J/A+A/687/A37 Variable eROSITA X-ray sources in SEP (Bogensberger+, 2024)
Characterisation of the X-ray point source variability in the
eROSITA south ecliptic pole field.
Bogensberger D., Nandra K., Salvato M., Liu T., Wolf J., Croom S.,
Starck H., Buchner J., Ponti G., Ider Chitham J., Maitra C., Robrade J.,
Merloni A., Krumpe M.
<Astron. Astrophys. 687, A37 (2024)>
=2024A&A...687A..37B 2024A&A...687A..37B (SIMBAD/NED BibCode)
ADC_Keywords: X-ray sources ; Redshifts
Keywords: accretion, accretion disks - black hole physics - time -
stars: variables: general - galaxies: active
Abstract:
During the Spectrum Roentgen Gamma (SRG)/eROSITA all-sky surveys,
X-ray sources close to the South Ecliptic Pole (SEP) are observed
almost every 4h. We aim to identify the sources exhibiting the most
significant long-term X-ray variability within 3° of the SEP in
the first three surveys, and investigate their properties.
We determined the variability significance of ∼104 sources observed
by eROSITA within 3° of the SEP by using thresholds on the Bayesian
excess variance (SCATT_LO) and the maximum amplitude deviation
(AMPL_SIG). Sources exhibiting a variability significance above
3σ were subdivided into likely Galactic and extragalactic
sources, by using spectral and photometric information of their
optical counterparts. We quantified the X-ray normalised excess
variances of all variable sources, and also calculated the
periodograms of the brightest ones.
Out of more than 104 X-ray sources detected by eROSITA within 3°
of the SEP, we identified 453 that exhibit significant X-ray
variability. SCATT_LO is significantly more sensitive to detecting
variable sources in this field, but AMPL_SIG helps to provide a more
complete variability sample. Of those variable sources, 168 were
classified as likely extragalactic, and 235 as likely Galactic. The
periodograms of most bright and variable extragalactic sources are
approximately described by an aliased power law
(P∝ν-α) with an index of α~=1. We identified
a potential tidal disruption event, and long-term transient sources.
The stellar X-ray variability was predominantly caused by bright X-ray
flares from coronally active stars.
Description:
The data were collected in the interval from December 13, 2019, until
June 16, 2021. Some X-ray sources were only detected in eRASS3, and
were not matched to optical counterparts, as that used the positions
in the eRASS:2 catalogue. Coordinates are based on the J2000 epoch. An
X-ray source was deemed to be variable if it exceeded the 3 sigma
variability thresholds defined in the paper in either the AMPL_SIG or
SCATT_LO variability quantifiers, in at least one of the three all-sky
surveys. NEVb adapts the normalised excess variance for the low count
rate regime. It employs Bayesian statistics and remains accurate
despite the varying fractional exposures of the eROSITA data. The
normalised intrinsic variance changes stochastically over time, even
if the process generating the variability is stationary, even without
measurement uncertainties. Therefore, when comparing the NEVb of
different sources or the same source at different times, the intrinsic
scatter in the normalised intrinsic variance should be included in the
error. The total error in the measurement of NIV_∞ is found
by adding the measurement error and the intrinsic scatter in
quadrature. The periodograms of the brightest variable extragalactic
sources were fitted with an aliased power-law model of the form
P∝nu-α. The periodogram index lists the best-fit values
of α.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 158 446 Catalog
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See also:
J/A+A/682/A34 : SRG/eROSITA all-sky survey catalogs (eRASS1) (Merloni+, 2024)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
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1- 23 A23 --- Name eRASSt Name (eRASSt JHHMMSS.s+DDMMSS)
(Name)
25- 32 F8.5 deg RAdeg Right ascension (J2000) of the X-ray
source (RAdeg_x)
34- 42 F9.5 deg DEdeg Declination (J2000) of the X-ray source
(DEdeg_x)
44- 52 F9.5 deg RAodeg ? Right ascension (J2000) of the most
likely optical counterpart (RAdeg_o)
54- 62 F9.5 deg DEodeg ? Declination (J2000) of the most
likely optical counterpart (DEdeg_o)
64 I1 --- Extragalactic [0/1]? Is the source likely
extragalactic (1), or
likely Galactic (0)? (Extragalactic)
66- 68 I3 --- D Galactic-extragalactic distinction
parameter (D)
70- 76 F7.4 --- z ? AAT optical redshifts (Redshift)
78 I1 --- Extended [0/1] Is the X-ray source extended (1),
or point-like (0)? (Extended)
80- 85 F6.3 ct/s CReRASS1 ? Average eRASS1 count rate
(Countrate_eRASS1)
87 I1 --- SigniSLeRASS1 [0/3]? SCATT_LO variability significance
in eRASS1 (SCATTLOsig_eRASS1)a (1)
89 I1 --- SigniASeRASS1 [0/3]? AMPL_SIG variability significance
in eRASS1 (AMPLSIGsig_eRASS1a (1))
91- 96 F6.3 ct/s CReRASS2 ? Average eRASS2 count rate
(Countrate_eRASS2)
98 I1 --- SigniSLeRASS2 [0/3]? SCATT_LO variability significance
in eRASS2 (SCATTLOsig_eRASS2a (1))
100 I1 --- SigniASeRASS2 [0/3]? AMPL_SIG variability significance
in eRASS2 (AMPLSIGsig_eRASS2a (1))
102-107 F6.3 ct/s CReRASS3 ? Average eRASS3 count rate
(Countrate_eRASS3)
109 I1 --- SigniSLeRASS3 [0/3]? SCATT_LO variability significance
in eRASS3 (SCATTLOsig_eRASS3a (1))
111 I1 --- SigniASeRASS3 [0/3]? AMPL_SIG variability significance
in eRASS3 (AMPLSIGsig_eRASS3a (1))
113-117 F5.3 --- NEVb Geometric mean normalised excess variance
NEVb of all 20-bin segments (NEV_b)
119-124 F6.3 --- e_NEVb [] NEVb measurement error in
- direction (NEVberror-)
126-131 F6.3 --- E_NEVb NEVb measurement error in
+ direction (NEVberror+)
133-138 F6.3 --- NEVbSct- Intrinsic scatter in NEVb in
- direction (NEVbscatter-)
140-144 F5.3 --- NEVbSct+ Intrinsic scatter in NEVb in
+ direction (NEVbscatter+)
146-151 F6.3 --- PeriodIndex ? Index of power-law fit to
periodogram (Periodogram_index)
153-158 F6.3 --- e_PeriodIndex ? Error of periodogram index
(Periodogramindexerror)
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Note (1): Variability significance classes as follows:
0 = variability significance < 1 sigma
1 - variability significance ≥ 1 sigma
2 = variability significance ≥ 2 sigma
3 = variability significance ≥ 3 sigma
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Acknowledgements:
David Bogensberger, dbogen(at)umich.edu
(End) Patricia Vannier [CDS] 04-Jun-2024