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Steve Drake HEASARC/GSFC & USRA/CRESST

2014 August 21 st , HEAD Meeting. Very Bright, Very Hot and Very Long: Swift Observations of the DG CVn " Superflare " of April 23rd, 2014. Steve Drake HEASARC/GSFC & USRA/CRESST. Collaborators. Rachel Osten ( STScI ) Adam Kowalski (ORAU/GSFC) Kim Page (U Leicester)

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Steve Drake HEASARC/GSFC & USRA/CRESST

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  1. 2014 August 21st, HEAD Meeting Very Bright, Very Hot and Very Long: Swift Observations of the DG CVn "Superflare" of April 23rd, 2014 Steve Drake HEASARC/GSFC & USRA/CRESST

  2. Collaborators • Rachel Osten (STScI) • Adam Kowalski (ORAU/GSFC) • Kim Page (U Leicester) • Jamie Kennea (Penn State) • Samantha Oates (IAA-CSIC) • Neil Gehrels (NASA/GSFC) • Hans Krimm (USRA/GSFC) • Mathew Page (UCL) • Kosmas Gazeas (U Athens) References Osten et al. (2010, ApJ, 721, 785) for the 2008 EV Lac flare Drake et al. (2014, Atel # 6121) Kowalski et al. (2014, in preparation) + Thanks to Rob Fender for permission to show the 15-GHz radio data, and to the Swift Project for their support for these observations

  3. Structure of Presentation • Coronae and flares 101 • The EV Lac ‘Superflare’ of April 25th 2008 • The Properties of the M4 V ‘Star’ DG CVn • The DG CVn ‘Superflare’ of April 23rd 2014 • Future Directions

  4. Our Friend: The Sun SOHO/EIT: Fe XII line @ 195 Å T ~ 1.5 MK Solar Corona Lx ~ 1026.5-27.3 erg/s T ~ 1 – 5 MK Solar Flares E ~ 1024-33erg T ~ 10 – 30 MK

  5. RHESSI spectrum of the impulsive phase of a solar flare between 6 – 250 keV Benz & Guedel 2010 ARAA, 48, 241

  6. Standard solar and stellar flare geometry B A A & C loop footpoints B loop-top source C Geometry of EV Lac flare (Osten et al. 2010) Hudson et al. (1994, ApJ, 422, 25) Yohkoh image of X1.5 solar flare

  7. So How Do you Observe Stellar X-Ray Flares? • Inactive stars similar tothe Sun flare for only a small fraction of the time => need either to observe 1 star for v. long time or multiple stars simultaneously as in Kepler field • Active stars can flare for a much larger fraction of the time (up to 20%), but most flares are small (N(E) ~ E-α, where α ~ 1.5 – 2.5) => to catch a big flare you still need to observe for pretty long time or be lucky • You can use `all-sky monitor’ X-ray observations, e.g., Swift BAT, MAXI, etc., => given present sensitivity levels ~10-9 erg cm-2 s-1, only the largest flares will be found: e.g., Swift detections of stellar flares from the active binary systems Algol, II Peg & HR 1099 and the M4.5V star EV Lac and MAXI detections of ~12 flares • You can ask for long pointed observations of your favorite targets with Chandra, XMM or Suzaku => hard to estimate flare rates for many stars, so no guarantee of success and hard to convince TACs to grant long enough observations

  8. X-Ray Flare from Very Nearby (5 pc) dM4.5 star EV Lac Caught by Konus/Wind & Swift on 2008 April 25 XRT UVOT Osten et al. 2010

  9. XRT and BAT Spectrum of Early Decay Phase of EV Lac Flare No significant non-thermal hard X-ray cpt Dominant ~100 MK thermal cpt He-like Fe K 6.7 keV line Cool Fe K 6.4 keV line behavior is affected by charge trapping Osten et al.2010

  10. The April 2008 Superflare of EV Lac • Proved that some (~ several) apparent GRBs are actually stellar flares • Peak X-ray temperature ~ 100 MK drops to ~30 MK after 45 minutes • Evidence for Fe K-α 6.4 keV in first few minutes after trigger: later detections likely spurious • No evidence for non-thermal power-law emission in BAT spectrum • Significant white-light flare (Delta V ~ 2.9 magnitudes over quiescence) seen by UVOT • Flare loop size of 0.1-0.4 R* inferred flare loop modeling & Fe K-α • Peak X-ray luminosity was 3 x Lbol= 104 x normal level: in fact for 500 s after trigger, Lx > Lbol • Total summed flare energy in soft X-ray band >~7 x 1034erg/s (but observations stopped after half a day when Lx was still 10 x normal level)

  11. The M4V Star(s) DG CVn= G 165-08 AB • DG CVn is a rapidly rotating (period < 1 day) pair of M4V stars (~ 0.17“ separation) that is fairly nearby (18 pc) but poorly studied • DG CVn is one of the most active stars in the solar neighborhood based on its levels of radio continuum, X-ray emission, and Hα emission • These properties explained by its very young age ~30 Myr (Riedel et al. 2014, AJ, 147, 85) • Its normal Lx ~ 1.5 x 1029 erg/s ~ 0.001 Lbol=> DG CVn star(s) at so-called ‘coronal saturation’ level • Thus, DG CVn is a plausible candidate for producing flares bright enough for Swift/BAT detections, albeit rather distant

  12. X-Ray Flare from DG CVn Caught by Swift BATon 2014 April 23

  13. XRT and BAT Spectrum of Early Decay Phase of DG CVn Flare No nonthermal hard X-ray cpt Dominant ~220 MK thermal cpt No Fe K 6.4 keV line

  14. Swift/XRT Light Curve of DG CVn F2 BFF

  15. Swift/UVOT & Ground-based R-band Light Curve of DG CVn F2 R BFF V XRT B U UVW1 UVM/W2

  16. AMI 15-GHz Observations of DG CVn Fender et al. (2014) BFF F2

  17. Soft X-Ray Energy Budget of DG CVn Flare Series

  18. Big Flares are Very Rare! BFF Courtesy of Gerry Doyle

  19. The April 2014 Superflare of DG CVn • Big first flare (‘BFF’) followed by >6 secondary flares: total time to drop back to usual soft X-ray level ~ 1 - 2 Ms • Peak X-ray temperature ~ 220 MK drops to ~43 MK after 90 minutes • No strong evidence for Fe K-α 6.4 keV in flare peak emission • No strong evidence for non-thermal power-law emission in BAT spectrum • Significant UV/optical emission from flare (Delta V ~ 2.5 & Delta UV >~ 5 magnitudes over quiescence) seen by ground-based obs. & UVOT • Peak X-ray luminosity was 2 x 1032 erg/s (1.5 x system Lbol): in fact for several minutes after trigger, Lx > Lbol • Total summed flare energy in soft X-ray band 2 x 1036 erg • Similar (1036erg) summed flare energy in ‘white light’ band coming from 0.5-5% of the stellar surface (cf. <0.05% for solar flares)

  20. Future Directions and Questions About DG CVn • Complete a full X-ray and optical analysis to infer geometry and physical properties of flaring plasma (Kowalski et al. 2014) • Continued Swift monitoring to build up flare vs energy statistics might be useful: suspect this star must have frequent smaller flares • New study of optical photometry: what is the cause of the periodic variation and what is the ‘true’ period? • Magnetic field measurement would be very desirable • Is the (wide) binarity of this system a factor in flare properties?

  21. Questions about Stellar Flares • Where is the Non-thermal Hard X-Ray Emission? - Hidden by the thermal emission: Current and previous instruments with sensitivity above10 keV range have observed many big stellar flares, but still only 1 plausible non-thermal detection: We need more sensitive hard X-ray instruments or must use simultaneous radio observations as proxy • How Hot Can Large Flares from Main-Sequence Stars Reach? • Peak X-ray temperatures can reach 10-20 keV (120 – 240 MK) • How Long Can Large Flares or ‘Flare States’ from Main-Sequence Stars Last? - Durations can be much longer than realized: weeks rather than hours to 1 day • How Energetic Are Flares from Main-Sequence Stars? • Peak X-ray flare luminosities in some stars can reach >~ 1032 erg/s, and can exceed Lbol(and these stars can stay active for ~ 108-9 years) => significant impact on habitable zones for extended time range • X-ray and optical-band flare energies can reach ~ 1036 erg

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