Quartile_Ratio_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

NH
The estimated Hydrogen column density along the line of sight to the source (Nishiyama et al. 2008, ApJ, 680, 1174), in atoms cm^-2.

Spectral_Index
The estimated photon index of the NuSTAR source using the median energy for an absorbed power-law model fit to its spectrum with N_H set to 6 x 10^22/cm2 (see Section 4.3 of the reference paper).

Spectral_Index_Error
The uncertainty in the estimated photon index of the NuSTAR source. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

Spectral_Index_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. In calculating the photon indices, the systematic error is estimated based on the selection of spectral model parameters (see Section 4.4 of the reference paper).

SB_Photon_Flux
The observed 3-10 keV photon flux of the NuSTAR source, in photon cm^-2 s^-1.

SB_Photon_Flux_Error
The uncertainty in the observed 3-10 keV photon flux of the NuSTAR source, in photon cm^-2 s^-1. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

SB_Photon_Flux_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

HB_Photon_Flux
The observed 10-40 keV photon flux of the NuSTAR source, in photon cm^-2 s^-1.

HB_Photon_Flux_Error
The uncertainty in the observed 10-40 keV photon flux of the NuSTAR source, in photon cm^-2 s^-1. The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

HB_Photon_Flux_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

SB_Lx
The observed 3-10 keV luminosity of the NuSTAR source, assumed to be at a distance of 8 kpc, in erg s^-1 (see Section 4.4 of the reference paper).

SB_Lx_Error
The uncertainty in the observed 3-10 keV luminosity of the NuSTAR source, assumed to be at a distance of 8 kpc, in erg s^-1 (see Section 4.4 of the reference paper). The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets.

SB_Lx_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error. This systematic error is given by the difference in the photometry results between two aperture sets.

HB_Lx
The observed 10-40 keV luminosity of the NuSTAR source, assumed to be at a distance of 8 kpc, in erg s^-1 (see Section 4.4 of the reference paper).

HB_Lx_Error
The uncertainty in the observed 10-40 keV luminosity of the NuSTAR source, assumed to be at a distance of 8 kpc, in erg s^-1 (see Section 4.4 of the reference paper). The errors quoted herein are the largest of three estimates: a statistical error and two different estimates of systematic errors. The statistical error is estimated from the uncertainty of the observed net counts after background subtraction. The systematic error is given by the difference in the photometry results between two aperture sets

HB_Lx_Flag
This flag parameter is set to 'S' to indicate that the corresponding error value that is quoted is the systematic error This systematic error is given by the difference in the photometry results between two aperture sets.

Aperture_Id_1
The source and background aperture radii code, for the first aperture set. The authors performed aperture photometry using two aperture sets for each source: the first set to provide the basic photometry results, and the second set to estimate the systematic errors originating from the aperture selection (Section 4.1 of the reference paper). The two baseline choices are (e) and (f). The source and background aperture radii that were used areas follows:

    Value Ap(1)/Ap(2)

      a = 20"/35"-42";
      b = 20"/30"-46";
      c = 20"/45"-75",
      d = 30"/45"-45";
      e = 30"/50"-80";
      f = 40"/60"-90";
      g = 8"/130"-145";
      h = 70"/145"-145";
      i = 70"/210"-230";
      j = 100"/210"-230".

Aperture_Id_1_Flag
This flag parameter is set 'a' to indicate that the specified aperture has additional exclusion zones (see Section 4.1 of the reference paper).

Aperture_Id_2
The source and background aperture radii code, for the second aperture set. The authors performed aperture photometry using two aperture sets for each source: the first set to provide the basic photometry results, and the second set to estimate the systematic errors originating from the aperture selection (Section 4.1 of the reference paper). The two baseline choices are (e) and (f). The source and background aperture radii that were used areas follows:

    Value Ap(1)/Ap(2)

      a = 20"/35"-42";
      b = 20"/30"-46";
      c = 20"/45"-75",
      d = 30"/45"-45";
      e = 30"/50"-80";
      f = 40"/60"-90";
      g = 8"/130"-145";
      h = 70"/145"-145";
      i = 70"/210"-230";
      j = 100"/210"-230".

Aperture_Id_2_Flag
This flag parameter is set 'a' to indicate that the specified aperture has additional exclusion zones (see Section 4.1 of the reference paper).

NuSTAR_Flags
The NuSTAR flags for the source, coded as follows:

   f = Sources showing the iron lines (Section 4.5);
   k = Sources with short-term variability according to a Kolmogorov-Smirnov (KS)
         test of individual observations (Section 4.6);
   r = Sources with long-term variability according to the maximum-to-minimum
         flux ratio of multiple observations (Section 4.6);
   c = The Chandra source flags (Muno et al. 2009, ApJS, 181, 110): sources
         confused with another nearby source;
   g = Sources that fell near the edge of a detector in one or more observations;
   b = Sources for which the source and background spectra have a >10% chance of
         being drawn from the same distribution according to the KS tests;
   s = Sources variable on short timescales, as indicated by probabilities of
         <1% that the event arrival times for at least one observation were
         consistent with a uniform distribution according to the KS test;
   l = Sources that were variable on long timescales, as indicated by a
         probability of <1% that the fluxes for all observations were consistent
         with a uniform distribution according to the KS test;
   t = Transients identified in Degenaar et al. (2012, A&A, 545, A49);
   p = Not explained in this paper.

Contact Person

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Last Modified: Monday, 16-Sep-2024 17:32:23 EDT