VLA Searches for Fast Radio Transients
at 1 TB hour\(^{-1}\)

C. J. Law (UCB), G. C. Bower (ASIAA, UCB), S. Burke-Spolaor (Caltech, JPL), B. Butler (NRAO), E. Lawrence (LANL), T. J. W. Lazio (JPL), C. Mattmann (JPL), M. Rupen (NRAO), A. Siemion (UCB), S. VanderWiel (LANL)

Fast Radio Transients

Fast radio transients are pulses of dispersed radio emission lasting less than 1 second. Slower radio transients originate predominantly in synchrotron emission, while faster transients are caused by coherent processes. Furthermore, at timescales faster than 1 second, propagation through the Galactic plasma induces dispersion, the frequency-dependent arrival time quantified by dispersion measure (DM), that begins to be detectable at MHz through GHz radio frequencies.

Fast, Extragalactic Bursts

Fast transients surveys at the Parkes Observatory has revealed a new population of radio transients: the FRB (Lorimer 2007, Thornton 2013). Their DMs range up to 1100 pc cm\(^{-3}\), an order of magnitude larger than expected from the Galaxy and consistent with propagation through the IGM from distances up to z\(\sim\)1.

Basic questions about FRBs remain open: What are they? and How can we use them? If they do in fact lie at cosmological distances, their dispersion can measure the baryonic mass of the IGM, much as Galactic pulsars of known distance have mapped the electron content of the Milky Way (Cordes 2002). Beyond using FRBs as probes, understanding the origin of FRBs may have relevance to gamma-ray bursts and sources of gravitational waves (Falcke 2013, Kashiyama 2013).

Pulsars, RRATs

Similar pulsar surveys have discovered a new class of Galactic radio transient: the rotating radio transient (McLaughlin 2006). It is unclear whether extreme objects like magnetars or ordinary pulsars can generate pulses detected as RRATs (Weltevrede 2006).

Moving slightly beyond our Galaxy, the most distant radio transients associated with a host galaxy are in M31 (Rubio-Herrera 2013). The dispersion measure of any pulses known to be in M31 would make the first constraint on the Milky Way and M31 halo baryon content, which would help address the “missing baryon problem” (citation not found: 2007ARA&A..45..221B).

Flare Stars, Ultracool Dwarfs, Exoplanets

Jupiter emits intense radio bursts that make it the brightest astronomical object in the solar system below 100 MHz. Coronal mass ejections (much as seen in the Sun), also drive radio fast, coherent radio flares. These processes could be used to measure magnetism and plasma properties of other stars (Hallinan 2007) and should profoundly affect the habitability of orbiting exoplanets (Tarter 2007). transients.

Fast Imaging with the VLA

While single-dish telescopes have pioneered the study of fast radio transients, interferometers are poised to transform the field. Interferometers form “synthetic” apertures many kilometers in diameter, which allows them to expand on every limitation of single-dish telescopes:

  • Precise localization: Interferometers image with arcsecond precision, as shown in the image of a pulsar pulse shown in Figure \ref{candplot}.

  • High survey speed: Interferometers have large fields of view and are powerful survey machines.

  • Robust calibration and interference rejection: Interferometers can measure fluxes more accurately and reject interference that complicates single-dish fast transient searches.

Interferometers are technically more demanding than single-dish telescopes because their fundamental measurement is the correlation of pairs of antennas. Thus, where a single-dish telescope has a single data stream (or a few, if using a multi-beam receiver), a comparable interferemeter like the VLA has 27 antennas and thus 351 data streams. An efficient algorithm for extracting transients from this massive data stream could revolutionize the study of fast transients by uniquely associating radio transients with multiwavelength counterparts (e.g., FRB host galaxies, RRAT NS hosts, stellar/planetary associations).

We have commissioned the Jansky Very Large Array (VLA) to observe with millisecond integrations and data rates of 1 TB hour\(^{-1}\) (Law 2012). We have also developed an extensive, parallelized software system to search visibility data for dispersed transients1. The pipeline is written in Python/Cython and run within the NRAO software package CASA 2.

  1. Portions of the code base are available at http://github.com/caseyjlaw/tpipe.

  2. See http://casa.nrao.edu.

VLA image of a dispersed millsecond pulse from pulsar B0355+54. Our VLA FRB survey has mostly observed in “B” configuration, which has a resolution of 4 arcsec and localization precision roughly 10 times better. \label{candplot}

Survey for Fast Radio Bursts

We are now conducting a large VLA survey for the highly-dispersed radio transients known as FRBs. The goal of the survey is to detect at least one FRB, localize it to arcsecond precision, and uniquely associate it with other objects. Assuming that the FRB has a host galaxy, unique associations can be made with arcsecond localizations out to a redshift of 1 (Bloom 2002).

This survey uses the VLA correlator to write an integration each 5 ms. Faster integrations would be more sensitive to the \(\sim1\) ms FRB pulse widths, but those data rates are not sustainable. Assuming that FRBs uniformly populate a cosmological volume, we expect to detect one FRB in roughly 35 hours of observing. We have targeted five locations at high Galactic latitudes to avoid confusion with Galactic dispersion.

Our goal is to observe 150 hours to detect 1–10 FRBs or exclude the published event rate with 99% confidence. At the time of this writing (January 2014), we have observed 78 hours and processed roughly half of that. No events have been found brighter than our confidence threshold of 8\(\sigma\), which is equivalent to a flux density of 130 mJy. At this threshold, we expect less than one false positive due to Gaussian noise in the entire survey.

The transient search pipeline is currently running on the NRAO Array Operations Center (AOC) cluster and on the “Darwin” cluster at Los Alamos National Lab (LANL). Data is transferred to LANL by mailing disks. We also have approved compute time and storage on the NERSC compute center. The search pipeline parallelizes DM trials over cores of a node and different time segments (“scans” in VLA parlance) are parallelized over nodes of the cluster.

The processing time and memory footprint are dominated by the FFT stage. The size of the image grid is determined by the VLA antenna configuration and ranges from 512–2048 on a side. In the more compact of these configurations (called “CnB”), the processing pipeline can search one hour of data in 70 hours on a single node, equivalent to roughly 340 images per second per node. The majority of our data were observed in a larger configuration (called “B”) and processed several times slower.