Swift detection of a superflare from CC Eridani

ATel #1787; P. A. Evans, K. L. Page, A. P. Beardmore, J. P. Osborne, O. Godet (U. Leicester), R. Osten (STScI), G. Tagliaferri (OAB), S. D. Barthelmy, J. R. Cummings, H. A.Krimm, T. Sakamoto (GSFC) and D. M. Palmer (LANL)
on 17 Oct 2008; 15:06 UT
Credential Certification: Phil Evans (pae9@star.le.ac.uk)

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At 11:22:52 UT on 2008 October 16, the Swift BAT triggered on a superflare from the the binary system CC Eridani. This system consists of a dK7 and dM3 star, with an orbital period of 1.57 days, and is at a distance of 11.5 pc. Swift slewed to the system immediately, and the XRT began observing the source 147.2 s after the trigger.

The BAT light curve shows a rise in intensity beginning near T-200s reaching a plateau around T=0 s. Emission continued at roughly the same level until approximately T+200 s before fading over the next 300 s. Very low level emission continued until at least T+800 s. Several shorter, very weak peaks are superimposed on this broad pattern: centered at T-150, T+150, T+275 and T+380 s. Nearly all of the emission is seen below 50 keV. The time-averaged spectrum from T-42.7 to T+413.6 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 3.36 +/-0.22. The fluence in the 15-150 keV band during this time interval is 2.7+/- 0.2 e-6 erg cm^-2 (all errors are at the 90% confidence level).

XRT obtained 2 observations of the source. The first contained one snapshot, beginning at T+147.2 s and continuing for 1.8 ks, entirely in Windowed Timing (WT) mode. The second observation began at T+10.9 ks, and contains 8.5 ks of Photon Counting (PC) mode data, spread across 4 snapshots.

The XRT light curve of the initial observation can be well described as a 2-segment power-law. The initial count-rate is 540 counts s^-1 with a decay index of 0.6 +/-0.1. At T+318 s the decay steepened to a slope of 1.17 +/-0.02. The second observation lies substantially below the extrapolated count rate from this decay, at a rate of ~2 counts s^-1 at T+10.9 ks. The rate was still decaying slightly by this time, with a power-law index of 0.29 +/-0.10.

We modelled the X-ray spectra with an absorbed APEC model; 3 temperature components were necessary to obtain an acceptable fit. In the WT mode data the temperatures were 45 /-12 keV, 3.3 +/-0.1 keV and 0.82 +/-0.01 keV; the PC-mode spectrum yielded lower temperatures of 2.1 +/-0.2 keV, 0.73 +/- 0.04 keV and 0.29 +/-0.04 keV. This softening is confirmed by the 1.5-10/0.3-1.5 keV keV hardness ratio, which shows a gradual softening with time which steepens between the two XRT observations. The abundances of the APEC model were tied between components, but could vary between the WT and PC spectra. In the WT mode data the abundance was 1.5 +/-0.2 solar, in the PC spectrum it had fallen to 0.30 (+/-0.06).

The average 0.5-10 keV flux from the PC mode spectrum is (3.52 +0.10,-0.08) e-11 erg cm^-2 s^-1, slightly above the range of values (1.52 -- 3.17 x 10-11 erg cm^-1 s^-1) reported by Crespo-Chacon et al. (2007, A&A, 471, 929) suggesting that star had not returned to quiescence. The average 0.5-10 keV flux from the WT mode observations is nearly 100 times brighter: (3.113 +0.036,-0.026) x 10-9 erg cm^-2 s^-1. If we create a WT spectrum covering just the early, shallow decay phase (to 324 s), the measured flux is (1.8 +7.2,-1.6) e-8 erg cm^-2 s^-1, 500 times the later level.

This flare is much larger than those reported by Crespo-Chacon et al. from XMM data, or by Pan & Jordan (1995, MNRAS, 272, 11), from ROSAT data which saw the X-ray flux increase by a factor of ~2. Chandra data reported by Nordon & Behar (2007, A&A ,464, 309) contained a larger flare, with the mean flux during the flare being eleven times that in quiescence, however this is increase is still nearly an order of magnitude smaller than that observed by Swift. Similar magnitude flares have been reported in other stars however, (e.g. II Peg, Osten et al., 2007, ApJ, 654; AB Dor, Maggio et al., 2000, A&A, 356, 627; Algol, Favata & Scmitt, 1999, A&A 350, 900), and Swift recently triggered on a superflare from EV Lac (Osten et al. ATEL #1499)