Searching for Low Frequency Radio Transients Steve Ellingson Virginia Polytechnic Institute & State University September 1, 2006
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
MHz38)s4.4(
MHz38)s6.0(
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⎜⎝⎛≤≤⎟
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⎜⎝⎛ υυ T
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315
MHz38MHz18pc/cm56.8DM)s155(
pc/cmDM)s103.8(
−
−
⎟⎠⎞
⎜⎝⎛⎟⎠⎞
⎜⎝⎛ ∆⎟⎟⎠
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⎛=
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υυ
υυτ
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(~
−
⎟⎠⎞
⎜⎝⎛ υT ?
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
erco
nnec
t Mat
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
5 G
b/s
Seria
l Int
erco
nnec
t Mat
rix
• 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