arXiv:1806.03610v1 [astro-ph.SR] 10 Jun 2018 pno h Dbcmslce ihtebnr ri due orbit ⋆ binary the with locked the becomes The that WD such disk. the synchronized, accretion also of an are spin polars of of formation low majority the vast a of prevents field from magnetic WD lines strong the The field secondary. sequence along main matter mass accreting wh magnetic (WD) a dwarf of consisting binaries interacting are Polars INTRODUCTION 1 5 cetdXX eevdYY noiia omZZZ form original in YYY; Received XXX. Accepted u .Babina, V. Ju. MNRAS © .V Baklanov V. A. snhoosplrV50Cg ria,si n beat and spin orbital, periods Cyg: V1500 Asynchronous 8 7 6 4 3 2 1 srnmclIsiueo lvkAaeyo cecs 0596 Sciences, of Academy Slovak of Institute Astronomical .P Pavlenko, P. E. nentoa etrfrAtooia,MdcladEcolog and o Medical Academy Astronomical, Russian for Astronomy, Center of International Institute of State Branch Moscow Terskol Lomonosov Cruc Institute, Las Astronomical D, Sternberg Suite 88003, Solano, NM, S. Cruces, 1025 Las 3DA, Observatory, Rocks MSC P Picture Vernadskogo University, 4 State Mexico University, Federal New Crimean Vernadsky I. V. eea tt ugtSinicIsiuinCienAstrop Crimean Institution Scientific Budget State Federal -al [email protected] E-mail: 07TeAuthors The 2017 000 , 1 – 7 21)Pern 2Jn 08Cmie sn NA L MNRAS using Compiled 2018 June 12 Preprint (2017) 1 1 1 .A Antonyuk, A. K. , 2 ⋆ .A Mason, A. P. yn:individual Cygni: h oln ftesilhtW srvae yteirdae companion irradiated dwarfs. words: the white Key magnetic by continues onto revealed Cyg flows as V1500 accretion of WD for asynchronism laboratory hot spin-orbit still and evolution the of cooling the negigascn hs upbtensossprtdb less by separated spots between jump phase second a undergoing ABSTRACT slkl h pnpro,wihi .31 i hre hnteobtlp interp we orbital jump the one-ha first by than every The separated jumps shorter regions polars. phase between min asynchronous displays 1.73(1) of maxima is characteristic curve a which light period, of s diagram spin intermittent O-C weak the re A likely accretion phase. emitting is beat cyclotron d with the systematically am track vary curves The they light and obtained. spin acc are WD changing curves the orb to the light of subtracting due dependent By is spin-phase flow. that the quencies, of period pole-switching beat magnetic night-to spin-orbit including from 9.6-d modulation the brightness measure Mean variations. ellipsoidal and h eeto ffc rmtehae oo a erae,bts but decreased, amplitude has an donor with heated Cy radiation the V1500 tical from 2 called covered effect now eruption reflection polar, post The years asynchronous 40 an occurring be campaign to photometric found was it Later h rgtNv yn 95i aenv namgei ht wr ( dwarf white magnetic a on nova rare a is 1975 Cygni Nova bright The tr:nve aalsi aibe tr:mgei ed– – field magnetic stars: – variables cataclysmic novae, stars: 1 3 , 4 .V Andreev, V. M. .A Sosnovskij, A. A. yia bevtr fRS acn,280,Rpbi fCr of Republic 298409, Nauchny, RAS, of Observatory hysical clRsac fNS,Urie2 kdmk aoonh Str Zabolotnoho Akademika 27 Ukraine NASU, of Research ical arnk onc,Slovakia Lomnica, Tatranska 0 ite nvriy nvriesyAe,1,Mso 192 Russi 119992, Moscow 13, Ave., Universitetsky University, cecs 665 ekTrkl aadn-akraRepu Kabardino-Balkaria Terskol, Peak 361605, Sciences, f opk,Smeoo,250,Rpbi fCie,Russia Crimea, of Republic 295007, Simferopol, rospekt, s M 80,USA 88001, NM, es, USA 180 n ftebihetnveo h 0hcnuy( century 20th the of novae brightest the of one , donor (1992). the Raine with and field King, magnetic Frank, the see of interaction the to ei oa ln ihG e n QHr ntecase the In Her. DQ and Per mag- GK rare Of a with of Ind. along site CD nova, the and is netic Aql, Cyg V1432 V1500 Cam, importance, particular BY po- are asynchronous polars an chronous to found ( later lar Cyg, V1500 binary the 1975 ◦ ∼ hnteso rfsduring drifts spot the Then . cmd,e l 1995 al. et Schmidt, oaCgi17 ece n antd n hswas thus and magnitude 2nd reached 1975 Cygni Nova ; 1 anr2006 Warner m . 5 The . 7 , 8 .V Pit, V. N. 1 0 m .Y.Shugarov, Yu. S. .Tenv curdo h Din WD the on occurred nova The ). . 3 eiulrvasccornemission cyclotron reveals residual .Tetreohrcnre asyn- confirmed other three The ). 1 .I Antonyuk, I. O. , ∼ ildmntsteop- the dominates till -ihs(2015-2017). 6-nights . etpaebefore phase beat 0.1 e spl switching pole as ret than ltd n profile and plitude ga t0.137613- at ignal in nteWD the on gions tladba fre- beat and ital A ob powerful a be to ngti sdto used is -night eingeometry retion .Ormultisite Our g. T 180 E fba period, beat lf 5 tl l v3.0 file style X , 6 a ◦ h post The . ro.The eriod. etrace We . ma Russia imea, lc Russia blic, 38 yv Ukraine Kyiv, 03680 . V1500 : Honda WD). 1 2 E. P. Pavlenko et al.

of V1500 Cyg, it is assumed that the expanding nova shell 0.5 0.5

slowed the orbit of the companion of a previously synchro- 1.0 1.0 nized polar (Stockman et al. 1988). A strong irradiation of the secondary by the hot WD dominated the total radiation 1.5 1.5

of this system for decades following the eruption. 2.0 2.0

While polarization was observed to track the WD spin, 2.5 2.5

optical photometry gave information on two periods: 1) the 3.0 3.0

0.5 0.5 orbital modulation caused by heating of the secondary com- 57212.4 57212.5 57212.6 57270.2 57270.3 57270.4

ponent by irradiation from the WD (Schmidt, et al. 1995) 1.0 1.0 that is still very hot, 40 years after the 1975 explosion and 2) the spin-orbital (synodic) beat period (Pavlenko and Pelt 1.5 1.5

1991). The orbital period has been detected every year 2.0 2.0 of observation and its amplitude has varied from year to year (Pavlenko and Shugarov 2005a,b). The beat period has 2.5 2.5

Relative magnitude systematically increased with time. Using the spin-orbit 3.0 3.0

0.5 0.5 beat period relation, orbital and beat periods were used 57226.4 57226.5 57226.6 57271.3 57271.4 57271.5 calculation for of the current value of the WD spin period 1.0 1.0 (Pavlenko and Shugarov 2005a) rather than direct measure- ments of the WD spin period which is possible only while 1.5 1.5

the accretion flow is stable, between pole switchings, and is 2.0 2.0 best done with polarimetric or X-ray observations.

2.5 2.5

Spin-orbit synchronization times of a hundreds to a few 3.0 3.0 thousand years are expected from theory, see e.g. Andronov 57245.3 57245.4 57245.5 57245.6 57368.6

(1987). Campbell and Schwope (1999) show that a range of HJD-2400000 synchronization times are possible with the fastest being Figure 1. Examples of nightly light curves. All the data are pre- about 50 years and requiring an unusually low-mass pri- sented in the same scale with the exception of the highly time mary of 0.5 M⊙. Such a low mass WD is unlikely given that resolved data for HJD 2457368 showing the fine structure of the evidence from the nova ejecta (Lance, McCall, and Uomoto . Light curves show significant night-to-night variabil- 1988) indicate a high mass WD. Photometry over the in- ity, typical of asynchronous polars. tervening years indicated that the WD was undergoing fast and nonlinear synchronization from measurements by Katz (1991) with a rate of PÛ = 1.8 × 10−6 for 1977- 8 8 1979, 2.7 × 10− for 1979-1987, and 2.4 × 10− for 2000- 2 OBSERVATIONS 2003 (Pavlenko and Shugarov 2005a). Schmidt, et al. (1995) Û −8 obtained P = 3.86 × 10 for 1987-1992 from polarime- CCD-photometry of V1500 Cyg was obtained during 25 try. The time of synchronization was estimated inde- nights within a 54-day interval in 2015, with a follow-up pendently by Schmidt, et al. (1995) as ∼ 170 years and observation in 2017. Observations were carried out using by Pavlenko and Pelt (1991) as 150-290 years. Recently five telescopes located at four observatories. See the jour- Harrison and Campbell (2016) suggest that V1500 Cyg is nal of observations given in Table 1. Exposure times varied already synchronized. Armed with extensive beat-phased re- from 10-s with no significant dead-time between exposures solved photometry we find that to the contrary, V1500 Cyg using the Otto Struve 2.1 m telescope at McDonald Ob- is still an asynchronous polar. servatory to 60-180 s at the Crimean Astrophysical Obser- vatory (CrAO) 1.25 m and Shajn 2.6 m telescopes as well This work aims to unravel the combination of several as the Terskol 60 cm telescope and Sternberg Astronomi- sources contributing to the optical radiation of V1500 Cyg cal Institute 1.25 m. All of the images were unfiltered ex- ∼ 40 years after the 1975 nova explosion and to establish cept for those obtained at McDonald Observatory, where the current status of V1500 Cyg in terms of synchronization a broad-band optical (BVR) filter was used, and on one and accretion geometry. First, one would expect a reduction night in 2017 where a B filter was used with the CrAO 2.6- of the reflection effect from cooling of the hot WD. Second, m telescope. Science frames were dark subtracted and flat- with reduced heating comes the appearance of significant, fielded in the usual manner. The comparison star USNO B1 and recently detected, optical cyclotron radiation from ac- 1381-0460911 (Monet 2003) which has B=19.05(5) and V= cretion columns (Harrison and Campbell 2018). Third, el- 17.83(5) was used for absolute calibration while the stars lipsoidal variations due to the aspherical Roche-lobe filling C2 and C3 (Kaluzhny and Semeniuk 1987) were used for companion may be observed. In particular, the test for asyn- differential photometry in 2015 and for B photometry in chronism is accomplished by examination of the variable 2017. During the time of these observations the brightness orientation of the WD magnetic field with respect to the of V1500 Cyg varied between V = 19m and V = 21m. Photo- secondary component. In other words, asynchronous polars metric uncertainties varied depended on the size of telescope, display light curves which vary according to the beat phase the brightness level, and weather conditions. The maximum between the binary orbit and the WD spin and is confirmed uncertainty was ∼ 0m.1. All of the data were converted to by the detection of these periods. Heliocentric Julian Day (HJD) for further analysis.

MNRAS 000, 1–7 (2017) V1500 Cygni 3

-1.0 Table 1. Journal of observations. 2.1

2.0

-0.5

HJD 2457000+ Observatory/ CCD N 1.9

1.8 (start - end) telescope 0.0

1.7

211.407 - 211.564 CrAO/2.6m APOGEE E47 105

1.6 0.5 212.405 - 212.555 CrAO/2.6m APOGEE E47 94 2xAbbe statistics Relative magnitude

1.5 221.420 - 221.560 CrAO/2.6m APOGEE E47 95

1.0 223.263 - 223.546 CrAO/2.6m APOGEE E47 247 1.4

1.3 225.380 - 225.553 SAI/1.25m VersArray-1300s 186 9.58d 1.5

0.06 0.08 0.10 0.12 0.14 0.16 226.379 - 226.557 SAI/1.25m VersArray-1300s 233 0.0 0.5 1.0 1.5 2.0 Frequency (1/day)

Beat phase 245.280 - 245.559 CrAO/2.6m APOGEE E47 341 270.228 - 270.386 CrAO/2.6m APOGEE E47 199 Figure 2. Left: periodogram for all of the data after orbital mod- 271.264 - 271.584 CrAO/2.6m APOGEE E47 392 ulation is subtracted. Right: residuals (gray points) folded on the 272.225 - 272.479 CrAO/2.6m APOGEE E47 300 9.58-d period. The mean per night values are marked by black 274.221 - 274.378 Terskol/60cm SBIG STL 1001 138 squares. The zero phase corresponds to the start of observations 278.264 - 278.421 Terskol/60cm SBIG STL 1001 100 (HJD =2457300.24118). For clarity the data are reproduced twice. 281.266 - 281.493 Terskol/60cm SBIG STL 1001 135 284.286 - 284.475 Terskol/60cm SBIG STL 1001 124 295.302 - 295.437 Terskol/60cm SBIG STL 1001 87 300.241 - 300.377 CrAO/1.25m ProLine PL23042 65 0.5 mag. So, the appearance of rapidly evolving light curves 301.231 - 301.419 CrAO/1.25m ProLine PL23042 69 along with the determination of a putative beat period is 311.191 - 311.350 CrAO/1.25m ProLine PL23042 74 strong evidence of spin-orbital asynchronism, still in 2015. 326.188 - 326.374 CrAO/1.25m ProLine PL23042 73 331.176 - 331.324 CrAO/1.25m ProLine PL23042 70 332.196 - 332.348 CrAO/1.25m ProLine PL23042 70 341.131 - 341.208 SAI/1.25m VersArray-1300s 56 344.133 - 344.344 SAI/1.25m VersArray-1300s 146 3.2 Orbital period: the reflection and possible 344.161 - 344.316 CrAO/1.25m ProLine PL23042 71 ellipsoidal effects 367.559 - 367.637 McDonald/2.1m ProEM 659 As we noticed that the dominant modulation is still caused 368.565 - 368.664 McDonald/2.1m ProEM 804 by a strong reflection component, see top panels of Fig. 3, 1013.389 - 1013.594 CrAO/2.6m APOGEE E47 127 our data reveled a period of 0.139617(6)-d that within errors coincides with the orbital period measurement of 0.139613-d 3 RESULTS (Semeniuk et al. 1995). In order to extract weaker signals in the light curve we construct a periodogram of the residuals Examples of nightly light curves are presented in Fig. 1. The of all data after both the beat and orbital modulation (which light curves commonly display smooth roundish maxima and we fit using a one-humped wave) were removed, see bottom sharp minima. Some light curves have step-like or teeth-like panels of Fig. 3. The result reveled some excess at the orbital variations which distort the smooth intensity modulation. A period in the form of a two-humped curve with near-equal majority of the light curves have an amplitude of about 1.5 maxima (amplitude of 0.35 mag). We suggest this is a light mag. However on some occasions, the nightly amplitude was curve produced by the ellipsoidal shape of the secondary as small as 1 mag., see for example the light curve on HJD component in a field of high-mass . However the 2457270, top right panel of Fig. 1. Nightly light curves also true ellipsoidal profile will show a small difference from the display mean brightness level variations. V1500 Cyg demon- one obtained due to some difference of a best-fitted curve strates complex brightness variability where the dominating from a properly modeled curve at a precise orbital inclina- signal is the orbital modulation caused by the strongly irra- tion. Likely, the observed profile of the orbital modulation diated secondary, hereafter described as a “reflection effect”. results from a superposition of reflection and ellipsoidal ef- Night-to-night variability is, in part, attributed to periodic fects. changes in our view of the accretion flow geometry which depends on the spin-orbit beat phase.

3.3 Spin period 3.1 Beat period Inspection of the periodogram of the beat and orbital re- In order to investigate possible spin-orbital beat modulation flection subtracted light curve, see Fig. 4, reveals no promi- we subtracted the orbital variability with a 0.13962-d period nent signal at the WD spin period, see arrows in left pan- from the observed light curve and constructed a periodogram els. Rather a residual signal is seen at the orbital period of the residual light curve using the ISDA package (Pelt of 0.139617(6)-d. This signal is interpreted as the ellipsoidal 1980). The periodogram in the range of the expected beat modulation, an orbital variation due to the changing view of period is shown as Fig. 2. The most significant feature is the the aspherical Roche-lobe filling companion, as discussed in minimum at a 9.58(3)-d period that is just a few percent the previous subsection. After the ellipsoidal modulation is longer than the predicted value of 9.1 - 9.3 d according to subtracted, a weak WD spin signal at 0.137613(7)-d remains the ephemeris of Schmidt, et al. (1995), using polarimetry (see Fig. 4). As expected, the spin signal is strongly diluted to track the spin of the WD and photometry for the orbital by changes in the accretion flow geometry likely including variability. The mean light curve has an amplitude of about pole switching. The marginally detected WD spin period is

MNRAS 000, 1–7 (2017) 4 E. P. Pavlenko et al.

-1.0 2.0

-0.5

1.5

0.0

2.00

0.5

1.0

1.0

1.5

0.5

P =0.139617 d

Reflection effect dom inates orb

2.05 2.0

7.10 7.15 7.20 7.25 7.30

0.0 0.5 1.0 1.5 2.0

1.95 -1.0

2.00 2xAbbe statistics 2xAbbe 2xAbbe statistics 2xAbbe Relative magnitude Relative

-0.5

1.95

0.0

1.90

0.137613 d

0.5

1.85

1.90 1.0 1.80

P

1.75 1.5

orb

Ellipsoidal m odulation dom inates

1.70 2.0 7.20 7.25 7.30

7.10 7.15 7.20 7.25 7.30 0.0 0.5 1.0 1.5 2.0

Frequency (1/day) Orbital phase Frequency (1/day)

Figure 3. Top left: Periodogram for all of the data of V1500 Cyg Figure 4. Periodogram for beat and orbit subtracted data in the after the beat modulation is subtracted reveals the strong signal region of the predicted spin period. This feature (recall arrows at the orbital period. Top right: Corresponding data folded on the in Fig. 3) remains as other signals have weakened. This dilution orbital period is consistent with a strong reflection effect. Bottom of the spin signal is due to pole switching of the magnetically left: Periodogram for residuals after both beat modulation and or- channeled accretion flow. bital modulation are subtracted. Arrows in left panels point to the feature consistent with the predicted spin period by calculation. % Bottom right: Corresponding data folded on the orbital period, 2015 its spin period was 173(1)-s or 0.88 shorter than its now showing the combined cyclotron modulation with ellipsoidal orbital period. variation from the aspherical companion. For clarity the data are reproduced twice in the right panels. Zero phase corresponds to the start of observations (HJD = 2457300.24118). 3.4 Accretion geometry The residuals (after the beat and orbital modulations were subtracted) are folded on the suggested WD spin period of consistent with the calculated spin period Pspin according 0.137613-d and are presented in Fig. 5 for each night. The to equation (1), namely, corresponding phases of the 9.58-d beat period are indicated. It is seen that the residual light curves still have a variable amplitude and profile. Several morphological structures are P−1 = P−1 − P−1 observed, including one-, two-, or even three-humped struc- beat spin orb, (1) tures. The shape of light curve even changes from cycle- where the observed orbital period is Porb = 0.139617-d to-cycle on some occasions (JD ...223). Sometimes, a high- and beat period is Pbeat = 9.58-d. The dilemma in measur- amplitude QPO-like structure is seen (JD ...221, JD ...270) ing the spin period directly is that one must either do so in superposed on some humps. The phases of highest maxima less than one-half of a beat cycle or face significant signal di- and deepest minima also change. Such a diversity of profiles lution due to accretion pole switching. The WD spin period could be explained by a complex structure of the WD mag- predicted using equation (1) is Pspin = 0.137611-d. netic field. Such a complex accretion geometry as has been It is not so obvious that we must detect the signal at the proposed for the asynchronous polar BY Cam (Mason et al. spin period. Its detectability probably depends on the accre- 1998). tion geometry in the V1500 Cyg system and uniformity of At first glance, changes in the nightly light curves ap- the data distribution. If the accretion stream switches from pear chaotic. A more attentive procedure was performed in one magnetic pole to the other one and back during the beat order to confirm or refute this suggestion. We determined cycle and if data are distributed uniformly in all beat phases, times of the highest maxima (see Table 2) assuming that we would expect to see no spin period. On the other hand, in they originate from the more intense accretion spot on the the case of no pole switching the spin signal must be strong WD and calculated a O-C diagram using the ephemeris: enough to be detected. As expected, the cooling of the WD HJD = 2457211.40731 + 0.137613E. has resulted in the reduction of the still dominant reflection max effect and the appearance of cyclotron emission producing a All available maxima are plotted against corresponding photometric signal modulated at the spin period. We there- phases of the beat period and are displayed in Fig. 6. This fore identify the 0.137613-d period as the WD spin period. plot clearly shows two clusters of points in O-C space. The Of particular significance, beat-phased resolved light curves first cluster occupies half of the beat period (beat phases indicate periodic changes in accretion geometry. We there- 0.0−0.5) and suggests that the observed period of the bright- fore conclude that while closer to being synchronous, the est accretion spot during that time is near constant and is white dwarf in V1500 Cyg is still not synchronized and in equal to the adopted spin period. The position of the rest

MNRAS 000, 1–7 (2017) V1500 Cygni 5

0.04

Beat ph =0.54-0.57

Beat ph =0.30-0.31 -0.5 -0.5 -0.5

JD ...245

JD ...367

0.03

0.0 0.0 0.0

0.02

0.01

Beat ph =0.00-0.16

0.5 0.5 0.5

JD ...211

0.00

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.5 -0.5 -0.5 Beat ph =0.56-0.57

-0.01 Porb

JD ...274

-0.02

0.0 0.0 0.0

-0.03

Beat ph = 0.05-0.06

Beat ph =0.35-0.37 -0.04

0.5 0.5 0.5

JD ...221

JD ...272

O-C (days) O-C

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 -0.05

Beat ph = 0.10-0.12 -0.5 -0.5 -0.5

JD ...212

-0.06

-0.07 0.0 0.0 0.0

-0.08

Beat ph =0.56-0.58

Beat ph =0.38-0.40

0.5 0.5 0.5

JD ...226

JD ...301 -0.09

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.10 -0.5 -0.5 -0.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0.0 0.0 0.0 Beat phase

Beat ph =0.40-0.41

Beat ph = 0.14-0.16 Beat ph =0.61-0.62 Figure 6. The O-C diagram for WD spin maxima versus beat 0.5 0.5 0.5

JD ...368

JD ...270

JD ...332 phase. The horizontal dotted red line is expected for an accre-

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.5 -0.5 -0.5

Beat ph = 0.24-0.27 Beat ph =0.41-0.43 Beat ph =0.61-0.63 tion spot that does not drift in position and thus varies at the

JD ...223 JD ...311 JD ...284 Pspin period of the WD. The vertical dotted lines correspond to

0.0 0.0 0.0 expected jumps in O-C at beat phases 0.0 and 0.5 that means

Relative magnitude Relative switching of accretion spot between poles separated by 180◦. The O-C values are consistent with this model and also display a drift- 0.5 0.5 0.5 ing behavior between beat phases 0.5 and 1.0.

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.5 -0.5 -0.5

0.0 0.0 0.0 Table 2. Timings of spin maxima for residuals after the beat and orbital modulations were removed.

Beat ph =0.46-0.48

0.5 0.5 0.5 Beat ph = 0.25-0.28 Beat ph =0.76-0.77

JD ...225

JD ...271 JD ...295 HJD

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.5 Beat ph = 0.27-0.29 -0.5 -0.5

JD ...300 2400000+ 2400000+ 2400000+ 2400000+

0.0 0.0 0.0

57211.448 57223.280 57225.495 57245.315 57245.416 57270.339 57271.311 57271.434

Beat ph =0.50-0.51

Beat ph =0.98-0.99 0.5 0.5 0.5 57272.272 57272.410 57274.278 57278.292

JD ...331

JD ...278 57284.332 57295.332 57300.345 57311.310

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

-0.5 -0.5

Beat ph = 0.29-32 57331.237 57332.199 57341.150 57368.596

JD ...281

0.0 0.0

cretion stream switches to the second region separated from

Beat ph =0.50-0.51 ◦ 0.5 0.5 180 JD ...341 the first accretion spot by less than . Unfortunately, a

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 lack of data does not allow complete coverage between beat

Spin phase phases 0.8 and 1.0.

Figure 5. Photometric data folded on the adopted spin period of 0.137613 d. The beat and orbital period modulation were re- moved. The zero phase corresponds to the start of observations 4 DISCUSSION (HJD = 2457300.24118). For clarity the data are reproduced twice. In order to further investigate the cooling time-scale for the WD in V1500 Cyg, we gathered all of the data on the amplitude of B-band photometry to fol- of the O-C values situated at beat phases 0.5 − 1.0 imply low the decrease in emission from the heated compan- that the accretion spot period is longer than the WD spin ion in the same manner as Somers and Naylor (1999). period, but shorter than orbital period, during that time. We included data from their Table 1 along with There are two jumps occurring at beat phases 0.0 and 0.5 data from Litvinchova, Pavlenko and Shugarov (2011) and suggesting that the accretion stream switches between ac- Harrison and Campbell (2016) and plotted them in Fig. 7. cretion spots at those phases. Further, during beat phases As it turns out, the new data lie on the extension of the ∼ 0.5-0.62 the accretion spot drifts in the opposite direction previous relation confirming that the orbital amplitude de- to the WD rotation with a period close to the orbital one. crease is indeed in accordance with theoretical predictions At later phases it probably remains relatively constant and for post-nova cooling. The best-fit power low index is η = close to the spin period. Finally, at beat phase 1.0 the ac- 1.13, which is remarkably consistent with the purely theoret-

MNRAS 000, 1–7 (2017) 6 E. P. Pavlenko et al.

dipole spot and an equatorial one, with some drift of an accretion spot in the opposite direction to the WD spin.

-14 10 MHD calculations around the beat cycle were also performed demonstrating the main effects of pole switching in the case ) -1

Å of a complex magnetic field structure (Zhilkin et al. 2012).

-1 The profiles of the 2015 spin light curves of V1500 Cyg s -2 are reminiscent of those of BY Cam in a high accretion state.

-15

10 Here for V1500 Cyg we find both the pole switching and a similar accretion spot drift as that observed in BY Cam. The

major difference being, in V1500 Cyg, the domination of the optical light is from the hot WD irradiating the companion producing a strong reflection effect.

-16

10

B band B amplitudeflux (ergscm 5 CONCLUSIONS

2,8 3,5 4,2 We have carried out a photometric investigation of the asyn-

LOG (TIME in days) chronous polar and post-nova V1500 Cyg ∼40 years after its

10 1975 eruption. Our extensive dataset consists of photomet- ric observations obtained during 25 nights within a 54-day Figure 7. Cooling of the white dwarf. The log of the ampli- interval in 2015 and during one night in 2017 at four ob- tude, A, of the reflection modulation is shown as a function of servatories. Several key features in the light curve argue for the log of the time T since the nova eruption in 1975. Data is continued asynchronous spin-orbital motion of the the bi- collected from the similar plot of Somers and Naylor (1999), see nary. These are the existence of nightly variations in the note in text, (black points) and more recent measurements of orbital light curve following the 9.6-d spin-orbital beat cy- Litvinchova, Pavlenko and Shugarov (2011) and our new B-band cle, the enigmatic appearance of the 0.137613-d spin period light curve (pink points) and those of Harrison and Campbell (2016) (blue points). Scatter is indicative of amplitude variation which is only seen during some beat phases, and ultimately as a function of beat phase. The best-fit matches the theoretical evidence of accretion stream flow switching between mag- prediction for WD cooling of Prialnik (1986) netic poles. Earlier it was suggested (Litvinchova, Pavlenko and Shugarov 2011) that the ical prediction of Prialnik (1986). Specifically, that the WD beat modulation in V1500 Cyg is caused by a periodic post-nova cooling should follow such a power-law with η = shading of the heated side of the secondary by an accretion 1.14, see the discussion in Somers and Naylor (1999). The ring around the WD. This explanation is in agreement with scattering around the best-fit line is caused by both photo- the interpretation of Schmidt, et al. (1995) of ”relatively metric accuracy and on the variable amplitude observed at broad optically thick accretion columns with lumps of dense different beat phases, recall the variations seen in Fig. 5. gas, and/or orbiting debris in a disk-like geometry”. We can There is some confusion in the paper of not distinguish between these possibilities, but we can say Somers and Naylor (1999) concerning the quantities η that such a structure would be a long-lived one, existing for and x. The discrepancy is between their formulae and what the last 40 years. More high density observations covering is stated in the text. According to their paper the response the complete beat cycle are needed to monitor the path of the secondary star to heating is x=0.75. Equating their towards synchronism and to further investigate the beat equations (2) to (3) we obtain η =-1.68, not 0.94 as quoted phase dependent peculiarities in accretion geometry of in the text. The value 0.94 could come from x=1/0.75, or V1500 Cyg. if the index in equation (3) is η/x instead of η × x. Our η, An important question remains. Why is the WD in plotted in Fig.7 is also not the actual η, but rather the slope V1500 Cyg still so hot? After 40 years it remains hot enough in that relation. We re-calculated this slope for Somers and for the irradiated secondary to dominate the optical flux, Naylor data and found that it is -1.16, not -1.26. Indeed, yet it CONTINUES TO FOLLOW THE THEORETICAL all the data give a slope of -1.13 that is reasonably close to COOLING LAW. The nova eruption was among the most -1.16. Despite this discrepancy, the slope of the amplitude luminous ever recorded and apparently heated the WD to is correctly measured and our new points are in agreement a higher temperature than is typical. Other magnetic CVs with the total trend. have also had luminous novae, namely GK Per and DQ Her, Finally we consider the new observations of V1500 suggesting that magnetic novae may be more luminous as a Cyg in comparison another asynchronous polar. BY sub-class. Further work is needed to properly address this Cam was the first asynchronous polar where evidence question. for accretion pole switching was found by polarimet- ric (Mason, Liebert, and Schmidt 1989) and photomet- ric (Mason et al. 1996; Silber et al. 1997; Mason et al. ACKNOWLEDGEMENTS 1998) studies. A multi-polar magnetic field model of an aligned dipole plus quadrupole field model was suggested P. Mason thanks the SDSS/FAST program for support. by Wu and Mason (1996); Mason et al. (1998). Pavlenko O. Antonyuk acknowledges the Russian Science Founda- (2006) found additional evidence of switching between a tion (grant N14-50-00043) and S. Shugarov thanks grants

MNRAS 000, 1–7 (2017) V1500 Cygni 7

VEGA2/0008/17 and APVV 15-0458 for partial financial support.

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