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
Stellar Flares Flaring is a common characteristic of magnetically active stars. Observations of stellar flares: • Provide constraints on stellar magnetic properties • Solar - Stellar connection • Habitability of discovered exoplanets https://blogs.stsci.edu/universe/2015/11/15/follow-the- photons-to-understand-the-effects-of-stellar-flares/ December 9 2016 Science at Low Frequencies III Meeting
100 - 200 MHz Stellar flares Spangler et al. (1976) 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 December 9 2016 Science at Low Frequencies III Meeting
100 - 200 MHz Stellar flares Spangler et al. (1976) 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 Where are all the flare stars? December 9 2016 Science at Low Frequencies III Meeting
Coherent Emission At <5 GHz emission dominated by coherent mechanism: 2 possible types 1. Electron Cyclotron Maser ‣ Emitted at local fundamental/2nd harmonic cyclotron frequency: ν c ~ 2.8 MHz (B Gauss ) → Constrain B-field ‣ Confirmed emission mechanism for radio bursts of brown dwarfs 2.Plasma Emission ‣ Emitted at local fundamental/2nd harmonic plasma frequency: ν p ~ 9.0 kHz (n cm-3 ) 1/2 → Constrain Density ‣ Different types of Solar flares due to plasma emission NOTE: Coherent emission expected to be highly (>50%) circularly polarised! December 9 2016 Science at Low Frequencies III Meeting
MWA Observations UV Ceti: ‣ Spectral types = dM5.5e ‣ Binary system w/ 26 yr period — both exhibit radio flares ‣ Distance = 2.7 pc •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 December 9 2016 Science at Low Frequencies III Meeting
Detection of UV Ceti December 11 2015: 30 min integrations 1 σ RMS = 80 mJy 1 σ RMS = 1 mJy Lynch et al. ( submitted to ApJ) December 9 2016 Science at Low Frequencies III Meeting
Light-curve analysis PERIODICITY: STOKES V (pos = RH, neg = LH) • 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 Lynch et al. ( submitted to ApJ) December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: A. Source size constrained by assuming periodic persistent source: ‣ L = Δ t vsin(i) ~ 10 9 cm ‣ T b ~ 10 14 K December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: A. Source size constrained by assuming periodic persistent source: ‣ L = Δ t vsin(i) ~ 10 9 cm ‣ T b ~ 10 14 K B. Source size constrained by VLBA: Benz et al. 1998 ‣ L ~10 10 cm VLBA/VLA @ 3.6 cm ‣ T b ~10 13 K ⤾ December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: T b ~ (10 13 - 10 14 ) K ⇒ Coherent Lynch et al. ( submitted to ApJ) December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: T b ~ (10 13 - 10 14 ) K ⇒ Coherent 2.Polarisation: A. Circular: Both right & left handed; >27% B. Linear: >18%; ϕ = + 3 rad m -2 Lynch et al. ( submitted to ApJ) December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: T b ~ (10 13 - 10 14 ) K ⇒ Coherent 2.Polarisation: A. Circular: Both right & left handed; >27% B. Linear: >18%; ϕ = + 3 rad m -2 3. Time/Frequency structure: 3. Time/Frequency structure: A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth A. For 6-min/10 MHz bins, constant across full 30.72 MHz bandwidth B. Due to low SNR cannot rule out finer structure B. Due to low SNR cannot rule out finer structure December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: T b ~ (10 13 - 10 14 ) 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 December 9 2016 Science at Low Frequencies III Meeting
Emission Type? 1. Brightness Temperature: T b ~ (10 13 - 10 14 ) 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 December 9 2016 Science at Low Frequencies III Meeting
100 - 200 MHz Flare rates Lynch et al. ( submitted to ApJ) 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 observational scheme (LST align observations) 2. Use results to constrain flare rates & coordinate multi-wavelength shadowing December 9 2016 Science at Low Frequencies III Meeting
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