List of variable X-ray sources within 3° of the South Ecliptic Pole observed by eROSITA in its first three all sky surveys

These sources were identified by Bogensberger et al. 2023B, by investigating the light curves of X-ray sources detected by eROSITA, in its 1st, 2nd, and 3rd eROSITA All Sky Surveys (eRASS), within 3° of the South Ecliptic Pole (SEP). Threshold conditions on the AMPL_SIG and SCATT_LO variability parameters were defined as a function of the count rate and number of bins by Bogensberger et al. 2023A. These were subsequently used to obtain a sample of significantly variable sources, with a false positive likelihood of 0.135%. Variable sources were further distinguished into likely galactic and likely extragalactic sources, which we describe with the distinction parameter, D. X-ray binaries in the LMC that lie in this field have been excluded from this table, as they will be described by Kaltenbrunner et al. in prep. The light curves of variable sources were split into segments within which each source was observed on almost every eroDay. The number of bins, the count rate, and the NEV is shown for each of these segments individually. We also show the NEV, averaged over all 20 bin continuous intervals of the light curves. The errors in the NEV are the total (measurement + sampling) error, under the assumption that the segments are all adjacent. These sources were sorted by their RA, starting from the smallest.

Description of acronyms in table:

X-ray coords: X-ray coordinates of the variable X-ray source, expressed in RA and Dec, in decimal degrees. This is a preliminary X-ray location, that uses an approximate astrometric correction, which will be determined more accurately in a future eROSITA catalogue.

Opt coords: The optical coordinates of the most likely counterpart in CatWISE2020 to the X-ray location of the X-ray variable source. The matching is performed using NWAY. The location in RA and Dec is expressed in decimal degrees. This should not be assumed to always be the correct counterpart to the X-ray variable source.

Z: The redshift, as determined by optical spectroscopic observations of the most likely counterpart of the eROSITA detected variable X-ray sources, by 2dF/AAOmega. Not all of these redshifts are reliable. See the parameter QOP.

Spectype: The spectral type of the optical spectrum of this variable source. This indicates which type of source has the most similar spectrum to the one observed. The following table lists what source spectrum type each value of the Spectype parameter refers to:

QOP: The quality of the optical spectrum, and the estimated certainty that the fitted type and redshift are reliable. A value of 1 indicates a source for which the redshift and spectral type cannot be determined. A value of 2 indicates a possible redshift and spectral type determination, with an approximately 50% certainty. 3 indicates a good spectrum, with 90% confidence that the redshift and spectral type are accurate. Finally, a value of 4 corresponds to a great spectrum, for which there is an estimated 99% confidence that the fitted redshift and spectral type are accurate. A QOP of 6 is used for spectra that can confidently be said to have been produced by a single star, rather than by a galaxy. The redshift and spectral type classification should only be used for sources that have QOP ≥3.

D: The galactic/extragalactic distinction parameter of the variable X-ray source. It is defined by combining the results of four different methods, to distinguish between sources that are most likely to be galactic, and sources that are most likely to be extragalactic. For a full description of this parameter, see Bogensberger et al. 2023B. Sources with 1 ≤ D ≤ 4, and D ≥ 9 are defined as likely extragalactic sources. Sources with D ≤-1, and 5 ≤ D ≤ 8 are defined as likely galactic sources. Sources with D = 0 could not be identified as either likely galactic or likely extragalactic sources. These are sources of unknown type.

Exgal/gal: The final source type classification, distinguishing between likely galactic, likely extragalactic, and unknown sources.

N_b: The number of eroDays of observation of the source, per segment of the light curve. This only includes eroDays in which the fractional exposure was ≥ 0.1.

R_S: The average source count rate per segment of the light curve. It is expressed in units of counts per second, for the 0.2-5.0 keV energy band. The average count rate is also calculated for very short segments, which are not included in other columns. Short gaps of a few days are ignored by the count rate, but still distinguish segments used for the variability analysis.

Varclass: This parameter describes how the source was classified as a variable or non-variable source in the three eRASSs. X-ray sources were selected as variable if the value of either SCATT_LO or AMPL_SIG of the light curve within at least one of the three eRASSs exceeded the 3σ thresholds. Due to possible issues with the matching of sources across the eRASSs, we investigated variability within individual eRASSs. If the source was detected in an eRASS, this parameter will contain "eRi:", with i=1, 2, or 3. This denotes the eRASS in question, and is followed by a number, which indicates whether this source was found to be variable by SCATT_LO and AMPL_SIG in that particular eRASS. A value of 0 indicates that the source was found below the AMPL_SIG and SCATT_LO 3σ thresholds, and was therefore not identified as a significantly variable source. A value of 1 refers to sources only identified as variable by AMPL_SIG. A value of 2 is assigned to sources only identified as variable by SCATT_LO. Finally, a value of 3 is used for sources that were identified as variable by both methods.

NEVb seg: The bexvar NIV estimate of all segments of the light curve. This is a Bayesian estimator, which is more reliable than the standard NEV method at low count rates, but provides the same values at high count rates.

NEVb20: The log average bexvar NEV of 20 bin segments within the entire eRASS1, 2, and 3 light curves.

Light curve: A link to the rebinned, and non-rebinned light curves of the variable source.

This table is also available as a fits file, with further in-depth information, which can be downloaded here.

In case of questions, please feel free to contact David Bogensberger (dbogen@umich.edu, david.bogensberger2@gmail.com).