We present high speed photometry of the Sloan Digital Sky Survey cataclysmic
variable SDSS J013701.06-091234.9 in quiescence and during its 2003 December
superoutburst. The orbital modulation at 79.71\pm0.01 min is double humped; the
superhump period is 81.702\pm0.007 min. Towards the end of the outburst late
superhumps with a period of 81.29\pm0.01 min were observed. We argue that this
is a system of very low mass transfer rate, and that it probably has a long
outburst interval.Comment: 5 pages, 8 figures. Accepted for publication in MNRA

Araujo-Betancor

B. Warner

E. Armstrong

G. Bolt

Harvey

Hellier

Hessman

J. Patterson

Kolb

Krzeminski

M. L. Pretorius

Mason

Murray

O'Donoghue

Osaki

P. A. Woudt

Patterson

Robinson

Rogoziecki

Rolfe

Schoembs

Skidmore

Szkody

Vogt

Warner

Whitehurst

Woudt

English

arXiv

We present high speed photometry of the Sloan Digital Sky Survey cataclysmic variable SDSS J013701.06-091234.9 in quiescence and during its 2003 December superoutburst. The orbital modulation at 79.71pm0.01 min is double humped; the superhump period is 81.702pm0.007 min. Towards the end of the outburst late superhumps with a period of 81.29pm0.01 min were observed. We argue that this is a system of very low mass transfer rate, and that it probably has a long outburst interval

Pretorius, M L

Woudt, P A

Warner, B

Bolt, G

Patterson, J

Armstrong, E

Cape Town University OpenUCT

Mon. Not. R. Astron. Soc. 352, 1056–1060 (2004) doi:10.1111/j.1365-2966.2004.08000.xHigh-speed photometry of SDSS J013701.06 − 091234.9M. L. Pretorius,1 P. A. Woudt,1 B. Warner,1 G. Bolt,2 J. Patterson3and E. Armstrong31Department of Astronomy, University of Cape Town, Private Bag, Rondebosch 7700, South Africa2Center for Backyard Astrophysics (Perth), Camberwarra Drive, Craigie, Western Australia3Department of Astronomy, Columbia University, 538 West 120th Street, New York, New York 10027, USAAccepted 2004 May 6. Received 2004 March 26; in original form 2004 March 1ABSTRACTWe present high-speed photometry of the Sloan Digital Sky Survey cataclysmic variable SDSSJ013701.06 − 091234.9 in quiescence and during its 2003 December superoutburst. Theorbital modulation at 79.71 ± 0.01 min is double-humped; the superhump period is 81.702 ±0.007 min. Towards the end of the outburst late superhumps with a period of 81.29 ± 0.01 minwere observed. We argue that this is a system of very low mass transfer rate, and that it probablyhas a long outburst interval.Key words: techniques: photometric – binaries: close – stars: dwarf novae – stars: individual:SDSS J013701.06 − 091234.9 – novae, cataclysmic variables.1 I N T RO D U C T I O NCataclysmic variable stars (CVs) are identified in the Sloan Dig-ital Sky Survey by spectroscopic classification of candidates se-lected on the basis of their colours (see Szkody et al. 2002). Thediscovery of SDSS J013701.06 − 091234.9 (hereafter abbreviatedSDSS0137) is announced in the second CV release of the SloanDigital Sky Survey (Szkody et al. 2003). The optical spectrum showsa blue continuum with broad absorption around narrow emissioncores of Hβ and higher members of the Balmer series. There is alsosome indication of broad TiO absorption features characteristic of anM-dwarf primary (Szkody et al. 2003). These features indicate thatit is a very low mass transfer rate (Ṁ) system where the accretiondisc is so faint that most of the luminosity of the system comes fromthe stars themselves.Szkody et al. (2003) obtained 2 h of time-resolved spectroscopyand reported an orbital period (Porb) of 84 min; the short Porb is alsoconsistent with a low Ṁ . The presence of TiO absorption, togetherwith our photometry, indicates that this spectrum was taken duringthe normal quiescent state, rather than, say, during a decline fromoutburst, where the hydrogen absorption may originate from thedisc.We observed SDSS0137 at a quiescent V magnitude near 18.6 onseven nights; the system then brightened by ∼6 mag and strong su-perhumps were seen. This qualifies SDSS0137 as an SU UMa-typedwarf nova [see e.g. Warner (1995a) for a review of SU UMastars].E-mail: retha@mensa.ast.uct.ac.za (MLP); pwoudt@circinus.ast.uct.ac.za(PAW); warner@physci.uct.ac.za (BW); gbolt@iinet.net.au (GB);jop@astro.columbia.edu (JP); armstrong@astro.columbia.edu (EA)In Section 2 we present our observations in quiescence and out-burst. These results are discussed in Section 3 and summarized inSection 4.2 O B S E RVAT I O N SWe obtained photometry of SDSS0137 at the Sutherland site of theSouth African Astronomical Observatory (SAAO) using the 1.0-and 1.9-m telescopes together with the University of Cape Towncharge-coupled device (UCT CCD) [O’Donoghue (1995) gives adescription of the UCT CCD photometer and its reduction software].In addition, the SAAO CCD was used on three nights; unlike theUCT CCD, this is not a frame-transfer CCD and hence there are afew seconds of deadtime between integrations.All the quiescent observations were made in white light; withthe UCT CCD this gives photometry with an effective wavelengthsimilar to Johnson V , but with a very broad bandpass. This, togetherwith the non-standard flux distribution of a CV, means that it is notpossible to transform the observations on to any standard photo-metric system. By observing hot subdwarf and white dwarf stan-dards, we are able to give V magnitudes that are accurate to only∼0.1 mag.Most of the outburst observations were made with telescopes ofthe Center for Backyard Astrophysics (CBA) – see Harvey et al.(1995).2.1 Quiescent light curvesSDSS0137 was observed in quiescence on seven nights spread overalmost four months; Table 1 is a log of these observations. We finda double-humped orbital modulation, but no coherent signals onshorter time-scales. The light curves are shown in Fig. 1. Note thatC© 2004 RASDownloaded from https://academic.oup.com/mnras/article/352/3/1056/1210039 by University of Cape Town user on 03 June 2021High-speed photometry of SDSS J013701.06−091234.9 1057Table 1. Log of the observations in quiescence.Run No. Date (at the start HJD of first obs. Length (h) t int (s) Telescope V (mag)of the night) (245 2000+)S7060 2003 August 27 879.54482 3.20 40 1.0-m 18.4∗S7063 2003 August 28 880.47545 4.74 40 1.0-m 18.4∗S7073 2003 August 30 882.51423 2.51 40 1.0-m 18.5∗S7088 2003 September 5 888.55917 2.57 40 1.0-m 18.5S7144 2003 October 5 918.44016 1.74 30 1.9-m 18.6S7168 2003 December 16 990.28114 2.32 60 1.0-m 18.5S7172 2003 December 17 991.27868 2.27 60 1.0-m 18.6:Notes: t int is the integration time; ‘:’ denotes an uncertain value; ∗taken with the SAAO CCD.Figure 1. The first four of the light curves of SDSS0137 in quiescence,phased using equation (1). Runs S7063, S7073 and S7088 are shifted alongthe horizontal axis by 17, 53 and 169 cycles and down by 0.35, 0.60 and1.00 mag respectively for display purposes.the relative strength of the fundamental and first harmonic changesfrom night to night, in run S7073, e.g. the first harmonic is onlybarely present.The lower scatter on the first three nights is caused in part by thefact that the SAAO CCD (used on these nights) gives a much largerfield of view, and hence includes a larger number of comparisonstars, than the UCT CCD, resulting in higher quality differentialphotometry. However, at least some of the scatter in run S7088 iscaused by intrinsic flickering. The quantum efficiency of the UCTCCD peaks at λ ∼ 600 nm, but is still 25 per cent at λ ∼ 350 nm(O’Donoghue 1995), whereas the SAAO CCD has very poor blueand ultraviolet (but high red and near-infrared) sensitivity; see fig. 17of Woudt & Warner (2002). Therefore the UCT CCD observationsare more receptive of short-wavelength flickering.Fig. 2 is a Fourier transform of the first four runs; the fundamentaland first harmonic give an unambiguous P orb = 79.71 ± 0.01 min.The ephemeris for the times of maximum isHJDmax = 245 2879.594 + 0.055 351(±7)E . (1)Fig. 3 shows the average light curve obtained by folding runsS7060, S7063, S7073 and S7088 on this ephemeris after removinga linear trend from each of the light curves individually.2.2 Outburst observationsIn a system with Ṁ as low as indicated by the the spectroscopicappearance of SDSS0137, one would expect to see only fairly infre-Figure 2. The Fourier transform of runs S7060, S7063, S7073 and S7088combined. There is only one choice of alias that gives the correct fundamen-tal and first harmonic relation; these peaks are marked by vertical bars at0.2091 mHz (79.71 min) and 0.4183 mHz (39.85 min).Figure 3. The mean quiescent light curve of SDSS0137 (plotted twice)using the first four runs folded on the ephemeris given in equation (1).quent dwarf nova outbursts. Nevertheless, after leaving SDSS0137at V = 18.6 mag on 2003 December 17, we were lucky enough tofind it at V = 12.5 mag four nights later. Table 2 is a log of theobservations during the outburst. This is the first recorded outburst.Smooth superhumps (with a peak-to-peak range of 0.27 mag)were present on December 21 already, implying that this was asuperoutburst, that therefore SDSS0137 is an SU UMa-type dwarfnova, and that the start of outburst was probably two or three daysearlier.The outburst light curves are shown in Fig. 4 (a few noisy runsare omitted). Linear trends were removed from most of these lightcurves. Combining all the data from the first eight nights of the out-burst gives the Fourier transform displayed in Fig. 5. The superhumpperiod (Psh) is 81.702 ± 0.007 min; the first and second harmonicare also clearly detected. The light curves in Fig. 4 are phased usingthis period.By the ninth night the superhumps were becoming noticeablyphase-shifted; another four nights later they had shifted by ∼0.5 cy-cle and could be classified as late superhumps (e.g. Vogt 1983).The observed minus calculated (O−C) diagram (Fig. 6) shows theC© 2004 RAS, MNRAS 352, 1056–1060Downloaded from https://academic.oup.com/mnras/article/352/3/1056/1210039 by University of Cape Town user on 03 June 20211058 M. L. Pretorius et al.Table 2. Log of the outburst observations.HJD of first obs. Length (h) t int (s) Telescope V (mag)(245 2000+)995.29032 2.10 5 1.00-m995.57788 4.93 2 1.30-m 12.4996.02674 3.48 15 0.25-m996.27143 0.96 6 1.00-m996.63321 2.70 5 1.30-m 12.6997.01334 3.97 15 0.25-m997.29209 3.76 28 0.36-m997.46431 3.62 80 0.25-m 12.7998.00842 4.16 15 0.25-m 12.8999.01274 3.81 30 0.25-m 12.81000.08011 2.21 30 0.25-m1000.61812 1.75 45 0.36-m 13.0:1001.45554 3.90 80 0.25-m1001.56973 2.33 60 0.36-m 13.0:1002.29981 3.21 28 0.36-m1002.45077 3.57 80 0.25-m1002.56369 4.24 5 1.30-m 13.11003.27273 1.93 6 1.00-m 13.11004.25491 2.60 28 0.36-m 13.21006.23953 0.88 60 0.41-m1007.08293 1.63 30 0.25-m 13.51008.01126 3.03 30 0.25-m 13.61011.01927 3.13 30 0.25-m1011.30694 0.87 28 0.36-m 13.91012.01266 2.87 60 0.25-m 14.7Notes: t int is the integration time; the magnitudes are nightly averages; ‘:’denotes an uncertain value; most observations were in white light.transition from superhumps to late superhumps; there is no cyclecount ambiguity in this diagram.Fig. 7 shows the observations on HJD = 245 3008 and one of theHJD = 245 3002 light curves plotted on the same intensity scale. Theamplitude and position of the late superhump maximum indicate thatit did not develop out of anything visible in the superhump profile.Rather, the structure in the superhump is its own repetitive profile.Figure 4. The outburst light curves after linear trends were removed from some of them; most of these light curves were binned to improve signal-to-noiseratio. The observations are phased according to the superhump period of 81.702 min. Individual runs are shifted along the horizontal axis by the number ofcycles indicated, and arbitrarily displaced vertically. A few of the lower quality runs are not shown.Figure 5. The Fourier transform of the first eight nights of the outburst.Vertical bars at 40.854 and 27.231 min mark the first and second harmonic.Figure 6. O−C diagram computed by comparing the observed times ofmaximum with the times calculated from a sine function with a period of81.702 min. This is consistent with a constant period up until HJD 245 3003;after that the phase starts changing.Fig. 8 is a Fourier transform of all the data taken after HJD =245 3004; it gives a late superhump period of 81.29 ± 0.01 min.All the outburst light curves were searched for rapid quasi-coherent oscillations, but with no success.C© 2004 RAS, MNRAS 352, 1056–1060Downloaded from https://academic.oup.com/mnras/article/352/3/1056/1210039 by University of Cape Town user on 03 June 2021High-speed photometry of SDSS J013701.06−091234.9 1059Figure 7. Superhump and late superhump profiles plotted in intensity units.There is no vertical displacement between the two light curves; they arephased using the superhump period and the time of the first observed super-hump maximum. The bottom light curve is transposed along the horizontalaxis by 95 superhump cycles.Figure 8. The Fourier transform of all observations of the late superhump.Peaks corresponding to the fundamental and first harmonic are marked.3 D I S C U S S I O NIt has often been noted that the standard model of CV evolutionpredicts the existence of a large number of short-period, low-Ṁ CVs,but that only very few such systems are known (e.g. Kolb 2002).Low-Ṁ CVs are intrinsically faint, but many are being discovered insurveys that reach faint limits, such as the Sloan Digital Sky Survey.The orbital modulation of SDSS0137 is very similar to the doublehump in WZ Sge (e.g. Patterson 1980). Three more examples of CVswith double-humped orbital modulations are WX Cet (Rogoziecki& Schwarzenberg-Czerny 2001), SDSS J161033.64 − 010223.3(Woudt & Warner 2004) and HS 2331+3905 (Araujo-Betancoret al. 2004). WZ Sge and WX Cet are both low-Ṁ dwarf novae oflong outburst interval; no outburst has yet been observed in SDSSJ161033.64 − 010223.3.The double-humped modulation in WZ Sge (and the other sys-tems like it) has not been convincingly modelled, but is probablycaused by the bright-spot. An early explanation was that the bright-spot has a large scaleheight and is thus visible above the inner accre-tion disc when it is on the far side of the disc (Krzeminski & Smak1971; Robinson, Nather & Patterson 1978). Since then, it has beendemonstrated that the disc in WZ Sge is optically thin (Skidmoreet al. 2000; Mason et al. 2000) so that the bright-spot can be seenthrough it.The second hump in SDSS0137 has lower amplitude during runS7073 than in any of our other runs. This has also been observed inWZ Sge (Krzeminski & Smak 1971); in WX Cet the second hump issometimes not detected at all (Rogoziecki & Schwarzenberg-Czerny2001). A possible explanation for this is that the optical thicknessof the disc can temporarily increase so that the spot is (at leastpartially) obscured when it is on the side of the disc facing awayfrom the observer (Krzeminski & Smak 1971). However, in the caseof our run S7073, an increase in optical depth of the disc cannot havebeen caused by an increase in Ṁ through the disc, since the systemwas 0.10 mag fainter than in run S7063 and 0.14 mag fainter thanin run S7060. Although there are a few features in the spectrumthat may be attributed to the secondary, it does not contribute alarge fraction of the total flux, therefore it seems very unlikely thatthe orbital modulation contains a substantial amount of ellipsoidalvariation from the secondary.Normal superhumps are thought to be caused by tidal stressesacting on an eccentric disc (e.g. Whitehurst 1988). Repetitive struc-ture in superhump profiles similar to what can be seen in Fig. 7,and in more of the light curves in Fig. 4, has been observed in VWHyi as well (Schoembs & Vogt 1980; Warner 1985). The fractionalperiod excess of the superhump is ε = (P sh − P orb)/P orb = 0.0250± 0.0001.The interval between supermaximum and the first appearance ofsuperhumps is less than 4 d. This indicates that the outburst intervalin SDSS0137 is probably years at most, rather than decades (Warner1995b).A variation in bright-spot brightness, resulting from the varyingdepth in the white dwarf potential well at which the stream impactsthe disc, was initially proposed as a model for superhumps (Vogt1982), but has since been used to explain late superhumps (e.g.Whitehurst 1988; Murray 1996; Rolfe, Haswell & Patterson 2001)(but see also Hessman et al. 1992). Hellier (2001) proposes an alter-ation to the tidal thermal instability (Osaki 1989), namely that thetidal and thermal instabilities are uncoupled. The tidal eccentricitycan persist after the disc has come down from the thermal high stateat the end of the outburst. The viscous dissipation – caused by tidalstresses – which results in superhumps then decreases because thedisc viscosity is now much lower. At this point the late superhumpsshould become detectable above the superoutburst light. It is there-fore perhaps surprising that the late superhump cannot be seen inthe HJD = 245 3002 light curves.There are two other systems for which the transition from super-humps to late superhumps has been carefully tracked: V1159 Ori(Patterson et al. 1995) and IY UMa (Patterson et al. 2000). In boththese examples O−C increases during the transition, in contrast towhat is shown in Fig. 6.4 S U M M A RYWe classify the CV SDSS0137 as an SU UMa-type dwarf nova andreport an orbital period of 79.71 min and a superhump period of81.702 min. SDSS0137 has a double-humped orbital modulation,which is compatible with the very low Ṁ indicated by the spectralappearance. Late superhumps developed ∼11 d after the start of theoutburst.AC K N OW L E D G M E N T SBW’s research is funded by the University of Cape Town. PAW issupported by the National Research Foundation and by funds madeavailable to BW from the University of Cape Town. MLP is fundedC© 2004 RAS, MNRAS 352, 1056–1060Downloaded from https://academic.oup.com/mnras/article/352/3/1056/1210039 by University of Cape Town user on 03 June 20211060 M. L. Pretorius et al.by the Department of Labour. We are very grateful to Berto Monard,David Messier, Don Starkey and Arto Oksanen for contributingobservations. The paper has benefited from constructive commentsby the referee, Dr Boris Gänsicke.R E F E R E N C E SAraujo-Betancor S., Gänsicke B. T., Hagen H.-J., Marsh T., Thorstensen J.,Harlaftis E., Fried R. E., Engels D., 2004, in Tovmassian G., Sion E.,eds, Proc. 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A., Warner B., 2004, MNRAS, 348, 599This paper has been typeset from a TEX/LATEX file prepared by the author.C© 2004 RAS, MNRAS 352, 1056–1060Downloaded from https://academic.oup.com/mnras/article/352/3/1056/1210039 by University of Cape Town user on 03 June 2021

High-speed photometry of SDSS J013701.06 − 091234.9

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High Speed Photometry of SDSS J013701.06-091234.9

High Speed Photometry of SDSS J013701.06-091234.9

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