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    We constrain the Hubble constant H0 using Fast Radio Burst (FRB) observations from the Australian Square Kilometre Array Pathfinder (ASKAP) and Murriyang (Parkes) radio telescopes. We use the redshift-dispersion measure (‘Macquart’) relationship, accounting for the intrinsic luminosity function, cosmological gas distribution, population evolution, host galaxy contributions to the dispersion measure (DMhost), and observational biases due to burst duration and telescope beamshape. Using an updated sample of 16 ASKAP FRBs detected by the Commensal Real-time ASKAP Fast Transients (CRAFT) Survey and localized to their host galaxies, and 60 unlocalized FRBs from Parkes and ASKAP, our best-fitting value of H0 is calculated to be $73_{-8}^{+12}$ km s−1 Mpc−1. Uncertainties in FRB energetics and DMhost produce larger uncertainties in the inferred value of H0 compared to previous FRB-based estimates. Using a prior on H0 covering the 67–74 km s−1 Mpc−1 range, we estimate a median ${\rm DM}_{\rm host}= 186_{-48}^{+59}\,{\rm pc \, cm^{-3}}$, exceeding previous estimates. We confirm that the FRB population evolves with redshift similarly to the star-formation rate. We use a Schechter luminosity function to constrain the maximum FRB energy to be log10Emax$=41.26_{-0.22}^{+0.27}$ erg assuming a characteristic FRB emission bandwidth of 1 GHz at 1.3 GHz, and the cumulative luminosity index to be $\gamma =-0.95_{-0.15}^{+0.18}$. We demonstrate with a samplemore »of 100 mock FRBs that H0 can be measured with an uncertainty of ±2.5 km s−1 Mpc−1, demonstrating the potential for clarifying the Hubble tension with an upgraded ASKAP FRB search system. Last, we explore a range of sample and selection biases that affect FRB analyses.

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  2. Abstract We investigate the fine-structure [C ii ] line at 158 μ m as a molecular gas tracer by analyzing the relationship between molecular gas mass ( M mol ) and [C ii ] line luminosity ( L [C II ] ) in 11,125 z ≃ 6 star-forming, main-sequence galaxies from the simba simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [C ii ] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) log L [ C II ] – log M mol relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies at z ≲ 6. We derive a median [C ii ]-to- M mol conversion factor of α [C II ] ≃ 18 M ⊙ / L ⊙ . This is lower than the average value of ≃30 M ⊙ / L ⊙ derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [C ii ] observations ofmore »low-mass galaxies. For our simulations, [C ii ] is a better tracer of the molecular gas than CO J = 1–0, especially at the lowest metallicities, where much of the gas is CO-dark . We find that L [C II ] is more tightly correlated with M mol than with star formation rate (SFR), and both the log L [ C II ] – log M mol and log L [ C II ] – log SFR relations arise from the Kennicutt–Schmidt relation. Our findings suggest that L [C II ] is a promising tracer of the molecular gas at the earliest cosmic epochs.« less
    Free, publicly-accessible full text available April 1, 2023
  3. We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z  = 6.312 and its luminous X-ray and optical afterglow. Following the detection by Swift and Konus- Wind , we obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of E iso = 1.27 −0.19 +0.20 × 10 54 erg, GRB 210905A lies in the top ∼7% of gamma-ray bursts (GRBs) in the Konus- Wind catalogue in terms of energy released. Its afterglow is among the most luminous ever observed, and, in particular, it is one of the most luminous in the optical at t  ≳ 0.5 d in the rest frame. The afterglow starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ∼46.2 ± 16.3 d (6.3 ± 2.2 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band duemore »to a constant contribution from the host galaxy and potentially from a foreground intervening galaxy. In particular, the host galaxy is only the fourth GRB host at z  > 6 known to date. By assuming a number density n  = 1 cm −3 and an efficiency η  = 0.2, we derived a half-opening angle of 8.4 ° ±1.0°, which is the highest ever measured for a z  ≳ 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of ≃1 × 10 52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent within 2 σ with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.« less
    Free, publicly-accessible full text available September 1, 2023
  4. Context. Fast radio bursts (FRBs) are extremely energetic pulses of millisecond duration and unknown origin. To understand the phenomenon that emits these pulses, targeted and un-targeted searches have been performed for multiwavelength counterparts, including the optical. Aims. The objective of this work is to search for optical transients at the positions of eight well-localized (< 1″) FRBs after the arrival of the burst on different timescales (typically at one day, several months, and one year after FRB detection). We then compare this with known optical light curves to constrain progenitor models. Methods. We used the Las Cumbres Observatory Global Telescope (LCOGT) network to promptly take images with its network of 23 telescopes working around the world. We used a template subtraction technique to analyze all the images collected at differing epochs. We have divided the difference images into two groups: In one group we use the image of the last epoch as a template, and in the other group we use the image of the first epoch as a template. We then searched for optical transients at the localizations of the FRBs in the template subtracted images. Results. We have found no optical transients and have therefore set limiting magnitudesmore »to the optical counterparts. Typical limits in apparent and absolute magnitudes for our LCOGT data are ∼22 and −19 mag in the r band, respectively. We have compared our limiting magnitudes with light curves of super-luminous supernovae (SLSNe), Type Ia supernovae (SNe Ia), supernovae associated with gamma-ray bursts (GRB-SNe), a kilonova, and tidal disruption events (TDEs). Conclusions. Assuming that the FRB emission coincides with the time of explosion of these transients, we rule out associations with SLSNe (at the ∼99.9% confidence level) and the brightest subtypes of SNe Ia, GRB-SNe, and TDEs (at a similar confidence level). However, we cannot exclude scenarios where FRBs are directly associated with the faintest of these subtypes or with kilonovae.« less