Characterisation of the X-ray point source variability in the eROSITA south ecliptic pole field^★

¹ Max Planck Institute für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
² Department of Astronomy, The University of Michigan, 1085 South University Avenue, Ann Arbor, Michigan, 48103, USA
e-mail: dbogen@umich.edu
³ Excellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
⁴ Max-Planck Institut für Astronomie, Königstuhl 17, 69177 Heidelberg, Germany
⁵ School of Physics, The University of Sydney, Camperdown NSW 2006, Australia
⁶ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁷ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁸ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 30 January 2024
Accepted: 30 March 2024

Abstract

Aims. During the Spectrum Roentgen Gamma (SRG)/eROSITA all-sky surveys, X-ray sources close to the South Ecliptic Pole (SEP) are observed almost every 4 h. 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.

Methods. 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.

Results. 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.