A rare outburst of the WZ Sge-type dwarf nova PQ Andromedae

2024 April 5

Jeremy Shears & Gary Poyner

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

We present a light curve of the 2020 outburst of PQ Andromedae, which was only the fourth confirmed on record, with a mean interval of 27 years. The outburst was detected at magnitude 10.5, representing an amplitude of ~8.6 magnitudes above mean quiescence of 19.1. The main part of the outburst lasted about 53 days, although it is likely that the first part was missed. The system underwent five dramatic rebrightening episodes during the decline phase, the longest appearing as an eight-day plateau in the light curve. Long photometry runs were not possible due to the short observing window. Humps were detected during one run which resembled superhumps. The strongest signal in the photometry was at 17.835(35) cycles/d, or 0.05607(11) d. The system was still slightly above quiescence more than 800 days after the outburst started. The observed characteristics of PQ And are consistent with it being a member of the WZ Sge subclass. Moreover, in terms of outburst frequency and amplitude, orbital period, and light curve characteristics, PQ And appears to be a close sibling of WZ Sge itself.

Introduction

Dwarf novae are a type of cataclysmic variable (CV) star in which a cool main-sequence secondary star loses mass to a white dwarf primary. Material from the secondary falls through the inner Lagrangian point, L1, and, because it carries substantial angular momentum, does not settle on the primary immediately but forms an accretion disc. From time to time, as material builds up in the disc, thermal instability drives the disc into a hotter, brighter state, causing an outburst in which the star brightens by several magnitudes.

Dwarf novae of the SU UMa family occasionally exhibit superoutbursts, which last several times longer than normal outbursts and may be up to a magnitude brighter. During a superoutburst, the light curve of a SU UMa star is characterised by superhumps. These are modulations which are a few per cent longer than the orbital period.^1,2 They are thought to arise from the interaction of the secondary star orbit with a slowly precessing, eccentric accretion disc. The eccentricity of the disc arises because of a 3:1 resonance between the secondary star orbit and the motion of matter in the outer part of the accretion disc.

WZ Sge-type objects are a subclass of SU UMa-type dwarf novae. Most SU UMa systems have an orbital period of between 2.5 hours and ~78 minutes. The WZ Sge objects have orbital periods at the lower end of the range, with superoutbursts that are unusually large-amplitude (~6 to 8 magnitudes) and rare (years to decades in frequency). At the beginning of a superoutburst, the light curve usually shows orbital humps, but these soon evolve into superhumps. Then, just when the outburst appears to be over and the system is returning to quiescence, one or more rebrightening events sometimes occur. The record holder is EZ Lyn, which showed 11 rebrightenings during its 2006 outburst.³ For a recent review of WZ Sge systems, and further details about classification, the reader is directed to Kato (2015).⁴

In this paper, we present analysis of the 2020 outburst of PQ And, the first for 32 years. Believed to be a WZ Sge system, PQ And is located near the eastern edge of Andromeda, not far from the border with Perseus and, to the south, the border with Triangulum. The field can be found some four and a half degrees ESE of gamma And, and two and a half degrees SE of the well-known galaxy NGC 891. Its position is RA 02^h 29^min 29.57^s, Dec. +40° 02ʹ 39.8ʺ (J2000.0). It lies at a distance of 150 ± 50 pc.⁵

We begin by summarising the outburst history of the star.

The 1988 outburst

The discovery of PQ And by former BAA Variable Star Section (VSS) secretary Dave McAdam, on 1988 Mar 21.900 UT, was a remarkable event which warrants retelling.

McAdam was engaged in photographic nova patrol work under the auspices of the UK Nova/Supernova Search Programme, organised in conjunction with The Astronomer magazine. His discovery was obtained with a 304 mm ƒ/4 lens on 35 mm film, which gave a 6.75 × 4.5° field. The field was centred on NGC 891, as there had been a query about variability of a foreground star near the galaxy.⁶ He developed the negatives the following evening and noted a star on the southern edge of the field which he had not recorded on his last visit to this area, on Jan 22. The magnitude was measured at 10.0 pv and the suspect was reported to Guy Hurst. Confirmation observations were obtained by Martin Mobberley on Mar 25.844 UT at magnitude 10.8 pv, and visually by Guy Hurst at the same time at magnitude 10.6. The new object was announced in IAU Circular 4570 on Mar 26. It was given the preliminary designation Nova And 1988. A report on the discovery was published by McAdam in The Astronomer.⁷ The front cover of the 1988 May edition, with McAdam’s discovery photograph showing just how close to the field edge the object was, is shown in Figure 1. A photograph of McAdam is shown in Figure 2.

A light curve of the outburst is presented in Figure 3. Observations of the ‘nova’ were understandably sparse at the time, as the field is awkward in the setting northwestern evening sky during the latter parts of March. PQ And faded by 1.6 magnitudes in eight days to magnitude 11.5 v, then entered a five-day ‘plateau’ before fading to 13.0 v by Apr 13, when the field was lost in twilight. The final positive detection was on Jun 21 (JD 2447333.9) at 17.0 pg.

A spectroscopic study in 1988 July by Wade & Hamilton (1988) revealed a lack of nebular features that novae would normally exhibit.⁸ They noted that the spectra were reminiscent of the recurrent nova WZ Sge at minimum light. Moreover, Richter (Sternwarte Sonneberg) searched 1,725 Sonneberg plates taken between 1928 and 1989, with a plate magnitude limit of 13–14 mag. Two further outbursts were detected in 1938 and 1967.⁹ Patterson et al. (2005) subsequently reported a photometric period during quiescence which they interpreted as P_orb ∼ 0.0560 d (80.6 min).⁵ Together, these features established that PQ And is a WZ Sge-type DN.

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