2021-11-24

Minutes for MWA Transient meeting on the theme: Polarised/Stokes V Transients

Attendees: Gemma Anderson, Chenoa Tremblay, Christene Lynch, Bryan Gaensler, Xiang Zhang, Laura Driessen, Sam McSweeney, John Morgan, George Heald, Jai Chauhan, Steve Prabu, Susmita Sett, Zhijun Xu

Declined due to timezone or other commitments: Joe Callingham (2am in Europe), Natasha Hurley-Walker (meeting clash), Tara Murphy, Joshua Pritchard + VAST members (OzGrav retreat clash) 

Proposed Agenda (see email sent on 23 Nov 2021):

• Xiang Zhang (Shanghai Astronomical Observatory) will present her work on a flaring M dwarf detected in GLEAM-X data.
• General updates from individual members on the Polarised/Stokes V transients theme - This does not need to be limited to MWA work but can include ASKAP, MeerKAT, LOFAR, optical, etc. Updates could include:

• Recent results
• Current/planned research
• New ideas for proposals/collaborations

• Stokes V/polarisation transient work planned during MWAX science commissioning
• Discussion regarding future meeting scheduling and flexibility

• Future discussion on this or a similar theme (particularly in a different time zone)
• General scheduling discussion

• Any other business

Minutes:

This is the first MWA Transients meeting of our new series. 

Discussions on exoplanet searches/detections with MWA

Christene:

• Christene's 2018 paper investigates this. This paper was using MWA data cross-matched with known exoplanets.
• LOFAR best bet - predicted this should be possible. 
• Joe's paper 1mJy - very challenging for MWA - LOTSS, cross-match with GAIA to find them, Stokes V selection 
• Phase II MWA - GLEAM-X and LOBES 1 sigma sensitivity of mJy - much longer integrations
• longer last emissions found so could try long integrations
• Some emissions only last 1 hour so won't gain anything with longer integrations.
• A few dozen sources may be possible with MWA (looked at Emil's polarisation survey - GLEAM data)
• Could pick your favourite targets and try stacking? Might need 10s of hours
• Highly circularly polarised (>70%)

Updates relating to the theme (including Xiang's presentation)

Christene: MWA archival search on targeted M dwarfs

• Continue working on objects that may produce bright bursts - e.g. UV Ceti see 2017 paper
• The following year did 100 hours but did not detect anything - so emission is not very stable
• Duty cycle about 1 %
• Processing all archival MWA data on UV Ceti to look at long term variability
• A list of 10 bright objects
• Stokes V RTS on these images - RTS very fast and don't need to clean in Stokes V
• Incorporate GMRT
• Archival data that has not been processed (not GLEAM data that Emil used for his paper).
• Targeted sources of specific objects need lots of extra data unless the objects are really flaring.
• John - emphasize Stokes V polarisation calibration improvements done with GLEAM-X 
  • Recommends transferring this knowledge between GLEAM-X, GeG, and this project.
  • Christene - Use RTS to see if there is a flare and then do proper imaging if there is a detection - see if there is a big improvement
• Chenoa - Quick RTS search may be only hitting the highest threshold
  • Christene- The sensitivity dependence is definitely in Stokes I but probably not a problem in Stokes V
  • Would definitely want to do GLEAM-X processing of any Stokes I images when a Stokes V detection is found
• Sammy - there is some Linear and Circular polarisation swapping if calibration is not done properly so there may be some sensitivity loss in Stokes V with RTS.
  • Christene: Worth looking at level this occurs (original work also done with analytic beam). Position in the primary beam will also have different effects on the leakage.

Xiang Zhang: Stokes V transient discovery in GLEAM-X data (FK Aqu)

• POGS-X circular: M dwarf detection - FK Aqu: double M dwarf binary
• 73% circular pol, 1/10th of observations covering sky region, no linear pol
• Known radio star - VLA detection
• Sits at edge of GAMA-23 field
• ATCA follow-up - source detected
• Stokes V light curve 0.02-0.08 Jy
• Xiang - Are there typical flare star light curves?
  • Christene - There no common light curve for flare stars as they depend on the different properties of the stars. Not sure what causes the flare - could be an episodic burst or a hot spot that looks to pulse due to stellar rotation. 
• Stokes I - Only looked at Stokes I image so haven't detected it yet. Should be detectable in a clean Stokes I image (snapshots). However, it is listed in Tim's GLEAM-X catalogue.
•  Laura - The periods of the binaries are on the order of a day. Could be radio emission from the interaction?
• Next step to make clean Stokes I snapshot images 
  • See if the fractional polarisation changes
• Christene - This is one of the 10 stars targeted in the above described project. Christene will send lots of info and references relating to this star to Xiang and Laura. 

Laura Driessen: Looking at ASKAP POSSUM

• Stokes I and V matches (similar to what Joe did with GAIA) many stellar matches/chance coincidences
• Something more similar with STOKES I - RS CVn low polarisation (chromospheric activity and gyrosynchrotron emission)
  • Stellar matches using machine learning
  • Need another filtering criterion as chance coincidences are still very high. (Not Stokes V)
  • Ultimate would be X-ray matches
  • One of the target sources is detected in ROSAT but not detected in the radio
• Gemma - eROSITA-AAL MoU - contact Time Domain team or a Galactic team - all-sky - a catalogue of X-ray sources (not just flares - want quiescent detections)
• Laura to be joint project scientist in VAST 
• Christene - VAST doing a lot of work (Josh and Mark have a paper). Look at the overlap with Mark - unbiased search for radio stars in RACS data - GAIA and colour-colour cross-match selections in Stokes I. 
  • Laura is hesitant to do colour cuts with binary systems
• Gemma - Is there an MWA synergy?
  • Laura - Straight forward MWA synergy - if found in ASKAP, then see if it is there in MWA
  • Laura - Low-frequency detections help to distinguish between coherent vs incoherent

Sam McSweeney

• The MWA PSR group is interested in closing the loop on the polarisation based on the GLEAM-X work
• The team wants to reproduce the same polarisation corrections for PSRs. 
• PSRs give more stringent tests on polarisation corrections.
• Working with John to make sure GLEAM-X and pulsar polarisation are consistent.

Future meetings and scheduling:

• It was agreed that it seems fair to swap between two different timezones, alternating each month.
• Due to Doodle Poll and scheduling fatigue, Gemma will organise the alternative time for the other timezone with enough notice for people to schedule around it.
• The next meeting will be in February.
• There is a strong interest to continue the "Polarisation/Stokes V Transients" theme conversation with VAST.
• Gemma and Tara will plan a meeting on "Polarisation/Stokes V Transients - VAST and MWA synergies" for February. This will likely be held at a similar time to the current meeting in order to best accommodate VAST collaborators.