* Astronomy

Title: Multiperiodicity, modulations and flip-flops in variable star light curves II. Analysis of II Peg photometry during 1979-2010
Authors: Marjaana Lindborg, Maarit J. Mantere, Nigul Olspert, Jaan Pelt, Thomas Hackman, Gregory W. Henry, Lauri Jetsu, Klaus G. Strassmeier

According to earlier Doppler images of the magnetically active primary giant component of the RS CVn binary II Peg, the surface of the star was dominated by one single active longitude that was clearly drifting in the rotational frame of the binary system during 1994-2002; later imaging for 2004-2010, however, showed decreased and chaotic spot activity, with no signs of the drift pattern. Here we set out to investigate from a more extensive photometric dataset whether such a drift is a persistent phenomenon, in which case it could be due to either an azimuthal dynamo wave or an indication of the binary system orbital synchronisation still being incomplete. We analyse the datasets using the Carrier Fit method (hereafter CF), especially suitable for analysing time series in which a fast clocking frequency (such as the rotation of the star) is modulated with a slower process (such as the stellar activity cycle). We combine all collected photometric data into one single data set, and analyse it with the CF method. As a result, we confirm the earlier results of the spot activity having been dominated by one primary spotted region almost through the entire data set, and the existence of a persistent, nearly linear drift. Disruptions of the linear trend and complicated phase behaviour are also seen, but the period analysis reveals a rather stable periodicity with P(spot)=6.71054d ± 0.00005d. After the linear trend is removed from the data, we identify several abrupt phase jumps, three of which are analysed in more detail with the CF method. These phase jumps closely resemble what is called flip-flop event, but the new spot configurations do not, in most cases, persist for longer than a few months.

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Maximum of the variable star II Pegasi at 02:28 UT, 20th June, 2011.

Magnitude range: 7.2  -7.8
Period: 6.7 days   
Type: RS       

Position(2000): R.A. = 23 55 04.05 Dec.= +28 38 01.2

Title: Spots, plages, and flares on lambda Andromedae and II Pegasi
Authors: A. Frasca (1), K. Biazzo (1), G. Tas (2), S. Evren (2), A. C. Lanzafame (3) ((1) INAF - Catania Astrophysical Observatory, Catania, Italy, (2) Ege University Observatory, Bornova, Izmir, Turkey, (3) Department of Physics and Astronomy, Astrophysics Section, University of Catania, Italy)

We present the results of a contemporaneous photometric and spectroscopic monitoring of lambda And and II Peg aimed at investigating the behaviour of surface inhomogeneities in the atmospheres of these active stars which have nearly the same temperature but different gravity. The light curves and the modulation of the surface temperature, as recovered from LDRs, are used to map the photospheric spots, while the H-alpha emission has been used as an indicator of chromospheric inhomogeneities. The spot temperatures and sizes were derived from a spot model applied to the contemporaneous light and temperature curves. We find larger and cooler spots on II Peg (T_sp ~ 3600 K) compared to lambda And (T_sp ~ 3900 K); this could be the result of both the different gravity and the higher activity level of the former. Moreover, we find a clear anti-correlation between the H-alpha emission and the photospheric diagnostics. We have also detected a modulation of the intensity of the HeI D_3 line with the star rotation. A rough reconstruction of the 3D structure of their atmospheres has been also performed by applying a spot/plage model to the light and temperature curves and to the H-alpha flux modulation. A close spatial association of photospheric and chromospheric active regions has been found in both stars. Larger and cooler spots have been found on II Peg, the system with the active component of higher gravity and higher activity level. The area ratio of plages to spots seems to decrease when the spots get bigger. Moreover, with the present and literature data, a correlation between the temperature difference Delta_T = T_ph - T_sp and the surface gravity has been also suggested. In addition, a strong flare affecting the H-alpha, the HeI D_3, and the cores of NaI D_1,2 lines has been observed on II Peg.

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Title: Nonthermal Hard X-ray Emission and Iron Kalpha Emission from a Superflare on II Pegasi
Authors: R. A. Osten, S. Drake, J. Tueller, J. Cummings, M. Perri, A. Moretti and S. Covino

Abstract. We report on an X-ray flare detected on the active binary system II~Pegasi with the Swift telescope. The trigger had a 10-200 keV luminosity of 2.2$ imes10^32 erg s^-1-- a superflare, by comparison with energies of typical stellar flares on active binary systems. The trigger spectrum indicates a hot thermal plasma with T~ 180 x 10^6 K. X-ray spectral analysis from 0.8--200 keV with the X-Ray Telescope and BAT in the next two orbits reveals evidence for a thermal component (T>80 x 10^6 K) and Fe K 6.4 keV emission. A tail of emission out to 200 keV can be fit with either an extremely high temperature thermal plasma (T ~ 3 x 10^8 K) or power-law emission. Based on analogies with solar flares, we attribute the excess continuum emission to nonthermal thick-target bremsstrahlung emission from a population of accelerated electrons. We estimate the radiated energy from 0.01--200 keV to be ~6 x 10^36 erg, the total radiated energy over all wavelengths sim10^38 erg, the energy in nonthermal electrons above 20 keV ~ 3 x 10^40 erg, and conducted energy 20 keV when compared to the upper and lower bounds on the thermal energy content of the flare. This marks the first occasion in which evidence exists for nonthermal hard X-ray emission from a stellar flare. We investigate the emission mechanism responsible for producing the 6.4 keV feature, and find that collisional ionisation from nonthermal electrons appears to be more plausible than the photoionisation mechanism usually invoked on the Sun and pre-main sequence stars.

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