Swift Detection of a Superflare from DG CVn
ATel #6121; S. Drake (CRESST/USRA/GSFC), R. Osten (STScI), K. L. Page (U. Leicester), J. A. Kennea (PSU), S. R. Oates (IAA-CSIC, UCL-MSSL), H. Krimm (CRESST/USRA/GSFC) and N. Gehrels (NASA/GSFC)
on 5 May 2014; 18:45 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Optical, Ultra-Violet, X-ray, Gamma Ray, Star, Transient
Referred to by ATel #: 7983
The Swift team reports the detection of a superflare from one of the stars in the close visual (0.17â) dM4e+dM4e flare star binary system DG CVn (G 165-8AB). The Burst Alert Telescope (BAT) triggered on DG CVn at 2014-04-23T21:07:08UT = T0 (trigger 596958 reported in GCN
Circ. 16158), resulting in an automatic slew to the source. The partial
coding was 93%. The hard X-ray source had a peak intensity in the BAT
15-50 keV band of ~300 mCrab or 0.06 count/cm^2/s. The BAT data
cover the period from T0-239 to 963 s. The mask-weighted lightcurve
shows a single peak from ~T0-40 s to 120 s and another weaker peak
from ~T0+200 to 240 s. The time-averaged spectrum from -29 to 337 s is
well fit by either a simple power-law or a bremsstrahlung model. The
power law index of the time-averaged spectrum for the former model is
2.62+/-0.33, while the temperature of the latter is 26(+12,-8) keV. The
fluence in the 15-150 keV band is (8.4+/-1.5)e-7 erg/cm^2 for the
power-law model.
When the Swift X-Ray Telescope (XRT) started observing at T0+117 s, the
soft X-ray 0.3-10 keV rate of DG CVn was ~100 count/s, corresponding to
5e-9 erg/cm^2/s, and then decayed moderately, reaching a count rate of ~50
count/s by ~328 s after the trigger. After a 4.2 ks gap in XRT
observations of this field, the soft X-ray emission had declined to a
level of 4-15 count/s, but, after a further gap, at T0+11 ks DG CVn was
observed to have had a second, smaller flare back to a level of ~30
count/s. The source then decayed monotonically for ~10 days, with a
power-law of alpha = 1.39+/-0.01 fitting the data after T0+10 ks, ignoring a third, much smaller flare (peak rate 0.7 count/s) which
occurred at T0+460 ks. By the end of this period, the count rate had
declined to ~0.07 count/s (swift.ac.uk/DGCVn.gif) or a soft X-ray flux of
DG CVn of 3e-12 erg/cm^2/s, similar to the levels of previous detections
of this source by ROSAT and XMM-Newton. Preliminary spectral analysis of
the XRT data from T0+120 to 600 ks using a 3T APEC fit yields a
temperature of 27(+6,-8) keV for the dominant high-T component in the
first observation (T0+117 to T0+328 s), in agreement with the initial BAT
measurement. The derived temperatures for this component decline rapidly
with time to ~4 keV during (T0+)4-20 ks, 2.7 keV for 20-50 ks and 2.3 keV
for 60-200 ks, except during the 2 later re-flares when they showed small
increases.
The Swift UV/Optical Telescope (UVOT) began settled observations of the
field of DG CVn at T0+125 s. The initial finding chart images in the white
and u filters are heavily saturated. After the 4.2 ks gap in XRT/UVOT
observations, UVOT observed in all 7 UVOT filters, with the optical
filters (v, b and u) being saturated until ~T0+20 ks. Several more smaller
flares were observed after the initial trigger, with the second and third
brightest occurring at T0+10 ks and T0+80 ks after the trigger. The flares
have decreased in peak brightness as the overall brightness has decreased.
The temporal evolution is most pronounced in the UV filters.
The peak XRT flux corresponds to an X-ray luminosity of 1.9e32 erg/s at
the 18 pc distance of this system (recently derived by Riedel et al. 2014,
AJ, 147, 85) which is 1.5 times the combined systemic bolometric
luminosity of 1.3e32 erg/s. Thus, like the 2008 EV Lac superflare detected
by Swift (Osten et al. 2008, ATel #1499; 2010, ApJ, 721, 785), for a
period of a few minutes the X-ray emission from this flare outshone all
the light from its parent star. As discussed by Riedel et al. (2014), this
binary system has kinematic, rotational and activity characteristics
indicative of membership in the young star population scattered throughout
the solar neighborhood rather than to the dominant Gyr-old thick disk
population. These authors suggest a likely age of 30 Myr for DG CVn. Thus,
the extreme activity of this system as manifested by this event is due to
its extreme youth.