Swift-XRT observations of short period oscillations seen in the Super Soft Source emission from Classical and Recurrent Novae
Abstract
We report short timescale, soft X-ray flux modulationsseen by the Neil Gehrels Swift Observatory X-ray Telescope (XRT) during thesuper soft source (SSS) emission phase from novae. A periodogramanalysis revealed oscillations were visible in the 0.3-10.0 keV XRT light curves obtained from RS Oph (period, P=35.0 s), KT Eri(P=34.9 s), V339 Del (P=54.0 s), and V5668 Sgr (P=69.7 s), withfractional rms variabilities ranging from 1-8 per cent.During day 32-59 of the RS Oph outburst, the oscillation centralfrequency appeared quite variable, ranging from 26.2-31.1 mHz, caused by a lower coherence at this time. However, after day 50 theoscillation became more coherent, with a frequency that slowlyincreased from 28.3-28.9 mHz over 9 days, before the trend reversed. The oscillation frequency was less variable in KT Eri and V339 Del. No other correlations, such as between the oscillation frequency or amplitude with source intensity, were apparent.A wavelet analysis of the variability seen in RS Oph, KT Eri and V339Del revealed the oscillations were sometimes visible for entire XRTsnapshots lasting 1.0-1.5 ks, yet on other occasions, they weredetected for only ∼ 120 s (i.e. ∼ 2-4 cycles). The modulation fractional amplitude was variable, occasionally reaching values of 15-20 per cent for a few cycles. During times when the coherence was low, the oscillation phase was seen to jump by ∼ 0.4-0.6 cycles in RS Oph, then remain stable for ∼ 10 cycles. KT Eri showed smaller phase jumps of ∼ 0.2 cycles.We detect a significant spectral variation through the 35 s oscillation seen in RS Oph, with the spectrum becoming harder at the time of the modulation maximum. Fits to the oscillation maximum and minima spectra suggest the increased flux between 0.6-0.75 keV in the former is caused by a 25 per cent reduction in the oxygen column density at this phase.We discuss the possible origins behind the oscillations.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.237B