Dr Lauren Rhodes

Abstract:

Abstract Relativistic Doppler boosting significantly affects the observed emission of astrophysical jets resulting in observational biases. In this work we investigate the observational biases and modelling opportunities which arise due to relativistic boosting using two X-ray binary case studies. Using the one-sided jet ejecta from MAXI J1535-571, we demonstrate that incorporating non-detections of the receding jet ejecta into kinematic modelling can significantly improve parameter estimation, reducing posterior uncertainties by over $40{{\ \rm per\ cent}}$. For the bipolar jets of MAXI J1820+070, we recover the intrinsic jet rest-frame emission of both approaching and receding jet components, demonstrating that they follow a common powerlaw evolution. Using this rest-frame emission profile as a base model, we show that current observational strategies strongly bias against detecting ejecta with high initial Lorentz factors ≳ 5 and receding ejecta components across a broad region of parameter space. These results highlight the importance of observational strategy selection, particularly early-time and late-time observations, and leveraging non-detections in the modelling of relativistic jets. More generally, quantifying observational biases and maximising modelling capabilities by incorporating the non-detection of receding jets can be employed to enhance interpretation of future gravitational-wave/optically-triggered observations of off-axis, extragalactic jetted transients.

Abstract:

We present ATLAS100 – a sample of 1729 supernovae and other explosive optical transients within ∼100 Mpc observed by the ATLAS survey over a span of 5.75 yr from 2017 September 21 to 2023 June 21. The volume-limited sample includes transients associated with galaxies with a spectroscopic redshift of , and spectroscopically classified transients within this redshift threshold where a host redshift was not available in existing catalogues. Our host galaxy list is constructed from aggregating all available galaxy redshift and distance catalogues. We carefully select all transients within a projected radius of 50 kpc of these hosts. The ATLAS100 transient sample has a host galaxy redshift completeness fraction of 83 per cent, consistent with expectations for the redshift completeness of local galaxy catalogues. Within this volume, the spectroscopic classifications are 87 per cent complete and we reclassify many ambiguous transients with joint light curve and spectroscopic considerations. Here, we release the catalogue together with compiled, binned, and cleaned ATLAS photometry for all transients. We fit the light curve data to derive peak luminosities and characteristic time-scales. We explore the sample characteristics, demographics, and discuss the completeness and purity of the sample. This is the first in a series of papers that will explore the rates and physical parameters of a complete and large sample of nearby supernovae and transients brighter than .

Abstract:

Abstract Scorpius X-1 (Sco X-1) is a neutron star X-ray binary in which the neutron star is accreting rapidly from a low mass stellar companion. At radio frequencies, Sco X-1 is highly luminous and has been observed to have jet ejecta moving at mildly relativistic velocities away from a radio core, which corresponds to the binary position. In this Letter, we present new radio observations of Sco X-1 taken with the Karl G. Jansky Very Large Array. Using a fast imaging method, we find that the 10 and 15 GHz data show a number of flares. We interpret these flares as the possible launching of fast jets (βΓ > 2), previously observed in Sco X-1 and called ultra-relativistic flows, and their interaction with slower moving jet ejecta. Using the period between successive flares, we find that it is possible for the fast jets to remain undetected, as a result of the fast jet velocity being sufficiently high to cause the jet emission to be beamed in the direction of the motion and out of our line of sight. Our findings demonstrate that the ultra-relativistic flows could be explained by the presence of fast jets in the Sco X-1 system.

Abstract:

We present the discovery of the radio afterglow of the most distant ultralong gamma-ray burst (GRB) detected to date, GRB 220627A at redshift z = 3.084. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ∼15 minutes, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the Fermi Gamma-ray Burst Monitor spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456 days postburst: an initial, steep decay (Fν ∝ t−2) is followed by a shallower decline (Fν ∝ t−1/2) after ∼20 days. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as n ∝ r−8/3; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via very long baseline interferometry (VLBI) observations at ∼12 days postburst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.

Abstract:

We present the early radio detection and multiwavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift z = 0.257. The Australia Telescope Compact Array automatically triggered a 9 hr observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hr post-burst. Splitting this observation into 1 hr time bins, the early radio afterglow exhibited flaring, scintillating and plateau phases. The scintillation allowed us to place the earliest upper limit (<10 hr) on the size of a GRB blast wave to date, constraining it to <1 × 1016 cm. Multiwavelength modeling of the full afterglow required a period of significant energy injection between ∼0.02 and 1 day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of p = 1.66 ± 0.01. Our model also indicates a jet break at ∼2 days, and a half-opening angle of θj=16.°6±1.°1 . Following the period of injection, the total energy is ζ ∼ 18 times the initial impulsive energy, with a final collimation-corrected energy of EKf ∼ 5.7 × 1049 erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of Γ > 35 to Γ > 5 between ∼0.1 and 1 day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour.