90 Acta Polytechnica CTU Proceedings 2(1): 90–93, 2015 90 doi: 10.14311/APP.2015.02.0090 Simultaneous UBVRI Observations of the AE Aquarii Blobs G. Latev, R. Zamanov, S. Boeva, K. Stoyanov 1Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarighradsko Shousse Blvd., 1784 Sofia, Bulgaria Corresponding author: glatev@astro.bas.bg Abstract We summarize the results of our study of the cataclysmic variable AE Aqr on the basis of simultaneous UBV RI observa- tions. For the flares, we estimated the average risetime of about 300 sec, and colours (U − B)0 ∼ 1.1 and (B − V )0 ∼ 0.1. We also calculated temperatures, sizes, masses and expansion velocities of a few individual fireballs. In a single night (16.08.2012), we detected ∼ 8 min quasi-periodic modulation. Keywords: Stars: individual: AE Aqr - novae, cataclysmic variables - binaries: close - Stars: flare. 1 Introduction AE Aquarii is a fascinating magnetic cataclysmic vari- able (CV) with orbital period of 9.88 h. Like the typ- ical CVs, it consists of a K4 IV/V star which trans- fers material toward a magnetic fast spinning white dwarf (WD). AE Aqr has a relatively long orbital pe- riod and is one of the largest CVs having semimajor axis a = 2.33 ± 0.02 R�, WD mass MWD = 0.63 ± 0.05 M�, secondary mass M2 = 0.37 ± 0.04 M� (the quantities obtained with high-dispersion time-resolved absorption line spectroscopy by Echevarŕıa et al. 2008). A review of AE Aqr was presented by P. Meintjes (this volume). AE Aqr has radio and millimeter synchrotron emis- sion (Bastian, Dulk & Chanmugam 1988), and could be source of TeV γ-rays (Oruru & Meintjes 2012 and references therein). However, MAGIC does not detect γ−ray emission from AE Aqr (Aleksić et al. 2014). Wynn, King & Horne (1997) demonstrated that the WD is acting as magnetic propeller and is ejecting most of the matter transferred through the inner Lagrangian point in the form of blobs (‘fireballs’). The SPITZER infrared spectrum above 12.5 µm can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution in expanding clouds (Dubus et al. 2007). AE Aqr hosts a rapidly rotating white dwarf. Its spin period is P = 33.08 s and spin down rate – 5.64 × 10−14 s s−1 (Patterson 1979, Mauche et al. 2011), corre- sponding to a spin-down luminosity of 6 × 1033 erg s−1. A part of this spin-down power goes for ejection of the blobs. The magnetospheric propeller is effective and only a small fraction (∼3%) of the trasferred mateial eventually accretes on to WD (Oruru & Meintjes 2012). 2 Observations Our observations of AE Aqr were started in 1998 with an electophotometer in V-band and are now extended to simultaneous multicolour (UBV RI) observations with the telescopes of the National Astronomical Observa- tory Rozhen (2.0m RCC, 50/70 cm Schmidt and 60 cm telescope) and the Belogradchick Astronomical Obser- vatory (60 cm telescope). The observational sessions can be summarized as follow: (i) 1998 - 1999: V-band electrophotometric observations. (ii) Aug 2010 - Aug 2011: simultaneous multicolor (UBV RI) CCD observa- tions. (iii) Sep 2011 - Aug 2013: new simultaneous mul- ticolor CCD observations. Observational methods and data reduction are described in Zamanov et al. (2012). 3 Photometric behavior Strong flickering and flaring activity was noted by Henize (1949). On time-scales of about 10 minutes, the light curve of AE Aqr displays flares with an amplitude up to ≈ 1 mag (see Fig.1 and Fig.2). Multicolour opti- cal photometry was performed by Chincarini & Walker (1981) in UBV bands. Later van Paradijs, van Ameron- gen, & Kraakman (1989), reported five-colour (in Wal- raven bands) observations and showed that the flares have rise time ∼ 100 − 200 s and occur throughout the whole orbit. In Fig.2 we plot the orbital light curve of AE Aqr using the orbital period of 0.411655530 d and the zero- orbital phase JD0 = 2449281.4222200 (Casares et al. 1996). In this figure a part of our data obtained in Au- gust 2013 are shown. The quiescent flux curve is rec- ognizable as smooth orbital variation with two maxima and two minima per orbital cycle. 90 http://dx.doi.org/10.14311/APP.2015.02.0090 Simultaneous UBVRI Observations of the AE Aquarii Blobs Figure 1: Optical light curve of AE Aqr obtained on September 27, 2011 in the BV RI bands. Figure 2: The B band light curve of AE Aqr (August 2013) folded with the orbital period (9.8797 h). 3.1 The flares as expanding fireballs Pearson, Horne & Skidmore (2003, 2005) formulated analytic expressions for the spectral evolution and con- tinuum light curves of flickering and flaring variability that occur over a wide range of astrophysical objects. They applyed these expressions to the observations of the cataclysmic variables AE Aqr and SS Cygni, and of the supernova SN 1987A, deriving physical parame- ters for the material involved. They have shown that the observed flare spectrum and evolution of AE Aqr is reproducible with an isothermal fireball with Popula- tion II abundances. The interested reader is directed to their papers for a full discussion. The basic assumptions are: (i) the flares of AE Aqr are due to the appear- ance and expansion of fireballs (blobs); (ii) the blobs are isothermal; (iii) they represent spherically symmet- ric expansion of a Gaussian density profile with radial velocity proportional to the distance from the center of the expansion. Figure 3: Upper panel: dereddened fluxes of a fire- ball at the maximum of the flare. The solid line is a black body fit. Lower panel: time evolution of the flare in the V band. The (red) solid line represents the isothermal fireball model. Following Pearson et al. (2005), the dimensionless time β is defined as β = 1 + H(t − tpk), where tpk is the time of the peak of the flare, H is an “expan- sion constant” setting the speed of the expansion. The dimensionless time β is also the expansion factor being the constant of proportionality between the current and peak scale length apk: β ≡ a/apk. The central density of the fireball is ρ = M (πa2)3/2 , (1) where M is the total mass of the material involved in the expansion (the fireball mass). The speed of expan- sion at a is v = Ha. The optical depth parallel to the 91 G. Latev et al. observer’s line of sight is τ(y) = − ∫ −∞ ∞ κ dx = τ0 e −2( y a )2 (2) where y is the impact parameter (the distance from the fireball center perpendicular to the line of sight), κ1 is the linear absorption coefficient, � is the correction for stimulated emission, � = 1 − e(hν/kT). Here T is the fireball temperature, a is the length scale (which we call the fireball size). The optical depth on the line of sight through the center of the fireball (y = 0) is τ0 = κ1 � M 2 21/2 T 1/2 0 ν 3 π5/2 a5 . (3) The emission of the fireball is: fν = π a2 Bν(T) 2 d2 S(τ0), (4) where S(τ0) is the ”saturation function” (see Fig. 1 of Pearson et al. 2005). To calculate the fireball parameters T,M,a,H,ρ and v, we performed the following: 1. We compute the peak flux of the fireball, Fpk, in the five optical bands (UBV RI). An example is given in Fig.3, where the calculated peak fluxes (corrected for the reddening) are plotted. 2. We derive the temperature of the fireball with a black body approximation applying IRAF nfit1d rou- tine. 3. We evaluate the size of the blob at the peak, apk, using τ0 = 6.8202 and Eq.4. 4. We calculate the mass of the fireball (Eq.3). The calculations are done adopting Population II abun- dances (κ1 = 1.27 × 1052 m−1). 5. Fitting the V band light curves, we derive the expansion constant H. An example is given in Fig.3 lower panel) . 6. We derive the speed of expansion and the central density. Part of the calculated parameters are given in Ta- ble 1. We reach lovely agreement between between the model and the light curves of the optical flares using fireballs with a temperature T ∼ 15000 K and mass M ∼ 1020 g. Blobs are detected in two other close binaries con- taining white dwarfs (the recurrent nova RS Oph and the symbiotic star CH Cyg). In future it will be intert- ing to understand is it the same mechanism (magnetic propeller), which generates the blobs. Figure 4: a) Light curve (in Johnson B band) of AE Aqr obtained on August 16, 2012. Quasi-periodic os- cillations are visible. b) Power spectrum of the light curve. The maximum indicates a period of about 8 min. 4 Unusual Behaviour on August 16, 2012 Usually, AE Aqr has light curve as shown in Fig.1. How- ever, in our observation obtained on August 16, 2012 (see Fig.4) a clear periodicity is visible. The power spec- trum has a maximum corresponding to T = 7.5 ± 0.2 min. Possible explanations of this unusual behavior are: (1) Beat modulation between the orbital period and the WD spin period (e.g. as supposed for BG CMi, Fig.7.3 of Warner 1995). The equation 1/Pbeat = 1/Pspin − 1/Porb (5) gives Pbeat = 33.03 s, which is too short in comparison with the observed value. (2) Blob rotating with Keplerian velocity at the bor- der of the magnetosphere: Rblob = (2π) −2/3 (GMwd) 1/3 T2/3, (6) The period T = 7.5 min corresponds to Keplerian rota- tion at distance Rblob ≈ 0.11 R�. The magnetospheric radius is estimated as Rm ≈ 4−8Rwd ≈ 0.05−0.10 R� (Zamanov et al. 2012 and references therein). In our opinion, we had the chance to observe a blob rotating at the magnetosphere boundary. 92 Simultaneous UBVRI Observations of the AE Aquarii Blobs Table 1: The computed parameters of the fireballs. The time is in format YYYYMMDD hh:mm. In the table are given as follows: rise time in seconds, colours of the peak emission of the fireball (corrected for the interstellar reddening), temperature of the fireball, its mass and size (at the peak of the flare). Quantity 20100813 23:40 20100814 19:20 20100814 19:48 20100814 20:10 20110831 21:43 rise time [sec] 260±20 230±25 290±30 440±20 260±30 (U − B)0 -1.36±0.06 -1.43±0.03 -0.93±0.04 -0.80±0.05 -1.02±0.07 (B − V )0 0.24±0.03 0.23±0.03 0.17±0.03 0.19±0.03 0.03±0.06 temperature T [K] 14 545±1000 27 292±1500 10 856±150 9 527±100 13 395±200 mass M [1019 g] 9.6±1.5 6.8±1.5 39 ±6 78±12 97±15 size apk [10 9 cm] 3.0±0.3 2.5±0.3 5.3±0.3 7.1±0.4 7.7±0.4 5 Conclusions Using 4 telescopes, we performed simultaneous obser- vations in 5 bands (UBV RI) of the flare activity of the cataclysmic variable AE Aqr. Adopting the model of an isothermically expanding ball of gas, we calculated parameters (temperature, size, mass) of a few individ- ual fireballs. In a single night, we detected ∼ 8 min quasiperiodic modulation, which might be due to a blob rotating at the magnetosphere boundary. In future we intent to measure more blobs and to search for correlations between their physical parame- ters. Also, we will try to do a follow-up study of the QPOs phenomenon. Acknowledgement The authors are very grateful to the anonymous referee for useful notes and comments. This work was sup- ported in part by the OP ”HRD”, ESF and the Bulgar- ian Ministry of Education and Science (BG051PO001- 3.3.06-0047). References [1] Aleksić, J., Ansoldi, S., Antonelli, L. A., et al. 2014, A&A, 568, AA109 [2] Bastian, T. S., Dulk, G. A., & Chan- mugam, G. 1988, ApJ, 324, 431 doi:10.1093/mnras/282.1.182 [3] Casares, J., Mouchet, M., Martinez-Pais, I. G., & Harlaftis, E. T. 1996, MNRAS, 282, 182 [4] Chincarini, G., & Walker, M. F. 1981, A&A, 104, 24 doi:10.1111/j.1365-2966.2008.13248.x [5] Dubus, G., Taam, R. E., Hull, C., Watson, D. M., & Mauerhan, J. C. 2007, ApJ, 663, 516 [6] Echevarŕıa, J., Smith, R. C., Costero, R., Zharikov, S., & Michel, R. 2008, MNRAS, 387, 1563 doi:10.1111/j.1365-2966.2012.20410.x [7] Henize, K. G. 1949, AJ, 54, 89 [8] Oruru, B., & Meintjes, P. 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LATEV: We will try to do this in our future work. 93 http://dx.doi.org/10.1093/mnras/282.1.182 http://dx.doi.org/10.1111/j.1365-2966.2008.13248.x http://dx.doi.org/10.1111/j.1365-2966.2012.20410.x http://dx.doi.org/10.1046/j.1365-8711.2003.06079.x http://dx.doi.org/10.1086/426582 http://dx.doi.org/10.1002/asna.201211718 Introduction Observations Photometric behavior The flares as expanding fireballs Unusual Behaviour on August 16, 2012 Conclusions