A Distant Fast Radio Burst Associated with Its Host Galaxy by the Very Large Array
Summary (2 min read)
- Fast radio bursts (FRBs) are millisecond-timescale radio transients of extremely high brightness originating at cosmological distances (Cordes & Chatterjee 2019; Petroff et al. 2019).
- Several FRBs have been localized by radio interferometers and associated with host galaxies of known distance; their luminosity distances range from 149Mpc to 4 Gpc (Tendulkar et al.
2.1. Program and Overall Description
- In 2018, the VLA and CHIME/FRB teams began collaborating to use the VLA for follow-up of repeating FRBs found by CHIME/FRB.
- The authors observed using the L-band system of the VLA, spanning 1–2 GHz , in 20 separate observations.
- Each observation had an onsource time of around 1.5 hr that was searched by the realfast system.
2.2. Search Technique
- The observations used a commensal correlator mode that generated visibilities with an integration time of 5ms to be searched by realfast.
- Prior to that, all visibilities were saved to the archive at their full time resolution, resulting in large data sets (of order 1.5 TB).
- After applying available online calibrations, the search pipeline dedispersed and integrated visibilities in time before forming images.
- Images were generated with a simple, custom algorithm that uses natural weighting and a pillbox gridding scheme.
- 4. Verification Tests and Significance Analysis Traditional fast transient surveys measure event significance based on a noise estimate that is local in time (e.g., a standard deviation of a time series).
- Noise-like events are expected to be sensitive to the image gridding parameters, so the authors ignore all events that cannot be reproduced in larger images.
- The authors visually inspected the 263 candidates detected above 7.5σ in observations of this field and removed those affected by unflagged interference to get a sample of 31 candidates.
- The FRB search pipeline also uses spectral brightness fluctuations to distinguish candidate events from noise (Law et al.
- The real-time FRB search software makes several assumptions to improve computational efficiency, and as a result images that are used within it are not optimal.
- For all observations, the flux density scale was set with an observation of the calibrator source 3C147, and at these frequencies is accurate to 1%–2% (Perley & Butler 2017).
- The imaging and self-calibration were then repeated using this self-calibrated data set, on a 9 s timescale—essentially selfcalibrating every visibility.
- The image has a 1σ sensitivity of3.6 μJybeam-1, consistent with expectations for the total on-source time and flagging.
- The sensitivity of CHIME/FRB varies with observing epoch, position along transit, and burst spectral shape.
2.7. Optical Associations
- The authors considered the significance of this candidate high enough to trigger observations designed to find an optical counterpart.
- On UT 2019 November 26, the authors obtained an additional set of ´18 200 s z-band images of the FRB field with the Alhambra Faint Object Spectrograph and Camera on the Nordic Optical Telescope (NOT).
- The images were processed with standard procedures and astrometrically calibrated to the Gaia-DR2 reference frame.
- The 1σ radio localization region overlaps with sourceB, but the 2σ (90% confidence interval) radio localization region overlaps with sourceA.
- In Table 2, the authors report estimates for the stellar mass and rest-frame u−r color with the latter reflective of the inferred star-forming properties of each galaxy.
3.1. Joint Probability of Radio Candidate with Optical Association
- The chance of randomly associating a point on the sky with a galaxy has previously been studied in the context of gammaray bursts.
- 19 Following the same approach the authors estimate chance association probabilities Note.
- Estimates for M* and u−r are based on the photometric redshift and bear great uncertainty.
- Under the assumption that an FRB should reside in a galaxy, the authors can use the host galaxy association to improve the confidence in the significance of the candidate event.
- This naming convention is consistent with a new standard developed by several groups in the FRB community.
3.2. FRB Host Galaxy and DM
- The identification of a specific FRB host galaxy can be critical for both estimating the likely host DM contribution to the total observed DM, and for identifying trends in FRB host galaxy types and environments, which can in turn help discriminate between FRB origin models.
- One galaxy is the foreground object to the other.
- Regarding local contributors to DM, for contribution from the interstellar medium of the Milky Way, the authors adopt the value of 83.5 -pc cm 3 predicted by Cordes & Lazio (2002).
- Figure 7 shows the expected probability distribution of nonlocal DM components using the model of Prochaska & Zheng (2019).
- The authors present the discovery of FRB 20190614D by VLA/ realfast, the first FRB discovered blindly via interferometric imaging.
- Data were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among Caltech, the University of California, and the National Aeronautics and Space Administration (NASA).
- The authors therefore must have a noise standard deviation estimate that is good to»1%, which has less than a factor of two uncertainty in the chance occurrence probability.
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Cites background from "A Distant Fast Radio Burst Associat..."
...…et al. 2019; Bhandari et al. 2020a), 181112 (Prochaska et al. 2019b), 190102 (Macquart et al. 2020; Bhandari et al. 2020a), 190523 (Ravi et al. 2019), 190608 (Macquart et al. 2020; Bhandari et al. 2020a; Chittidi et al. 2020), and 20190614D (hereafter referred to as FRB 190614; Law et al. 2020)....
...Realfast FRB 190614: Law et al. (2020) report the first putative detection of an FRB from the realfast collaboration (Law et al. 2018), FRB 20190614D (here referred to as FRB 190614)....
...For the host of FRB 190614, no constraints could be placed on the SFR due to the uncertain nature of the host galaxy and redshift (Sample D; Law et al. 2020)....
...For FRB 190614, Law et al. (2020) identifies two potential host galaxy candidates, for which only photometric redshifts have been obtained, placing it in Sample D....
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Frequently Asked Questions (2)
Q1. What have the authors contributed in "A distant fast radio burst associated with its host galaxy by the very large array" ?
The authors present the discovery and subarcsecond localization of a new fast radio burst ( FRB ) by the Karl G. Jansky Very Large Array ( VLA ) and realfast search system. The authors describe a suite of statistical and data quality tests they used to verify the significance of the event and its localization precision. Comparing the host distance to that implied by the dispersion measure suggests a modest ( ~ 50 pc cm 3 ) electron column density associated with the FRB environment or host galaxy/galaxies.
Q2. What have the authors stated for future works in "A distant fast radio burst associated with its host galaxy by the very large array" ?
In the future, the system will transition to a community service mode, in which real-time alerts are distributed automatically. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. The authors describe the detection of an event with an S/N estimate on the border between statistically significant and not. Their case was further complicated by the fact that the candidate FRB was discovered while the realfast system was turning on, so the number of recorded visibilities changes as a function of frequency/baseline/time.