1No.76

Jul. 2021

ISSN 0917-2211

Variable Star Bulletin
V606 Aql (Nova Aquilae 1899) is now a dwarf nova
Taichi Kato1 and Naoto Kojiguchi1
1Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Jul. 15

Abstract
We found that the 1899 nova V606 Aql currently shows dwarf nova outbursts with a typical cycle lengthof 270 d and amplitudes of ∼1.5 mag using Public Data Release of Zwicky Transient Facility observations.
The low mass-transfer rate in quiescence has been suggested to explain the large eruption amplitude (Tappertet al., 2016), and the present detection of dwarf nova outbursts supports this interpretation. The transition
to the dwarf nova state more than 100 yr after the nova eruption gives credence to the hibernation scenario.
The absolute magnitude estimated from dwarf nova outbursts suggests that V606 Aql should have been a fast
nova and the presence of high excitation lines in quiescence would be explained by the presence of a massive
white dwarf.

V606 Aql is a nova discovered by W. Fleming. The nova was ﬁrst seen on a plate on 1899 April 21. The
early stage of the nova eruption was not well covered by observations. The brightest magnitude recorded was 6.7(pg). The light curve showed a steep decline, followed by a plateau of 100 d duration (Duerbeck, 1987). Duerbeck(1987) extrapolated the light curve and estimated the peak of 5.5 and t3 of 65 d.
The identiﬁcation of the post-nova remained a mystery for a long period despite the bright magnitude during
the nova eruption. Khatisov (1971) proposed an identiﬁcation of a blend of stars. Duerbeck (1987) listed threestars composing this blend. The exact identiﬁcation, however, was ﬁnally made by Tappert et al. (2016) bymulticolor photometry and spectroscopy.
The post-nova had brightness of V =20.4 and R=20.1 (Tappert et al., 2016). The amplitude of the nova
eruption was one of the largest among the sample of Tappert et al. (2016) and there were only ∼10 novae withsimilar amplitudes in the sample of pre-1986 eruptions (Tappert et al., 2016). Tappert et al. (2016) discussed
that these large amplitudes of eruptions may be associated with the low mass-transfer rates (Ṁ ) in the quiescentdisk.
We used Public Data Release 6 of the Zwicky Transient Facility (ZTF, Masci et al. 2019) observations1 andfound that V606 Aql is currently in dwarf nova state (ﬁgure 1).
There were four dwarf nova outbursts between 2018 June and 2021 April (table 1). Note that ZTF g andr observations were not simultaneous. The intervals of the second, third and the fourth outbursts were 269 dand 272 d. The interval between the ﬁrst and second outbursts was 484 d, suggesting that one outburst wasmissed between them near the solar conjunction. The relatively stable intervals between the outbursts also
support the dwarf nova-type instability. The outbursts lasted for ∼10 d and the fading rates were around 0.1
mag d−1 . Although the fading rates suggest and object with a relatively long orbital period [i.e. more thanseveral hours, see e.g. Warner (1987); Warner (1995)], these fading rates were likely severely underestimated due
1 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>.

2V606 Aql

18ZTF r
ZTF g

1920
2158200

58400

58600

58800

59000

59200

59400

Figure 1: ZTF light curve of V606 Aql
Table 1: List of dwarf nova outbursts
Peak dateJD

2018 June 23
2019 October 20
2020 July 152021 April 132458293
245877724590462459318

ZTFg
19.04
18.84
19.03
19.27

ZTF
r
19.11
–
18.85
18.89

to the contribution from the bright quiescent disk. The amplitudes of the outbursts were about 1.5 mag. ZTF
made time-resolved observations on two nights on 2018 August 17 and 18 (less than 3 hr each) without detecting
an eclipse.
The dwarf nova state is what is expected for a post-nova with a reduced Ṁ , as suggested by Tappert et al.
(2016). This ﬁnding is in line with the “hibernation” scenario (Shara et al. 1986; Patterson 1984), which suggeststhat Ṁ in post-novae is reduced long time (∼100 yr or more) after the nova eruption. The most recent detectionof the dwarf nova activity was in the old nova BC Cas (1929) (Kato and Kojiguchi, 2020). These recent ﬁndings
give credence to the hibernation scenario.
The typical absolute magnitude of outbursting dwarf novae is +4 (Warner, 1987) with a dependence on theorbital period. Since the orbital period of V606 Aql is unknown, we used +4. Using the estimated E(B −V )=0.35(Tappert et al., 2016) and the standard A(V )/E(B − V ) = 3.1, the distance modulus is estimated to be 13.8. The
observed brightest magnitude of the 1899 nova eruption corresponds to Mpg = −8.5 (corrected for extinction).
Using the Gaia-calibrated maximum magnitude - rate of decline (MMRD) relation (Özdönmez et al., 2018), thisvalue corresponds to a fast nova with t3 of ∼15 d. The value appears to be consistent with the “steep decline”
of the nova eruption. The presence of a plateau phase, however, may have been unusual for a fast nova. Thisdiscussion needs to be treated with caution since the true maximum was not observed and since it is unclear
whether the entire disk becomes hot during these dwarf nova outbursts in post-novae.
The presence of He II and Bowen emission lines (Tappert et al., 2016) is not typical for a dwarf nova.
Considering the large eruption amplitude and steep decline of the nova outburst, the mass of the white dwarf is
expected to be large (e.g. Hachisu and Kato 2006). The existence of these high excitation lines may be a resultof a massive white dwarf as in Kato et al. (2021).

3Acknowledgments
This work was supported by JSPS KAKENHI Grant Number 21K03616.
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).

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Editor Seiichiro Kiyota
e-mail: skiyotax@gmail.com