Visualizing Multiwavelength Properties of Classified X-Ray Sources from Chandra Source Catalog

X-ray sources can be classified in a number of different ways. For the brightest sources, one would typically study the X-ray spectrum (e.g., identifying Fe lines, fitting a set of models) and variability properties (e.g., flares, periodicity), in combination with any multiwavelength (MW) counterpart properties. Such comprehensive investigations become unfeasible for the much more numerous population of faint sources lacking high-quality X-ray spectra and light curves. In such cases X-ray properties are less certain and less informative while the MW counterpart properties (e.g., optical colors, X-ray to optical flux ratios) must be more heavily relied upon to classify a given source (see e.g., Laycock et al. 2005; Lin et al. 2012; Rivera Sandoval et al. 2018; Tomsick et al. 2020). Collecting the MW properties and being able to visualize them is an important first step.

The Chandra Source Catalog 2.0 (Evans et al. 2020; hereafter CSC 2.0) contains detailed information about positional, photometric, spectral, and temporal properties for 317,167 unique point and extended X-ray sources. CSC 2.0 is the best catalog for collecting MW properties of X-ray sources located in the Galactic plane due to the subarcsecond positional uncertainties ⁴ (PUs) which result in a much lower chance of confusion when cross-matching.

We compiled X-ray sources belonging to 9 broad astrophysical classes: active galactic nuclei (AGNs), pulsars and isolated neutron stars (NS), non-accreting X-ray binaries (NS BIN), ⁵ cataclysmic variables (CV), high mass X-ray binaries (HMXB), low-mass X-ray binaries (LMXB), high mass stars (HM-STAR), ⁶ low mass stars (LM-STAR) and young stellar objects (YSO). The classifications were based on the following sources: Veron Catalog of Quasars & AGN 13th Edition (Véron-Cetty & Véron 2010), the ATNF Pulsar Catalog (Manchester et al. 2005), the CV Catalog 2006 Edition (Downes et al. 2001), the HMXB Catalog in the Galaxy 4th Edition (Liu et al. 2006), the LMXB Catalog 4th Edition (Liu et al. 2007), the Catalogue of Stellar Spectral Classifications (Skiff 2014), the VIIth Catalog of Galactic WR Stars (van der Hucht 2001), the YSO catalogs from multiple molecular clouds and open clusters (Ozawa et al. 2005; Giardino et al. 2007; Delgado et al. 2011; Povich et al. 2011; Rebull et al. 2011; Megeath et al. 2012).

We cross-matched sources from CSC 2.0 with these catalogs using error circles with the radius equal to the semimajor axis of the 2σ error ellipse in CSC 2.0. The X-ray sources were then cross-matched, using X-ray error circles, to the optical Gaia eDR3 catalog (Gaia Collaboration et al. 2020), the near-infrared Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006), the CatWISE2020 catalog (Marocco et al. 2021), the unWISE catalog (Schlafly et al. 2019), and the AllWISE catalog (Cutri et al. 2021) in the infrared. In our cross-matching, several criteria were applied to filter out unreliable sources, including some crowded environments such as globular clusters and the Galactic center as well as sources strongly affected by complex extended emission around them (e.g., bright pulsar wind nebulae), sources with large X-ray PUs, X-ray sources having NaNs and/or zero fluxes in multiple X-ray bands or having true sat_src_flag and/or streak_src_flag in CSC 2.0, YSOs or STARs with no matched MW counterparts (possibly due to large proper motions), and MW counterparts matched with isolated NSs (by chance coincidence). As a result, we are left with 2,687 X-ray sources. The number of sources for each source class is given at the top of Figure 1.

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The visualization tool allows for plotting of various permutations of two of the following features:

• 1.  
  master-level X-ray properties ⁷ from CSC 2.0 including the energy fluxes in the broad (${F}_{{\rm{b}}};$ 0.5–7 keV), hard (${F}_{{\rm{h}}};$ 2–7 keV), medium (${F}_{{\rm{m}}};$ 1.2–2 keV), and soft (${F}_{{\rm{s}}};$ 0.5–1.2 keV) bands, intra-observation Kuiper's test variability probability (P_intra), inter-observation variability probability (P_inter), and X-ray flux significance (Signif.);
• 2.  
  three X-ray hardness ratios derived from the energy fluxes using HR ${}_{\mathrm{ms}}=({F}_{{\rm{m}}}-{F}_{{\rm{s}}})/({F}_{{\rm{m}}}+{F}_{{\rm{s}}})$, HR ${}_{\mathrm{hm}}=({F}_{{\rm{h}}}-{F}_{{\rm{m}}})/({F}_{{\rm{h}}}+{F}_{{\rm{m}}})$ and HR ${}_{{\rm{h}}(\mathrm{ms})}=({F}_{{\rm{h}}}-({F}_{{\rm{m}}}+{F}_{{\rm{s}}}))/({F}_{{\rm{h}}}+{F}_{{\rm{m}}}+{F}_{{\rm{s}}});$
• 3.  
  G, ${G}_{\mathrm{BP}}$ (BP) and ${G}_{\mathrm{RP}}$ (RP) magnitudes from the Gaia eDR3;
• 4.  
  J, H, and K magnitudes from the 2MASS;
• 5.  
  W1 and W2 magnitudes from the CatWISE2020 and the unWISE and W3 magnitude from the AllWISE;
• 6.  
  a selection of colors (e.g., G − J) using various MW magnitudes;
• 7.  
  X-ray to optical flux ratio, ${F}_{{\rm{X}}}/{F}_{{\rm{o}}}$, with ${F}_{{\rm{X}}}={F}_{{\rm{b}}}$ and ${F}_{{\rm{o}}}$ based on Gaia's G band magnitude.

The tool, publicly available on a dedicated webpage, ⁸ allows for easy interactive plotting of MW properties of the sources for a user-selected subset of source classes. The Graphical User Interface (GUI) is written in Java while plotting is done on-the-fly using the Bokeh Python library. ⁹ As shown in Figure 1, the user can choose from multiple MW features by clicking the corresponding buttons to the left of the Y-axis and below the X-axis. At the top there are buttons corresponding to the distinct astrophysical classes with the numbers of sources within each class. Users can choose to display different combinations of the classes by toggling/untoggling the class names. Whenever appropriate, a logarithmic scale is used in the plot (which is reflected in the axis labels). The user can select a source by clicking on it and some information about the source will be shown. A lasso tool can be used to select a group of sources which then remain selected for any combination of features plotted. The plot can be easily saved as a PNG file by clicking the corresponding button to the right of the plot.

An example, with HR${}_{\mathrm{ms}}$ hardness ratio plotted versus X-ray to optical flux ratio, is shown in Figure 1. One can see a clear separation between AGN and HM-STAR/LM-STAR/YSO while other types of sources are overlapping, requiring an investigation of different sets of features to distinguish between them.

We provide an online tool, based on previously published X-ray source classifications and data from CSC 2.0 and multiple all-sky surveys, to visualize MW properties of X-ray sources with known classes. We ask anyone who will be using this tool in their work to cite this publication.

Support for this work was provided by NASA through CXO Awards AR8-19009B, AR9-20005A, AR0-21007X, and NASA ADAP award 80NSSC19K0576.