NICER monitoring of supersoft X-ray sources

Abstract

We monitored four supersoft sources - two persistent ones, CAL 83 and MR Vel, and the recent novae YZ Ret (Nova Ret 2020) and V1674 Her (Nova Her 2021) - with NICER. The two persistent SSS were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 years after the last observations. Short period modulations of the supersoft X-ray source (SSS) appear where the spectrum of the luminous central source was fully visibl (in CAL 83 and V1674 Her) and were absent in YZ Ret and MR Vel, in which the flux originated in photoionized or shocked plasma, while the white dwarf (WD) was not observable. We thus suggest that the pulsations occur on, or very close to, the WD surface. The pulsations of CAL 83 were almost unvaried after 15 years, including an irregular drift of the $\simeq$67 s period by 2.1 s. Simulations, including previous XMM-Newton data, indicate actual variations in period length within hours, rather than an artifact of the variable amplitude of the pulsations. Large amplitude pulsations with a period of 501.53$\pm$0.30 s were always detected in V1674 Her, as long as the SSS was observable. This period seems to be due to rotation of a highly magnetized WD.We cannot confirm the maximum effective temperature of ($\simeq$145,000 K) previously inferred for this nova, and discuss the difficulty in interpreting its spectrum. The WD appears to present two surface zones, one of which does not emit SSS flux.

in press in the Astrophysical Journal

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Cited by in corpus (8)

• The multi-wavelength view of shocks in the fastest nova V1674 Her
• A multiwavelength light curve analysis of the classical nova YZ Ret: An extension of the universal decline law to the nebular phase
• XMM-Newton and SRG/eROSITA observations of the isolated neutron star candidate 4XMM J022141.5-735632
• Mixing of hot shocked plasma with cold gas in Nova YZ Ret 2020
• Application of hydrostatic local thermodynamic equilibrium atmosphere models to interpretations of supersoft X-ray source spectra
• Revisiting multiwavelength data on the supersoft X-ray source CAL 83
• Multi-wavelength study for gamma-ray nova V1405 Cas
• A TESS View of Post-Eruption Variability in the Novae V1405 Cas,V1716 Sco, and V1674 Her