1
No.115

Apr. 2023

Variable Star Bulletin
Eclipse observations of V838 Her (Nova Her 1991) during nova eruption
Taichi Kato1
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
Received 2023 Apr. 30
revised 2023 May. 16
Abstract
I observed the very fast nova V838 Her (Nova Her 1991, optical peak at 5–5.4 mag) during the fading
phase of the nova eruption in 1991. I detected eclipses, for the ﬁrst time in the world in any nova during
eruption, and the epochs of the eclipses were reported to IAU Circular No. 5262. Although these epochs
have been referenced in the literature, the light curves of these eclipses remained unpublished. Here, I present
these light curves. The phase-averaged light curve around 1991 April 21 (mean V =13.1, 27 d after the opticalpeak) showed an 0.14 mag primary eclipse and an 0.03 mag secondary eclipse. Combined with the subsequent
literature, the eclipses likely appeared after 1991 April 14 (V =12.5). It has been suggested that the accretion
disk had already been re-established before this epoch and I found no strong argument against this. The early
appearance of the secondary minimum appears to be a phenomenon common to very fast novae and it looks
likely to be explained, at least partly, by a strongly heated secondary. This observation reinforces the possible
interpretation of the early presence of a transient luminous donor for the fastest nova V1674 Her (Nova Her2021). As a comparison and my motivation of the observation of V838 Her, I brieﬂy review the early historyof V1500 Cyg (Nova Cyg 1975).

V838 Her (Nova Her 1991) was discovered by Matsuo Sugano on 1991 March 24.781 (JD 2448340.281) at a
photovisual magnitude of 5.4 and by George Alcock on 1991 March 25.19 (JD 2448340.69) at a visual magnitudeof 5 (Sugano et al. 1991). Although this was a naked-eye nova at its peak, very few people saw the nova when it
was visible to the naked eyes or with small binoculars. Due to the rainy or cloudy weather in Japan following theSugano’s discovery, the ﬁrst visual observation reported to the VSOLJ database1 from Japan was 10.2 mag on
1991 April 1 by Hiroaki Narumi. Infrared photometry and spectroscopy, and optical spectroscopy conﬁrming avery rapidly expanding nova were reported in Harrison et al. (1991). At that time, this nova showed Feii emission
lines. Wagner et al. (1991) obtained optical spectra and also detected Hei, Nii and other lines. Wagner et al.
(1991) noted that the spectrum was strikingly similar to that of Nova V1500 Cyg (fastest classical nova knownat that time and later shown as a nova eruption from a polar) obtained on 1975 September 6–7 and suggested
that the nova eruption in V838 Her occurred on a strongly magnetized white dwarf.
V1500 Cyg is a famous naked-eye nova which erupted in 1975. Upon knowing this nova in a television news
program, I immediately got out to ﬁnd a bright nova changing the shape of the constellation. The object waseven suspected to be a supernova from a large amplitude of more than 18 mag (see e.g., Lindegren and Lindgren1975). No nova of comparable apparent brightness has been recorded since V1500 Cyg. P. Tempesti reported
periodic variations with a period of 3.2 hr already when the nova declined to V =6 (∼4 mag below the optical
peak; report in Kozai et al. 1975) and conﬁrmed it again at V =6.5 (report in Gieren et al. 1975). R. Kochand C. Ambruster also detected the same short-period variations (reports in de Vegt et al. 1975; Harevich et al.
1975). Semeniuk (1975) determined the period to be 0.1410 d (=3.38 hr). The period was then observed todecrease to a mean value of 0.1384 d (Tempesti 1976; Semeniuk et al. 1976; Young et al. 1977). Chia et al.
(1977) remarked a resemblance to TT Ari and considered the hot spot illuminating an overlying shell or shells.
Patterson (1978) reported an increase of this period in 1977. Although Patterson (1978) stated that the period
1 <http://vsolj.cetus-net.org/database.html>.

2

−2

0

2

50

60

70

80

Figure 1: Light curve of V838 Her in 1991 April–May. Magnitudes are given relative to the comparison star.

and phase variations have a common origin in the central object, which powers the expanding nebula, the originof the variations remained unknown [see also Kleine and Kohoutek (1979)]. Although Patterson (1979) [andlater by Kaluzny and Semeniuk (1987)] reported the reﬁned period, the mechanism of the variations was not yet
clariﬁed. In the meantime, Campbell (1976) recorded proﬁle variations on a time scale of hours of the Hα emission
during the early decline phase and suggested a model considering the light-travel-time eﬀects. Hutchings and
McCall (1977) proposed a searchlight illumination-type model and was reﬁned further in Hutchings et al. (1978).
Hutchings (1979) pointed out the similarity of the light amplitude, phasing, velocity amplitude and phasing
and the phasing of emission-line strengths to the polar AM Her, although intrinsic linear polarization had not
yet been detected (Kemp and Rudy 1976; McLean 1976; Kemp et al. 1977; these observations were apparently
aimed to detect a signature of non-spherical expansion and were apparently not related to the possibility of apolar). Stockman et al. (1988) ﬁnally detected circular polarization and optical cyclotron emission, conﬁrming
the strongly magnetized nature of the white dwarf. Stockman et al. (1988) suspected that coupling between thewhite dwarf and the expanded envelope and the interaction of the orbiting secondary star were responsible forthe asynchronism and photometric variations.
This story of V1500 Cyg from 1975 to 1988 was still fresh in my mind in 1991, and the suggestion of anova on a strongly magnetized white dwarf by Wagner et al. (1991) strongly attracted my attention. I was amaster course student of astronomy then, and immediately wrote a proposal to use the 60-cm telescope at Ouda
Station, Kyoto University (Ohtani et al. 1992).2 The time-allocating committee of Ouda Station was, however,skeptical whether I could obtain any scientiﬁcally meaningful result by time-resolved photometry of a nova ineruption. This was understandable — the V1500 Cyg case required more than 10 years to understand even by
top experts of thie ﬁeld, and this showed how unpredictable to detect periodic variations from a nova in eruption.
I was, however, allocated some telescope time sharing with another student. As people who have visited or lived
in Japan in this season (April–May) would know well, the weather is dominated by frequently passing fronts,
and rain. It is usually very diﬃcult to encounter “photometric” nights in this season, particularly for a visiting
astronomer staying for observation only for a short time. These fronts and rain, however, bring migratory birds
from the south, and the epoch of my observations incidentally matched the best season for meeting arriving
summer migrants. Even if the night was ruined by clouds, the chorus of birds at dawn was not what I had heard
2 This “historical” telescope was translocated to the Museum of Astronomical Telescopes was established in Sanuki, Kagawa, Japan(<https://www.telescope-museum.com/telescope/>).

3
in urban life, and this experience was suﬃcient to bring me fascination to the world of birds (e.g., Kato 2022).
Even without detailed knowledge about birdsong, I could immediately recognize the very characteristic song ofJapanese Paradise Flycatcher (Terpsiphone atrocaudata)3 — what a surprise!
Let’s move to astronomical observations. Instead of detecting V1500 Cyg-like variations, I unexpectedly
discovered eclipses (report in Dopita et al. 1991). These observations were probably one of the ﬁrst time-resolved
CCD photometry of novae in eruption (Kato et al. 2004).4 The data were obtained using a CCD camera (Thomson
TH 7882, 576 × 384 pixels, on-chip 2 × 2 binning adopted) attached to the Cassegrain focus of the 60-cm RitcheyChrétien telescope at Ouda Station, Kyoto University. I mainly used V band for time-resolved photometry. I also
obtained some I-band data and initially narrow and wide band interference ﬁlters (if I correctly remember, they
were for Hβ and Ca lines to study the line strengths) used by another student with whom I shared some nights.
The frames were analyzed using a custom C code running on NEC PC9801 personal computers. The detailsof the observations, however, were lost or relocated to a place diﬃcult to ﬁnd after more than 30 years. The
only relatively easily accessible data were V -band measurements posted to the VSOLJ database and diﬀerential
magnitudes stored in my computer. At that time, the images were stored in magneto-optical disks, which were
expensive for us to keep all the images. Some images should be still present, but I’m not aware where the media
are placed and are these media should be diﬃcult to access even if they are still present. For this reason, I only
use V -band measurements posted to the VSOLJ database and diﬀerential magnitudes stored in my computer.
The data in the VSOLJ database were converted to real (not diﬀerential) magnitudes, and the diﬀerence from thediﬀerential magnitudes indicates that I used GSC 1034.3147 (Gaia DR3 4504548475874019840, Gaia BP =12.46
and RP =10.98: Gaia Collaboration et al. 2022) as the comparison star with a magnitude of V =11.80, as judged
from the ﬁeld of view of the CCD. I performed aperture photometry, and a star close to V838 Her (Gaia DR34504548029197366272, Gaia BP =14.74 and RP =13.81: Gaia Collaboration et al. 2022) contaminated the results.
In this paper, I use diﬀerential magnitudes. As already stated, the weather in April–May was very variable andthere were many observations aﬀected by clouds. Outlier data (usually more than 0.1-mag diﬀerent from therest) have been removed (one can refer to the original data in the VSOLJ database). This manual removal was
unavoidable since the data were often aﬀected by passages of clouds (diﬀerent parts of the images were diﬀerently
aﬀected), rather than photon noise-limited. The data used here are summarized in table 1. The magnitudes are
given relative to the comparison.
I show the overall V -band light curve in 1991 April–May in ﬁgure 1. Although I observed this object in later
seasons and detected eclipses, they are not included since the nova faded below the nearby star and the spatial
resolutions of the images were rather insuﬃcient for reliable PSF photometry.
I used the same ephemeris as in Ingram et al. (1992)
Min(BJD) = 2448369.227(1) + 0.297635(6)E(1)

The most important part of the observations showing the appearance of eclipses is given in ﬁgure 2. Before
the initial epoch shown in this ﬁgure, there was no hint of orbital modulations. Although Leibowitz et al. (1992)
suggested the presence of an eclipse already on 1991 April 13 (BJD 2448360.59, at +20 d after the opticalpeak at 2448340.5), our data on BJD 2448361.3 (+20.8 d) seems to exclude an eclipse with a depth reported in
Leibowitz et al. (1992). Considering that Leibowitz et al. (1992) recorded only the expected ingress phase of an
eclipse and not the egress part, it looks premature to conclude the presence of an eclipse before 1991 April 14(object at V =12.5). A safe conclusion is that eclipses started to appear sometime between 1991 April 14 (BJD
2448361) and April 22 (BJD 2448369). A phase-averaged light curve of the combined three consecutive nights(BJD 2448367.1–2448369.3, +26.6–28.8 d) is shown in ﬁgure 3. On these nights, the object was around V = 13.1,
8 mag below the optical peak and 7 mag above the quiescence. As judged from this orbital proﬁle, a secondary
minimum appears to have already present in addition to the primary eclipse. Eclipses were probably present on
3 The reason why I was already familiar with this bird was that it had been introduced as an “astronomy fan among
birds”.
Its Japanese name is “sankōchō”, which can be literally translated to “three-light bird”.
The three lights here
are the Moon (“tsuki” in Japanese), the Sun (“hi” in one of expressions in Japanese) and stars (“hoshi”). The bird sings“tski-hi-hoshi hoi-hoi-hoi” (Brazil 2009). I had learned this literal expression of the birdsong in a popular astronomy magazine. This species is a summer migrant to Japan and the exceptionally long tails of male birds not only attract female birds (the long tail is therefore a target of sexual selection) but also birdwatchers. This species once became rarein the 1990s (around my observations at Ouda Station), together with other summer migrants. Deforestation in the wintering grounds and big Asian Forest Fires in 1997–1998 (<http://datazone.birdlife.org/sowb/casestudy/in-indonesia-humaninitiated-ﬁres-are-responsible-for-massive-losses-of-rainforest->) were suspected to be some of the causes. The population has
apparently increased (<https://www.yamashina.or.jp/hp/yomimono/hanshokubunpu_chosa_report.html>, <https://www.birdatlas.jp/news/j_bird_atlas_2016-21.pdf>) and we can now sometimes hear the voice in the suburbs of Kyoto.
4 See also the statement by Schaefer (2022) in relation to the eruption of U Sco in 2010.

4

Table 1: Log of observations of V838 Her.
Start∗End∗
Mean mag.
48348.2738 48348.3463−1.89848349.2406 48349.3230−1.61648350.2527 48350.3388−1.27748351.2477 48351.3061−0.87148356.2649 48356.3283−0.03448359.1710 48359.3312
0.56548361.1548 48361.3298
0.823
48362.1956 48362.3276
0.837
48363.1765 48363.2359
0.979
48366.1520 48366.1589
1.225
48367.1983 48367.3114
1.261
48368.1389 48368.2852
1.310
48369.1359 48369.2578
1.42048370.2725 48370.2751
1.487
48374.1498 48374.3175
1.677
48377.1975 48377.2804
1.800
48378.1737 48378.1872
1.767
48380.1298 48380.3117
1.902
48381.1031 48381.3096
1.906
48382.1542 48382.2104
1.953
48384.0819 48384.3069
2.038∗
BJD−2400000.
†
Number of observations.

Error
0.001
0.009
0.005
0.011
0.021
0.003
0.005
0.008
0.007
0.011
0.002
0.002
0.007
0.033
0.009
0.015
0.043
0.005
0.005
0.004
0.010

N†
335
10
62
51
17
364
245
75
30
5
135
174
147
4
188
40
8
212
257
64
108

Filter
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V

April 20 (BJD 2448367). There was no suggestion of a secondary eclipse on April 14.
Leibowitz et al. (1992) reported the presence of an accretion disk three weeks after the nova eruption. I here
examine whether the light curve can be expressed by an eclipse of an accretion disk. In modeling the eclipse light
curve, I used an orbital inclination (i) of 78–90◦ (Szkody and Ingram 1994). The estimate of the primary (whitedwarf) mass by Szkody and Ingram (1994) was too small for a very fast nova, and I used 1.35 M⊙ from Kato et al.
(2009) instead. The secondary mass was assumed to be 0.86 M⊙ considering that the secondary is an unevolved
main-sequence star and was extrapolated from the table in Knigge (2006, 2007). This value is not very diﬀerent
from the one (0.73–0.75 M⊙ ) in Szkody and Ingram (1994). The duration of the observed eclipse (0.2 orbital
phase) could be reproduced for i=78–90◦ assuming a ﬂat, standard accretion disk (yes, this is an awfully roughapproximation) with a size limited by tidal truncation. The eclipse depth was 3.6 mag for i=90◦ and 2.0 mag
for i=78◦ . The observed depth (ﬁgure 3) of 0.14 mag. roughly suggests the fractional contribution from the disk
to be 0.04 (i=90◦ ) to 0.07 (i=78◦ ). Considering that the object was V = 13.1 around these observations, theapparent brightness of the disk can be estimated to be V =16.6 (i=90◦ ) and V =16.0 (i=78◦ ). Using the distancemodulus of 12.2(4) mag and E(B − V )=0.53(5) by Kato et al. (2009), converted to AV =1.6(2), the brightnessof the disk corresponds to MV =+2.8(4) (i=90◦ ) and +2.2(4) (i=78◦ ). These values are ∼2 mag brighter thannovalike systems observed by Gaia (Abril et al. 2020). The diﬀerence becomes even larger if the geometricaleﬀect of the high orbital inclination of this system is taken into account. These observed values, however, would
be explained by a strongly irradiated disk. I used a geometrically thin disk for modeling, which is probably not
a very good approximation of the real disk, if present, and these values and discussions would better be regarded
as a rough approximation.
The secondary appears on day +9 according in the model by Kato et al. (2009). The presence of eclipses
18 d after the appearance of the secondary appears to be consistent. Leibowitz et al. (1992) gave an upper limitof ∼0.05 mag for the secondary eclipse. The present data, however, suggest a detectable secondary eclipse witha duration comparable to the primary eclipse (ﬁgure 3). The depth was ∼0.03 mag.
Let’s compare with modern observations of the eclipsing, fast recurrent nova U Sco. During its 2010 eruption,
Worters et al. (2010) suggested that an accretion disk had already been re-established 7 d after the optical peak50

(BJD−2400000)
48359.3

48361.3

148362.3

48367.3

48368.2
248369.2

48374.248380.2
348381.2

48382.2

48384.2

4

−0.5

0.0

0.5

1.0

1.5

Figure 2: Variation of the orbital proﬁles of V838 Her. The data were binned to 0.01 phase.

6

−0.1

0.0

0.1

0.2
−0.5

0.0

0.5

1.0

1.5

Figure 3: Phase-averaged light curve of V838 Her on the initial three nights (1991 April 20–22) when eclipses
appeared. The data were binned to 0.01 phase.

based on appearance of ﬂickering. U Sco was around V =12.5 then, which was ∼100 times brighter than theobject in quiescence. The observed amplitude of 0.2 mag was too large (∼20 times of the quiescent luminosity)
to be considered as genuine ﬂickering, which is produced by the variable release of the gravitational potential at
the hot spot. Munari et al. (2010) casted doubts on the interpretation by Worters et al. (2010) due the absenceof such variations at 15.7 d after the optical peak. Schaefer et al. (2011) interpreted such variations in the earlyphase of decline as ﬂares (of unknown nature). Schaefer et al. (2011) suggested from eclipse mapping that thelight source at the primary was spherically symmetric 15–26 d after the optical peak and that it was a rim-brightdisk 26–41 d after the peak. Although re-establishment of the accretion disk 7 d after the optical peak would havebeen too early in U Sco, I can make a comparison whether the disk could be re-established in V838 Her at +27 d.
Drake and Orlando (2010) considered the case of U Sco and a mass-transfer rate of 10−8 to 10−6 M⊙ yr−1 was
suﬃcient to build up a disk in 7 d, if the dynamic pressure of the radiatively-driven outﬂow from the evolving
central object could be overcome. One should also note that the pre-eruption surface densities of the disk (and,consequently, the disk mass) are not absolutely required when the disk is re-established. Before the eruption, thedisk should have been in a hot, ionized state due to viscous heating, which requires high surface densities. Afterthe nova eruption when the white dwarf is suﬃciently hot, the disk can be maintained in hot, ionized state due
to radiation even if the surface density is low and a lower-mass disk is expected to explain the observation. The
pre-eruption mass-transfer rate of V838 Her is expected to be comparable to that of a novalike star: 10−8 M⊙
yr−1 or larger (Knigge et al. 2011). Irradiation by the primary would probably increase the mass-transfer rate
and the condition given by Drake and Orlando (2010) appears to have easily been achieved, if only the dynamic
pressure of the radiatively-driven outﬂow from the evolving central object could be overcome. I therefore could
not ﬁnd a strong argument against the presence of an accretion disk in V838 Her at +27 d.
The presence of black body emission at ∼20000 K was reported in infrared observations of U Sco (Evanset al. 2023), This component was attributed to the irradiated secondary or a combination with free-free emissions
from the nebula. In the case of V838 Her, the secondary ﬁlling the Roche lobe and having a temperature of
20000 K corresponds to MV =+4.7 (compare with MV =+7.1 for a non-irradiated secondary: Kato et al. 2009).
The secondary with this temperature comprises 0.7% of the luminosity of the nova at V =13.1. Although thiscontribution (0.007 mag) is much lower than the depth (0.03 mag) of the secondary minimum I recorded, thecontribution of the secondary may become an observable one assuming an even higher temperature (0.02 mag

7
for a full eclipse of a 40000 K object). Considering the observational uncertainty, the secondary eclipse suggested
by my observations could have been a real feature arising from the eclipse of the secondary. The fastest knownclassical nova, but non-eclipsing, V1674 Her showed orbital variations starting from +5 d, 4 mag below the
optical peak, often accompanied by secondary minima (Patterson et al. 2022), in which a transient luminous
donor (=irradiated secondary, as in I my suggestion for V838 Her) was given as a hypothesis.

Supplementary Data
The V -band diﬀerential data used in this analysis is given as a plain text v.bjd.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. I am grateful to Dr. Ryuko Hirata,
who was my supervisor when I was a student and assisted me when writing a report to IAU Circ. No. 5262,
and to Thomas Djamaluddin, who shared observation and lived together at Ouda Station when I was a beginnerobserver.

List of objects in this paper
V1500 Cyg, AM Her, V838 Her, V1674 Her, Nova Her 1991, U Sco, Gaia DR3 4504548029197366272,
Gaia DR3 4504548475874019840, GSC 1034.3147

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