NICER Observations of the 2020 X-ray Minimum of eta Carinae

ATel #13516; Michael F. Corcoran (CUA & NASA/GSFC), Kenji Hamaguchi (UMBC & NASA/GSFC), David Espinoza (CUA), Ted Gull (NASA/GSFC & STScI), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), A. F. J. Moffat (U. Montreal), Gerd Weigelt (MPIfR), Noel Richardson (ERAU), Augusto Damineli (IAG/USP), Felipe Navarete (IAG/USP)
on 23 Feb 2020; 18:39 UT
Credential Certification: Michael Corcoran (michael.f.corcoran@nasa.gov)

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X-ray emission from the massive colliding wind binary eta Carinae has been monitored by NASA's Neutron Star Interior Composition Explorer (NICER) X-ray observatory since 20 July 2017, through the system's recent X-ray minimum and periastron passage (ATEL #13508). The X-ray minimum is associated with periastron passage and superior conjunction of the companion star. The NICER observations cover nearly half the recent orbital cycle, starting at a phase of 0.536, when the stars were just past the apastron point. The NICER observations show the gradual increase in X-ray flux as the two stars in the binary approach each other, rising from about 15 to 44 NICER counts/s (corresponding to a flux of about 1.1 to 3.4e-10 ergs/cm2/s in the 2-10 keV band), along with the strong, rapid, apparently stochastic X-ray variability starting about 2 months before the X-ray minimum state (ATEL #13327). This behavior is similar to the X-ray variability seen in previous X-ray monitoring campaigns carried out using RXTE and Swift (Corcoran et al., 2017 ApJ, 838, 45) over 4 previous binary orbits. NICER shows that the X-ray minimum state (at a net count rate of approximately 0.5 NICER net counts/s, or a flux of about 3e-12 ergs/cm2/s in the 2-10 keV band) was reached near 2020-02-13. This timing is consistent with the ephemeris derived from previous X-ray observations. A periodogram analysis using the Plavchan algorithm (https://exoplanetarchive.ipac.caltech.edu/cgi-bin/Pgram/nph-pgram) with fixed period step sizes in the 2000-2100 day interval finds a maximum power peak at 2023.04, close to the period previously derived, 2023.40+/-0.71 d (Corcoran et al., 2017, ibid.), and comparable to the period of 2022.7+/- 3 days determined from He II 4686 line variations (Teodoro et al., 2016, ApJ, 819, 131). Initial analyses of the NICER spectra show increasing X-ray absorption up to X-ray minimum, with a maximum absorbing column density about a factor of 100 greater than the column when the stars are well separated. Recovery from the X-ray minimum should begin within 1-2 months, and we encourage observations during this interval through the full X-ray recovery to supplement ongoing space-based and ground-based monitoring campaigns. This work is supported by NASA grant #80NSSC19K1451. NICER is a large area, high time resolution 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.