MeerKAT

Abstract:

We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival Green Bank Telescope (GBT), Murchison Widefield Array, and Very Large Array (VLA) images at 333, 88, and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud associated with the halo of the Sgr B complex on a scale of ∼60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic ray electrons. Correlation of the Fe I 6.4 keV K α line and synchrotron emission provides compelling evidence that the low energy cosmic ray electrons are responsible for producing the K α line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has a mean spectral index α ∼ −1 ± 1, which gives the magnetic field strength ∼100 μG for cloud densities nH = 104–105 cm−3, and estimated cosmic ray ionization rates between 10−13 and 10−14 s−1. Furthermore, the energy spectrum of primary cosmic ray electrons is constrained to be E−3 ± 1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and γ -ray emission detected from this cloud. These measurements have important implications on the role that high cosmic ray electron fluxes at the Galactic centre play in production of radio synchrotron emission, the Fe I K α line emission at 6.4 keV, and ∼GeV γ -ray emission throughout the Central Molecular Zone.

Abstract:

Blueshifts - or, more accurately, blue asymmetries - in broad emission lines such as C iv λ1550 are common in luminous quasars and correlate with fundamental properties such as Eddington ratio and broad absorption line (BAL) characteristics. However, the formation of these blueshifts is still not understood, and neither is their physical connection to the BAL phenomenon or accretion disc. In this work, we present Monte Carlo radiative transfer and photoionization simulations using parametrized biconical disc-wind models. We take advantage of the azimuthal symmetry of a quasar and show that we can reproduce C iv blueshifts provided that (i) the disc-mid-plane is optically thick out to radii beyond the line formation region, so that the receding wind bicone is obscured; and (ii) the system is viewed from relatively low (that is, more face-on) inclinations (≤40°). We show that C iv emission-line blueshifts and BALs can form in the same wind structure. The velocity profile of the wind has a significant impact on the location of the line formation region and the resulting line profile, suggesting that the shape of the emission lines can be used as a probe of wind-driving physics. While we are successful at producing blueshifts/blue asymmetries in outflows, we struggle to match the detailed shape or skew of the observed emission-line profiles. In addition, our models produce redshifted emission-line asymmetries for certain viewing angles. We discuss our work in the context of the C iv λ1550 emission blueshift versus equivalent-width space and explore the implications for quasar disc wind physics.

Abstract:

As more Fast Radio Bursts (FRBs) are being localized, we are learning that some fraction have persistent radio sources (PRSs). Such a discovery motivates an improvement in our understanding of the nature of those counterparts, the relation to the bursts themselves and why only some FRBs have PRSs. We report on observations made of FRB 20121102A with the MeerKAT radio telescope. Across five epochs, we detect the PRS associated with FRB 20121102A. Our observations are split into a cluster of four epochs (MJD 58732-58764) and a separate single epoch about 1000 d later. The measured flux density is constant across the first four observations but then decays by more than one-third in the final observation. Our observations on MJD 58736 coincided with the detections of 11 bursts from FRB 20121102A by the MeerTRAP backend, seven of which we detected in the image plane. We discuss the importance of image plane detections when considering the commensal transient searches being performed with MeerKAT and other radio facilities. We find that MeerKAT is so sensitive that within a 2-s image, we can detect any FRB with a flux density above 2.4 mJy at 1.3 GHz and so could localize every FRB that has been detected by CHIME to date.

Abstract:

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs)¹, sources of high-frequency gravitational waves (GW)² and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process)³. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers^4-6, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817^7-12. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.