MeerKAT

Abstract:

Blast wave models are commonly used to model relativistic outflows from ultra-relativistic gamma-ray bursts (GRBs), but are also applied to lower Lorentz factor ejections from X-ray binaries (XRBs). Here, we revisit the physics of blast waves and reverse shocks in these systems and explore the similarities and differences between the ultra-relativistic () and moderately relativistic () regimes. We first demonstrate that the evolution of the blast wave radius as a function of the observer frame time is recovered in the on-axis ultra-relativistic limit from a general energy and radius blast wave evolution, emphasizing that XRB ejections are off-axis, moderately relativistic cousins of GRB afterglows. We show that, for fixed blast wave or ejecta energy, reverse shocks cross the ejecta much later (earlier) on in the evolution for less (more) relativistic systems, and find that reverse shocks are much longer lived in XRBs and off-axis GRBs compared to on-axis GRBs. Reverse shock crossing should thus typically finish after 10–100 of days (in the observer frame) in XRB ejections. This characteristic, together with their moderate Lorentz factors and resolvable core separations, makes XRB ejections unique laboratories for shock and particle acceleration physics. We discuss the impact of geometry and lateral spreading on our results, explore how to distinguish between different shock components, and comment on the implications for GRB and XRB environments. Additionally, we argue that identification of reverse shock signatures in XRBs could provide an independent constraint on the ejecta Lorentz factor.

Abstract:

We report on the detection of optical/near-infrared (O-IR) quasi-periodic oscillations (QPOs) from the black hole (BH) X-ray transient Swift J1727.8–1613. We obtained three X-ray and O-IR high-time-resolution observations of the source during its intermediate state (2023 September 9, 15, and 17) using NICER, HAWK-I@VLT, HIPERCAM@GTC, and ULTRACAM@NTT. We clearly detected a QPO in the X-ray and O-IR bands during all three epochs. The QPO evolved, drifting from 1.4 Hz in the first epoch, up to 2.2 Hz in the second, and finally reaching 4.2 Hz in the third epoch. These are among the highest O-IR QPO frequencies detected for a BH X-ray transient. During the first two epochs, the X-ray and O-IR emission are correlated, with an optical lag (compared to the X-rays) varying from +70 to 0 ms. Finally, during the third epoch, we measured, for the first time, a lag of the band with respect to the band at the QPO frequency ( +10 ms). By estimating the variable O-IR SED we find that the emission is most likely non-thermal. Current state-of-the-art models can explain some of these properties, but neither the jet nor the hot flow model can easily explain the observed evolution of the QPOs. While this allowed us to put tight constraints on these components, more frequent coverage of the state transition with fast multiwavelength observations is still needed to fully understand the evolution of the disc/jet properties in BH low-mass X-ray binaries.

Abstract:

The impact of active galactic nuclei (AGNs) on the evolution of galaxies explains the steep decrease in the number density of the most massive galaxies in the Universe. However, the fuelling of the AGN and the efficiency of this feedback largely depend on their environment. We use data from the Low Frequency Array Two-metre Sky Survey Data Release 2 (DR2), the Dark Energy Spectroscopic Instrument Legacy Imaging Surveys, and the Sloan Digital Sky Survey DR12 to make the first study of the orientations of radio jets and their optical counterpart in relation to the cosmic web environment. We find that close to filaments (), galaxies tend to have their optical major axes aligned with the nearest filaments. On the other hand, radio jets, which are generally aligned perpendicularly to the optical major axis of the host galaxy, show more randomized orientations with respect to host galaxies within of filaments. These results support the scenario that massive galaxies in cosmic filaments grow by numerous mergers directed along the orientation of the filaments while experiencing chaotic accretion of gas on to the central black hole. The AGN-driven jets consequently have a strong impact preferentially along the minor axes of dark matter haloes within filaments. We discuss the implications of these results for large-scale radio jet alignments, intrinsic alignments between galaxies, and the azimuthal anisotropy of the distribution of circumgalactic medium and anisotropic quenching.

Abstract:

We analyze and model rapid rotations of polarization orientations in PSR B1919+21’s single pulses based on Five-hundred-meter Aperture Spherical radio Telescope observation data. In more than one-third of B1919+21’s single pulses, the polarization position angle (PA) is found to rotate quasi-monotonically with pulse longitude over 180° or even 360°. Some single pulse PA even rotates by over 540°. Most of these quasi-monotonic PA curves have negative slopes with respect to pulse longitude. Oscillations of circular polarization fraction accompany these PA rotations. This rapid rotation could be induced by a quick change of phase lag between two normal wave modes within an individual pulse. We propose a phenomenological model to reproduce the observed polarization rotations in single pulses, and calculate phase lags in a dipolar magnetic field of an aligned rotating pulsar, with a dispersion relation of orthogonal wave modes in strongly magnetized electron-positron plasma. According to the dispersion relation, the weak frequency dependence of observed polarization rotation requires small angles between the radio wavevector and local magnetic fields, which requires the radio emission height to be low, on the order of 10 times neutron star radius.