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 supersoft X-ray sources (SSS) were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 yr after the last observations. Short-period modulations of the SSS appear where the spectrum of the luminous central source was fully visible (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 yr, including an irregular drift of the ≃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 ± 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 (≃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.

A Multiwavelength Light-curve Analysis of the Classical Nova YZ Ret: An Extension of the Universal Decline Law to the Nebular Phase

“…The WD photosphere is occulted by the edge of an elevated accretion disk (e.g., Ness et al 2013;Sokolovsky et al 2022). It is consistent with the small X-ray fluxes during the SSS phase (see also Orio et al 2022); the flux at the SSS phase is about 10 times smaller than that at the X-ray flash phase on t 0 + 0 days (filled red square in Figure 1). Izzo et al (2020) observed YZ Ret with the UVES at ESO on t 0 + 72 days (JD 2,459,110.5) in the mid-SSS phase.…”

The multi-wavelength view of shocks in the fastest nova V1674 Her

“…A remarkable feature of V1674 Her is the emergence of orbital (3.67 h = 0.153 d; Schmidt et al 2021) and white dwarf spin periods (8.36 min = 0.00580 d; Patterson et al 2021) shortly after the eruption. The two periods are seen in X-rays in addition to optical data (Maccarone et al 2021;Pei et al 2021;Page et al 2022;Orio et al 2022a;Lin et al 2022). The spin period was present before the eruption according to the Zwicky Transient Facility photometry reported by Mroz et al (2021).…”

“…The spin period change may be caused by some combination of magnetic coupling between the rotating white dwarf and the ejecta, non-rigid rotation or substantial radial expansion of the heated white dwarf. Following the spin-down associated with the eruption, a spin-up in the post-eruption phase is reported by (Patterson et al 2022) on the basis of optical photometry, while the presence of changes in the X-ray derived period deserves further investigation (Drake et al 2021;Orio et al 2022a).…”

“…An individual Swift pointing is shorter than the orbital period. Investigations of X-ray orbital periodicity based on multiple Swift and NICER pointings are presented by Drake et al (2021), Orio et al (2022a), andLin et al (2022).…”

XMM-Newton and SRG/eROSITA observations of the isolated neutron star candidate 4XMM J022141.5−735632

“…We searched for periodic signals that could be associated with the rotation period of the X-ray source (hundreds of milliseconds to tens of seconds); alternatively, to identify flux modulations of the order of tens of seconds to hours as observed in novae and supersoft X-ray sources (Orio et al 2022). We considered only the two ∼ 30 ks XMM-Newton observations of J0221 (Table 2), the individual eRASS exposures being too shallow for a meaningful timing analysis.…”