MeerKAT

Abstract:

Transitional millisecond pulsars (tMSPs) bridge the evolutionary gap between accreting neutron stars in low-mass X-ray binaries and millisecond radio pulsars. These systems exhibit a unique subluminous X-ray state characterized by the presence of an accretion disk and rapid switches between high and low X-ray emission modes. The high mode features coherent millisecond pulsations spanning from the X-ray to the optical band. We present multiwavelength polarimetric observations of the tMSP PSR J1023+0038 aimed at conclusively identifying the physical mechanism powering its emission in the subluminous X-ray state. During the high mode, we report a probable detection of polarized emission in the 2–6 keV energy range, with a polarization degree of (12 ± 3)% and a polarization angle of −2∘ ± 9∘measured counterclockwise from the north celestial pole toward the east (99.7% confidence level, c.l.; uncertainties are quoted at 1σ). At optical wavelengths, we find a polarization degree of (1.41 ± 0.04)% and a polarization angle aligned with that in the X-rays, suggesting a common physical mechanism operating across these bands. Remarkably, the polarized flux spectrum matches the pulsed emission spectrum from optical to X-rays. The polarization properties differ markedly from those observed in other accreting neutron stars and isolated rotation-powered pulsars and are also inconsistent with an origin in a compact jet. Our results provide direct evidence that the polarized and pulsed emissions both originate from synchrotron radiation at the boundary region formed where the pulsar wind interacts with the inner regions of the accretion disk.

Abstract:

Flares produced following the tidal disruption of stars by supermassive black holes can reveal the properties of the otherwise dormant majority of black holes and the physics of accretion. In the past decade, a class of optical-ultraviolet tidal disruption flares has been discovered whose emission properties do not match theoretical predictions. This has led to extensive efforts to model the dynamics and emission mechanisms of optical-ultraviolet tidal disruptions in order to establish them as probes of supermassive black holes. Here we present the optical-ultraviolet tidal disruption event AT 2022dbl, which showed a nearly identical repetition 700 days after the first flare. Ruling out gravitational lensing and two chance unrelated disruptions, we conclude that at least the first flare represents the partial disruption of a star, possibly captured through the Hills mechanism. Since both flares are typical of the optical-ultraviolet class of tidal disruptions in terms of their radiated energy, temperature, luminosity, and spectral features, it follows that either the entire class are partial rather than full stellar disruptions, contrary to the prevalent assumption, or some members of the class are partial disruptions, having nearly the same observational characteristics as full disruptions. Whichever option is true, these findings could require revised models for the emission mechanisms of optical-ultraviolet tidal disruption flares and a reassessment of their expected rates.

Abstract:

We present the first results of the JWST Emission Line Survey (JELS). Utilizing the first NIRCam narrow-band imaging at 4.7 m, over 63 arcmin in the PRIMER/COSMOS field, we have identified 609 emission line galaxy candidates. From these, we robustly selected 35 H star-forming galaxies at , with H star-formation rates () of . Combining our unique H sample with the exquisite panchromatic data in the field, we explored their physical properties and star-formation histories, and compared these to a broad-band selected sample at which has offered vital new insights into the nature of high-redshift galaxies. UV-continuum slopes () were considerably redder for our H sample () compared to the broad-band sample (). This was not due to dust attenuation as our H sample was relatively dust-poor (median ); instead, we argue that the reddened slopes could be due to nebular continuum. We compared and the UV-continuum-derived to SED-fitted measurements averaged over canonical time-scales of 10 and 100 Myr ( and ). We found an increase in recent SFR for our sample of H emitters, particularly at lower stellar masses (). We also found that strongly traces SFR averaged over 10 Myr time-scales, whereas the UV-continuum overpredicts SFR on 100 Myr time-scales at low stellar masses. These results point to our H sample undergoing ‘bursty’ star formation. Our F356W sample showed a larger scatter in across all stellar masses, which has highlighted how narrow-band photometric selections of H emitters are key to quantifying the burstiness of star-formation activity.

Abstract:

Flat radio spectra of compact jets launched by both supermassive and stellar-mass black holes (BHs) are explained by an interplay of self-absorbed synchrotron emission up to some distance along the jet and optically thin synchrotron at larger distances. Their spatial structure is usually studied using core shifts, in which the position of the peak (core) of the emission depends on the frequency. Here, we propose a novel and powerful method to fit the spatial dependence of the flux density at a given frequency of the jet and counterjet (when observed), using the theoretical spatial dependencies provided as simple analytical formulae. We apply our method to the spatial structure of the jets in the luminous hard spectral state of the BH X-ray binary Swift J1727.8−1613. It was the most resolved continuous jet from an X-ray binary ever observed. We find that the observed approaching jet is significantly intrinsically stronger than the receding one, which we attribute to an increase in the emission of both jets with time (observationally confirmed), together with the light travel effect, causing the receding jet to be observed at an earlier epoch than the approaching one. The jets are relatively slow, with a velocity of ∼(0.3–0.4)c. Our findings imply that the magnetic field strength increased with time. Additionally, the magnetic flux is significantly lower than in jets launched by “magnetically arrested disks.” Our method is general, and we propose that it be applied to jets launched by both stellar-mass and supermassive BHs.