Searching for Low FrequencyRadio Transients

Steve EllingsonVirginia Polytechnic Institute & State University

September 1, 2006

Transients

Discovery of astronomical events occurring over short timeframes tends to be a surprise. Examples:– GRBs– Pulsars (periodic emission, then giant pulses, then nanoGPs)– Recent (many low-frequency) transient detections

Discovery of the Crab PulsarStaelin & Reifenstein 1968

Transients

Discovery of astronomical events occurring over short timeframes tends to be a surprise. Examples:– GRBs– Pulsars (periodic emission, then giant pulses)– Recent (many low-frequency) transient detections

Interesting!– “Extreme physics”– Probes for exploring the interstellar / intergalactic medium

(Inoue 2004)– Ready-made laboratories for exploring the frontiers of physics?

Seeking Out Transient Sources

Reasonable to expect there are lots of new things to find

But, existing telescopes not great for this– Collecting area is important, but spatial resolution is not a big deal– In a blind search for rare events, FOV is a big deal

New instruments operating at low frequencies (< 300 MHz) perhaps better place to start– Interesting science case for sources in this wavelength regime– Ae ∝ λ2, so individual dipoles deliver serious collecting area – Big FOV possible using either single dipoles or multibeaming arrays– Galactic synchrotron emission dominates system temperature

(cheap front ends deliver best possible sensitivity)

Temperature Measured by a Dipole

~300,000 K at 10 MHz

~800 K at 100 MHz

Instrument-DominatedTsys

Ionosphere becoming

opaque

Galactic Noise-Dominated Tsys

Possible Sources of Low Frequency Transients

Exploding primordial black holes (Rees 1977)

– Current density bound < 4.8 x 10-3 pc-3 yr-1(Benz & Paesold 1998)

GRB prompt emission– Pulse mechanisms (Benz & Paesold 1998, Usov & Katz 2000)

– Maser-type emission (Sagiv & Waxman 2002)

Supernovae prompt emission (Colgate 1975, Meikle & Colgate 1978)

Coalescing exotic binary systems– A few NS–NS mergers per year (Hansen & Lyutikov 2001)

– NS–BH merger rate higher, but emission weaker

UHECRs (air showers)

All the other stuff we can’t imagine yet…

Propagation of Pulses at Low FrequenciesPlasma delay

Scatter broadening

Scintillation – ISM: Non issue (very narrow coherence bandwidth)– Interplanetary: Possibly significant; 30-50% over seconds and MHz– Ionosphere: Probably only a few %

4.44

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For Crab using PLFM or ETA

For Crab

Crab GPs at 23 MHz

Large scatter is computed spectral indices

Still lots to do…

23 MHz Detection of a Crab GPs

by UTR-2 (Popov et al. 2006, astro-ph/0606025)

Empirical scatter broadening expression

5.3

MHz38)s6.0(~

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VT Pilot Experiment (PLFM)D. Wilson MS Thesis (2005)

Pisgah Astronomical Research Institute, Western North Carolina

NRL (LWA prototype) “fat dipole” + active balun (c. 2003)

Direct sampling, 8 bits @ 200 MSPS

Off-line RFI mitigation & de-dispersion search

PLFM Detections > 5σ

Second Look: Detections > 6.5σ

Detection DM minimizes TAssociations…?

PLFMSite

ETASite

To Asheville

RFI Comparison: B28 vs. B17

Strong signals on ridge (B28)

Strong signals at proposed array site (B17)

…10-15 dB reduction in strong (linearity-threatening) RFI

TerrainShielding!

Eight meter wavelength Transient Array (ETA)

Continuous, all-sky, low-frequency, “source agnostic” search for single dispersed pulses with 10 < DM < 1000 pc cm-3

Array of 12 dual-polarized dipoles, Galactic noise-limited in 29-47 MHz(Ae ~ 476 m2 @ 38 MHz)

Sufficient collecting area to routinely obtain 5σ detections on Crab GPs with rate at least one a day

About 2-3 orders of magnitude improvement in over previous searches

PCPCPCPC

NodeNodeNodeNode

A/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IF

RFRFRFRFRFRFRFRFRFRFRFRF

Dip

ole

Arr

ayETA System Design

AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2

NodeNodeNodeNodeNodeNodeNodeNodeNodeNodeNodeNode

3.12

5 G

b/s

Seria

l Int

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nnec

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rix

ActiveBalun

LongCoax

120 MSPS x 12-bit(Digital Receiver,Channelization,RFI Mitigation,

Calibration)

432 Mb/sSerialLVDS

ReconfigurableComputer Cluster

(RCC)

(Beamforming,RFI Mitigation,Dedispersion)

4-NodePC Cluster

ParallelLVDS

Eventually,forms about 10 fixed beams covering sky

Antenna / Front End

•Front End:•T: < 400 K (250K)•Gain: 24 dB •P1dB: -3 dBm @ 38 MHz

“Sky-to-Front EndTransfer Characteristic”

-13 dBm at antenna terminals!

What an ETA A/D Sees

Galactic background convolved with antenna IME

Confirmation of Galactic Noise-Limited Sensitivity

• Total power in 1 MHz bandwidth• No RFI Mitigation applied !

29.5 MHz

34.5 MHz

29.0 MHz

41.5 MHz

46.0 MHzMinima at ~11:00 LSTMaxima at ~ 18:00 LST

Confirmation of Galactic Noise-Limited Sensitivity

2 observations 24 h apart @ Galactic max

2 observations 24 h apart @ Galactic min

Static sky model

• No RFI Mitigation applied !

ETA Search Range(29-47 MHz)

Digital Signal Processing / Data Recording120 MSPS A/Ds,FPGA-baseddigital receivers

“RCC”

PCCluster

Reconfigurable Computing Cluster (RCC)

• 16-node “Virtual FPGA”

• Each node is a development board with Xilinx XC2VP30 FPGA

• Edge nodes (“E”) catch streaming LVDS from digital receivers

• 3.125 Gb/s Infiniband-like interconnects

• Center nodes (“C”) route between RCC nodes & push results to PC cluster

• PPCs internal to FPGAs run Linux, perform GPP-type functions

XilinxML310

PCPCPCPC

NodeNodeNodeNode

A/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IFA/D-IF

RFRFRFRFRFRFRFRFRFRFRFRF

Dip

ole

Arr

ayETA PC Cluster

AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2AB x2

NodeNodeNodeNodeNodeNodeNodeNodeNodeNodeNodeNode

3.12

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• 4 Dell SC430 Linux PCs

• Each PC has ~700 GB HDD space organized as software RAID array(1-4 hours of streaming acquisition for DF1)

• Array of 400 GB LTO3 tape drives for archiving

First “All the Way Through” Test

• No RFI Mitigation applied !

Stand 12 (Outrigger)[Red]

Stand 1 (Core)[Blue]

NRL Sky Model(crudely scaled)

(no data)

RFI “whiteout”

Each cluster is 100 integrations,

5 MHz x 35 ms each

Wideband junk

Wideband junk

Wideband junk

Self-

Gen

erat

ed (P

C)

6-m

Am

ateu

r Rad

io

Ionospheric enhancement

Ionospheric enhancement

Citi

zen’

s B

and,

oth

er H

F

NC

Sta

te P

olic

e

Sneaky RFI Mechanisms…

Spectra (10 s)

Max Hold

Mean

Stability

Impulsive RFI

Quiet Period

Noisy Period

(Not really a problem unless time resolution of search

approaches ~100 µs)

Dedispersed Time Series (Crab GP Search)

Project Milestones

May 2004 PLFM (Pilot experiments on ridge)Aug 2005 NSF Project StartOct 2005 Demonstration of new Galactic noise-

limited antenna / front endNov 2005 Demonstration of direct sampling of

search bandwidth from the first four dipolesApr 2006 First array streaming / 2 hour acquisitionJuly 2006 Lightning strike – array damage Summer 2006 Repairing lightning damage

Commissioning / Algorithm (Re)developmentReceiver upgrade

August 2006 Anticoincidence system funded!Fall 2006 Commence routine observing at NC site

Develop anti-coincidence site

“ETA-2” Anti-Coincidence Site

Virginia Tech Campus Farm Operation / Kentland Farm – Near Radford, VA

• Map showing Kentland Farms and Interior

• For interior, show FSH3 comparison data (maybe on map)

West West VirginiaVirginia

SWSWVirginiaVirginia

BlacksburgBlacksburg

Candidate siteCandidate site(Interior, VA)(Interior, VA)

Candidate siteCandidate site(Kentland Farms)(Kentland Farms)

Project StatusCurrently operating with 1/3 of array; other 2/3 being rebuilt following lightning strike

“Commissioned” operating mode: 5 MHz bandwidth, acquisition of coherent time series from all dipoles direct to storage.

About 6 hours of observation on tape; about half sufficiently RFI-free to be useable; currently being analyzed for short pulses from 10-100 pc/cm3 (Nothing interesting yet.)

Plan to collection ~100 hrs on tape over next few months

Acknowledgements

Cameron Patterson (CpE)

John Simonetti(Phys)

Vivek Venugopal(CpE) Colin Ellingson

Sean Cutchin(Phys)

Brian Martin (CpE)

Wyatt Taylor (EE)

Anthony Lee (EE)

Zach Boor (Phys)

Caleb Magruder (EE)

Supported by the National Science Foundation

(AST-0504677)

Supported by the Virginia Tech Dept. of Physics