1
No.98

Apr. 2022

ISSN 0917-2211

Variable Star Bulletin
Romanov V48: unusual intermediate polar in the period gap
Taichi Kato1 ,tkato@kusastro.kyoto-u.ac.jp
Filipp D. Romanov2
ﬁlipp.romanov.27.04.1997@gmail.com, https://orcid.org/0000-0002-5268-7735
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
2 Pobedy street, house 7, ﬂat 60, Yuzhno-Morskoy, Nakhodka, Primorsky Krai 692954, Russia

Received 2022 Apr. 05, Revised 2022 May 01

Abstract
We studied Romanov V48, which had been considered as a possible polar below the period minimum(orbital period of 0.0420991 d). We analyzed the publicly available Zwicky Transient Facility data and foundthat this object is an intermediate polar with an orbital period of 0.102312 d (in the period gap) and a spin
period of 0.0420991 d. The amplitude of the spin variation was very large (0.6 mag) and the proﬁle has been
conﬁrmed to vary with the beat period of 0.071534 d. The ratio between the spin and orbital periods waslarge and the system resembles the intermediate polar DW Cnc below the period gap. Infrared emission from
Romanov V48, however, could not be explained by radiation from the secondary. The infrared excess and theshape of the spectral energy distribution resembled those of polars, suggesting that the emission mechanism inRomanov V48 is similar to those of polars. Romanov V48 may be an intermediate object between intermediate
polars and polars.

The object Gaia EDR3 2236896418906579072 at 20h 11m 16.s 825, +60◦ 04′ 28.′′ 04 (J2000.0) has BP =17.407,
RP =17.000 and a parallax ϖ=0.693(65) mas (Gaia Collaboration et al. 2021). The variability of this object was detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) as ATO
J302.8201+60.0744 with a classiﬁcation of “dubious” (unspeciﬁed variable stars not classiﬁed into the 12 categories including pulsating, eclipsing or other representative variables) (Heinze et al. 2018). Heinze et al. (2018)gave a period of 0.081328 d (by Lomb-Scargle method: Scargle 1982). This object has an X-ray counterpartof 1RXS J201117.9+600421. The Zwicky Transient Facility (ZTF: Masci et al. 2019) also detected the variability of this object and listed it as a candidate variable star with a range of 16.783–18.188 mag and a possible
period of 0.0404 d (Ofek et al. 2020). Using Gaia DR2 (Gaia Collaboration et al. 2018), Mowlavi et al. (2021)
selected candidate variable stars and listed this object as a short time-scale candidate (DR2_STS). The catalog
by Mowlavi et al. (2021) was released in 2020 September in arXiv.
One of the authors (Filipp Romanov, hereafter FR) noticed this object when comparing X-ray sources(such as ROSAT sources) and ultraviolet-excess objects (GALEX: Martin et al. 2005) with blue stars on the
Aladin Sky Atlas to search for cataclysmic variables (CVs). FR was the ﬁrst to correctly identify the period of
0.0420991 d using ZTF DR4 and the Pan-STARRS1 (PS1: Chambers et al. 2016) surveys. Sebastián Otero, thechief supervisor of the American Association of Variable Stars (AAVSO) Variable Star Index (VSX: Watson et al.
2006) suggested a classiﬁcation of a candidate polar (AM:). FR reported these results to the AAVSO VSX with
the name of Romanov V48 on 2021 February 231 . We hereafter use this designation for this object.
1 <https://www.aavso.org/vsx/index.php?view=detail.top&oid=2215537>.

2

16

17

18
ZTF r

ZTF g

ZTF i

58200 58400 58600 58800 59000 59200 59400 59600
Figure 1: Long-term ZTF light curve of Romanov V48.

Since the period is far shorter than the “period minimum” of CVs (Kolb 1993; Howell et al. 1997; Gänsicke et al.
2009; Knigge 2006; Knigge et al. 2011; Kato 2022), we made a more detailed analysis and tried to clarify thenature of the object. We analyzed the light variation using the ZTF public data2 . The long-term variation isshown in ﬁgure 1. There was no diﬀerence between g, r and i light curves and there was no high/low states which
are usually seen in many polars. There was one sequence of time-resolved photometry in the ZTF data (ﬁgure 2).
The presence of ﬂickering is characteristic to a CV. The box-shaped, large-amplitude (0.7 mag) orbital variation
is similar to that of a polar and does not resemble the orbital variation of a dwarf nova. A phase dispersion
minimization (PDM: Stellingwerf 1978) analysis of the ZTF data conﬁrmed the period detected by FR (ﬁgure 3).
The period was determined to be 0.04209909(2) d, whose error was estimated by the methods of Fernie (1989)
and Kato et al. (2010). The light curve phased with this period is shown in ﬁgure 4. There is no diﬀerence in the
proﬁle between the three ZTF bands and the colors were almost zero. The presence of two maxima of diﬀerent
amplitudes within one phase excludes the possibility of the double period being the true one.
Baran et al. (2021) reported on a search for pulsating subdwarf B star using the Transiting Exoplanet
Survey Satellite (TESS) full frame images and Romanov V48 was included in their sample (under the nameof 2236896418906579072). Baran et al. (2021) included this object among 30 sdBV candidates that were notspectroscopically classiﬁed (in their table 3 and ﬁgure 7). Although the power spectrum by Baran et al. (2021)
detected the period we determined at 275µHz, Baran et al. (2021) did not mention it. The signal at 113µHz in
Baran et al. (2021) is indeed present in the ZTF data. The period has been determined to be 0.1023116(4) d(ﬁgure 5). The amplitude of the 0.1023116-d period was 0.4 mag, smaller than that of the 0.04209909-d period.
We could not detect a signal around 12µHz (=0.96 d) in the ZTF data. We refer to the 0.04209909-d period as
P1 0.1023116-d period one as P2 . The maxima can be expressed as
Max (P1 ) (BJD) = 2458718.664(1) + 0.04209909(2)E(1)

Max (P2 ) (BJD) = 2458718.657(1) + 0.1023116(4)E.

(2)

and
The presence of two periods P1 and P2 suggests an intermediate polar (IP). In this interpretation, P2 is theorbital period (Porb ) and it is in the period gap. In this case, we can expect a signal at the beat period between
2 The ZTF data can be obtained from IRSA <https://irsa.ipac.caltech.edu/Missions/ztf.html> using the interface <https://irsa.ipac.caltech.edu/docs/program_interface/ztf_api.html> or using a wrapper of the above IRSA API<https://github.com/MickaelRigault/ztfquery>.

3

17.0

17.5

P1
18.0
0.87

0.88

0.89

0.90

0.91

0.92

0.93

0.94

Figure 2: ZTF light curve of Romanov V48 on 2020 July 5. Flickering and large-amplitude orbital variation arepresent.

P1 (spin period) and P2 . This is indeed the case and a period P3 =0.0715335(2) d was detected (ﬁgure 6). Thissignal was not apparent in the ﬁgure of Baran et al. (2021). The ephemeris is
Max (P3 ) (BJD) = 2458718.668(1) + 0.0715335(2)E.

(3)

This period reﬂects the period in which the beamed light from the magnetic poles on the white dwarf sweeps
the surface of the secondary. The existence of this period is a strong support to the IP interpretation. We could
detect systematic variations of pulse proﬁles depending on the beat phase (ﬁgure 7). These variations reﬂect thegeometry of the magnetic poles against the accretion stream from the secondary.
IPs inside or below the period gap are relatively rare and well-established ones having Porb shorter than Romanov V48 are: V455 And (Araujo-Betancor et al. 2005), HT Cam (Tovmassian et al. 1998; Kemp et al. 2002),
V1025 Cen (Buckley et al. 1998; Hellier et al. 2002), DW Cnc (Patterson et al. 2004), EX Hya (Sterken et al.
1983; Jablonski and Busko 1985), CC Scl (Woudt et al. 2012; Kato et al. 2015), AX J1853.3−0128, IGR J18173−
2509 (Thorstensen and Halpern 2013) and Swift J0503.7−2819 (Halpern and Thorstensen 2015). According to
Koji Mukai’s “The Intermediate Polars” page3 , only one IP was plotted on his diagram [V515 And: (Butters et al.
2008; Kozhevnikov 2012); Porb has not been directly determined]. Many IPs follow the relation between spin
period (Pspin ) and Porb on the line Pspin /Porb ≃ 0.1 or less (Patterson 1994). Among the objects listed, V1025
Cen, DW Cnc, EX Hya and IGR J18173−2509 are the exceptions. Norton et al. (2008) explained a larger fractionof IPs having large Pspin /Porb in short-Porb systems considering the evolution and the spin equilibrium. RomanovV48 appears to be most similar to DW Cnc although Romanov V48 is in the period gap and has larger amplitudesof spin pluses. The absolute magnitude of Romanov V48 (MV =+6.5) is the brightest among these objects except
IGR J18173−2509 whose distance is unknown.
Romanov V48 has an excess infrared emission in 2MASS J, H (Cutri et al. 2003) and the Wide-ﬁeld Infrared
Survey Explorer (WISE: Wright et al. 2010) W1 and W2 bands (the object was below the detection limit in
2MASS Ks , WISE W3 and W4). Even a donor with an evolved core [such as in QZ Ser (Thorstensen et al.
2002)] cannot explain the spectral energy distribution (SED) in the infrared, while the infrared SED of DW Cnc
is on the Rayleigh-Jeans tail of a hot object (ﬁgure 8). The infrared excess resembles that of a polar as shownin the ﬁgure. IPHAS J052832.69+283837.6 was chosen as an example of a polar (Borisov et al. 2016) [see e.g.
Harrison and Campbell (2015) for infrared SEDs of other polars]. The infrared excess in Romanov V48 may
suggest that the emission mechanism in this system is similar to those in polars. The large amplitudes of the spin
pulses may also suggest a magnetic ﬁeld stronger than the majority of IPs. Having a large Pspin /Porb , RomanovV48 may be an intermediate object between IPs and polars. Polarimetric, X-ray and spectroscopic observations
are encouraged.

3 <https://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html>.

4

θ
1.0

0.9

0.8

0.7

P=0.04210
0.042080

0.042100

0.042120

(d)

−0.6

−0.4

−0.2

0.0

0.2

0.4

−0.5

0.0

0.5

1.0

1.5

Figure 3: PDM analysis of Romanov V48 using the ZTF data of the shorter period (P1 ) = spin period. (Upper):
PDM analysis. The bootstrap result using randomly contain 50% of observations is shown as a form of 90%
conﬁdence intervals in the resultant θ statistics. (Lower): mean proﬁle. 1σ error bars are shown.

516

P=0.0420991 d

17

18
ZTF r

−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

ZTF g

ZTF i

1.0

1.2

1.4

Figure 4: The light curve of Romanov V48 using the ZTF data folded by P1 . The zero epoch was chosen as BJD
2458718.664.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF
team for making their data available to the public. We are grateful to Naoto Kojiguchi for helping downloading
the ZTF data. This research has made use of the AAVSO Variable Star Index and NASA’s Astrophysics Data
System.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as
part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant
No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar
Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches
Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortiumof Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations
are conducted by COO, IPAC, and UW.
The ztfquery code was funded by the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement n◦ 759194 – USNAC, PI: Rigault).
This publication makes use of data products from the Wide-ﬁeld Infrared Survey Explorer, which is a joint
project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute ofTechnology, funded by the National Aeronautics and Space Administration.
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint projectof the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

List of objects in this paper
V455 And, V515 And, HT Cam, V1025 Cen, DW Cnc, EX Hya, CC Scl, QZ Ser, 1RXS J201117.9+600421,
ATO J302.8201+60.0744, AX J1853.3−0128. Gaia EDR3 2236896418906579072, IGR J18173−2509, IPHAS
J052832.69+283837.6, Romanov V48, Swift J0503.7−2819

6

θ
1.00

0.98

0.96

0.94

0.92

0.90

P=0.10231
0.10210.10220.10230.10240.10250.1026

(d)

−0.2

0.0

0.2

−0.5

0.0

0.5

1.0

1.5

Figure 5: PDM analysis of the longer period (P2 ) = orbital period of Romanov V48 using the ZTF data. (Upper):
PDM analysis. (Lower): mean proﬁle.

7

θ
1.00

0.99

0.98

0.97

P=0.07153
0.96
0.071450.07150

0.07155

0.07160

(d)

−0.2

0.0

0.2

−0.5

0.0

0.5

1.0

1.5

Figure 6: PDM analysis of the beat period (P3 ) of Romanov V48 using the ZTF data. (Upper): PDM analysis.
(Lower): mean proﬁle.

80

φ=0.00−0.252

φ=0.25−0.50

4

φ=0.50−0.75

68

φ=0.75−1.00

−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Figure 7: Variation of pulse proﬁles of Romanov V48 depending on the beat phase (ϕ). The beat phases ϕ were
determined using the equation (3). The pulse phases were determined using the equation (1). All bands of the
ZTF data were combined.

9

10−10

DW Cnc (IP)λF λ (erg cm−2s−1)

10−11(polar)
IPHAS J052832.69+283837.6

10−12

−13

T =6500K, q=0.2

Romanov V48

10

T =3300K, q=0.2
10−14

T =4500K, q=1.0

10−15
1

Wavelength (µ)

10Figure 8: The spectral energy distribution (SED) of Romanov V48 (ﬁlled squares). The ﬁgure is based on SDSS(York et al. 2000; Ahumada et al. 2020) u′ ,g ′ ,r′ ,i′ ,z ′ magnitudes, 2MASS J, H, Ks magnitudes and WISE W1–
W4 magnitudes. Zero-point calibrations of the 2MASS magnitudes used Cohen et al. (2003). The short-Porb
intermediate polar DW Cnc (green circles) and the short-Porb polar IPHAS J052832.69+283837.6 (red circles)
are shown for a comparison. The arrows represent upper limits. The error bars represent 1σ errors arising fromphotometric errors only. 1σ errors arising from the uncertainty in the distance of Romanov V48 are 0.20 mag.
The curves represent the expected black-body radiation for the secondary ﬁlling the Roche lobe of a binary
with a period of 0.10231 d at the distance of Romanov V48. Three combinations of the mass ratios (q) andtemperatures are shown as a typical M-type star and extreme limits. Any donor ﬁlling the Roche lobe cannot
explain the infrared SED of Romanov V48.

10

References
Ahumada, R. et al. (2020) The 16th Data Release of the Sloan Digital Sky Surveys: First release from the
APOGEE-2 Southern Survey and full release of eBOSS spectra. ApJS 249, 3
Araujo-Betancor, S. et al. (2005) HS 2331+3905: The cataclysmic variable that has it all. A&A 430, 629
Baran, A. S., Sahoo, S. K., Sanjayan, S., & Ostrowski, J. (2021) A search for variable subdwarf B stars in TESS
full frame images – II. Variable objects in the northern ecliptic hemisphere. MNRAS 503, 3828
Borisov, N. V., Gabdeev, M. M., & Afanasiev, V. L. (2016) Photopolarimetric observations of the sample of polar
candidates. Astrophys. Bull. 71, 95
Buckley, D. A. H., Cropper, M., Ramsay, G., & Wickramasinghe, D. T. (1998) The new intermediate polar RX
J1238−38: a system below the period gap? MNRAS 299, 83
Butters, O. W., Norton, A. J., Hakala, P., Mukai, K., & Barlow, E. J. (2008) RXTE determination of theintermediate polar status of XSS J00564+4548, IGR J17195−4100, and XSS J12270−4859. A&A 487, 271
Chambers, K. C. et al. (2016) The Pan-STARRS1 surveys. arXiv e-prints arXiv:1612.05560
Cohen, M., Wheaton, W. A., & Megeath, S. T. (2003) Spectral irradiance calibration in the Infrared. XIV. The
absolute calibration of 2MASS. AJ 126, 1090
Cutri, R. M. et al. (2003) 2MASS All Sky Catalog of point sources (NASA/IPAC Infrared Science Archive)
Fernie, J. D. (1989) Uncertainties in period determinations. PASP 101, 225
Gaia Collaboration et al. (2018) Gaia Data Release 2. Summary of the contents and survey properties. A&A
616, A1
Gaia Collaboration et al. (2021) Gaia Early Data Release 3. Summary of the contents and survey properties.
A&A 649, A1
Gänsicke, B. T. et al. (2009) SDSS unveils a population of intrinsically faint cataclysmic variables at the minimum
orbital period. MNRAS 397, 2170
Halpern, J. P., & Thorstensen, J. R. (2015) Optical studies of 13 hard X-ray selected cataclysmic binaries fromthe Swift-BAT survey. AJ 150, 170
Harrison, T. E., & Campbell, R. K. (2015) The WISE light curves of polars. ApJS 219, 32
Heinze, A. N. et al. (2018) A ﬁrst catalog of variable stars measured by the Asteroid Terrestrial-impact Last
Alert System (ATLAS). AJ 156, 241
Hellier, C., Wynn, G. A., & Buckley, D. A. H. (2002) On the accretion mode of the intermediate polar V1025
Centauri. MNRAS 333, 84
Howell, S. B., Rappaport, S., & Politano, M. (1997) On the existence of low-luminosity cataclysmic variables
beyond the orbital period minimum. MNRAS 287, 929
Jablonski, F., & Busko, I. C. (1985) EX Hya – the slowest DQ Her star? MNRAS 214, 219
Kato, T., Hambsch, F.-J., Oksanen, A., Starr, P., & Henden, A. (2015) CC Sculptoris: Eclipsing SU UMa-type
intermediate polar. PASJ 67, 3
Kato, T. (2022) Evolution of short-period cataclysmic variables: implications from eclipse modeling and stage asuperhump method (with New Year’s gift). VSOLJ Variable Star Bull. 89, (arXiv:2201.02945)
Kato, T. et al. (2010) Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. II. The Second
Year (2009-2010). PASJ 62, 1525
Kemp, J., Patterson, J., Thorstensen, J. R., Fried, R. E., Skillman, D. R., & Billings, G. (2002) Rapid oscillations
in cataclysmic variables. XV. HT Camelopardalis (=RX J0757.0+6306). PASP 114, 623

11Knigge, C. (2006) The donor stars of cataclysmic variables. MNRAS 373, 484
Knigge, C., Baraﬀe, I., & Patterson, J. (2011) The evolution of cataclysmic variables as revealed by their donor
stars. ApJS 194, 28
Kolb, U. (1993) A model for the intrinsic population of cataclysmic variables. A&A 271, 149
Kozhevnikov, V. P. (2012) An extensive photometric study of the recently discovered intermediate polar V515
And (XSS J00564+4548). MNRAS 422, 1518
Martin, D. C. et al. (2005) The Galaxy Evolution Explorer: A space ultraviolet survey mission. ApJ 619, L1
Masci, F.-J. et al. (2019) The Zwicky Transient Facility: Data processing, products, and archive. PASP 131,018003
Mowlavi, N. et al. (2021) Large-amplitude variables in Gaia Data Release 2. Multi-band variability characterization. A&A 648, A44
Norton, A. J., Butters, O. W., Parker, T. L., & Wynn, G. A. (2008) The accretion ﬂows and evolution of magnetic
cataclysmic variables. ApJ 672, 524
Ofek, E. O., Soumagnac, M., Nir, G., Gal-Yam, A., Nugent, P., Masci, F., & Kulkarni, S. R. (2020) A catalogueof over 10 million variable source candidates in ZTF Data Release 1. MNRAS 499, 5782
Patterson, J. et al. (2004) Rapid oscillations in cataclysmic variables. XVI. DW Cancri. PASP 116, 516
Patterson, J. (1994) The DQ Herculis stars. PASP 106, 209
Scargle, J. D. (1982) Studies in astronomical time series analysis. II – statistical aspects of spectral analysis of
unevenly spaced data. ApJ 263, 835
Stellingwerf, R. F. (1978) Period determination using phase dispersion minimization. ApJ 224, 953
Sterken, C., Vogt, N., Freeth, R., Kennedy, H. D., Marino, B. F., Page, A. A., & Walker, W. S. G. (1983) EX
Hydrae – a coordinated campaign of photoelectric photometry from four observatories. A&A 118, 325
Thorstensen, J. R., Fenton, W. H., Patterson, J. O., Kemp, J., Halpern, J., & Baraﬀe, I. (2002) QZ Serpentis:
A dwarf nova with a 2-hour orbital period and an anomalously hot, bright secondary star. PASP 114, 1117
Thorstensen, J. R., & Halpern, J. (2013) Optical and X-ray studies of 10 X-ray-selected cataclysmic binaries. AJ
146, 107
Tonry, J. L. et al. (2018) ATLAS: A High-cadence All-sky Survey System. PASP 130, 064505
Tovmassian, G. H. et al. (1998) A new cataclysmic variable RX J0757.0+6306: candidate for the shortest periodintermediate polar. A&A 335, 227
Watson, C. L., Henden, A. A., & Price, A. (2006) The International Variable Star Index (VSX). Society for
Astronom. Sciences Ann. Symp. 25, 47
Woudt, P. A. et al. (2012) CC Sculptoris: A superhumping intermediate polar. MNRAS 427, 1004
Wright, E. L. et al. (2010) The Wide-ﬁeld Infrared Survey Explorer (WISE): Mission description and initial
on-orbit performance. AJ 140, 1868
York, D. G. et al. (2000) The Sloan Digital Sky Survey: Technical summary. AJ 120, 1579

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12

VSOLJ
c/o Keiichi Saijo National Science Museum, Ueno-Park, Tokyo Japan
Editor Seiichiro Kiyota
e-mail: skiyotax@gmail.com