1

Probing the dynamic Universe with ASKAP

Shivani Bhandari

ASA 2018Swinburne University

Melbourne

Transients

2

• 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

3

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

Goals

4

• 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.

Early Science Data

5

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

Imaging challenges

6

7

ASKAP early science data

Bhandari et al. 2018 (In press)

• Synthesis beam 17’’X12’’

• ~1400 sources above 10-sigma

8

THE VAST PIPELINE

9Murphy et al. 2013 Banyer et al. 2012

The Variable And Slow Transient pipeline

10

11

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’’)

12

Variability criteria

Who are variables?

Chi square > 24.3 for p < 0.001

M > 0.1

13

We are variables!

Modulation index: 0.1 - 0.2

14

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.

15

We are variables!

16

J220833-453600

Intrinsic variability of a quasar?

17

Multi-wavelength counterparts

Infrared catalogues• WISE• GALEX• 2MASSOptical catalogues• GSC• UNSO-B1.0• Sky-mapper

Interstellar Scintillation ofActive Galactic Nuclei or galaxiesHosting AGNs

18

Transient phase space

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!

19

Exploring the unexplored transient phase space

20