The Square Kilometre Array (SKA)

Abstract:

Abstract Previous work detected transient ultraviolet outflow features for the short-period (Porb ≈ 111 min), low-mass X-ray binary (LMXB) UW CrB, suggesting the presence of a disc wind in the system. However, because of the transient nature of the outflow features, and the limited amount of data available, the features were challenging to interpret. To follow up on this work, we present a comprehensive multi-wavelength campaign on UW CrB. We observe complex phenomenology and find several features that could be naturally interpreted as being associated with a persistent disc wind. Moreover, we identify a blue-shifted absorption in the Hβ line during one of the epochs, which might be the signature of such an outflow. We present an X-ray to radio campaign of the source, discuss our results in the context of accretion disc wind outflows, present a ‘toy model’ interpretation of the outflow scattering the X-ray emission into our line of sight, and explore the implications for binary evolution models. If correct, our preferred scenario of a persistent disc wind suggests that mass transfer for LMXBs can be non-conservative down to short orbital periods, and thereby opens an important parameter space for angular momentum loss in compact binaries.

Abstract:

Abstract A tidal disruption event (TDE) occurs when a star travels too close to a supermassive black hole. In some cases, accretion of the disrupted material onto the black hole launches a relativistic jet. In this paper, we present a long-term observing campaign to study the radio and submillimeter emission associated with the fifth jetted/relativistic TDE: AT 2022cmc. Our campaign reveals a long-lived counterpart. We fit three different models to our data: a nonthermal jet, a spherical outflow consisting of both thermal and nonthermal electrons, and a jet with thermal and nonthermal electrons. We find that the data are best described by a relativistic spherical outflow propagating into an environment with a density profile following R −1.8. Comparison of AT 2022cmc to other TDEs finds agreement in the density profile of the environment but also that AT 2022cmc is twice as energetic as the other well-studied relativistic TDE, Swift J1644. Our observations of AT 2022cmc allow a thermal electron population to be inferred for the first time in a jetted transient, providing new insights into the microphysics of relativistic transients jets.

Abstract:

A key science goal of large sky surveys such as those conducted by the Vera C. Rubin Observatory and precursors to the Square Kilometre Array is the identification of variable and transient objects. One approach is analyzing time series of the changing brightness of sources, namely, light curves. However, finding adequate statistical representations of light curves is challenging because of the sparsity of observations, irregular sampling, and nuisance factors inherent in astronomical data collection. The wide diversity of objects that a large-scale survey will observe also means that making parametric assumptions about the shape of light curves is problematic. We present a Gaussian process (GP) regression approach for characterizing light-curve variability that addresses these challenges. Our approach makes no assumptions about the shape of a light curve and, therefore, is general enough to detect a range of variable and transient source types. In particular, we propose using the joint distribution of GP amplitude hyperparameters to distinguish variable and transient candidates from nominally stable ones and apply this approach to 6394 radio light curves from the ThunderKAT survey. We compare our results with two variability metrics commonly used in radio astronomy, namely ην and Vν, and show that our approach has better discriminatory power and interpretability. Finally, we conduct a rudimentary search for transient sources in the ThunderKAT data set to demonstrate how our approach might be used as an initial screening tool. Computational notebooks in Python and R are available to help deploy this framework to other surveys.

Abstract:

Abstract We investigate the clustering of Lyman-break galaxies at redshifts of 3 ≲ z ≲ 5 within the COSMOS field by measuring the angular two-point correlation function. Our robust sample of ~60,000 bright (mUV ≲ 27) Lyman-break galaxies was selected based on spectral energy distribution fitting across 14 photometric bands spanning optical and near-infrared wavelengths. We constrained both the 1- and 2-halo terms at separations up to 300 arcsec, finding an excess in the correlation function at scales corresponding to <20 kpc, consistent with enhancement due to clumps in the same galaxy or interactions on this scale. We then performed Bayesian model fits on the correlation functions to infer the Halo Occupation Distribution parameters, star formation duty cycle, and galaxy bias in three redshift bins. We examined several cases where different combinations of parameters were varied, showing that our data can constrain the slope of the satellite occupation function, which previous studies have fixed. For an MUV-limited sub-sample, we found galaxy bias values of $b_g=3.18^{+0.14}_{-0.14}$ at z ≃ 3, $b_g=3.58^{+0.27}_{-0.29}$ at z ≃ 4, $b_g=4.27^{+0.25}_{-0.26}$ at z ≃ 5. The duty cycle values are $0.62^{+0.25}_{-0.26}$, $0.40^{+0.34}_{-0.22}$, and $0.39^{+0.31}_{-0.20}$,respectively. These results suggest that, as the redshift increases, there is a slight decrease in the host halo masses and a shorter timescale for star formation in bright galaxies, at a fixed rest-frame UV luminosity threshold.

Abstract:

Abstract Using deep observations across three of the LOFAR Two-metre Sky Survey Deep Fields, this work measures the angular clustering of star forming galaxies (SFGs) and low-excitation radio galaxies (LERGs) to z ≲1.5 for faint sources, S144 MHz ≥200 μJy. We measure the angular auto-correlation of LOFAR sources in redshift bins and their cross-correlation with multi-wavelength sources to measure the evolving galaxy bias for SFGs and LERGs. Our work shows the bias of the radio-selected SFGs increases from $b=0.90^{+0.11}_{-0.10}$ at z ∼0.2 to $b=2.94^{+0.36}_{-0.36}$ at z ∼1.2; faster than the assumed b($z$)∝1/D($z$) models adopted in previous LOFAR cosmology studies (at sensitivities where AGN dominate), but in broad agreement with previous work. We further study the luminosity dependence of bias for SFGs and find little evidence for any luminosity dependence at fixed redshift, although uncertainties remain large for the sample sizes available. The LERG population instead shows a weaker redshift evolution with $b=2.33^{+0.28}_{-0.27}$ at z ∼0.7 to $b=2.65^{+0.57}_{-0.55}$ at z ∼1.2, though it is also consistent with the assumed bias evolution model (b($z$)∝1/D($z$)) within the measured uncertainties. For those LERGs which reside in quiescent galaxies (QLERGs), there is weak evidence that they are more biased than the general LERG population and evolve from b=$2.62^{+0.33}_{-0.33}$ at z ∼0.7 to $b=3.08^{+0.85}_{-0.84}$ at z ∼1.2. This suggests the halo environment of radio sources may be related to their properties. These measurements can help constrain models for the bias evolution of these source populations, and can help inform multi-tracer analyses.