The supersoft X-ray phase of RS Oph

ATel #14926; M. Orio (INAF-Padova and University of Wisconsin), K. Gendreau (NASA-GSFC), S. Pei (Padova University), M. Giese (University of Wisconsin), R. Ignace(East Tennessee State University), P. Pragati (MIT Kavli Institute for Astrophysics and Space Research), E. Ferrara (NASA/GSFC, CRESST and University of Maryland College Park), T. Enoto (RIKEN), H. Maehara (National Astronomical Observatory of Japan), Z. Azourmanian (NASA/GSFC), A. Fabian (IoA, Cambridge), 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)
on 20 Sep 2021; 21:26 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Marina Orio (orio@astro.wisc.edu)

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RS Oph is being monitored with NICER almost daily. In the most recent observations, we detected a luminous supersoft X-ray source, in agreement with recent measurements (ATel #14894, ATel #14901) with very few counts above 1.5 keV. The count rates were 942.0 ± 0.5 cts/s on 2021 September 19 (averaged over a net exposure time of 6580 s, in different snapshots lasting up to about 1000 s each), 898.5 ± 0.6 cts/s on 2021 September 19 (with a total net exposure of 5950 s), and 2072.0 ± 0.9 cts/s on 2021 September 20 (with a total net exposure of 2460 s). We estimate an average absorbed flux 7.7 x 10^-10 erg/cm²/s on September 19, 7.3 x 10^-10 erg/cm²/s on September 20, and a large increase to approximately 1.6 x 10^-9 erg/cm²/s on September 21. On all these dates, the spectrum can be approximately fitted with a blackbody at a temperature around 40 eV, absorbed with a column density of in the range 5-6 x 10 ²² cm^-2 and a component of thermal plasma at 1.1 keV. Pulsations with a period of 35.10 ± 0.01 s and amplitude of up to 20% were measured on September 18, and with a period 34.95 ± 0.01 s and smaller amplitude on September 19. The amplitude of the pulsations varied, and an irregular drift of the period around the mean value of about 35 s was also apparent.