The Transient Radio Sky to be Revealed by the SKA

Jim Cordes Cornell University

AAS MeetingWashington, DC8 January 2002

TRANSIENT SOURCES TRANSIENT SOURCES

Sky Surveys:Sky Surveys:The X-and-The X-and--ray sky has been monitored highly -ray sky has been monitored highly successfully with wide FOV detectors successfully with wide FOV detectors (e.g. RXTE/ASM, CGRO/BATSE).

Neutrino/gravitational wave detectors are ‘all sky.’Neutrino/gravitational wave detectors are ‘all sky.’

The transient radio sky (e.g. t < 1 month) is largely The transient radio sky (e.g. t < 1 month) is largely unexplored.unexplored.

New objects/phenomena are likely to be discovered New objects/phenomena are likely to be discovered as well as predictable classes of objects.as well as predictable classes of objects.

TRANSIENT SOURCES (2)TRANSIENT SOURCES (2)

TARGET OBJECTS: 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

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

W = light travel time

brightness temperature:

SpkD2

Tb = ------------- 2k (W)2

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

Lines of constant brightness temperature

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

Solar system

+

local galactic sources

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

OH masers

+

Pulsars (including giant pulses)

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

Cosmological sources:

AGNs (including IDV sources)

+

GRB afterglows

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

Phase Space for Transients: SpkD2 vs. W

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

Interstellar scintillations = apparent fast variations of IDVs & GRBs

The SKA will thoroughly explore this phase space

W

W

Pulse

Process

Spk

log

Sp

kD

2

log W

TRANSIENT SOURCES (3)TRANSIENT SOURCES (3)TARGET PROCESSES:TARGET PROCESSES:

• Intrinsic:incoherent:

( inverse Compton brightness limit)coherent: (virtually no limit)continuum: low frequencies favored

spectral line: masers

• Extrinsic:scintillationmaser-maser amplificationgravitational lensingabsorption events

TRANSIENT SOURCES (4)TRANSIENT SOURCES (4)Sure detectionsSure 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 .

PossibilitiesPossibilities::

• -ray quiet bursts and afterglows?-ray quiet bursts and afterglows?

• Intermittent ETI signals?Intermittent ETI signals?

• Planetary flares?Planetary flares?

Crab Pulsar

Giant Pulses from Nearby GalaxiesGiant Pulses from Nearby Galaxies

SCIENTIFIC RETURNSCIENTIFIC RETURN

• Many objects Many objects map out IGM as well as ISM of map out IGM as well as ISM of

galaxiesgalaxies

• IGM: electron density and magnetic fieldIGM: electron density and magnetic field

• NS birth rates in other galaxiesNS birth rates in other galaxies

• Constraints on IMFConstraints on IMF

• Census of young pulsars, clues about magnetars?Census of young pulsars, clues about magnetars?

M33

Beam 2

J1907+0918

226 ms

DM = 358

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

Methods with the SKAMethods with the SKA

I. Target individual SNRs in galaxies to5-10 Mpc

II. Blind Surveys: trade FOV against gain by multiplexing SKA into

subarrays.

III. Exploit coincidence tests to ferret out RFI, use multiple beams.

Primary beam & station Primary beam & station synthesized beamssynthesized beams

Station subarrays for Station subarrays for larger FOV (mosiacing)larger FOV (mosiacing)

One station of many in SKAOne station of many in SKA

Summary • Transient science is unexplored territory for radio astronomy:

New looks at known sources, entirely new classes of sources: LOFAR will survey transients at f < 240

MHz; SKA for 240 MHz < f < 22 GHz (or more)• Implications for SKA design: 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 from pulsar science