高能暂现源 · 每日文献综述

The observational determination of the lower limit of neutron star masses is crucial for the physics of core-collapse supernovae. In this light, SMC X-1 is an important object because of its estimated pulsar mass lying near or potentially below the theoretical lower limit. SMC X-1 exhibits a double-peaked optical orbital light curve due to the tidal distortion of the donor star, and analysis of this allows us to constrain the binary parameters. In this study, we analyzed optical and X-ray light curves of SMC X-1 obtained by Transiting Exoplanet Survey Satellite and Monitor of All-sky X-ray Image. We found the systematic variations in the optical orbital light curves synchronized with the X-ray super-orbital modulation, regarding the following two aspects: the minimum at inferior conjunction and the double-peak asymmetry. To explain this behavior, we developed a modified ellipsoidal modulation model in which the precessing accretion disk changes the geometry of X-ray irradiation on the donor and that of optical irradiation on the disk. As a result, this model succeeded in reproducing the observed optical and X-ray light curves. Furthermore, we discovered that intense X-ray irradiation could cause the optical emission center to shift away from the gravitational center, potentially leading to an underestimation of the radial velocity of the donor by approximately 20%. Correcting for this effect yields an updated pulsar mass estimation of about <inline-formula><tex-math>$1.35\, M_{\odot}$</tex-math></inline-formula>.

In the 2021 outburst of RS Ophiuchi, the gamma-ray and the X-ray flux were measured quasi-simultaneously from day 1 after the optical peak, offering the first comprehensive view of shocks in a nova occurring in a symbiotic system. We present a previously unpublished observation done with NuSTAR in the 3─79 keV range, 9 days after maximum, and we review the complex history of the evidence of shocks in the previous outbursts of this nova in light of the intensive X-ray monitoring of 2021. We find evidence that the shock causing the particle acceleration measured with the Cherenkov telescopes also produced the thermal flux detected in the 0.2─30 keV X-ray range, while the large gamma-ray flux observed with Fermi after about a day is not consistent with the X-ray observations. We conclude that an initial, strong shock, with a particle─particle loss timescale shorter than the timescale of particle acceleration at energy higher than a few GeV, occurred close to the red giant atmosphere, where either the X-rays' emitting volume was reduced by turbulence, or—perhaps less likely—the X-rays were completely absorbed by large column density near the giant and by the accretion wake along the line of sight. We compare RS Oph with other novae in long-period systems with evolved companions, discussing how the shocks' phenomenology is a powerful tool to derive other physical parameters. Finally, we discuss predictions that in T CrB, expected to have a new nova outburst within the next few years, the shocks may not be as energetic as in RS Oph.