Photometry of the SW Sex-type nova-like BH Lyncis in high state
Astronomy & Astrophysics manuscript no. 4530 c© ESO 2018
November 5, 2018
Photometry of the SW Sex-type nova-like BH Lyncis in high state⋆
V. Stanishev1,2⋆⋆, Z. Kraicheva2⋆⋆, and V. Genkov2⋆⋆
1 Department of Physics, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
2 Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarighradsko Shousse Blvd., 1784 Sofia, Bulgaria
Received ; accepted
ABSTRACT
Aims. We present a photometric study of the deeply eclipsing SW Sex-type nova-like cataclysmic variable star BH Lyn.
Methods. Time-resolved V-band CCD photometry was obtained for seven nights between 1999 and 2004.
Results. We determined 11 new eclipse timings of BH Lyn and derived a refined orbital ephemeris with an orbital period of
0.d155875577(14). During the observations, BH Lyn was in high-state with V ≃ 15.5 mag. The star presents ∼ 1.5 mag deep eclipses
with mean full-width at half-flux of 0.0683(±0.0054)Porb . The eclipse shape is highly variable, even changing form cycle to cycle.
This is most likely due to accretion disc surface brightness distribution variations, most probably caused by strong flickering. Time-
dependent accretion disc self-occultation or variations of the hot spot(s) intensity are also possible explanations. Negative superhumps
with period of ∼ 0.d145 are detected in two long runs in 2000. A possible connection between SW Sex and negative superhump phe-
nomena through the presence of tilted accretion disc is discussed, and a way to observationally test this is suggested.
Key words. accretion, accretion discs – binaries: eclipsing – stars: individual: BH Lyn – novae, cataclysmic variables
1. Introduction
BH Lyncis is an eclipsing novalike (NL) cataclysmic variable
(CV) with an orbital period of ∼ 3.h74 (Andronov et. 1989).
Thorstensen et al. (1991a), Dhillon et al. (1992), and Hoard &
Szkody (1997) have shown that spectral behavior of BH Lyn
resembles that of SW Sex-type novalikes. SW Sex stars are
spectroscopically defined sub-class of novalikes (Thorstensen
et al. 1991b). Most of them are eclipsing, but show single-
peaked emission lines contrary to the expected double-peaked
from high-inclined accretion discs. Other distinctive character-
istics are high-velocity emission components, narrow absorp-
tion components superimposed over emission lines around or-
bital phase 0.5, and a large phase offset of the emission line ra-
dial velocities, with respect to the photometric conjunction. The
eclipse profiles are V-shaped rather that U-shaped, and the ac-
cretion discs brightness temperature distribution derived from
eclipse mapping is much flatter than expected for a steady-state
accretion disc (e.g. Rutten et al. 1992). Patterson (1999) re-
ports that most of the SW Sex stars show both negative and
positive superhumps. Besides, some of the members show low
states (Honeycutt et al. 1993). Currently, there is no widely ac-
cepted model of SW Sex stars. In most of the CVs, the accre-
tions stream from secondary hits on the outer disc edge, and a
hot spot is formed at the impact. In the most elaborated model
of the SW Sex stars, Hellier (1998) suggested that part of the
gas in the stream does not stop in the vicinity of the hot spot.
Instead, it continues moving above the disc surface, hits the disc
close to the white dwarf, and thus forms a second spot. Recently,
Rodriguez-Gil et al. (2001) discovered variable circular polariza-
tion in LS Peg and suggested that SW Sex stars are intermediate
polars with the highest mass accretion rates.
⋆ Based on observations obtained at Rozhen National Astronomical
Observatory, Bulgaria
⋆⋆ E-mail: vall,#zk,#vgenkov@astro.bas.bg (VS,#ZK,#VG)
Table 1. V band observations of BH Lyn. The eclipse timings
are also given.
UT date HJD Start Duration HJD mid-eclipse
-2451000 [hour] -2451000
Feb. 20, 1999 230.3833 3.23 230.45114
Jan. 08, 2000 552.2748 5.61 552.33385
552.48983
Jan. 09, 2000 553.2240 9.16 553.26945
553.42513
553.58089
Mar. 12, 2000 616.2509 4.21 616.39900
Feb. 28, 2003 1699.3052 7.44 1699.42318
1699.57916
Dec. 19, 2003 1993.5075 3.77 1993.56007
Jan. 18, 2004 2023.5165 3.86 2023.64410
The object of this study, BH Lyn, is mostly studied spectro-
scopically, and the existing photometric data are generally used
to obtain the eclipse ephemeris and to supplement the spectral
observations. In this paper, we report the results of our photom-
etry of BH Lyn obtained in 1999-2004.
2. Observations and data reduction
The photometric observations of BH Lyn were obtained with
the 2.0-m telescope in the Rozhen Observatory. A Photometrics
10242 CCD camera and a Johnson V filter were used. The CCD
camera was 2×2 pixels binned, which resulted in ∼13 s of read-
out dead-time. In total, 7 runs of photometric data were obtained
between 1999 and 2004. The exposure time used was between 30
and 60 s. Some details of the observations are given in Table 1.
After bias and flat-field corrections, the photometry was done
with the standard DAOPHOT aperture photometry procedures
(Stetson 1987). The magnitude of BH Lyn was measured relative
http://arxiv.org/abs/0704.1302v1
2 V. Stanishev et al.: Photometry of the SW Sex-type nova-like BH Lyncis in high state
230.4 230.5
02-20-1999
552.3 552.4 552.5
01-08-2000
553.3 553.4 553.5 553.6
01-09-2000
616.3 616.4
03-12-2000
1699.3 1699.4 1699.5 1699.6
02-28-2003
HJD-2451000
1993.5 1993.6
12-19-2003
2023.5 2023.6
01-18-2004
Fig. 1. V-band observations of BH Lyn. The solid line shows the sinusoidal fit with the period of the superhumps detected in the
2000 data.
to the star BH Lyn-5 (V = 14.47), and BH Lyn-4 (V = 15.30)
served as a check (Henden & Honeycutt 1995). The runs are
shown in Fig. 1, and it appears that BH Lyn was in high state
during all observations.
3. Results
The eclipse timings given in Table 1 were determined by fitting
a parabola to the lower half of the eclipses. To refine the orbital
ephemeris of BH Lyn, we also used the eclipse timings collected
by Hoard & Szkody (1997). The O −C residuals with respect to
the best linear ephemeris are shown in Fig. 2. Clearly, the linear
ephemeris does not describe the eclipse times well and, as Hoard
& Szkody (1997) point out, this is mainly due to the anoma-
lously large, positive residual of the first eclipse timing. Hoard
& Szkody (1997) suggested that the first eclipse timing was in
error and calculated a linear ephemeris without it. The O − C
residuals of our new eclipse timings are rather large, ∼0.d006,
and increasingly positive. Together with the first two timings,
whose O − C residuals are also positive, this suggests the pres-
ence of a curvature in the O−C residuals. The dashed line is the
second-order polynomial fit to all eclipse timings. The quadratic
term is 7.6 × 10−12 and implies that the orbital period of BH
Lyn increases on a time scale of ∼ 4.2 × 106 yrs. In most of the
CVs, the mass donor star is the less massive one, and hence, if
the mass transfer is conservative, the orbital period of the sys-
tem will increase. For plausible component masses in BH Lyn,
MWD ∼ 0.73 and M2 ∼ 0.33 (Hoard & Szkody 1997), the mass
transfer rate should be Ṁ ∼ 5 × 10−8 M⊙ yr
−1 to be compati-
ble with the putative orbital period increase. However, there are
several arguments against this scenario. First, there is a bulk of
evidence that CVs evolve toward shorter orbital periods due to
the angular momentum loss of the secondary by magnetic break-
ing (Warner 1995). Second, Ṁ ∼ 5 × 10−8 M⊙ yr
−1 is probably
too high and generally not typical for CVs. Third, the eclipse
timings presented by Andronov et al. (1989) have been deter-
mined by the phase folding of observations with photographic
plates with rather long exposure times of 8, 12, and 30 min. It
is not surprising then, that those timings exhibit relatively large
-60000 -40000 -20000 0 20000 40000
Cycle number
-0.01
Fig. 2. O − C residuals of the minima with respect to the
best linear ephemeris. The second-order polynomial fit to the
O − C residuals is also shown. The solid line is our best linear
ephemeris. The filled circles show our new timings.
scatter (the timings with cycle numbers ∼ 3000). The second
timing has been determined from plates with exposure time 30
min, only slightly shorter than the total eclipse duration, and its
large positive O − C of this timing may be a statistical fluctua-
tion. Because the first two timings are the ones that determine
the curvature in the O − C residuals, one may question whether
the curvature is real. Future observations may prove that the or-
bital period of BH Lyn increases, however, our opinion is that
only two timings determined from patrol plates do not provide
enough evidence for this. We therefore determined an updated
linear ephemeris without using the first two timings:
HJDmin = 2447180.33600(28)+ 0.
d155875577(14)E. (1)
This ephemeris is shown by the solid line in Fig. 2. It is very
similar to the ephemeris of Hoard & Szkody (1997); the orbital
period is only slightly larger and the reference times differ by
≤ 1 min.
V. Stanishev et al.: Photometry of the SW Sex-type nova-like BH Lyncis in high state 3
0 5 10
Frequency [cycle/day]
 Jan. 8 & 9, 2000 Porb      Psh=0
d.145
Fig. 3. Periodogram of the January 2000 data. The negative su-
perhump and the orbital periods are indicated.
The light curves show prominent humps whose maxima oc-
cur at different orbital phases in the different runs (Fig. 1). We
interpret this as an indication of the presence of superhumps.
Because our data are sparse, they are clearly not enough for
an in-depth study of superhumps in BH Lyn. After removing
the data during eclipses, we computed the Lomb-Scargle peri-
odogram (Scargle 1982) of the two January 2000 series only
(Fig. 3). The strongest peak around the expected frequency of the
superhumps corresponds to a period of ∼0.d1450±0.0065, which
is close to the negative superhumps period 0.d1490±0.0011 re-
ported by Patterson (1999). The least-squares fit gives the semi-
amplitude of the signal of 0.084±0.005 mag.
We have also searched all runs for periodic variations on the
minute time-scale. The power spectra show many peaks with fre-
quencies below ∼ 150 cycle day−1, but the attempts to fit the
runs with periods corresponding to any of the peaks in the peri-
odograms were not satisfactory. Thus, most probably no coher-
ent oscillations are present. The individual power spectra show a
typical red noise shape characterized by a power-law decrease of
the power with frequency P( f ) = f γ. The mean power spectrum
of BH Lyn has power-law index γ = −1.77. Because the red
noise processes have strong low-frequency variability, it is most
likely that the peaks in the periodograms are due to the red noise.
Nevertheless, the peak at ∼ 32 cycles day−1 is present in most
periodograms, and it is also noticeable in the mean power spec-
trum (Fig. 4). This might indicate the presence of quasi-periodic
oscillations like the ones discussed by Patterson et al. (2002),
however, a study based on more data is needed to confirm this.
The red noise in the power spectra of CVs is a result of
flickering (Bruch 1992). BH Lyn light curves show strong flick-
ering activity; flickering peaks with typical durations of 5–10
min and amplitudes reaching ∼ 0.2 mag can be recognized in
Fig. 1. The mean standard deviation in the light curves after the
low-frequency signals have been subtracted is ∼ 0.06 mag. This
value is consistent with the standard deviation found in the light
curves of the NLs TT Ari, MV Lyr and PX And (Kraicheva et
al. 1999a,b; Stanishev et al. 2002).
The depth of the eclipses in BH Lyn during our observa-
tions is ∼ 1.5 mag, and their average full-width at half-flux is
0.0683(±0.0054)Porb. The out-of-eclipse magnitudes were fit-
ted with low-order polynomial functions to account for bright-
ness variations that are not due to the eclipse, but most prob-
ably arise from the superhumps. The eclipses were normalized
to the fits and are shown in Fig. 5. As can be seen, there is a
substantial variability of the eclipse shape, even during a sin-
gle night. The variations are most notable in the upper half of
the eclipse profiles. Half of the eclipses appear to be fairly sym-
1 10 100 1000
Frequency [cycle/day]
0.001
0.010
0.100
1.000
Psh=0
d.145
QPOs ∼ 47 min?
P(f)∝ f-1.77
Fig. 4. The mean power spectrum of BH Lyn light curves.
02-20-1999 01-08-2000 No.1
01-08-2000 No.2
01-09-2000 No.1
01-09-2000 No.2
-0.1 0.0 0.1
01-09-2000 No.3
03-12-2000
02-28-2003 No.1
-0.1 0.0 0.1
02-28-2003 No.2
Orbital phase
12-19-2003
-0.1 0.0 0.1
01-18-2004
Fig. 5. Normalized eclipses of BH Lyn. The dashed lines are
guide to the eye to see the difference of the eclipse profile eas-
ier. The symbols used for the eclipses in Fig. 6 are shown in the
lower left corners.
metric, while the rest are clearly asymmetric. More interestingly,
though, the egress of the eclipses on Mar. 3, 2000 and No.1 on
Jan. 9, 2000, and possibly the ingress of some other eclipses, are
not monotonic. To highlight the differences, in Fig. 6 we show all
the eclipses together. Except for the single eclipse in 1999, the
ingress of all eclipses are very similar. The egress of the eclipse
are however very different, and the eclipses could be split into
three sequences. In Fig. 6, each of these groups is plotted with a
different symbol.
4 V. Stanishev et al.: Photometry of the SW Sex-type nova-like BH Lyncis in high state
-0.1 0.0 0.1
Orbital phase
Fig. 6. The three eclipse groups plotted together with different
symbols.
4. Discussion
Because of the large variability of the eclipse profiles in BH Lyn,
we are reluctant to attempt eclipse mapping or to try to estimate
the system parameters from the eclipse width. Clearly, such an-
alyzes could give false results. The rather rapid changes in the
eclipse profiles, even during a single night, could be explained
by temporal variations of the AD surface brightness distribution.
Large flickering peaks can be seen before or after some of the
eclipses (Fig. 5). If such a peak occurs during an eclipse, it could
alter its shape, even to cause the eclipse not to be monotonic.
Another explanation could be that the amount of overflowing
gas varies, and as a consequence the intensity of the two hot
spots could also change, causing variations in the eclipse profile.
Variations of the area of the eclipsing body with time will
also cause variations of the eclipses. Given the time scale of
the observed changes, the secondary is ruled out. On the other
hand, the SW Sex stars most likely possess very complex accre-
tion structures, and it may be that the AD is self-occulting. Self-
occultation seems to be the most reasonable explanation of the
UV observations of another SW Sex star, DW UMa (Knigge et
al. 2000), hence giving support for this in BH Lyn. Variations of
the effective area of the occulting parts may cause the observed
eclipse profile changes.
The presence of negative superhumps in eclipsing SW Sex
stars is very interesting. The origin of negative superhumps is
still a puzzle, but they are believed to be caused by a retro-
grade precession of an accretion disc (AD) that is tilted with
respect to the orbital plane (Bonnet-Bidaud et al. 1985). If neg-
ative superhumps do arise from the precession of tilted ADs,
then the accretion stream overflow would easily occur (Patterson
et al. 1997). Therefore, the SW Sex and negative superhumps
phenomena should have the same origin. Due to the presence
of precessing tilted AD, the amount of gas in the overflowing
stream will be modulated on the negative superhump period.
Hence, the intensity of the second hot spot will change and
may produce superhumps (Patterson et al. 1997; Stanishev et
al. 2002). This scenario can be observationally tested. In this
model, the negative superhumps should manifest themselves in
spectra in two ways: 1) the intensity of the high-velocity emis-
sion components in spectra, which are thought to arise from
the second spot, should be modulated with the superhumps pe-
riod; 2) since the orientation of the tilted disc with respect to
the observer will change over the precession cycle, at certain
precession phases, the SW Sex signatures should disappear. To
test these predictions, time-resolved high signal-to-noise spec-
trophotometry over several consecutive nights is needed, since
the precession periods are of the order of a few days. We en-
courage such studies.
Acknowledgements. The work was partially supported by NFSR under project
No. 715/97.
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List of Objects
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyncis’ on page 1
‘BH Lyn’ on page 1
‘SW Sex’ on page 1
‘SW Sex’ on page 1
‘SW Sex’ on page 1
‘LS Peg’ on page 1
‘SW Sex’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 1
‘BH Lyn’ on page 2
‘BH Lyn’ on page 2
‘BH Lyn’ on page 2
‘BH Lyn’ on page 2
‘BH Lyn’ on page 2
‘BH Lyn’ on page 2
‘BH Lyn’ on page 3
V. Stanishev et al.: Photometry of the SW Sex-type nova-like BH Lyncis in high state 5
‘BH Lyn’ on page 3
‘BH Lyn’ on page 3
‘TT Ari’ on page 3
‘MV Lyr’ on page 3
‘PX And’ on page 3
‘BH Lyn’ on page 3
‘BH Lyn’ on page 3
‘BH Lyn’ on page 3
‘BH Lyn’ on page 4
‘DW UMa’ on page 4
‘BH Lyn’ on page 4
	Introduction
	Observations and data reduction
	Results
	Discussion
