The X-ray lightcurve and spectrum observed with NICER

ATel #14954; M. Orio (INAF-Padova and University of Wisconsin), K. Gendreau (NASA-GSFC), S. Pei (Padova University), M. Giese (U Wisconsin), D. Altamirano (Southampton), Z. Azourmanian (NASA/GSFC), T. Enoto (RIKEN), A. Fabian (IoA, Cambridge), E. Ferrara (NASA/GSFC, CRESST and University of Maryland College Park), 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), C. Markwardt (NASA-GSFC), J. Miller (Univ. of Michigan), J. Nichols (CfA), M. Parker (IoA, Cambridge), P. Pragati (MIT Kavli Institute for Astrophysics and Space Research), T. Strohmayer (NASA-GSFC), M. Wolff (US Naval Research Laboratory))
on 4 Oct 2021; 17:00 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Marina Orio (orio@astro.wisc.edu)

Tweet

RS Oph was observed with NICER in the 0.2-12 keV energy range, with several short exposures (usually lasting up to ~1000 s) done every day during 2021 September (see also ATel #14926, ATel #14901, ATel #14864). The average count rate showed a rapid increase on 9/18/2021, followed by a moderate decrease on the following day, then it increased again reaching a peak on 9/21/2021. Then, a rapid decrease occurred within few hours, but we measured another rise up to a maximum average count rate of ~3500 cts/s on exposures taken on 9/26/2021. After small oscillations, a constant decrease was measured since 9/27/2021. On 9/30/2021 the average count rate was ~1100 cts/s, with irregular fluctuations. In the last snapshot, the count rate had moderately risen again, to an average 1300 cts/s. We estimate that the absorbed flux was about 3 x 10^-9 erg/cm²/s at maximum count rate, and 10^-9 erg/cm²/s at minimum. The spectral shape did not change significantly during the decline, with a strong peak at 0.5 keV and almost no counts above 1 keV. Although the spectrum is complex, the soft flux can be fitted with a blackbody at about 45 eV. Moreover, the ~35 s quasi periodic oscillations remained detectable in several of the short snapshot exposures. This phenomenology suggests that the supersoft X-ray source may still be hot and luminous, but may be partially obscured at times by large clumps in the ejecta, that are optically thick to supersoft X-rays. The observations will resume tomorrow (October 5). NICER is a 0.2-12 keV X-ray instrument operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.