Spectral and flux variability in NICER observations and optical spectra of RS Oph

ATel #14901; S. Pei (Padova University), M. Orio (INAF-Padova and University of Wisconsin), K. Gendreau (NASA-GSFC), J. Mikolajewska (Nicolaus Copernicus Astronomical Center, N. Ospina (INFN and Padova University), E. Aydi (Michigan State University), Z. Azourmanian (NASA/GSFC), D. Buckley (South African Observatory), R. Carrera (INAF-Padova), J. Carbajo-Hijarrubia (University of Barcelona), T. Enoto (RIKEN), A. Fabian (IoA, Cambridge), E. Ferrara (NASA/GSFC, CRESST and University of Maryland College Park) M. Giese (University of Wisconsin), R. Ignace(East Tennessee State University), N. Islam Syeda (NASA/GSFC and University of Baltimore), K. Mukai, (NASA/GSFC, CRESST and University of Baltimore, Baltimore County), H. Maehara (National Astronomical Observatory of Japan), J. Miller (Univ. of Michigan), J. Nichols (CfA), M. Parker (IoA, Cambridge), P. Pragati (MIT Kavli Institute for Astrophysics and Space Research).
on 6 Sep 2021; 21:12 UT
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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NICER monitoring of RS Oph in the 0.2-12 keV energy range is continuing with daily observations, with non-continuous exposure times ranging between about 1000 s and 9000 s each day. After 2021 August 17 (ATel #14864) we measured a steady, slow decrease of count rate in the 0.2-12 keV range as the X-ray source became softer (increasing in hardness ratio, namely count rate in the 0.2-1 keV band divided by count rate in the 1-12 keV band), but there were soft X-ray flares on 2021 August 26 and 27. On 2021 September 1st, the average count rate was 126.9 ± 0.5 cts/s in the 0.2-12 keV range. The spectrum was structured, with multiple components. A preliminary fit for the total exposure of ~3000 s on this date can be obtained with a column density N(H)=1.45 x 10x²² cm^-2, a blackbody component at a temperature of 59 eV, and two components of collisionally ionized plasma in thermal equilibrium (APEC) at 0.2 and 1 keV, respectively. Although an alternative fit three APEC components, lower column density and lower luminosity cannot be ruled out, the blackbody luminosity in these exposures would be 7.4 x 10³⁸ erg/s assuming a distance of 1.6 kpc. Even if a blackbody fits tends to overestimate the luminosity, this acceptable fit indicates that the central supersoft X-ray source, namely the hydrogen burning white dwarf, may have already emerged and be the origin of the supersoft flux reported also in ATel #14894. Optical spectra taken with both the SALT High Resolution Spectrograph and the Nordic Optical Telescope and the FIES spectrograph on 2021 September 1st show a strong He II emission line, that increased in flux by a factor of about 10 since a SALT spectrum was previously taken on 2021 August 28, reaching 5-6 x 10^-11 erg/cm²/s on 2021 September 1st. This line was hardy measurable, and sometimes not even detectable, in several spectra taken every night with The Mercator 1.2m telescope and the HERMES spectrograph (see also ATel #14894) for eight previous nights in a row. This sudden increase in He II line flux is consistent with the ionizing flux observed in the supersoft X-rays with NICER. NICER observations continued and on 2021 September 4th, the soft X-ray flux had the largest flare. The average count rate on that day was 127.4 ± 0.2 cts/s in the 0.2-12 keV range, but in the 0.2-0.6 keV range, it increased from an average about 35 cts/s, to 130 cts/s during a period of time of only 5 minutes. On 2021 September 5th, the soft range count rate decreased again and was more stable. In the 0.2-0.6 keV soft band, we measured a pulsation with a period of 36.7 s on 2021 September 4, and a more definite modulation was measured in this energy range on September 5, with a period of 34.88 ± 0.02 s. Soft X-ray flares coincident with the onset of short period modulations were also found in the X-ray data obtained for the nova in 2006, see Nelson et al. 2008 (ApJ 673, 1067). NICER observations are ongoing, and so is the optical spectroscopic monitoring.