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Probing the dynamic Universe with ASKAP
Shivani Bhandari
ASA 2018Swinburne University
Melbourne
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Transients
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• Typically Synchrotron emission • TB < 1012 K• Variable on timescales: seconds to years• Typically discovered in image data
Fast Transients• Flavours of coherent emission• TB > 1012 K• Vary on timescales: nano-milli sec• Typically discovered in time-series data
Slow Transients
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Explosion VASTScience
Explosions
• Supernovae• Gamma ray bursts• Orphan afterglows
Propagation• Extreme scattering events• Intra-day variables
Accretion• Neutron stars• Black hole• Quasars• X-ray binaries
Magnetism
• Magnetars• Flare stars• Planetary variability
Unknown
• known unknowns• unknown unknowns
Credits: space.com
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Goals
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• Investigate continuum data quality from ASKAP early science data.• Test ASKAPSoft from user perspective.• Test VAST on ASKAP data products.• Put limits on transient rate at 1.4 GHz.• Characterise the variability of the 1.4 GHz sky.
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Early Science Data
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8 epochs on NGC 7232 field —12 ASKAP dishes — 36 beams covering 30 deg2 area on sky
— 48 MHz of bandwidth centred at 1.4 GHz
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Imaging challenges
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ASKAP early science data
Bhandari et al. 2018 (In press)
• Synthesis beam 17’’X12’’
• ~1400 sources above 10-sigma
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THE VAST PIPELINE
9Murphy et al. 2013 Banyer et al. 2012
The Variable And Slow Transient pipeline
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RA: -1.8’’ to 3.4’’DEC: 2.0’’ to 4.1’’
Image Analysis
Flux calibration was within 10%
Small fraction of the synthesised beam (17’’X12’’)
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Variability criteria
Who are variables?
Chi square > 24.3 for p < 0.001
M > 0.1
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We are variables!
Modulation index: 0.1 - 0.2
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Transient Search
Experiment 1: Search for single epoch transients on the timescale of days.Result:• No transient detections. • We place an upper limit on the transient source surface density to be⍴ < 0.01 deg-2 above flux limit > 1.5 mJy with 95% confidence for non-detections.
Experiment 2: Search for transients on the timescale of a decade.Result: • J220833-453600! • SUMSS: 3-sigma detection, ASKAP ~ 28 mJy• m > 0.5 between SUMSS and 2017 ASKAP observations.
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We are variables!
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J220833-453600
Intrinsic variability of a quasar?
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Multi-wavelength counterparts
Infrared catalogues• WISE• GALEX• 2MASSOptical catalogues• GSC• UNSO-B1.0• Sky-mapper
Interstellar Scintillation ofActive Galactic Nuclei or galaxiesHosting AGNs
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Transient phase space
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VAST
• Detect and monitor orphan GRB afterglows
• Conduct studies of ESEs• Discover flaring magnetars and other
pulsed emissions• Investigate intrinsic variability of AGNs• Exploring the unknowns!
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Exploring the unexplored transient phase space
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