高能暂现源 · 每日文献综述

The recent candidate gravitational-wave (GW) alert from a compact object merger involving at least one subsolar mass (SSM) object has prompted questions about their origins. S251112cm is reported by LIGO/Virgo with a false-alarm rate of 1 per 6.2 yr, nearby luminosity distance of 93 ± 27 Mpc, and probability of containing an SSM object of 100%. Such a system, if astrophysical, likely did not involve the supersolar neutron stars or black holes invoked to explain kilonovae. One must then also invoke hitherto unobserved and speculative models to produce SSM mergers that may have electromagnetic (EM) counterparts. We introduce a framework that vets and scores candidate counterparts to SSM GW events to inform follow-up in search of any among the zoo of potential EM transients: kilonovae, kilonovae-within-supernovae, superkilonovae, or active galactic nucleus (AGN) flares from binary black hole mergers. We use a suite of telescopes to perform tiling, galaxy-targeted observations, and photometric/spectroscopic follow-up of promising candidates. In near-real time, we ingest candidates reported by the community, including some of the first observations reported by the Vera C. Rubin Observatory. We vet and score a total of 456 candidates, including 67 from Rubin, but find no likely counterpart. We nonetheless highlight candidates that demonstrate the ability of our framework to distinguish between different transient types and describe strategies to maximize the chances of detecting a counterpart to the next SSM event. Our framework will be implemented in the forthcoming Multimessenger Tool for Rapid Object Vetting and Examination.

Swift J1727.8─1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass (<inline-formula><tex-math>$\sim 10\,\mathrm{ M}_\odot$</tex-math></inline-formula>) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He II flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He II remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption (<inline-formula><tex-math>$v_w\sim -750\, \mathrm{km\, s^{-1}}$</tex-math></inline-formula>) and the suppressed blue peak of He II suggests a unified origin in a massive, cool (<inline-formula><tex-math>$T\lesssim 10^{4}\, \mathrm{K}$</tex-math></inline-formula>) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He II profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be <inline-formula><tex-math>$\dot{M}_w\gtrsim 10^{-9}\, \mathrm{ M}_\odot \, \mathrm{yr^{-1}}$</tex-math></inline-formula>, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system's secular evolution.