A NICER View of Cygnus X-3 During a Period of Gamma-Ray Emission

A NICER View of Cygnus X-3 During a Period of Gamma-Ray Emission Michael L. McCollough (SAO/CXC/CfA), M. Corcoran (CUA/GSFC), T. Kallman (GSFC) , K. Koljonen (FINCA/Unv. Of Turku), M. Gurwell (CfA), R. Corbet (GSFC), K. Gendreau (GSFC), Z. Arzoumanian (GSFC), and the Cygnus X-3 NICER Team Abstract In 2018, Cygnus X-3 underwent active periods which produced several major radio flares and gamma-ray emission. As part of a multi-wavelength campaign, Cygnus X-3 was observed in the X-ray (NICER, MAXI, Swift/BAT), submillimeter (SMA), radio (RATAN-600, AMI-LA, MRO), IR (Faulkes), and gamma-ray (Fermi, AGILE) bands. Here we present results of the the NICER observations made during this campaign. The NICER spectra are used to provide high timesampled measurements of the variations in the continuum, line and absorption features as a function of time and phase, and to compare the spectra to higher spectral resolution observations previously obtained by the Chandra HETG. We briefly discuss the variations in the NICER spectra taken during a gamma-ray emission and the time of launch of Jy level flares (seen at submillimeter wavelengths). Cygnus X-3 NICER Observations Cygnus X-3 is an unusual X-ray binary containing a compact object and a Wolf-Rayet (WR) companion ( van Kerkwijk et al. 1996, Koljonen & Maccarone 2018) , making it a high mass X-ray binary system. But its orbital period (4.8 hrs) is typical for a low mass X-ray binary system. It is a strong radio source routinely producing radio flares of over ~1 Jy and up to ~20 Jy in rarer major radio (at 2.25 GHz). Even during Cygnus X-3's quiescence state it can be relatively bright in the radio (60-100 mJy). It has been shown to produce radio jets (Mioduszewski et al. 2001) and also exhibits correlated behavior of the radio and X-ray emission indicative of an intimate connection between these wavelength regions (McCollough et al. 1999, Szostek et al. 2008, and Koljonen et al. 2010). NICER is mounted on the International Space Station (ISS). It uses X-ray (0.212 keV) "concentrator" optics and silicon-drift detectors. Because of being on the ISS, when and for how long a source can be observed can vary depending on the where the ISS is in its orbit, typically observations are ~1 ksec. For Cygnus X-3 during the 2018 campaign we had 153 observations for a combined total observing time of 153 ksec. The observation durations varied between 0.5 ksec up to 3 ksec. Fig. 1 show the count rates for the various observations as a function of Cygnus X-3 orbital phase. . Fig. 1: 2018 Cygnus X-3 NICER observations are shown as a function of Cygnus X-3 orbital phase. The observations cover fairly well the Cygnus X-3 orbit. There is a large scatter in X-ray flux which is result of flaring that routinely happen during the observed quenched/flaring state. Cygnus X-3’s NICER Spectrum Fig. 2: The NICER measured flux of Fe XXV line (He-like Fe) as a function of Cygnus X-3 orbital phase. Note the scatter which is a result of flaring activity. We fit the NICER spectra with a model which successfully fits the Chandra HETG data (Kallman, McCollough, Koljonen, et al. 2019), with sufficient fit quality. The model Components were: 1. cold absorption 2. notch absorbing iron 3. 6.4 keV Gaussian emission 4. photoionized nebular emission with log(ξ) ~ 2 – 3 5. photoionized nebular emission with log(ξ) ~ 4 – 5 6. wind emission 7. Diskbb 8. Blackbody XSTAR was used to compute nebular and wind components. Fig. 2 show the Fe XXV line measurements as a function of orbital phase. To study the orbital changes of various lines we do phase averages of the measurements. We found the optimal bin, to average out the flaring was 8 phase bins. Fig. 3: A Cygnus X-3 Chandra HETG spectrum (Obsid: 7268) taken during a quenched state is shown. The spectrum has a number of H-like and He-like emission lines from Fe down to Mg. Also S and Si RCC can be detected as well. Fig. 5: This a plot of the orbital modulation of spectral lines from the Cygnus X-3 Chandra HETG observation (Obsid: 7268), taken from Vilhu, et al. 2009. The diamonds with a dotted line is the total flux and the X are the line measurements. The top panel is Si XIV, middle panel is Fe XVV, and the bottom panel is Fe XVVI. The values have been divided by their orbital maximum. Fig. 6: This is the same plot as Fig. 5 using phase averaged (8 bins) NICER data. We see the same basic behavior as was observed with the Chandra data. Fig. 4: This is a NICER spectra of Cygnus X-3 during the quenched state. Given are tentative line identifications which corresponds to the lines that were confirmed by observations made with the Chandra HETG (Kallman et al. 2019) for features above 1 keV. Fig. 7: This an SMA observation (225.6 GHz) taken during this campaign and during which NICER observations were being made. During this time Fermi detectied gamma-ray emission from Cygnus X-3. It is of interest to note that the minima in the SMA light curve correspond to the 0 orbital phase of Cygnus X-3. Cygnus X-3 NICER Results Spectra: We have shown that the NICER spectral resolution and sensitive is good enough to perform a spectral analysis of the emission line rich spectra of Cygnus X-3. Wind Lines: The lines of the light elements correspond to the lower ionization component of the spectrum and are consistent will arising in unshadow wind of the WR companion (see Fig. 8). This is consistent with the Chandra results (Kallman, et al. 2019). The asymmetry seen in the light curve is likely relate to asymmetry in the wind due to the orbital motion of the compact object. Fe Lines: The highly ionized Fe lines are tied to the higher ionizational component which is likely related to the compact object. They also a different orbital phase relation then the wind related lines (see Fig. 7). These results are also consistent will what was found from the Chandra analysis (Kallman, et al. 2019). Fig. 8: This a plot of the orbital modulation of lines in the Fe line region (Fe K⍺, Fe XXV, and Fe XXVI). The H-like and He-like Fe lines appear to be anti-correlated with the continuum with a small offset in the maximum between H-like and He-like lines. The weak Fe K⍺ line appears to anti-correlate with the high ionization Fe lines. Fig. 9: This a plot of the orbital modulation of the lighter elements (Ar XVIII, S XVI, and Si XIV) and the continuum. Overall the lines track the continuum emission. The main difference is the line emission is lower relative to the continuum on the rise to the maximum and higher relative to the continuum on the drop from the maxim. Submillimeter & Gamma-Ray: The submillimeter and gamma-ray emission is currently been examined in term of the spectral variation that is seen in the NICER data (see Fig.9) .