We report photometric observations of the hidden symbiotic star SU Lyn in the optical bands. In five nights we detect a weak flickering in U band with amplitude of about 0.05 magnitudes. No intranight variations are found in B, V, g' and r' bands. This is one more indication that the secondary component is a white dwarf accreting at low accretion rate. We also searched for intranight variability of a dozen related object (RR Boo, RT Boo, AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, XX Oph, FS Cet and Y Gem) in which no variability above the observational errors is detected.This work is part of the project KP-06-H28/2 08.12.2018 ”Binary
stars with compact object” (Bulgarian National Science Fund). DM and BB acknowledge
project RD-08-100/2022. JM and PLE acknowledge support from Programa Operativo
FEDER 2014-2020 and Consejer´ıa de Econom´ıa y Conocimiento of Junta de Andaluc´ıa (Ref.
1380270)

Kurtenkov, A.

Sánchez Ayaso, Estrella

Zamanov, R. K.

English

Arias Montano: Institutional Repository of the University of Huelva

arXiv:2206.10151v2  [astro-ph.SR]  12 Aug 2022The hidden symbiotic star SU Lyn - detection offlickering in U bandR. K. Zamanov1, A. Kostov1, M. Moyseev1, N. Petrov1, Y. M. Nikolov1, G.Y. Latev1, D. Marchev2, S. Boeva1, K. A. Stoyanov1, M. S. Minev1,3, J.Mart́ı4, V. Radeva5, E. Sánchez-Ayaso6, M. F. Bode7,8, K. I lkiewicz9, G.Nikolov1, P. L. Luque-Escamilla10, B. Spassov1, B. Borisov2, V. D.Marchev1, A. Kurtenkov11 Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy ofSciences, Tsarigradsko Shose 72, BG-1784 Sofia, Bulgaria2 Department of Physics and Astronomy, Shumen University ”Episkop KonstantinPreslavski”, 115 Universitetska Str., BG-9700 Shumen, Bulgaria3 Department of Astronomy, Faculty of Physics, Sofia University ”Saint KlimentOhridski”, 5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria4 Departamento de F́ısica (EPSJ), Universidad de Jaén, Campus Las Lagunillas s/n, A3,E-23071 Jaén, Spain5 Naval Academy, 73 Vasil Drumev str., BG-9026 Varna, Bulgaria6 Departamento de Ciencias Integradas, Centro de Estudios Avanzados en F́ısica,Matemática y Computación, Universidad de Huelva, E-21071, Huelva, Spain7 Astrophysics Research Institute, Liverpool John Moores University, IC2, 149 BrownlowHill, Liverpool, L3 5RF, UK8 Office of the Vice Chancellor, Botswana International University of Science andTechnology, Private Bag 16, Palapye, Botswana9 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa,Poland10 Departamento de Ingenieŕıa Mecánica y Minera (EPSJ), Universidad de Jaén, CampusLas Lagunillas s/n, A3, E-23071 Jaén, Spain(Submitted on 21 June 2022; Accepted on 11 August 2022)Abstract. We report photometric observations of the hidden symbiotic star SU Lyn in theoptical bands. In five nights we detect a weak flickering in U band with amplitude of about0.05 magnitudes. No intranight variations are found in B, V, g’ and r’ bands. This is onemore indication that the secondary component is a white dwarf accreting at a low accretionrate.We also searched for intranight variability of a dozen related objects - RR Boo, RT Boo,AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, XX Oph, FS Cet and Y Gem -however no variability above the observational errors is detected.Key words: stars: binaries: symbiotic – accretion, accretion discs – white dwarf – stars:individual: SU Lyn1 IntroductionThe symbiotic stars are interacting binaries with long orbital periods in therange from 100 days to more than 100 years. They consist of a red or yellowstar transferring mass to a hot compact object (e.g. Miko lajewska 2012). Themass donor is a giant or supergiant of spectral class G-K-M. If the giant is anAsymptotic Giant Branch star, the system usually is a strong infrared source.More than 250 symbiotic systems are known in our Galaxy (Akras et al. 2019,Merc et al. 2019).Bulgarian Astronomical Journal 38, 20232 Zamanov et al.Flickering is a non-periodic variability (on minutes-to-hour timescales) typ-ical for the accreting white dwarfs in cataclysmic variables (e.g. Bruch 1992,2021). Systematic searches for flickering in symbiotic binaries (Dobrzycka etal. 1996; Sokoloski et al. 2001) showed that flickering is rarely detected in theclassical symbiotic stars. Until now, only 11 of the symbiotic stars are observedto vary in the optical bands and do so on a timescale of ∼ 10 minutes withamplitude > 0.1 magnitude – RS Oph, T CrB, MWC 560, Z And, V2116 Oph,CH Cyg, RT Cru, o Cet, V407 Cyg, V648 Car, EF Aql, and ZZ CMi. The lastthree were identified as flickering sources during the last decade: V648 Car(Angeloni et al. 2012), EF Aql (Zamanov et al. 2017), and ZZ CMi (Zamanovet al. 2021).Here we report (i) photometry of SU Lyn and detection of intranight vari-bility in U band and (ii) photometry of 11 related objects in which no vari-ability is detected.Table 1. Photometric observations of SU Lyn. In the table are given date (informat YYYY-MM-DD), telescope, band, UT start-end, number of the framesand exposure time, typical observational error (merr), type of variability.date telescope band UT start-end frames mag merr variability2020-01-18 50/70 Roz U 23:53 - 02:23 173 x 30 sec 10.24 0.007 flickering 0.11 mag2020-04-07 2.0m U 18:21 - 20:24 60 x 120 sec 10.90 0.002 flickering 0.05 mag2020-04-08 2.0m U 18:04 - 20:38 260 x 30 sec 10.87 0.004 flickering 0.05 mag2020-04-17 2.0m U 18:12 - 20:28 170 x 45 sec 11.22 0.020 flickering 0.03 magV 18:14 - 20:27 648 x 0.02 sec 8.15 0.005 no variability2021-03-13 50/70 Roz U 23:42 - 01:41 98 x 30 sec 10.24 0.008 smooth 0.2 magB 23:43 - 01:42 97 x 10 sec 9.96 0.005 no variability2022-03-27 60cm U 20:02 - 22:42 273 x 15 sec 11.22 0.015 flickering 0.03 magB 20:02 - 22:42 273 x 2 sec 10.05 0.007 no variability2022-04-27 40cm Shu g’ 18:31 - 21:15 282 x 10 sec 9.26 0.010 no variabilityr’ 18:30 - 21:15 295 x 4 sec 7.68 0.010 no variability2 ObservationsWe have observed SU Lyn with (i) the 50/70 cm Schmidt telescope of theNational Astronomical Observatory (NAO) Rozhen, Bulgaria, (ii) the 2.0 mRCC telescope of the NAO Rozhen, (iii) the 60 cm telescope of the NAORozhen, and (iv) the 40 cm telescope of the Shumen University “EpiskopKonstantin Preslavski”, Bulgaria (Kjurkchieva et al. 2020). All the telescopesare equipped with CCD cameras.The observations are obtained in broad bands in Johnson and SDSS sys-tems: U band – effective central wavelength 4671 Å, B band – 4380 Å, V band– 5445 Å, g’ band – 4671 Å, r’ band – 6141 Å. More details about the filtertransmission can be found in Rodrigo, Solano & Bayo (2012). As comparisonFlickering of the symbiotic-like binary star SU Lyn 3Fig. 1. Detection of intranight variability of SU Lyn with the 50/70 cmSchmidt telescope of NAO Rozhen in U band.Fig. 2. Low amplitude flickering with ∆U ≈ 0.05 is visible in the observationswith the 2.0m telescope in April 2020.Fig. 3. Simultaneous observations in two bands. A low amplitude variability isvisible in U band, with amplitude ∆U ≈ 0.03 mag. No variability is detectedin B and V bands.4 Zamanov et al.Table 2. Log of observations on which no variability is detected. In the Tableare given object, date of observation, telescope, band, UT start - UT end, num-ber of the frames and exposure time, average magnitude, typical observationalerror (merr), and the result (no variability).object date-obs telescope band UT start-end frames mag merr resultRR Boo 2019-06-21 41cm Jaen B 21:10 - 22:01 67 x 20 sec 11.99 0.01 negativeV 66 x 10 sec 10.21 0.01 negativeRT Boo 2019-06-22 41cm Jaen B 20:59 - 21:35 34 x 30 sec 14.03 0.03 negativeV 32 x 20 sec 12.09 0.01 negativeAM Cyg 2019-08-27 2.0m Roz V 23:04 - 23:59 281 x 10 sec 14.29 0.01 negativeAG Peg 2017-09-26 41cm Jaen B 18:44 - 19:51 145 x 10 sec 9.71 0.01 negativeV 137 x 5 sec 8.56 0.01 negativeBF Cyg 2017-09-18 41cm Jaen B 18:59 - 20:06 87 x 20 sec 10.11 0.01 negativeV 87 x 20 sec 9.66 0.01 negativeNQ Gem 2018-03-22 41cm Jaen B 20:04 - 21:01 79 x 15 sec 10.16 0.01 negativeV 20:33 - 21:01 52 x 5 sec 8.05 0.01 negativeStHa190 2017-10-02 41cm Jaen B 19:02 - 20:07 53 x 30 sec 11.37 0.01 negativeV 53 x 30 sec 10:54 0.01 negativeV627 Cas 2017-09-18 50/70 Roz B 01:03 - 02:34 160 x 30 sec 14.75 0.03 negativeY Gem 2018-10-09 60 cm Bel B 02:17 - 03:21 71 x 30 sec 10.90 0.01 negativeV 71 x 5 sec 9.51 0.01 negativeFS Cet 2015-08-17 50/70 Roz B 00:54 - 01:47 130 x 20 sec 12.44 0.01 negative2016-12-19 50/70 Roz V 18:05 - 19:01 240 x 10 sec 12.35 0.01 negativeXX Oph 2018-09-14 50/70 Roz U 17:36 - 18:15 39 x 30 sec 10.25 0.01 negative50/70 Roz B 17:38 - 18:12 35 x 5 sec 9.87 0.01 negativeFlickering of the symbiotic-like binary star SU Lyn 5Fig. 4. Root mean square deviation versus the average U-band magnitude.The green squares refer to the night 2020-01-18, and the plus signs refer to2021-03-13. The rms of SU Lyn deviates from the behavior of the field stars,which indicates that it is variable.stars we used HD47726 (U=9.36∗, B=8.69, V=7.72) and HD47954 (U=8.90,B=8.90, V=8.41), HD237533 (U=9.88∗, B=9.78, V=9.20). The magnitudesare taken from SIMBAD, except those marked with ∗, which are our estima-tions using HD47954.The observations are plotted on Fig. 1, Fig. 2, and Fig. 3. It has to benoted that on the left panel of Fig. 1 are plotted two other stars – HD 237533and TYC 3770-1348-1. HD 237533 has an average U=9.88 and is brighterthan SU Lyn. TYC 3770-1348-1 has an average U=10.82 and is fainter thanSU Lyn. Both are shifted to appear on the same scale as SU Lyn.Table 1 is a journal of observations of SU Lyn, where are given the datewhen the observations begin, the telescope, the filter, UT-start and UT-end ofthe run, number of the frames and exposure time, typical observational error(in magnitudes), and the type of variability visible in the run.We also observed another 11 objects – three variable stars of Mira-type(RR Boo, RT Boo and AM Cyg), a sample of symbiotic stars (AG Peg, BFCyg, NQ Gem, StHa190, V627 Cas) as well as three other objects (Y Gem,FS Cet, XX Oph). These observations are performed also with:– (1) the telescope of the University of Jaén (Spain). This is an astronomicalfacility located within the University of Jaén premises and mostly devoted toeducational purposes. It hosts an automated 41 cm telescope equipped witha CCD camera with Johnson-Cousins filters. Differential photometry in the Band V bands was conducted as described in Mart́ı et al. (2017).– (2) the 60 cm telescope of the Belogradchik Astronomical Observatory, Bul-garia (Strigachev & Bachev 2011). The log of observations with non-detectionsis contained in Table 2.6 Zamanov et al.3 Intranight variability of SU Lyn in U bandVariability on a time scale of 1 hour is detected in all U band observations. In4 nights, it is a low amplitude flickering, with amplitude in the range 0.03 <∆U ≤ 0.11 mag. In one night we observe a smooth increase of the brightness(2021-03-13, right panel of Fig. 1) in U band of 0.2 mag over 2 hours, whileno variability is visible in the simultaneous observations in B band.The intranight variability is detected only in U band. No variability on atime scale of 1 hour is visible on our runs at longer wavelengths (B, V, g’, r’).It is likely that the red giant is the dominating source in these bands.For the two runs obtained with the Rozhen Schmidt telescope (presentedon Fig. 1) we measure the standard deviation of SU Lyn and of about 20 otherstars in the field and plot it on Fig. 4. These two runs are appropriate becausethe field of view of the Schmidt telescope is wide and and a sufficient numberof stars can be measured. The standard deviation is calculated asσrms =√1Npts − 1∑i(mi −m)2, (1)where m is the average magnitude in the run, Npts is the number of the datapoints. The σrms calculated in this way includes the variability of the star(if it exists) and the measurement errors. For non-variable stars it representsthe precision of the photometry. The rms deviation of SU Lyn is 5-7 timeslarger than that expected from observational errors, indicating that SU Lynis variable on a timescale of ∼ 1 hour.SU Lyn is a binary system composed of a white dwarf and a red giant.Lopes de Oliveira et al. (2018) estimate that the mass of the white dwarf is≥ 0.7 M⊙. In the optical wavelengths there are no prominent emission linestypical for symbiotic stars – Hα is visible as an weak emission, with equivalentwidth of about −0.5 Å. SU Lyn is bright and variable at X-ray wavelengths,with periods of high X-ray luminosity and strong UV variability and classifiedas a hidden symbiotic star (Mukai et al. 2016). I lkiewicz et al. (2022) proposedthat SU Lyn can be a progenitor of a classical, persistent symbiotic system.The presence of flickering in U band is one more piece of evidence thatthe hot component is a white dwarf surrounded by an accretion disc (seeSect. 4.1 in Sokoloski & Bildsten 2010). For a comparison, the amplitudes ofthe flickering in U band of a few symbiotic stars are: RS Oph 0.3 - 0.4 mag,CH Cyg 0.4 - 0.5 mag, ZZ CMi 0.05 - 0.25 mag. SU Lyn has a lower amplitude,which means that the accretion disc is not bright and therefore the white dwarfis accreting at a lower accretion rate.4 Search for intranight variability of related objectsWe also searched for intranight variability of a few related objects – RR Boo,RT Boo, AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, FS Cet,XX Oph and Y Gem.Rapid brightness variations of the Miras RR Boo, RT Boo and AM Cygare detected in Hipparcos photometry with amplitude ≈ 0.3 mag (de Lavernyet al. 1998). However we do not detect a similar variability in our runs.Flickering of the symbiotic-like binary star SU Lyn 7AG Peg made a transition from a slow symbiotic nova (which drove the1850 outburst) to a classical symbiotic star (Ramsay et al. 2016). The UVemission of AG Peg shows stochastic variability (flickering) on timescales ofminutes and hours (Zhekov & Tomov 2018). We do not detect variability in Band V bands.BF Cyg is a symbiotic star which displays signatures of a variable mass-outflow and formation of a highly-collimated bipolar mass ejection (Shchurovaet al. 2019; Tomov et al. 2019). No variability is detected in B and V bands.NQ Gem displays optical spectra similar to that of the symbiotic recurrentnova T CrB (Greene & Wing 1971) and X-ray emission similar to that of CHCyg (Luna et al. 2013). Both T CrB and CH Cyg are known flickering sources,however we do not detect similar variability of NQ Gem in our B and V bandobservations.StHa190 is the first yellow symbiotic star with rapidly variable bipolarmass outflow (Munari et al. 2001). We do not detect variability on a timescaleof 1 hour in B and V bands.V627 Cas is a symbiotic star (Vrašťák 2018). We do not detect variabilityon a timescale of 1 hour in B band.For Y Gem, Sahai et al. (2018) find strong and variable UV and X-rayemission and strong flickering in the UV continuum. Snaid et al. (2018) ob-served flickering in u’ and g’ bands. We do not detect variability on a timescaleof 1 hour in B and V bands.FS Cet is included in the Catalog of cataclysmic variables selected from theLarge Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) DataRelease 5 (Hou et al. 2020). Although the flickering is typical for cataclysmicvaraibles, we do not detect variability on time scale of 1 hour in B and Vbands.XX Oph is known as Merrill’s iron star (Howell et al. 2009). We do notdetect variability on a timescale of 1 hour in U and B bands.Conclusions: We report photometric observations of the symbiotic-likestar SU Lyn performed with the 2.0 m and the 50/70 cm Schmidt telescopesof NAO Rozhen. In five nights we detect a low amplitude flickering in U band(amplitude in the range 0.05 - 0.10 mag). This indicates that the secondarycomponent is a white dwarf accreting at low accretion rate.For a few related objects (RR Boo, RT Boo, AM Cyg, AG Peg, BF Cyg,NQ Gem, StHa190, V627 Cas, Y Gem, FS Cet, and XX Oph), the search forrapid optical variability (in the most cases in B and V bands) gives a negativeresult.Acknowledgements: This work is part of the project KP-06-H28/2 08.12.2018 ”Bi-nary stars with compact object” (Bulgarian National Science Fund). DM and BB acknowl-edge project RD-08-100/2022. JM and PLE acknowledge support from Programa OperativoFEDER 2014-2020 and Consejeŕıa de Economı́a y Conocimiento of Junta de Andalućıa (Ref.1380270).8 Zamanov et al.ReferencesAkras, S., Guzman-Ramirez, L., Leal-Ferreira, M. L., & Ramos-Larios, G. 2019, ApJS, 240,21Angeloni, R., Di Mille, F., Ferreira Lopes, C. 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The hidden symbiotic star SU Lyn -- detection of flickering in U band

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The hidden symbiotic star SU Lyn -- detection of flickering in U band

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