Transcript aas.hashville.28may03 - Cornell Astronomy

The Dynamic Radio Sky
and Future Instruments
Jim Cordes
Cornell University
AAS Meeting
Nashville
28 May 2003
Dynamic Radio Sky
• We know enough about the DRS to know that there
is a great deal yet to be discovered
• c.f. the high energy universe, optical, etc.
• What is in the DRS?
• What are the prospects for new discoveries?



Astrophysical parameters
Extrinsic effects
RFI
• Instruments & surveys that will reveal the DRS
TRANSIENT SOURCES
Sky Surveys:
The X-and--ray skies have been monitored highly successfully
with wide FOV detectors (e.g. RXTE/ASM, CGRO/BATSE).
Neutrino/gravitational wave detectors are ‘all sky.’
Optical transient surveys (ROTSE, RAPTOR, LSST) are/will
revolutionalize our knowledge of the optical transient sky and will
drive the trend toward data mining of » petabyte databases.
The transient radio sky (e.g. t < 1 month) is largely unexplored.
New objects/phenomena are likely to be discovered as well as
extreme cases in predictable classes of objects.
Ingredients for transient detection
AT needs to be “large”
A = collecting area
 = solid angle covered (instantaneous FOV)
T = time per sky position
Issue: to dwell (stare), or tile the sky, or be triggered?
Successes in transient astronomy
Vela
ROTSE
RAPTOR
RXTE/ASM
Why is the Dynamic Radio Sky
Largely Uncharted?
• Large collecting areas, A, needed for sensitivity
• Typically A is small enough that telescope
throughput is small
• Telescope time is expensive so dwell times are short
• Sources cover a wide range of time scale and sky
density
 insufficient sky and temporal coverage
Arecibo
Giant pulse from the
Crab pulsar
S ~ 160 x Crab Nebula
~ 200 kJy
Detectable to ~ 1.5 Mpc
with Arecibo
2-ns giant pulses from
the Crab: (Hankins et
al. 2003)
Giant Pulses seen
from B0540-69 in
LMC (Johnston &
Romani 2003)
Giant pulses are the fastest known transients
• Giant pulses from Crab detectable to
•
•
•
•
•
~1.5 Mpc with Arecibo @ 1/hour
2-ns wide `nano-Giant pulses’
identified from Crab (Hankins et al.
2003)
GPs seen from Crab clone in LMC
(B0540-69) by Johnston & Romani
(2003) w/ similar intrinsic amplitude
GPs from two millisecond pulsars
Radio GPs in pulse components also
seen in X-rays
GP-emitting objects have ~ same B
fields at their light cylinders
Nano-giant pulses (Hankins et al. 2003)
Arecibo
5 GHz
0.5 GHz bw
coherent
dedispersion
Solar Radio Bursts
STARE
611 MHz
3-station radio
transient
detector (Katz,
Hewitt, Corey,
Moore 2003)
GRB 980519 variability (Frail et al. 2000)
Interstellar
scintillations
TRANSIENT SOURCES
TARGET OBJECTS:
• Atmospheric/lunar pulses from neutrinos &
cosmic rays
• Accretion disk transients (NS, blackholes)
• Neutron star magnetospheres
• Supernovae
• Gamma-ray burst sources
• Brown dwarf flares (astro-ph/0102301)
• Planetary magnetospheres & atmospheres
• Maser spikes
• ETI
TRANSIENT SOURCES
TARGET PROCESSES:
• Intrinsic:
incoherent:
( inverse Compton brightness limit)
coherent: (virtually no limit)
continuum: low frequencies favored
spectral line: masers
• Extrinsic:
scintillation
maser-maser amplification
gravitational lensing
absorption events
Phase Space for Transients:
W=
light travel time
log SpkD2
Pulse
W
Process
SpkD2 vs. W
W
log W
Spk
brightness
temperature:
SpkD2
Tb = ------------2k (W)2
Phase Space for Transients:
log SpkD2
Pulse
W
Process
W
log W
Spk
SpkD2 vs. W
Lines of
constant
brightness
temperature
Phase Space for Transients:
log SpkD2
Pulse
W
Spk
SpkD2 vs. W
Solar
system
+
Process
W
log W
local
galactic
sources
Phase Space for Transients:
log SpkD2
Pulse
OH masers
W
Process
SpkD2 vs. W
W
log W
Spk
+
Pulsars
(including
giant pulses)
Phase Space for Transients:
log SpkD2
Pulse
W
Spk
SpkD2 vs. W
Cosmological
sources:
AGNs
(including
IDV sources)
Process
W
+
GRB
afterglows
log W
Phase Space for Transients:
log SpkD2
Pulse
W
Process
W
log W
Spk
SpkD2 vs. W
Phase Space for Transients:
log SpkD2
Pulse
W
Process
W
log W
Spk
SpkD2 vs. W
Interstellar
scintillations =
apparent fast
variations of
IDVs & GRBs
New instruments can cover this phase space
log SpkD2
Pulse
W
Process
W
log W
Spk
Exploring the Transient Radio Sky:
Striving for large AT
• Pilot observations:





Arecibo: single pixel and multibeam (ALFA)
STARE and similar multisite arrays
GBT: single pixel and multibeam arrays
ATA: 2.5 deg FOV, ~8 array beams
EVLA (wideband, high sensitivity & spatial resolution)
• LOFAR: low frequencies (< 240 MHz)
• SKA: broad frequency range (0.15 to 25 GHz)
Giant pulses from M33
Arecibo observations
(Maura Mclaughlin & Cordes, submitted to ApJ, astro-ph
Galactic Center Transients
VLA 0.33 GHz
Hyman et al. 2002
Exploring the Transient Radio Sky:
Covering the Sky
• Staring vs. mosaicing (tiling)?
• Radio sky needs both;

Fast transients: too fast to raster scan the sky
(< hours to months) (e.g. GPs)

Slower transients:


raster scan (e.g. for objects showing radio only)
trigger from other wide-field instruments (GRB
afterglows)
TRANSIENT SOURCES
Sure detections:
• Analogs to giant pulses from the Crab pulsar out
to ~5 – 10 Mpc
• Flares from brown dwarfs out to at least 100 pc.
• GRB afterglows to 1 µJy in 10 hours at 10 .
Possibilities:
• -ray quiet bursts and afterglows?
• Intermittent ETI signals?
• Planetary flares?
Isolated pulsar
Re-ignition of pulsar
action in mergers?
Hansen & Lyutikov 2000
RFI Editing in the f-t plane
RFI
dynamic
spectra
(from AO
monitoring
program)
Dynamic
spectrum
of pulsar
scintillation
Working Around Radio
Frequency Interference
• Single-dish/single-pixel transient detection:
• Very difficult to separate terrestrial & astrophysical
transients (significant overlap in signal parameter
space)
• Multiple beam systems (Parkes, Arecibo, the
GBT):
•
Simultaneous on/offs  partial discrimination
Multiple site systems (a la LIGO, PHOENIX)
•
Very powerful filtering of RFI that is site specific or
delayed or Doppler shifted between sites
LOFAR = Low Frequency Array
Stations of dipoles
30 to 240 MHz
Large AT
Optimal for coherent
continuum transients
China KARST
SKA = Square Kilometer Array
Canadian
aerostat
US Large N
Current Concepts
(cf. Allen Telescope Array,
Extended VLA)
Australian
Luneburg
Lenses
Dutch fixed
planar array
(cf. LOFAR = Low Freqency
Array)
Current Baseline Specifications
Parameter
Design Goal
Comments
Sensitivity
Surface brightness
Point sources
Frequency range
Redshift coverage
A/T = 2 x 104 m2 / K
1K at 0.1 arcsec (cont)
0.5 Jy
0.15 – 22 GHz
Z < 8.5 HI,
Z > 4.2 CO (10)
Zmax ~ 2 HI, ~ 20 CO
1 degree2 at 1.4 GHz
> 100
40 mas at 1.4 GHz
20x Arecibo, 75x VLA
L* galaxies
FOV (imaging)
Multibeams
Ang. Resolution
Pixels
Instantaneous
bandwidth
Spectra channels
Image Dynamic
Range
Polarization
isolation
108
20% at high frequencies
104
106 at 1.4 GHz
-40 dB
10 in 1 day, 100 MHz
VLBI: SKA enables all-sky
phase referencing
Methods with LOFAR & SKA
I.
Target individual objects
II.
Blind Surveys: trade FOV against
gain by multiplexing SKA into
subarrays.
III.
Allow rapid response to triggers
IV. Exploit coincidence tests to ferret out
RFI, use multiple beams.
Primary beam & station
synthesized beams
Station subarrays for
larger FOV
One station of many in SKA
Blind Surveys with SKA
• Number of pixels needed
to cover FOV:
Npix~(bmax/D)2 ~104-109
• Number of operations
Nops~ petaops/s
• Post processing per beam:
e.g. standard pulsar
periodicity analysis
Summary
Transient science is unexplored territory for
radio astronomy:
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New looks at known sources
Entirely new classes of sources:
LOFAR will survey transients at f < 240 MHz;
SKA for 0.15 GHz < f < 25 GHz (or more)
Implications for SKA design:

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
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Rapid imaging/mosaicing of sky (days)
Large instantaneous FOV desired for short time scales (e.g.
hemispheric).
US Plan: Subarrays to allow coincidence tests and maximal
sky coverage.
Versatile imaging/beamforming/signal processing modes.
Similar implications for pulsar science
Radio Pulsars
• ~1400 known (doubled by Parkes MB)
• ~100 millisecond pulsars
• 2 to 3 with planets
• ~5 NS-NS binaries (Porb > 8 hr)
• MSPs have exceedingly stable spins,
suitable for seeking gravitational wave
perturbations
Why more pulsars?
• Extreme Pulsars:
• P < 1 ms
P > 8 sec
• Porb < hours
B > 1013 G (link to magnetars?)
• V > 1000 km s-1
• NS-NS & NS-BH binaries
• Population & Stellar Evolution Issues
• The high-energy connection (e.g. GLAST)
• Physics payoff (GR, Gwaves, EOS, LIGO, GRBs…)
• Serendipity (strange stars, transient sources)
• New instruments (AO, GBT, LOFAR, SKA) will dramatically
increase the volume searched (galactic & extragalactic)
Parkes MB Feeds
Parkes MB Feeds
ALFA Science Goals:
Massive Surveys

Drift scan surveys
(14 sec across 3.5 arcmin)

Deep Galactic plane survey (GPS)
(5-10min, |b| < 5 deg, 30 < l < 80 + anticenter)

Medium latitude surveys
( 5 < |b| < 25 deg)

Targeted: globular clusters, high EM/DM
HII regions, SNRs, Galactic chimneys,
M33, X/ -ray selected objects
(long dwell times, up to 2.5 hr)
Surveys
with Parkes,
Arecibo &
GBT.
Simulated &
actual
Yield ~ 1000
pulsars.
ALFA Surveys at Arecibo
• ALFA surveys can be viewed as part of a long-
term, grander effort (“Full Galactic Census”)
(LOFAR, SKA, )
• RFI mitigation required and provides general
purpose tools
• Data & data products = long term resources
 data management policy & resources
~ 1 petabyte of survey raw data
~ 1 petabyte of data products
• Exploit telescope time fully (transients,
piggybacking)
SKA pulsar
survey
600 s per
beam
~104 psr’s