SDSS J102146.44+234926.3: New WZ Sge-type dwarf nova
COMMISSIONS 27 AND 42 OF THE IAU
INFORMATION BULLETIN ON VARIABLE STARS
Number 5763
Konkoly Observatory
Budapest
10 April 2007
HU ISSN 0374 – 0676
SDSS J102146.44+234926.3: NEW WZ SGE-TYPE DWARF NOVA
GOLOVIN, ALEX1,2,3; AYANI, KAZUYA4; PAVLENKO, ELENA P.5; KRAJCI, TOM6; KUZNYETSOVA,
YULIANA2,7; HENDEN, ARNE8; KRUSHEVSKA, VICTORIA2; DVORAK, SHAWN9; SOKOLOVSKY,
KIRILL10,11; SERGEEVA, TATYANA P.2; JAMES, ROBERT12; CRAWFORD, TIM13; CORP, LAURENT14;
Kyiv National Taras Shevchenko University, Kyiv, UKRAINE
e-mail: astronom 2003@mail.ru, astron@mao.kiev.ua
Main Astronomical Observatory of National Academy of Science of Ukraine, Kyiv, UKRAINE
Visiting astronomer of the Crimean Astrophysical Observatory, Crimea, Nauchnyj, UKRAINE
Bisei Astronomical Observatory, Ibara, Okayama, JAPAN
Crimean Astrophysical Observatory, Crimea, Nauchnyj, UKRAINE
AAVSO, Cloudcroft, New Mexico, USA
7 International Center of Astronomical and Medico-Ecological Researches, Kyiv, UKRAINE
AAVSO, Clinton B. Ford Astronomical Data and Research Center, Cambridge, MA, USA
Rolling Hills Observatory, Clermont, FL, USA
Sternberg Astronomical Institute, Moscow State University, Moscow, RUSSIA
11 Astro Space Center of the Lebedev Physical Institute, Russian Academy of Sciences, Moscow, RUSSIA
AAVSO, Las Cruses, NM, USA
AAVSO, Arch Cape Observatory, Arch Cape, OR, USA
AAVSO, Rodez, FRANCE
The cataclysmic variable SDSS J102146.44+234926.3 (SDSS J1021 hereafter; α2000 =
10h21m46.s44; δ2000 = +23
◦49′26.′′3) was discovered in outburst having a V magnitude of
13.m9 by Christensen on CCD images obtained in the course of the Catalina Sky Survey
on October 28.503 UT 2006. In an archival image there is a star with V ∼ 21m at this
position (Christensen, 2006) and there is an object in the database of the Sloan Digital Sky
Survey Data Release 5 (Adelman-McCarthy et al., 2007; SDSS DR5 hereafter) with the
following magnitudes, measured on January 17.455 UT, 2005: u = 20.83, g = 20.74, r =
20.63, i = 20.84, z = 20.45. In the USNO-B1.0 catalog this object is listed as USNO-B1.0
1138-0175054 with magnitudes B2mag = 20.79 and R2mag = 20.35. The large amplitude
and the blue color imply that the object could be a dwarf nova of SU UMa or WZ Sge
type (Waagen, 2006).
Fig. 1 (left) shows the 8′ × 8′ image of the SDSS J1021 vicinity, generated from SDSS
DR5 Finding Chart Tool (http://cas.sdss.org/astrodr5/en/tools/chart/chart.asp).
Time resolved CCD photometry has been carried out from different sites by the authors
since November 21, 2006 (the first night after the discovery was reported) until 2006
December 06 (Data available for download at http://www.aavso.org/data/download
and from IBVS server; See Table 1 for log of observations). The photometry was done in
the V and Rc bands as well as unfiltered; this did not affect the following period analysis.
http://arxiv.org/abs/0704.1276v2
http://www.aavso.org/data/download
2 IBVS 5763
The error of a single measurement can be typically assumed to be ±0.m02. Fig. 1 (right)
shows the overall light curve of the object. Here we assume mR = munfiltered. The light
curve could be divided into three parts, denoting the plateau stage, dip and long-lasting
echo-outburst (rebrightening).
Before carrying out Fourier analysis for the presence of short-periodic signal in the light
curve (superhumps), each observer’s data set was individually transformed to a uniform
zero-point by subtracting a linear fit from each night’s observations. This was done to
remove the overall trend of the outburst and to combine all observations into a single data
From the periodogram analysis (Fig. 2, left) the value of the superhump period Psh
= 0 .d05633 ± 0.00003 was determined. Such a value is typical for the WZ Sge-type
systems and is just 58.7 seconds shorter than Psh of another WZ Sge-like system: ASAS
002511+1217.2 (Golovin et al., 2005).
The superhump light curve (with 15-point binning used) folded with 0d.05633 period
is shown on Fig. 2 (right). It is plotted for two cycles for clarity. Only JD 2454061.0-
2454063.6 data was included. Note the 0.m1 amplitude of variations and the double-
humped profile of the light curve. There remain many questions concerning the nature
of a double-humped superhumps in the WZ Sge-type stars. The explanation of a double-
humped light curve could lie in a formation of a two-armed precessional spiral density
wave in the accretion disk (Osaki, 2003) or a one-armed optically thick spiral wave, but
with the occurrence of a self-eclipse of the energy emitting source in the wave (Bisikalo,
2006).
Other theories concerning a double-peaked superhumps can be found in Lasota et al.
(1995), Osaki & Meyer (2002), Kato (2002), Patterson et al. (2002), Osaki & Meyer
(2003).
Table 1. Log of observations
JD Duration of
(mid of observational Observatory Telescope CCD Filter
obs. run) run [minutes]
2454060.9 214 Rolling Hills, FL, USA Meade LX200-10 SBIG ST-9 V
2454061.0 158 Cloudcroft, NM, USA C-11 SBIG ST-7 none
2454062.0 259 Cloudcroft, NM, USA C-11 SBIG ST-7 none
2454062.9 288 Cloudcroft, NM, USA C-11 SBIG ST-7 none
2454063.6 115 CrAO, UKRAINE K-380 SBIG ST-9 R
2454064.6 222 CrAO, UKRAINE K-380 SBIG ST-9 R
2454066.7 S.D.P. * Pic du Midi, FRANCE T-60 Mx516 None
2454067.6 90 CrAO, UKRAINE K-380 Apogee 47p R
2454067.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454069.0 S.D.P. Arch Cape, USA SCT-30 SBIG ST-9 V
2454069.0 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454069.6 63 CrAO, UKRAINE K-380 Apogee 47p R
2454071.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454072.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454073.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454074.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454075.9 S.D.P. Las Cruses, NM, USA Meade LX200 SBIG ST-7 V
2454166.8 S.D.P. Sonoita Observatory, USA 0.35 m telescope SBIG STL-1001XE V
2454167.7 S.D.P. Sonoita Observatory, USA 0.35 m telescope SBIG STL-1001XE V
* S.D.P. - Single Data Point
IBVS 5763 3
Figure 1. Left: SDSS image of the SDSS J1021 vicinity;
Right: Light curve of SDSS J1021 during the outburst;
Applying the method of ”sliding parabolas” (Marsakova & Andronov, 1996) we deter-
mined, when it was possible (JD 2454061.0 - 2454063.6), the times of maxima of super-
humps (with mean 1σ error of 0 .d0021) and calculated O-C residuals based on founded
period. The moments of superhump maximua are given in Table 2. No period variations
reaching the 3σ level were found during the time of observations.
Another prominent feature of the SDSS J1021 light curve is the echo-outburst (or re-
brightening - another term for this event) that occurs during the declining stage of the
superoutburst. On Nov. 27/28 2006 (i.e. JD 2454067.61-2454067.68) a rapid brighten-
ing with the rate of 0.m13 per hour was detected at Crimean Astrophysical Observatory
(Ukraine; CrAO hereafter), that most probably was the early beginning of the echo-
outburst. Judging from our light curve, we conclude that rebrightening phase lasted at
least 8 days. Similar echo-outbursts are classified as ”type-A” echo-outburst according to
classification system proposed by Imada et al. (2006) as observed in the 2005 superout-
burst of TSS J022216.4+412259.9 and the 1995 superoutburst of AL Com (Imada et al.,
2006; Patterson et al., 1996).
Rebrightenings during the decline stage are observed in the WZ Sge-type dwarf novae
(as well as in some of the WZ Sge-type candidate systems). However, their physical
mechanism is still poorly understood. In most cases, just one rebrightening occurs (also
observed sometimes in typical SU UMa systems), though a series of rebrightenings are also
possible, as it was manifested by WZ Sge itself (12 rebrightenings), SDSS J0804 (11) and
EG Cnc (6) (Pavlenko et al., 2007). There are several competing theories concerning what
causes an echo-outburst(s) in such systems, though all of them predict that the disk must
be heated over the thermal instability limit for a rebrightening to occur. See papers by
Patterson et al. (1998), Buat-Menard & Hameury (2002), Schreiber & Gansicke (2001),
Osaki, Meyer & Meyer-Hofmeister (2001) and Matthews et al. (2005) for a discussion of
the physical reasons for echo-outbursts.
Recent CCD-V photometry manifests that SDSS J1021 has a magnitude of 19.m72±0.07
and 19.m59 ± 0.07 as of 06 March and 07 March, 2007 (HJD = 2454165.80 and HJD =
2454167.74) respectively, at Sonoita Research Observatory (Sonoita, Arizona, USA) using
a robotic 0.35 meter telescope equipped with an SBIG STL-1001XE CCD camera.
Spectroscopic observations were carried out on November 21.8 UT with the CCD spec-
4 IBVS 5763
Figure 2. Left: Power spectrum, revealing the Psh of SDSS J1021;
Right: Superhump profile of SDSS J1021
Table 2. Times of superhump maximums
HJD E O-C σ(O−C)
2454061.03380 0 0 0.00120
2454061.88103 15 0.00228 0.00130
2454061.93507 16 -0.00001 0.00368
2454061.99121 17 -0.00020 0.00099
2454062.89325 33 0.00056 0.00179
2454062.94709 34 -0.00193 0.00214
2454063.00533 35 -0.00002 0.00156
2454063.62385 46 -0.00113 0.00464
trograph mounted on the 1.01-m telescope of Bisei Astronomical Observatory (Japan).
The preliminary discussion of the spectra can be found in (Ayani & Kato, 2006). The
spectral range is 400-800nm, and the resolution is 0.5 nm at Hα. HR 3454 (α2000 =
08h43m13.s475; δ2000 = +03
◦23′55.′′18) was observed for flux calibration of the spectra.
Standard IRAF routines were used for data reduction.
Spectrum (Fig. 3) shows blue continuum and Balmer absorption lines (from Hǫ to
Hα) together with K CaII 3934 in absorption. Very weak HeI 4471, Fe 5169, NII 5767
absorption lines may be present. Hǫ 3970 is probably blended by H Ca II 3968. The FeIII
5461 line resembles weak P-Cygni profile. Noteworthy, FeIII 5461 and NII 5767 may be
artifacts caused by imperfect subtraction of city lights: HgI 5461 and 5770 (spectrum of
the sky background which was subtracted, is available upon request). The HeI 5876 line
(mentioned for this object in Rau et al., 2006) is not detectable on our spectrum. It is
remarkable that Hα manifests a ”W-like” profile: an emission component embedded in
the absorption component of the line.
Table 3 represents EWs (equivalent widths) of detected spectral lines. EW was calcu-
lated by direct numerical integration over the area under the line profile.
The archive photographic plates from the Main Astronomical Observatory Wide Field
Plate Archive (Kyiv, Ukraine; MAO hereafter) and Plate Archive of Sternberg Astro-
nomical Institute of Moscow State University (Moscow, Russia; SAI hereafter) and plate
from Crimean Astrophysical Observatory archive (Ukraine) were carefully scanned and
inspected for previous outbursts on the plates dating from 1978 to 1992 from MAO, 1913
IBVS 5763 5
Table 3. Equivalent widths of spectral lines
Line EW [Å]
K CaII 3934 -5.8
Hǫ 3970 / H CaII 3968 -8.7
Hδ 4101 -6.4
Hγ 4340 -8.5
Hβ 4861 -6.4
Hα 6563 -7.7
Hα 6563 (emission) 2.3
HeI 4471 -0.95
FeII 5169 -0.65
NII 5767 -0.7
- 1973 from SAI and 1948 from CrAO archives. The number of plates from each archive
is 22 for SAI, 6 for MAO and 1 for CrAO archives. For all plates the magnitude limit
was determined (this data as well as scans of plates are available upon request). The
selection of plates from MAO archive was done with the help of the database developed
by L.K. Pakuliak, which is accessible at http://mao.kiev.ua/ardb/ (Sergeeva et al., 2004;
Pakuliak, L.K. & Sergeeva, T.P., 2006;). No outbursts on the selected plates from the
MAO, SAI and CrAO archives were detected. This implies that outbursts in SDDS J1021
are rather rare, which is typical for the WZ Sge-type stars.
4000 6000 8000
H / HCaII
K CaII H H
HFeIII NIIFeII
Wavelength
Figure 3. Spectra of SDSS J1021 obtained on November 21.8 UT on 1.01-m telescope of Bisei
Astronomical Observatory (Japan)
Table 4 (available only electronically from IBVS server or via AAVSO ftp-server at
ftp://ftp.aavso.org/public/calib/varleo06.dat) represents BV RcIc photometric calibration
of 52 stars in SDSS J1021 vicinity, which have a V-magnitude in the range of 11.m21-17.m23
and can serve as a comparison stars. Calibration (by AH8) was done at Sonoita Research
Observatory (Arizona, USA).
The large amplitude of the SDSS J1021 outburst of 7m, superhumps with a period
6 IBVS 5763
below the ”period gap”, rebrightening during the declining stage of superoutburst, rarity
of outbursts and obtained spectrum allow to classify this object as a WZ Sge type dwarf
nova.
Acknowledgements: AG is grateful to Aaron Price (AAVSO, MA, USA) for his
great help and useful discussions during the preparation of this manuscript. Authors are
thankful to A. Zharova and L. Sat (both affiliated at SAI MSU, Moscow, RUSSIA) for the
assistance on dealing with SAI Plate Archive and to V. Golovnya for the help concerning
MAO Plate Archive (Kyiv, Ukraine). It is a great pleasure to express gratefulness to Dr.
N. A. Katysheva, Dr. S. Yu. Shugarov (SAI MSU both) and Dr. D. Bisikalo (Institute of
Astronomy RAS, Moscow, Russia) for useful discussions concerning the nature of SDSS
J1021. IRAF is distributed by the National Optical Astronomy Observatories, which
are operated by the Association of Universities for Research in Astronomy, Inc., under
cooperative agreement with the National Science Foundation.
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