高能暂现源 · 每日文献综述

Recent evidence from Pulsar Timing Arrays (PTAs) for a nanohertz gravitational wave background is broadly consistent with theoretical expectations from a population of massive black hole binaries (MBHBs), although the inferred amplitude appears somewhat higher than predicted by standard models. Interpreting these observations requires a robust understanding of the merger timescales of MBHBs, and of their connection to host galaxy properties. In this work, we investigate the evolution of MBHBs selected from cosmological galaxy mergers in the ILLUSTRISTNG simulation. We re-simulate these systems at high resolution using the N-body code GRIFFIN to accurately resolve the dynamical friction and stellar hardening phases, and follow their evolution to coalescence with a semi-analytical model. We find that cosmological galaxy encounters and the resulting MBHBs are typically highly eccentric. We characterise the distribution of binary eccentricities at formation and at entry into the PTA band, and quantify the corresponding residence times. We identify the key parameters governing the duration of the different stages of MBHB evolution, and derive scaling relations linking galaxy and orbital properties to dynamical friction, hardening, and total coalescence times. These relations provide a framework for subgrid prescriptions in cosmological simulations. Applying these scaling relations to the full ILLUSTRISTNG merger population, we infer the probability distributions of galaxy merger and black hole coalescence times. We find that galaxy mergers typically complete within ~0.7 Gyr, while the total black hole coalescence time is ~1.0 Gyr. These short timescales imply efficient binary evolution, consistent with current PTA constraints.

The Dynamic Eclipse Broadcast (DEB) Initiative citizen science program observed coronal visible continuum brightness during the 2024 April 8 total eclipse from locations across North America. We present results from 11 DEB sites spanning 2700 km of distance and showing 49 minutes of evolution. The coronal brightness radial profiles from these telescopes are tightly correlated from 1.2 to 4.0 solar radii and comparable to published photometric coronal intensities. The coronal flattening parameter is measured from 1.4 to 2.8 solar radii. A Ludendorff index of 0.0761 +/- 0.0007 is computed but the extrapolation techniques used by some to calculate this index are shown to disagree with this direct measurement, and alternate structure parameters are suggested. Measured radial velocities are compared with an MHD model of the corona during the eclipse from Y. Li et al. (2026). A polar downflow is measured with an average radial velocity of -37 +/- 3 km s-1 and a deceleration of 14 +/- 3 m s-2 at a speed and position which agrees with the model. The predicted mixture of outflows and downflows at low heights is seen, as well as outflows in two western regions of the corona. The fastest observed outflow has a radial speed of 105 km s-1 and is likely associated with a transient event not predicted by the model. Future DEB Initiative eclipse experiments can more tightly constrain models of the inner corona by using both coronal intensity and radial velocity measurements.