Galactic eROSITA Sources

First eROSITA all-sky map
eROSITA all-sky X-ray image from the early data release
Credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

The all-sky map from eROSITA will identify X-ray point sources, whose nature can be discerned by follow-up optical or infrared spectroscopy. Galactic X-ray sources are expected to be coronally active stars, young stellar objects, flare stars, and accreting compact objects.)


Useful follow-up BOSS or APOGEE spectroscopy requires Identifying the correct optical or infrared counterpart. The eROSITA Compact Objects and Stars teams provided the Gaia DR2 or 2MASS IDs cross-matched with an early version of the eROSITA eRASS-1 catalog. (eRASS-1 will be released in 2023) for use in targeting generation v0.5.3.

Galactic eROSITA cartons for Targeting Generation v0.5.3

For this targeting generation, there are three cartons:

  • mwm_erosita_stars
  • mwm_erosita_compact_gen
  • mwm_eroisita_compact_var

For mwm_erosita_stars, the targets were taken from the optical and/or infrared counterparts identified by the eROSITA Stars group from the catalog known internally as EROSITASupersetStars. This catalog was created using the techniques described in Freund et al (2022).

Similarly, the eROSITA compact objects group produced the catalog known internally as EROSITASupersetCompactobjects. Briefly, the catalog was constructed by taking all sources from eROSITA that meet the following criteria:


  • p: Gaia DR2 parallax (mas)
  • perr: Gaia DR2 parallax error (mas)
  • G: Gaia DR2 mean photometric G magnitude
  • varproxy = log10(sqrt(phot_g_n_obs/phot_g_mean_flux_over_error))  where phot_g_n_obs and phot_g_mean_flux_over_error are taken from Gaia DR2
  • L=-ln(P), where P is the probability that the source is caused by a random background fluctuation
  • fx= total X-ray flux from eROSITA
  • fopt: flux in the optical, determined by G/2.5 + 4.86
  • radec_err: position uncertain in eROSITA position
  • pm: the total proper motion (mas/yr)
  • pmerr: the total error in proper motion (mas/yr)
  • logpmdpm = log10(pm/pmerr)

Criteria for Gaia DR2 counterpart identification

  • Point source
  • G > 16
  • L > 8 in at least one of the three eROSITA energy bands.Detections in three energy bands
  • log(fx/fopt) < 2.7
  • Matches a Gaia DR2 source within 30 arcsec AND the source has radec_err > 0 AND angdist/radec_err < 2.1
  • Neither of the two following conditions are met:
    • (logpmdpm < 0.301) AND (p/perr > -1.4996 * logpmdpm – 4.05) AND (p/perr < 1.4995 * logpmdpm + 4.05)
    • (logpmdpm – 0.301)2/0.397942 + (p/perr)2/4.52 ≤ 1

Because there is often more than one possible compact object counterpart in Gaia, two distinct cartons were generated. In mwm_erosita_compact_gen, the closest Gaia DR2 source was selected. In mwm_erosita_compact_var, the Gaia DR2 source showing the largest variability was selected. Variability was defined as

  • varproxy = log10(sqrt(phot_g_n_obs/phot_g_mean_flux_over_error)) 


The cadence and instrument adopted depend on the Gaia DR2 G and 2MASS PSC H magnitudes.

  • H < 13 and G < 13 — bright_1x1 on APOGEE
  • 13 < G < 17, — bright_1x1 on BOSS
  • 17 < G < 19 — dark_1x2 on BOSS
  • G < 19 — dark_1x3 on BOSS
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