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

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Presentation on theme: "Very Bright, Very Hot and Very Long: Swift Observations of the DG CVn "Superflare" of April 23rd, 2014 Steve Drake HEASARC/GSFC & USRA/CRESST 2014 August."— Presentation transcript:

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

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) + Thanks to Rob Fender for permission to show the 15-GHz radio data, and to the Swift Project for their support for these observations 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)

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

4 SOHO/EIT: Fe XII 195 Å Our Friend: The Sun T ~ 1.5 MK L x ~ erg/s Solar Corona T ~ 1 – 5 MK Solar Flares E ~ erg 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 Hudson et al. (1994, ApJ, 422, 25) Yohkoh image of X1.5 solar flare A B C Geometry of EV Lac flare (Osten et al. 2010) A & C loop footpoints B loop-top source

7 So How Do you Observe Stellar X-Ray Flares?  Inactive stars similar to the 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 Osten et al XRT UVOT

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 R * inferred flare loop modeling & Fe K- α Peak X-ray luminosity was 3 x L bol = 10 4 x normal level: in fact for 500 s after trigger, L x > L bol Total summed flare energy in soft X-ray band >~7 x erg/s (but observations stopped after half a day when Lx was still 10 x normal level)

11 The M4V Star(s) DG CVn = G 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 L x ~ 1.5 x erg/s ~ L bol => 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 BAT on 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 BFF F2 R V B U UVW1 UVM/W2 XRT

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

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

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

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 ~ 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 erg/s (1.5 x system L bol ): in fact for several minutes after trigger, L x > L bol Total summed flare energy in soft X-ray band 2 x erg Similar (10 36 erg) 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 above 10 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 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 >~ erg/s, and can exceed L bol (and these stars can stay active for ~ years) => significant impact on habitable zones for extended time range -X-ray and optical-band flare energies can reach ~ erg


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