SRG/eROSITA observations of a bright X-ray outburst from Circinus X-1

ATel #13557; R. Arcodia (MPE Garching), F. Haberl (MPE Garching), V. Doroshenko (IAAT Tuebingen), P. Predehl (MPE Garching), J. Wilms (ECAP/FAU), V. Grinberg (IAAT Tuebingen)
on 16 Mar 2020; 21:19 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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We report the detection of a new outburst from the enigmatic X-ray binary Circinus X-1 by the eROSITA instrument onboard of the Russian/German Spektrum-Roentgen-Gamma (SRG) satellite during its ongoing all-sky survey. eROSITA first scanned over the position of Cir X-1 on February 29 2020, detecting a bright source (250-300 cts s^-1) located at RA(2000) = 15:20:40.68, Dec(2000) = -57:09:56.4 with an estimated positional uncertainty of less than about 4'', consistent with the position of Cir X-1. The source continued to be detectable up until March 5 2020 when the last scan over the source was conducted. Cir X-1 is known to exhibit regular outbursts and has been active recently as indicated by the MAXI monitoring ( http://maxi.riken.jp/pubdata/v6l/J1520-571/index.html ), including the detection during the current outburst, which appears to be gradually decaying also after the end of the eROSITA observations.

The spectrum of the source in the soft X-ray band is consistent with that previously reported (ATel #2650, ATel #9547) and can be described with a combination of a power law with photon index of 3.4 and a blackbody with kT~1.1 keV, absorbed by an equivalent Hydrogen column density of ~4x10²² cm^-2. The observed 0.5-2 keV flux is ~2.7x10^-10 erg cm^-2 s^-1, which implies an unabsorbed source flux of ~3x10^-8 erg cm^-2 s^-1 in the 0.3-10 keV band, consistent with previous reports (7x10^-8 erg cm^-2 s^-1, ATel #9547; but the flux is variable during and between individual outbursts). The observed source flux implies thus an indicative estimate for the unabsorbed 0.3-10keV luminosity to be ~3x10³⁸ erg s^-1, assuming a distance of 9.4 kpc (Heintz et al., 2015). The source brightness remained remarkably constant throughout the period covered by the eROSITA observations. The light curves obtained during individual scans also exhibit little variability and no evidence for X-ray bursts, which are sometimes observed from the source.

The timing and duration of the outburst are also as expected. In particular, assuming ephemerides by d'Ai et al. (2012), we estimate the expected periastron passage and thus outburst start time as MJD 58905(2), i.e. Feb 25 2020 (corresponding to orbital cycle 957). It is thus likely that eROSITA started to observe the source close to the peak of the outburst lasting at least 9 days, which is consistent with the typical duration reported previously (Asai et al. 2014). We finally note that Cir X-1 typically exhibits several outbursts in a row, so one could expect the X-ray activity to continue.