Peremennye Zvezdy (Variable Stars) 46, No. 5, 2026 Received 23 October; accepted 27 October.	Article in PDF
	DOI: 10.24412/2221-0474-46-49-53

Observations of HZ Her in B and V Photometric Bands

Alexandra S. Volkova, Igor M. Volkov, Nikolai I. Shakura

Sternberg Astronomical Institute, Lomonosov Moscow University, Universitetsky Ave., 13, 119992 Moscow, Russia

1. Introduction

Since the discovery of the bright X-ray source Her-X1 by Uhuru satellite and its optical identification as the variable star HZ Her using the Moscow photographic plate archive (Kurochkin 1972), the object has been monitored continuously for half a century. Right from the start of observations, the key characteristics were revealed: rapid ( ) rotation of the neutron star, orbital periodicity of 17, and a precessional period corresponding to 20.5 orbital periods (3485). The latter two periods are clearly visible in the optical and X-ray ranges. A continuous count of the precessional cycles has been maintained ever since, reaching the 600th cycle.

To explain all the observed phenomena, a binary star model has been adopted, consisting of an A-type giant star that fills its Roche lobe, a rapidly rotating neutron star, and an accretion disk.

The most striking event was the disappearance (or fading below the detection threshold) of X-ray emission during cycles 284-302 and 334-340. At the same time, the star's behavior in the optical range remained unchanged. This fact prompted intensive optical observations in modern times, awaiting the next shutdown of the X-ray source.

To date, an archive of photometric observations has been compiled through the efforts of some thirty groups using various instruments at different observatories. Information regarding the observational data we use from third-party sources is provided in Kolesnikov et al. (2020). We decided to significantly supplement and expand the optical data set with observations obtained at a single observatory using one instrument and identical detection equipment.

2. Observations

The observations were obtained at the INASAN observatory in Simeiz, on Mount Koshka, using the Zeiss-1000 telescope and an FLI16803 CCD. The Zeiss-1000 telescope is described in Nikolenko et al. (2019). The details of the instrumental photometric system are given in Barabanov et al. (2021). So far, we have obtained 13213 -band measurements and 10008 -band measurements. For comparison, the existing archive of -band observations by other authors comprised approximately 10,000 measurements at the time we began our own observations.

We used two comparison stars, GSC 2598 1267 and GSC 2598 1274, which are stars C2 and C4 from Lyutyi (1972). We normalized the brightness of the variable star to the combined brightness of the comparison stars. This permitted a slight improvement in the signal-to-noise ratio. We processed all observations using the Maxim DL program.

During data processing, we assumed that the star's brightness at the bottom of the primary minimum remained constant throughout the entire observation interval. This assumption does not contradict our consistent measurements using the same equipment. Thus, we were able to incorporate all available photometric observations of other observers into our analysis. All data were normalized to the same brightness at the bottom of the primary minimum, see Figs. 1, 2. In these plots, observations in which we detected fast changes in the brightness of the stellar disk are highlighted in red. Such changes in the brightness of the star occur in about two hours and are associated precisely with a change in the overall brightness of the disk rather than with some bright detail on the disk or on the star. The smooth course of the light curve in the eclipse after brightening supports this conclusion.

The magnitudes of HZ Her at the bottom of the primary minimum were measured relative to the equatorial standard star 107 998 = GSC 351 177 from Moffett & Barnes (1979) using the Zeiss-1000 telescope with a VersArray 512UV CCD in the photometric system. They are presented in Table 1. The transformation coefficients for VersArray 512UV are given in Volkova & Volkov (2026).

3. Periodicity analysis

To address this problem, the entire dataset of observations spanning 51 years was consolidated into a single collection. The volume of available data permits us to subdivide the precessional period into 20 phase intervals, each with a step of 0.05. Consequently, each interval contains between 1500 () and 2000 () data points. Residual deviations were analyzed using the moving average method (Volkov 2022). This method is applicable to light curves of any type; it is independent of the choice of initial phases and employs a sorting algorithm, which, while computationally intensive, is not a significant drawback given the current state of computing technology. We adapted this algorithm specifically to search for the precessional period, while the orbital period was assumed to be known with high precision and set to (Staubert et al. 2009). The precessional period was varied within specified limits. The entire precessional cycle was divided into 20 segments, for each of which a phase-folded light curve was constructed based on the orbital period . For each of such curves, the root-mean-square deviation from the mean instantaneous brightness was calculated; the 20 resulting sigma values were then averaged to yield a single error value for a given precessional period. This process was then repeated for the next value of the precessional period. The result was unexpected. For the and light curves, similar precession period values were obtained, namely 34883 for the band and 34885 days for the band, see Fig. 3. Both period values exceed the previously estimated precession period of 20.5 orbital periods, or 3485. Our precession periods differ from 20.5 orbital periods by . Other peaks are also visible in Fig. 3. The most significant of them corresponds to the periods of and . They are linked to the primary period over the 41-year interval. This is almost precisely the time interval between the richest sets of photoelectric observations, effectively representing a single precession period spanning the full observation interval. The presence of these related (alias) periods confirms the validity of the determined precession period. Ultimately, we arrive at the following value for the precession period: . This result can indicate that the continuous count of precession cycles should be reduced by one after the first disappearance of the X-ray emission.

4. Conclusions

Our photometric observations have significantly expanded the observational archive of photometric data. We found that the disk's brightness can experience a transition from a low state to a high state on timescales of two hours.

It was found that the mean precession period over the observation interval differs from the generally accepted value of 20.5 orbital periods.

Acknowledgements

This work was supported by RSF grant No. 251200012.

This work was carried out under the State Assignment of the Sternberg Astronomical Institute, Moscow State University. Observations were conducted with the Zeiss-1000 telescope of the Terskol Observatory Collective Use Center of INASAN.

The study utilized the SIMBAD database of the Strasbourg Astronomical Data Center (France) and the ADS bibliographic service (NASA, USA).

References:

Barabanov, S. I., Potanin, S. A., Savvin, A. D., Volkov, I. M., Kravtsova, A. S., & Nikolenko, I. V. 2021, INASAN Science Reports, 6, 92

Kolesnikov, D. A., Shakura, N. I., Postnov, K. A., Volkov, I. M., et al. 2020, Monthly Notices Roy. Astron. Soc., 499, No. 2, 1747

Kurochkin, N. E. 1972, Astronomicheskii Tsirkulyar, 717, 1

Lyutyi, V. M. 1972, Perem. Zvezdy, 18, No. 5, 417

Moffett, T. J. & Barnes, T. G. III 1979, Astron. J., 84, No. 5, 627

Nikolenko, I. V., Kryuchkov, S. V., Barabanov, S. I., & Volkov, I. M. 2019, INASAN Science Reports, 4, 85

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