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1. Comprehensive Analysis of a Dense Sample of FRB 121102 Bursts

   https://doi.org/10.3847/1538-4357/ac2577  

   Aggarwal, Kshitij ; Agarwal, Devansh ; Lewis, Evan F. ; Anna-Thomas, Reshma ; Tremblay, Jacob Cardinal ; Burke-Spolaor, Sarah ; McLaughlin, Maura A. ; Lorimer, Duncan R. ( November 2021 , The Astrophysical Journal)

   Abstract We present an analysis of a densely repeating sample of bursts from the first repeating fast radio burst, FRB 121102. We reanalyzed the data used by Gourdji et al. and detected 93 additional bursts using our single-pulse search pipeline. In total, we detected 133 bursts in three hours of data at a center frequency of 1.4 GHz using the Arecibo telescope, and develop robust modeling strategies to constrain the spectro-temporal properties of all of the bursts in the sample. Most of the burst profiles show a scattering tail, and burst spectra are well modeled by a Gaussian with a median width of 230 MHz. We find a lack of emission below 1300 MHz, consistent with previous studies of FRB 121102. We also find that the peak of the log-normal distribution of wait times decreases from 207 to 75 s using our larger sample of bursts, as compared to that of Gourdji et al. Our observations do not favor either Poissonian or Weibull distributions for the burst rate distribution. We searched for periodicity in the bursts using multiple techniques, but did not detect any significant period. The cumulative burst energy distribution exhibits a broken power-law shape, with the lower- andmore »higher-energy slopes of −0.4 ± 0.1 and −1.8 ± 0.2, with the break at (2.3 ± 0.2) × 10 37 erg. We provide our burst fitting routines as a Python package burstfit 4 4 https://github.com/thepetabyteproject/burstfit that can be used to model the spectrogram of any complex fast radio burst or pulsar pulse using robust fitting techniques. All of the other analysis scripts and results are publicly available. 5 5 https://github.com/thepetabyteproject/FRB121102« less
2. The First CHIME/FRB Fast Radio Burst Catalog

   https://doi.org/10.3847/1538-4365/ac33ab  

   Amiri, Mandana ; Andersen, Bridget C. ; Bandura, Kevin ; Berger, Sabrina ; Bhardwaj, Mohit ; Boyce, Michelle M. ; Boyle, P. J. ; Brar, Charanjot ; Breitman, Daniela ; Cassanelli, Tomas ; et al ( December 2021 , The Astrophysical Journal Supplement Series)

   We present a catalog of 536 fast radio bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project between 400 and 800 MHz from 2018 July 25 to 2019 July 1, including 62 bursts from 18 previously reported repeating sources. The catalog represents the first large sample, including bursts from repeaters and nonrepeaters, observed in a single survey with uniform selection effects. This facilitates comparative and absolute studies of the FRB population. We show that repeaters and apparent nonrepeaters have sky locations and dispersion measures (DMs) that are consistent with being drawn from the same distribution. However, bursts from repeating sources differ from apparent nonrepeaters in intrinsic temporal width and spectral bandwidth. Through injection of simulated events into our detection pipeline, we perform an absolute calibration of selection effects to account for systematic biases. We find evidence for a population of FRBs—composing a large fraction of the overall population—with a scattering time at 600 MHz in excess of 10 ms, of which only a small fraction are observed by CHIME/FRB. We infer a power-law index for the cumulative fluence distribution of$\alpha =-1.40\pm 0.11(\mathrm{stat.}{)}_{-0.09}^{+0.06}(\mathrm{sys.})$, consistent with the −3/2more »expectation for a nonevolving population in Euclidean space. We find thatαis steeper for high-DM events and shallower for low-DM events, which is what would be expected when DM is correlated with distance. We infer a sky rate of$[820\pm 60(\mathrm{stat.}{)}_{-200}^{+220}(\mathrm{sys.})]/\mathrm{sky}/\mathrm{day}$above a fluence of 5 Jy ms at 600 MHz, with a scattering time at 600 MHz under 10 ms and DM above 100 pc cm⁻³.

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3. A Toy Model for the Time–Frequency Structure of Fast Radio Bursts: Implications for the CHIME/FRB Burst Dichotomy

   https://doi.org/10.3847/1538-4357/ac3b4a  

   Metzger, Brian D. ; Sridhar, Navin ; Margalit, Ben ; Beniamini, Paz ; Sironi, Lorenzo ( February 2022 , The Astrophysical Journal)

   We introduce a toy model for the time–frequency structure of fast radio bursts, in which the observed emission is produced as a narrowly peaked intrinsic spectral energy distribution sweeps down in frequency across the instrumental bandpass as a power law in time. Though originally motivated by emission models that invoke a relativistic shock, the model could in principle apply to a wider range of emission scenarios. We quantify the burst’s detectability using the frequency bandwidth over which most of its signal-to-noise ratio is accumulated. We demonstrate that, by varying just a single parameter of the toy model—the power-law indexβof the frequency drift rate—one can transform a long (and hence preferentially time-resolved) burst with a narrow time-integrated spectrum into a shorter burst with a broad power-law time-integrated spectrum. We suggest that source-to-source diversity in the value ofβcould generate the dichotomy between burst duration and frequency-bandwidth recently found by CHIME/FRB. In shock models, the value ofβis related to the radial density profile of the external medium, which, in light of the preferentially longer duration of bursts from repeating sources, may point to diversity in the external environments surrounding repeating versus one-off FRB sources.
4. Molecular Gas Reservoirs in Massive Quiescent Galaxies at z ∼ 0.7 Linked to Late-time Star Formation

   https://doi.org/10.3847/1538-4357/ac9af7  

   Woodrum, Charity ; Williams, Christina C. ; Rieke, Marcia ; Leja, Joel ; Johnson, Benjamin D. ; Bezanson, Rachel ; Kennicutt, Robert ; Spilker, Justin ; Tacchella, Sandro ( November 2022 , The Astrophysical Journal)

   We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies atz∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges fromf_burst≈ 0.3%–6% and ended betweent_end-burst≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr ($f_{M_{1\mathrm{Gyr}}}=2.6\%\pm 1.8\%$) compared to the CO-undetected galaxies ($f_{M_{1\mathrm{Gyr}}}=0.2\%\pm 0.1\%$). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven bymore »recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFR_UV+IRmeasurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

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5. Relativistic ocean r-modes during type-I X-ray bursts

   https://doi.org/10.1093/mnras/stz3449  

   Chambers, F. R. N. ; Watts, A. L. ( December 2019 , Monthly Notices of the Royal Astronomical Society)

   Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. These frequencies can be offset from the NS spin frequency by several Hz (where known independently) and can drift by 1–3 Hz. One plausible explanation is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset. The frequency of the wave should decrease (in the rotating frame) as the burst cools hence explaining the drift. A strong candidate is a buoyant r-mode. To date, models that calculated the frequency of this mode taking into account the radial structure neglected relativistic effects and predicted rotating frame frequencies of ∼4 Hz and frequency drifts of >5 Hz; too large to be consistent with observations. We present a calculation that includes frame-dragging and gravitational redshift that reduces the rotating frame frequency by up to $30 \, {\rm per\, cent}$ and frequency drift by up to $20 \, {\rm per\, cent}$. Updating previous models for the ocean cooling in the aftermath of the burst to a model more representative of detailed calculations of thermonuclear X-ray bursts reduces the frequency of the mode still further. This model, combined withmore »relativistic effects, can reduce the rotating frequency of the mode to ∼2 Hz and frequency drift to ∼2 Hz, which is closer to the observed values.

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