NICER X-ray Observations of Cyg X-3 During the Recent Gamma Ray Bright State

ATel #11821; M. F. Corcoran (CUA & NASA/GSFC), D. Pasham (MIT), K. Gendreau, Z. Arzoumanian, T. Strohmayer (NASA/GSFC), D. Altamarano (U. Southampton), and A. Stevens (U. Michigan) for the NICER team
on 6 Jul 2018; 12:46 UT
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Credential Certification: Michael Corcoran (michael.f.corcoran@nasa.gov)

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We report X-ray observations of the microquasar Cyg X-3 during its gamma ray bright state (as detected by AGILE, ATels #11804, 11814) with the X-ray Timing Instrument (XTI) on the NICER X-ray observatory in the 0.4-12 keV band. The NICER observations began on 2018-07-03 23:30:29.184 UT (orbital phase = 0.423 according to the ephemeris of Bhargava et al. 2017, ApJ, 849, 141) and are continuing. NICER initially observed the source at a count rate of 345 XTI counts/s, declining to 270 XTI counts/s within 38 minutes. The NICER observations show phase-dependent variability of approximately a factor of 4 in count rate, from a maximum of 1069 counts/s to a minimum of 252 counts/s. The spectrum shows significant line emission, especially between 1 and 7 keV. The spectrum can be simply approximated by a combination of an absorbed power-law plus an absorbed thermal emission component. Sulfur line emission near 2.6 keV is particularly strong, and the spectrum also shows strong emission from the Fe K complex near 6.7 keV. The power-law component has a photon index of approximately 1.2 with a column density of about 8e22/cm^2. The thermal component has a temperature of 1-2 keV, with a somewhat lower column (about 4e22/cm^2). The initial NICER observation had a flux of 8.4e-9 ergs/cm^2/s in the 0.4-12 keV band. A power spectrum of a portion of the initial NICER observation indicates the appearance of a transient QPO near 140 Hz at a significance near the 4-sigma level. This QPO has not, however, been reliably detected again in subsequent observations. NICER is a 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.