Recent X-ray Activity of Eta Car as Seen by the Swift/XRT

ATel #6298; Michael F. Corcoran (NASA/GSFC-CRESST/USRA), Jamar Liburd (U. VI), Kenji Hamaguchi (NASA/GSFC-CRESST/UMBC), Theodore Gull (NASA/GSFC), Thomas Madura (NASA/GSFC/NPP), Mairan Teodoro (NASA/GSFC/CNPq), Anthony Moffat (U. Montreal), Chris Russell (U. Delaware), Andrew Pollock (ESA), Stan Owocki (U. Delaware)
on 7 Jul 2014; 18:55 UT
Credential Certification: Michael Corcoran (michael.f.corcoran@nasa.gov)

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Swift/X-ray Telescope (Swift/XRT) monitoring of the 2-10 keV emission from the extremely massive colliding wind binary Eta Car shows that the flux in this band is increasing as expected as the two stars approach periastron passage of the 2024-day orbit (Corcoran 2005, AJ, 129, 2018; Damineli et al., 2008, MNRAS, 386, 2330) when the X-ray emission will subsequently fall to a minimum state. As in previous cycles, the X-ray emission shows transient variations (Moffat & Corcoran, 2009, ApJ, 707, 693M) superimposed on the overall X-ray increase. The maximum flux observed by the XRT to date occurred on June 21 2014, at a level of 3.53±0.13×10^-10 ergs s^-1cm^-2 (0.049 photons s^-1cm^-2). This is the brightest that Eta Car has been observed to be in the 2-10 keV band in over 18 years of monitoring with the Rossi X-ray Timing Explorer (RXTE) and Swift (see Corcoran et al., 2010, ApJ, 725, 1528), 25% brighter than the previous observed maximum (2.79×10^-10 ergs s^-1cm^-2, which was observed on Dec 6, 2008 by RXTE). The XRT peak flux is about a factor of 3 lower than the flux peak in this band reported by MAXI on June 17 (0.15 +/- 0.03 s^-1cm^-2, ATEL #6289). The Swift/XRT obtained measures of the 2-10 keV band X-ray flux of Eta Car on June 14 and on June 18, just before and just after the peak reported by MAXI. The Swift/XRT measured 2-10 keV fluxes of 2.56±0.1×10^-10 ergs s^-1cm^-2 (0.037 photons s^-1cm^-2) and 3.17±0.11×10^-10 ergs s^-1cm^-2 (0.045 photons s^-1cm^-2), on June 14 and on June 18, respectively, well below the reported peak flux seen by MAXI on June 17. The Swift/XRT observations (3 times per week) are planned throughout the X-ray minimum phase (expected to begin on July 30, 2014). A plot of the current Swift XRT fluxes in the 2-10 keV band compared to previous RXTE measures shows that the June 21 flare is nearly coincident with a similar variation seen at the same orbital phase in the last orbital cycle, but in general there is no strong evidence that the flaring behavior is locked to the orbital phase (Moffat & Corcoran 2009).