154 MHz detection of faint, polarised flares from UV Ceti Christene - - PowerPoint PPT Presentation

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154 mhz detection of faint polarised flares from uv ceti
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154 MHz detection of faint, polarised flares from UV Ceti Christene - - PowerPoint PPT Presentation

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154 MHz detection of faint, polarised flares from UV Ceti Christene Lynch University of Sydney/CAASTRO Collaborators: Emil Lenc , University of Sydney/CAASTRO Tara Murphy, University of Sydney/CAASTRO David Kaplan, University of Wisconsin

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SLIDE 1

154 MHz detection of faint, polarised flares from UV Ceti

Christene Lynch University of Sydney/CAASTRO

Collaborators: Emil Lenc, University of Sydney/CAASTRO Tara Murphy, University of Sydney/CAASTRO David Kaplan, University of Wisconsin — Milwaukee Gemma Anderson, Curtin University

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December 9 2016 Science at Low Frequencies III Meeting

Stellar Flares

Flaring is a common characteristic

  • f magnetically active stars.

https://blogs.stsci.edu/universe/2015/11/15/follow-the- photons-to-understand-the-effects-of-stellar-flares/

Observations of stellar flares:

  • Provide constraints on stellar

magnetic properties

  • Solar - Stellar connection
  • Habitability of discovered

exoplanets

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December 9 2016 Science at Low Frequencies III Meeting

Spangler et al. (1976)

100 - 200 MHz Stellar flares

Single dish observations (1960’s - 1980’s) measured:

  • Flare rates = 0.03 - 0.8 flares/hour
  • Duration = 0.5 - 3 hours
  • Intensities = 0.8 - 20 Jy
  • Possible association with optical flares

Non-detections in MWA blind surveys for transients:

  • Tingay et al. (2016): Kepler K2 field,

5.9 hours

  • Rowlinson et al. (2016): 100 hrs of

MWA EoR field

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December 9 2016 Science at Low Frequencies III Meeting

Spangler et al. (1976)

Where are all the flare stars?

100 - 200 MHz Stellar flares

Single dish observations (1960’s - 1980’s) measured:

  • Flare rates = 0.03 - 0.8 flares/hour
  • Duration = 0.5 - 3 hours
  • Intensities = 0.8 - 20 Jy
  • Possible association with optical flares

Non-detections in MWA blind surveys for transients:

  • Tingay et al. (2016): Kepler K2 field,

5.9 hours

  • Rowlinson et al. (2016): 100 hrs of

MWA EoR field

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December 9 2016 Science at Low Frequencies III Meeting

Coherent Emission

NOTE: Coherent emission expected to be highly (>50%) circularly polarised!

At <5 GHz emission dominated by coherent mechanism: 2 possible types

  • 1. Electron Cyclotron Maser
  • Emitted at local fundamental/2nd harmonic cyclotron frequency:

νc ~ 2.8 MHz (BGauss) → Constrain B-field

  • Confirmed emission mechanism for radio bursts of brown dwarfs

2.Plasma Emission

  • Emitted at local fundamental/2nd harmonic plasma frequency:

νp ~ 9.0 kHz (ncm-3)1/2 → Constrain Density

  • Different types of Solar flares due to plasma emission
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December 9 2016 Science at Low Frequencies III Meeting

MWA Observations

  • Pointed observations — phase center moved to keep source

within primary beam

  • Total observation time = 8.8 hours — split over 4 days in Dec 2015
  • Frequency = 154 MHz; bandwidth 30.72 MHz

UV Ceti:

  • Spectral types = dM5.5e
  • Binary system w/ 26 yr

period — both exhibit radio flares

  • Distance = 2.7 pc
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December 9 2016 Science at Low Frequencies III Meeting

Detection of UV Ceti

1σ RMS = 80 mJy 1σ RMS = 1 mJy

December 11 2015: 30 min integrations

Lynch et al. (submitted to ApJ)

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December 9 2016 Science at Low Frequencies III Meeting

Light-curve analysis

PERIODICITY:

  • Mid-flare time &

amplitudes fit using top- hat function (only RH)

  • Minimize phase difference

for set of trial periods

  • P = 5.4492 ± 0.0002 hrs

(95% confidence)

  • v sin(i) ~ 30 km/s -> P <

6.5 hrs

STOKES V (pos = RH, neg = LH)

Lynch et al. (submitted to ApJ)

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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

  • 1. Brightness Temperature:
  • A. Source size constrained by assuming periodic persistent source:
  • L = Δt vsin(i) ~ 109 cm
  • Tb ~ 1014 K
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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

⤾

VLBA/VLA @ 3.6 cm

Benz et al. 1998

  • 1. Brightness Temperature:
  • B. Source size constrained by VLBA:
  • L~1010 cm
  • Tb ~1013 K
  • A. Source size constrained by assuming periodic persistent source:
  • L = Δt vsin(i) ~ 109 cm
  • Tb ~ 1014 K
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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

  • 1. Brightness Temperature:

Tb ~ (1013 - 1014) K ⇒ Coherent

Lynch et al. (submitted to ApJ)

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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

2.Polarisation:

  • A. Circular: Both right & left handed; >27%
  • B. Linear: >18%; ϕ = + 3 rad m-2
  • 1. Brightness Temperature:

Tb ~ (1013 - 1014) K ⇒ Coherent

Lynch et al. (submitted to ApJ)

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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

  • 1. Brightness Temperature:

Tb ~ (1013 - 1014) K ⇒ Coherent 2.Polarisation:

  • A. Circular: Both right & left handed; >27%
  • B. Linear: >18%; ϕ = + 3 rad m-2
  • 3. Time/Frequency structure:
  • A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth
  • B. Due to low SNR cannot rule out finer structure
  • 3. Time/Frequency structure:
  • A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth
  • B. Due to low SNR cannot rule out finer structure
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SLIDE 14

December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

  • 1. Brightness Temperature:

Tb ~ (1013 - 1014) K ⇒ Coherent 2.Polarisation:

  • A. Circular: Both right & left handed; >27%
  • B. Linear: >18%; ϕ = + 3 rad m-2
  • 3. Time/Frequency structure:
  • A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth
  • B. Due to low SNR cannot rule out finer structure
  • 4. Optical/X-ray counterpart:
  • A. No optical coverage during MWA observations
  • B. Both Swift Burst Alert & Monitor of All-sky X-ray Image monitor UV

Ceti; no bright X-ray flares detected

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December 9 2016 Science at Low Frequencies III Meeting

Emission Type?

  • 1. Brightness Temperature:

Tb ~ (1013 - 1014) K ⇒ Coherent 2.Polarisation:

  • A. Circular: Both right & left handed; >27%
  • B. Linear: >18%; ϕ = + 3 rad m-2
  • 3. Time/Frequency structure:
  • A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth
  • B. Due to low SNR cannot rule out finer structure
  • 4. Optical/X-ray counterpart:
  • A. No optical coverage during MWA observations
  • B. Both Swift Burst Alert & Monitor of All-sky X-ray Image monitor UV

Ceti; no bright X-ray flares detected

Cannot determine type of emission

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December 9 2016 Science at Low Frequencies III Meeting

100 - 200 MHz Flare rates

Lynch et al. (submitted to ApJ)

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December 9 2016 Science at Low Frequencies III Meeting

Summary & Future work:

  • 1. Observed 4 flares from M dwarf UV Ceti during 8.8 hr observation
  • 2. Flares only detected in Stokes V images due to order of magnitude

higher noise in confusion limited Stokes I images.

  • 3. Flares are coherent but cannot distinguish between different emission
  • types. Need wider bandwidths, higher sensitivity, and multi-wavelengths.
  • 4. First flare rates for low intensity (<100 mJy) flares at 100 - 200 MHz —

consistent with previous (brighter) detected flares.

NEXT STEPS:

  • 1. Detect more stellar flares! Target larger sample of M dwarfs with similar
  • bservational scheme (LST align observations)
  • 2. Use results to constrain flare rates & coordinate multi-wavelength

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