Revisiting the nova-like variable HS 0229+8016 with data from a BAA VSS campaign & TESS

2023 April 6

Jeremy Shears & Stewart Bean

A report of the Variable Star Section. Director: J. Shears

We present the light curve of HS 0229+8016 between 2019 and 2021, which includes data from a campaign organised by the BAA Variable Star Section, as well as from the TESS space telescope. The striking feature of the light curve is the presence of continuous 0.6mag cyclic variations (49 in total were detected). This behaviour was also observed during a 15-year study: each brightening episode lasted 8–11 days and recurred every 13–14 days. We interpret these as ‘stunted’ outbursts, which have also been observed in many nova-like cataclysmic variables. Analysis of TESS data from two 50-day observational windows reveals a photometric period of 0.161290(114)d, which we interpret as the orbital period of the system. Orbital humps of ~0.05mag peak-to-peak amplitude are evident throughout the TESS light curve. During the first window, there were regular and well-defined stunted outbursts, where the rise was faster than the decline. By contrast, during the second observational window the behaviour was less regular.

Introduction

HS 0229+8016 is a nova-like cataclysmic variable (CV) in Cepheus,¹ discovered during follow-up observations of optically selected CV candidates from the Hamburg Quasar Survey (HQS) by Aungwerojwit et al. (2005).² Its orbital period of 232.550 ± 0.049min (0.16149d) was determined from radial velocity and photometric variability studies.

An earlier paper by one of the current authors (hereafter, Shears (2020)) presented the results of the 13-year study of HS 0229+8016,³ including its long-term light curve from 2006 to early 2019. This study revealed 0.6mag cyclic variations, each lasting ~9.5 days, which Honeycutt calls ‘stunted’ outbursts and have been seen from time to time in several nova-like cataclysmic variables.⁴

Shears (2020) included some data from an observational campaign by the BAA Variable Star Section (VSS) between 2018 November and 2019 February.³ Many observers continued to monitor the star beyond the official end of the campaign, resulting in an unprecedentedly detailed light curve. In addition, high-time-resolution photometry became available from TESS (NASA’s Transiting Exoplanet Survey Satellite),⁵ covering epochs in 2019 and 2020. This paper discusses these new results, covering the 27-month interval from the beginning of 2019 to 2021 Mar 31.

HS 0229+8016 is in Cepheus at RA 02^h 35^m 58.21^s, Dec. +80° 29ʹ 44.3ʺ (J2000.0). A finder chart is available from the American Association of Variable Star Observers (AAVSO) Variable Star Plotter (aavso.org/vsp/).

Observations

To produce a long-term light curve of HS 0229+8016, we used data from the following databases: BAA VSS, AAVSO, ZTF (Zwicky Transient Facility, g and r band) and TESS.⁶ These data are mainly CCD measurements, either unfiltered or V-band using V-band comparison stars, although some visual estimates are also included. The databases include some measurements with other filters, but these were not used in this study, the exception being a small number of TG measurements (tricolour digital imaging using data from the green channel only). Observers whose data were included in the study are listed in Table 1.

TESS is pointed at a fixed location for a period of about 27 days, with each of the four cameras viewing a different 24×24° field of view (FOV). Each 27-day period is called a sector. During each sector, images of the FOV are collected with a two-minute cadence and stored on board. Every 13 days, the satellite is pointed towards Earth and the data downloaded. During download, no observing is possible and this period appears as a gap in each light curve. The cameras are sensitive to light from 600 to 1,000nm.

Results

Outburst characteristics

The light curve of HS 0229+8016 between 2019 Jan 1 and 2021 Mar 31 is presented in Figure 1. The star varied between magnitude 13.5 and 14.7. We added lines joining each data point to guide the eye since without these lines it is troublesome to see whether there is a regular variation or merely scatter. This reveals that for much of the time there are continuous low-amplitude cyclic variations: the stunted outbursts discussed in Shears (2020).³

The time of maximum of each stunted outburst, its duration, amplitude, maximum brightness and the time between consecutive maxima were extracted. The data for the 49 stunted outbursts which were measured are listed in Table 2. In Table 3 we present some statistical data for these episodes.

The mean duration of each stunted outburst was 8.3 days (range 4–12 days) and the mean amplitude was 0.6mag (range 0.4–0.8mag). The mean interval between each maximum was 12.7 days, with a standard deviation of 3.6 days; the shortest time between brightening events was 5.9 days and the longest 22.7 days. Similar results were obtained considering all available outburst data from 2006 to 2021, also shown in Table 3.

Figure 2 presents a histogram of the number of stunted outbursts observed at each outburst duration for the 2006 to 2021 data. The range of stunted outburst length was 5–13 days. The greatest number of outbursts were eight days in duration, followed by 11 days.

TESS data & the orbital period

The TESS data are presented as fluxes, which we transformed into magnitudes according to Equation 1:

mag = −2.5 × log (flux) + 20.7    [Equation 1]

The constant of 20.7 is arbitrary and was selected by trial and error, so that the derived TESS magnitudes would approximately coincide with the ground-based data comprising the long-term light curve. We then took the mean of each sequential 10 TESS datapoints, i.e., we binned the data 10-fold, giving a time resolution of 20min.

TESS light curves are presented in Figure 3, where TESS data are blue and data from the other ground-based databases are red. Figure 3a shows TESS data covering 50 days, although there were several gaps due to the 13-day cycle of data downloads. During this first observational window there were four stunted outbursts. Comparing the first three, it appears that each episode was slightly stronger than the previous one (amplitude ~0.3, 0.4 and 0.5mag). After the third brightening there was a dip in the light curve, shaped much like an inverted outburst. The brightness then recovered, although this coincided with a gap in photometry. The amplitude of the fourth stunted outburst was ~0.4mag; between the third and fourth episode there was a small hump in the light curve.

There was then a gap of 142 days until TESS observed the field again, this time for a further 51 days. The light curve in this second observational window (Figure 3b) showed the star’s behaviour had changed. There was a multiplicity of stunted outbursts, but they were of lower amplitude and less well defined. We note that the amplitude of some of the stunted outbursts observed by TESS appear to be slightly smaller than in the data from the ground-based data. This might be due to the different photometry systems used.

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