1. Steve Drake HEASARC/GSFC & USRA/CRESST
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 References 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
195 Å
T ~ 1.5 MK
Solar Corona
Lx ~ erg/s
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. stellar flare geometry
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 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
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 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 1034 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 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 ~ 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 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
BFF

15. Swift/UVOT & Ground-based R-band Light Curve of DG CVn
BFF
XRT V
Swift/UVOT & Ground-based R-band Light Curve of DG CVn 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 ~ 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 (1036 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 >~ 1032 erg/s, and can exceed Lbol (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 ~ 1036 erg