Optical Photometry of Dwarf Nova QZ Serpentis in Quiescence

Erica D. Jones Center for Astrophysics, Space Physics and Engineering Research at Baylor University

Dr. Dwight Russell Department of Physics at Baylor University

Richard Campbell Department of Mechanical Engineering at Baylor University

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Abstract—We present quiescent photometry of dwarf nova QZ Serpentis. Given the limited amount of published information These binary systems are known to have periods of on this object, we present photometry at different wavelengths quiescence, outbursts, and superoutbursts. In quiescence, and comment on the spectral class. We collect data in four SS Cygni has an average magnitude of 11.2; however, in filters, BVRI, and calibrate data using bias subtraction, dark outburst, there is a reported magnitude of 9 [4]. Dwarf subtraction, and flat division. We also use multiple aperture photometry in the AstroImageJ software to obtain the novae also exhibit larger changes in magnitude during brightness of our target object and our comparator star. We outburst. An example of this is dwarf nova V725 Aquilae. calculate the average magnitude for QZ Serpentis in each With an outburst magnitude of 13.7 and a quiescent filter. We present light curves and folded light curves for each magnitude of 19.32, the object increases over 100 times in observing run. We use differential photometry to yield an brightness [5]. average R magnitude of 17.399 ± 0.007 and B-V = 0.747 ± 0.030. We report a secondary spectral class ranging from an The dwarf nova QZ Serpentis was discovered in 1998 by early G-type star to an early K-type star. Light curves exhibit Katsumi Haseda [6]. QZ Ser is located at right ascension sinusoidal features, which is typical of contact binaries. Our 15h56m54.47s and declination 21˚07’19.0”. QZ Ser has an findings are consistent with previously published data. orbital period, Porb, of 0.08316 days or 119.752 minutes [6]. Keywords—dwarf novae, accretion disk, cataclysmic variables, Thorstensen et. al (2002) report a the magnitude of the variable star photometry secondary star as V = 17.9 ± 0.4 [6].

I. INTRODUCTION In this paper, we present optical photometry of QZ Ser in the BVR and I filters. We discuss observing runs, data ATACLYSMIC variables (CVs) are close binary star calibration techniques, data analysis, and light curve C systems divided into subcategories based on features. The majority of the data were collected using the frequencies and amplitudes of luminosity variations [1]. R filter. We create folded light curves using the orbital These categories are novalike variables (NL), novae (N), period, Porb = 0.08316 days, and T0 = 2452328.044 HJD recurrent novae (RN), and dwarf novae. The CV of interest published in [6] in order to analyze the features of QZ Ser in this paper is the dwarf nova. based on the position, or phase, of the binary.

II. METHODOLOGY Dwarf novae are close binaries with a “late-type main sequence” secondary star that fills its Roche lobe, transferring material through the Langrangian point L1, “the We collected data on 6 nights at the Paul and Jane Meyer inner Lagrangian point” [2]. This transfer of material forms Observatory using the 0.6 m Ritchey-Chretien telescope. an accretion disk around the white dwarf primary star [2]. Throughout the various observing runs, data were collected These objects exhibit interesting effects due to the presence in four filters: BVRI. On June 10, 2013, we collected 10 of an accretion disk, and various theories exists to explain exposures in the V filter with 60 s exposure time. Similar these effects observed in dwarf novae. Theories such as data were collected the next night in the I filter. change in mass-transfer rate and disk instability, offer explanations for effects observed when studying dwarf Data in the R filter were collected on both June 13, 2013 novae [3]. and June 14, 2013. On the second night, we were able to

NSF CASPER RET 2013 – E. Jones observe QZ Ser over one complete orbital period. We collected data in three filters (BVR) on the night of June 19, To plot the data, we used an IDL program. The IDL 2013. One July 9, 2013, we observed two full orbital program uses the HJD to calculate the phase of an object. periods of QZ Ser in the R band filter using 60 second As mentioned in the introduction, we use an orbital period, exposures. More details of the observing runs are included Porb, of 0.08316 days and a T0 of 2452328.044 HJD. These in Table I. values were reported in [6]. The phase was calculated based on this initial time correlating to a phase of 0.0 and this orbital period. The HJD of each data point was user to TABLE I. calculate the phase of QZ Ser at that time, with phase OBSERVATION LOG FOR QZ SER ranging from 0.0 to 1.0. We used an IDL program to convert the Julian date (JD) to the heliocentric Julian date UT Start Time UT End Time # of Exposures UT Date (HJD). (first exposure) (last exposure) /Filter (exptime)

2013 June 10 07:09:33 07:35:14 10/V(60s) By converting to HJD, we take into account the motion of the Earth around the sun and its effect on the movement 2013 June 11 04:21:11 04:52:40 10/I(60 s) towards or away from the object, depending on the right 2013 June 13 03:57:45 04:08:22 11/R(60 s) ascension and declination of the object. “In order to determine the HJD, astronomers must consider the time it 2013 June 14 04:39:44 06:57:43 130/R(60 s) would take light to travel from a celestial object to the 6/V(90 s) 2013 June 19 03:24:41 03:51:15 center of the Sun rather than to the Earth” [7]. This 6/R(90 s) calculation provides a reference for the amount of time it 16/B(180 s) 2013 June 19 03:38:32 05:58:06 16/V(120 s) takes light to reach one point, regardless of the motion of 16/R(120 s) the Earth. 2013 July 9 03:48:08 08:01:59 240/R(60 s) We also use phased average binning to calculate the mean magnitude at a phase step of 0.05, and we calculated the error using the standard deviation of the mean. The data were calibrated using the AstroImageJ software. AstroImageJ software outputs a flux error for the flux Bias frames, darks frames, and flat fields were median measurement of both the target and the comparator star. combined to create masters using the software. The master The flux measurements are converted to magnitude and dark with the same exposure time as the flats was subtracted magnitude error. The average magnitude of QZ Ser in each from all flats before the master flat using the appropriate filter is calculated using the average of the magnitude filter was created. Images were bias subtracted, dark measurements and error of this average is calculated using subtracted, and flat divided. The CCD was kept at -35˚ C traditional error analysis on the sum of values in the for all exposures. numerator of the average and on the constant in the denominator. Multi-aperture photometry was completed using the AstroImageJ software. Photometry was measured for the target, QZ Ser, and various comparator stars in the field of III. LIGHT CURVES view. Comparator star 4, or C4 as shown in Figure 1, was used for differential photometry. In this section we present light curves from each observing run. Figures 2 through 11 contain a light curve with magnitude versus time in the upper panel and magnitude versus phase in the lower panel. The lower panel of Figures 2-11 shows the changes in the brightness of QZ Ser over two periods, with data from one period repeated for continuity.

The light curve in Figure 10 contains data collected using the B filter. The brightness of QZ Ser in the B magnitude ranges from 19.2 to 18.0 within error bars. Figures 2, 6, and 8, represent the brightness of QZ Ser in the V filter. The V magnitude ranges from 18.2 to 17.4, including error bars. The R magnitude ranges from 17.6 to 17.1 in Figures 4, 7, 9, and 1l. Figure 5 shows the R magnitude ranging from 18.0 to 16.8. In the I filter, the magnitude of QZ Ser Figure 1. Finding chart of QZ Ser based on data taken in the R band Filter (60 s exposure) including the target and various ranges from 17.2 to 16.9 (Figure 3). The brightness of QZ comparator stars. The filter of view is with north at the Ser appears to increase at redder wavelengths. bottom and east to the left. NSF CASPER RET 2013 – E. Jones

Figure 4. Upper panel – light curve of data set collect 2013 June 13. Figure 2. Upper panel – light curve of data set collect 2013 June 10. Data were collected using the R filter. Lower panel – Data were collected using the V filter. Lower panel – folded light curve of the same data plotted twice to view folded light curve of the same data plotted twice to view over two periods. over two periods.

Figure 3. Upper panel – light curve of data set collect 2013 June 11. Figure 5. Upper panel – light curve of data set collect 2013 June 14. Data were collected using the I filter. Lower panel – Data were collected using the R filter. Lower panel – folded light curve of the same data plotted twice to view folded light curve of the same data plotted twice to view over two periods. over two periods for continuity.

NSF CASPER RET 2013 – E. Jones

Figure 8. Upper panel – light curve of data set collect 2013 June 19. Figure 6. Upper panel – light curve of data set collect 2013 June 19. Data were collected using the V filter with an exposure Data were collected using the V filter with an exposure time of 120 s. Lower panel – folded light curve of the time of 90 s. Lower panel – folded light curve of the same same data plotted twice to view over two periods for data plotted twice to view over two periods. continuity.

Figure 7. Upper panel – light curve of data set collect 2013 June 19.

Data were collected using the R filter with an exposure Figure 9. Upper panel – light curve of data set collect 2013 June 19. time of 90 s. Lower panel – folded light curve of the same Data were collected using the R filter with an exposure data plotted twice to view over two periods. time of 120 s. Lower panel – folded light curve of the same data plotted twice to view over two periods for continuity.

NSF CASPER RET 2013 – E. Jones

Observing run #5 was separated in Figures 6 to 11 based on filter selection and exposure time. In the 5a observing run, the V and R data was taken using 90s exposures. In the 5b observing run, the data in V and R were taken using 120s exposures. The 5a data in the B filter was excluded due to the exposure time being insufficient to detect QZ Ser.

Figure 12 contains all the data collected in the R filter during the summer of 2013. Data were collected in R during multiple observing runs. By calculating the HJD and using it to determine the phase of QZ Ser, we were able to plot the brightness of the object versus the phase. Because we used the phase, we are able to plot all data points from different observing runs and view them on one plot, as opposed to using the light curves in the upper panels of Figures 2 through 11.

The plot of hundreds of data points and corresponding errors in Figure 12 presents a challenge when trying to extract basic features in the light curve of an eclipsing binary. We used phase binning to help address this problem. Based on the phase, data points were binned, and the magnitude was averaged. The error on each point was Figure 10. Upper panel – light curve of data set collect 2013 June 19. calculated using the standard deviation of the mean. In the Data were collected using the B filter with an exposure lower panel of Figure 12, we see a much cleaner light curve time of 180 s. Lower panel – folded light curve of the with features that were washed out in the plot with all data same data plotted twice to view over two periods for continuity. points and error bars.

Figure 12. Upper panel – folded light curve of all data set collected Figure 11. Upper panel – light curve of data set collect 2013 July 9. Data throughout the observing runs during the summer of 2013 in were collected using the R filter. Lower panel – folded light the R filter. Lower panel – phased binned light curve of the curve of the same data plotted twice to view over two periods same data plotted twice to view over two periods for for continuity. continuity.

NSF CASPER RET 2013 – E. Jones

TABLE II. AVERAGE MAGNITUDE OF QZ SER, V. CONCLUSION IN FILTERS BVRI

Quiescent photometry of dwarf nova QZ Ser yield Average Filter(s) Error various information about the brightness, temperature, and Magnitude physical nature of the system. Data analysis was conducted using differential photometry to yield an average R B 18.572 ± 0.026 magnitude of 17.399 ± 0.007 and B-V = 0.747 ± 0.030. V 17.825 ± 0.015 Based on this calculate B-V, QZ Ser has a secondary spectral class ranging from an early G-type star to an early R 17.399 ± 0.007 K-type star. The light curve in Figure 12 exhibits sinusoidal features. The features in an eclipsing binary are consistent I 17.031 ± 0.001 with a contact binary, in which the secondary star fills its Roche lobe. Our findings are consistent with previous B-V 0.747 ± 0.030 findings and classifications of QZ Ser.

ACKNOWLEDGMENTS IV. DISCUSSION

We would like to thank Aubrey Brickhouse, president of For this system, there are limitations when attempting to the Central Texas Astronomical Society, for giving us determine the spectral class of the secondary star. Although access to the Paul and Jane Meyer observatory. Special QZ Ser is an eclipsing binary, it has an inclination of 67˚[6]. thanks to our telescope operator Willie Strickland who Both objects are visible at every phase. The two eclipses taught us how to navigate AstroImageJ. Thanks to Dr. present in the lower panel of Figure 12 represent the dip in Hyde, Dr. Matthews, and everyone involved in the magnitude when part of each component of the binary CASPER REU/RET program. This research was supported system is eclipsed. The accretion disk and white dwarf are by the NSF. never completely eclipsed by the secondary, so the spectral class is determined using the B-V based on the average REFERENCES magnitudes of QZ Ser which include all components. B-V [1] E. L. Robinson, "The structure of cataclysmic variables," Annual is 0.747 ± 0.030, implying a spectral class between an early Review of Astronomy and Astrophysics, vol 14, pp. 119-142, 1976. G-type star and an early K-type star. [2] D. Nogami, S. Masuda, and T. Kato, "The 1994 super-outburst of the new su uma-type dwarf nova sx leonis minoris," Publications of The average magnitude of QZ Ser in quiescence is the Astronomical Society of the Pacific, pp. 1114-1121, 1997. [3] L. Kotko, J. P. Lasota, G. Dubus, and J. M. Hameury, “Models of am reported for various filters in Table II. These values near canum venaticorum star outbursts,” Astronomy and 17 and 18 magnitudes are consistent with previous report of Astrophysics, vol. 544, p. 13, 2012. QZ Ser in quiescence [6]. Based on the larger magnitudes [4] W. B. Honey, G. T. Bath, P. A. Charles, R. Whitehurst, D. H. P. at bluer wavelengths, represented by values in Table II and Jones, J. Echevarria, M. J. Arevalo, J. E. Solheim, G. Tovmassian, and K. Takagishi, “Quiescent and outburst photometry of the dwarf by our B-V measurement, we can conjecture that the system nova SS Cygni,” Monthly Notices of the Royal Astronomical is composed of objects with temperatures closer to that of Society,vol. 236, pp. 727-734, 1989. the Sun than new O-type stars. [5] M. Uemura, T. Kato, E. Pavlenko, A. Baklanov, and J. Pietz, “Photometric observation of a new in-the-gap su uma-type dwarf nova v 725 qquilae during the 1999 superoutburst,” Publications of The folded light curve features in Figure 12 are consistent the Astronomical Society of Japan, vol. 53, pp. 539-545, 2001. with the physical nature of a contact binary system with the [6] J. R. Thorstensen, W. H. Fenton, J. Patterson, J. Kemp, J. Halpern, secondary filling its Roche lobe. The sinusoidal nature and I. Baraffe, “Qz serpentis: a dwarf nova with a 2 hour orbital implies a physical system in which one component of the period and an anomalously hot, bright secondary star,” Publications binary is always transiting another component with a short of the Astronomical Society of the Pacific, vol. 114, no. 800, pp. 1117-1123, 2002. time period where we see both object. This short period of [7] B. W. Carroll and D. A. Ostlie, An Introduction to Modern time is represented by the peak in magnitude, consistent Astrophysics, 2nd ed. Pearson Education, Inc., 2007, p. 15. with observing all components of the binary system.

Future research on the dwarf nova QZ Ser may include obtaining light curves over full orbital periods at multiple wavelengths. This paper presents a complete light curve in the R filter. A similar light curve in the B and V filters would add to the very limited amount of published data for this object.