11institutetext: Leibniz-Institut für Astrophysik Potsdam (AIP), An der
Sternwarte 16, 14482 Potsdam, Germany
11email<EMAIL_ADDRESS>22institutetext: South African Astronomical
Observatory, P.O. Box 9, Observatory, 7935, Cape Town, South Africa
33institutetext: Department of Astronomy, University of Cape Town, Private Bag
X3, Rondebosch 7701, South Africa 44institutetext: International Centre for
Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845,
Australia 55institutetext: Dr. Karl Remeis-Sternwarte & Erlangen Centre for
Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Sternwartstr. 7, 96049 Bamberg, Germany 66institutetext: Space Research
Institute (IKI) of Russian Academy of Sciences, Profsoyuznaya ul 84/32, 117997
Moscow, Russian Federation 77institutetext: Max-Planck-Institut für
extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
88institutetext: Department of Physics and Astronomy, Texas Tech University,
USA

# Identification of SRGt 062340.2-265715 as a bright, strongly variable,
novalike cataclysmic variable

Axel Schwope 11 David A.H. Buckley 22 3 3 Adela Kawka 44 Ole König 55
Alexander Lutovinov 66 Chandreyee Maitra 77 Ilya Mereminskiy 66 James Miller-
Jones 44 Manuel Pichardo Marcano 88 Arne Rau 77 Andrei Semena 66 Lee J.
Townsend 22 Jörn Wilms 55

We report the identification and follow-up of the transient SRGt
062340.2$-$265715 detected with both instruments on board the Spektrum-
Roentgen-Gamma mission. Optical spectroscopy of the $G=12.5$ counterpart
firmly classifies the object as a novalike cataclysmic variable (CV) at a
distance of 495 pc. A highly significant TESS period of 3.941 hours,
tentatively identified with the orbital period of the binary, could not be
found when the object was reobserved with TESS two years later. The newer
high-cadence TESS data revealed quasi-periodic oscillations around 25 min,
while ground-based photometry indicated periodic variability at 32 min.
Located in very sparsely populated regions of color-magnitude diagrams
involving X-ray and optical magnitudes and colors, the new object could be an
X-ray underluminous magnetic CV, an intermediate polar, or an overluminous
nonmagnetic CV. The lack of uniquely identified spin and orbital periods
prevents a final classification. The site of X-ray production in the system,
$L_{\rm X,bol}=4.8\times 10^{32}$ erg $\mathrm{s}^{-1}$, remains to be
understood given its high variability on long and short timescales.

###### Key Words.:

stars: cataclysmic variables – X-rays: stars - stars: individual: SRGt
062340.2-265715

##  1 Introduction

In the course of the second all-sky survey, on 2020 October 12, the two
instruments on board the Spektrum-Roentgen-Gamma mission (SRG, Sunyaev et al.,
2021) detected a bright transient at galactic coordinates
$l^{II}=234\aas@@fstack{d}7,b^{II}=-17\aas@@fstack{d}6$. The comparison of
data obtained during the first and second all-sky surveys with the two
instruments on board the mission, the Mikhail Pavlinsky ART-XC (Pavlinsky et
al., 2021) and eROSITA (Predehl et al., 2021), revealed an increase in X-ray
flux of about a factor 2, which triggered some fast-turnaround follow-up
observations.

An initial announcement of the source was published by Schwope et al. (2020)
and some analysis of the TESS data was reported by Pichardo-Marcano (2020). In
this paper we describe the initial results in more detail, together with
follow-up spectroscopy and photometry from the ground and from space. The
transient was found to be coincident with cataloged ROSAT, Swift/XRT, and Gaia
sources (1RXS J062339.8$-$265744 and 2SXPS J062339.9$-$265751) and the optical
transient ZTF19aaabzuh, respectively. It was mentioned earlier as a
cataclysmic variable by Denisenko on his
webpage111http://scan.sai.msu.ru/~denis/VarDDE.html, but no further supporting
information was available.

The X-ray and optical observations uniquely identify the transient with a
nova-like cataclysmic variable (CV). CVs are close binaries with white dwarfs
that accrete from a Roche-lobe filling star. In approximate terms, they may
physically be divided into magnetic and nonmagnetic objects (disk accretion
vs. quasi-radial accretion), and phenomenologically, they are often sorted
into dwarf novae (DN, which are nonmagnetic disk-accreting CVs that may become
unstable and cause the DN outburst) and so-called novalikes. Historically,
both magnetic and nonmagnetic CVs were sorted into this last class. The
nonmagnetic disk-accreting novalikes (often referred to as UX UMas, although
the VY Scl, Z Cam, and SW Sex objects and possibly other subtypes may also
fall in this class) are objects with high-accretion rates whose disks are
permanently found in the hot state so that no dwarf nova outbursts are
launched. CVs of all subtypes were expected to be found in large number
through systematic spectroscopic follow-up of point-like sources
serendipitously discovered in the ongoing all-sky surveys with eROSITA
(Schwope, 2012). The case considered here is unusual through its very high
optical brightness and its high and strongly variable X-ray brightness in the
hard SRG band as well, which allowed its discovery with both instruments on
the spacecraft.

##  2 Observations and analysis

###  2.1 SRG

The transient of 2020 October 12 was observed at the eROSITA position
$\alpha_{\mathrm{J2000.0}}=06^{\mathrm{h}}23^{\mathrm{m}}40\aas@@fstack{s}2$
($95\aas@@fstack{\circ}91754$),
$\delta_{\mathrm{J2000.0}}=-26^{\circ}57^{\prime}15^{\prime\prime}$
($-26\aas@@fstack{\circ}96425$) with a positional uncertainty of
$1\aas@@fstack{\prime\prime}6$ ($0\aas@@fstack{\prime\prime}6$ statistical,
$1\aas@@fstack{\prime\prime}5$ systematic). The Mikhail Pavlinsky ART-XC
telescope detected the source at the position of
$\alpha_{\mathrm{J2000.0}}=06^{\mathrm{h}}23^{\mathrm{m}}39\aas@@fstack{s}7$,
$\delta_{\mathrm{J2000.0}}=-26^{\circ}57^{\prime}53^{\prime\prime}$ with an
uncertainty of $15^{\prime\prime}$ (90%).

During the second SRG survey, the object had a total exposure time of 85 s (an
effective exposure time of 25 s) with ART-XC and was discovered at a mean flux
of $1.3_{-0.3}^{+0.4}\times 10^{-11}$
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ in the 4–12 keV energy band.
It was below the detection threshold during the first survey, when it had an
effective exposure time of 21 s, corresponding to an upper limit for the 4–12
keV flux of $F<10^{-11}$ $\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$.
With eROSITA, the source was detected in both surveys, in eRASS1 (2020 April
8), where it received a total exposure time of 228 s, with a mean count rate
of $1.38\pm 0.11$ $\mathrm{s}^{-1}$, and in eRASS2 (2020 October 12) with a
mean rate of $5.84\pm 0.24$ $\mathrm{s}^{-1}$ for a total exposure of 208 s.

More than 1200 photons were collected during eRASS2, which allowed generating
the eRASS2 light curve that is displayed in Fig. 1. Circular regions with
radii of 75″ and 100″ were used for the source and background extraction,
respectively. SRGt 062340.2$-$265715 was observed during seven erodays222An
eroday lasts 4 hours and corresponds to the revolution of the spacecraft
around the scan axis. The exposure for a given sky position is different for
each eroday and may last up to 40 s dependent on the off-axis angle and showed
variability between $10.7\pm 0.7$ $\mathrm{s}^{-1}$ and $1.3\pm 0.2$
$\mathrm{s}^{-1}$. Most of the photons registered with ART-XC were registered
during the fourth and fifth eroday (see Fig. 1).

Figure 1: X-ray light curve of SRGt 062340.2$-$265715 obtained for eRASS2 (0.2
- 10 keV). Each data point represents the mean rate of one individual scan
over the source position.

Figure 2: Mean combined spectrum of ART X-C and eROSITA of SRGt
062340.2$-$265715 obtained during the second survey with SRG with the best-
fitting thermal model spectrum. The lower panel shows the residuals in units
of the error per spectral bin.

The combined spectrum from the two instruments on board SRG is displayed in
Fig. 2. It was fit using XSPEC version 12.11 with a one-component emission
model, a thermal plasma absorbed by cold interstellar matter. A scaling factor
between the two spectral groups was also fit that accounts (a) for calibration
uncertainties between the two involved instruments and (b) for the fact that
the data were not taken strictly simultaneously, which is important given the
high variability of the source. The C-statistic was used for fitting. The
best-fit value found was 180 for 230 bins. This provides a temperature
$kT=5.7^{+2.6}_{-1.6}$ keV (errors given for a 90% confidence interval) and
fluxes of $F_{\rm eRO}\,\mbox{(}0.5-2.0\,{\rm keV})=4.3^{+0.3}_{-0.4}\times
10^{-12}$ $\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$, $F_{\rm
eRO}\,\mbox{(}0.5-10\,{\rm keV})=1.2^{+0.5}_{-0.4}\times 10^{-11}$
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$, and $F_{\rm
ART}\,\mbox{(}4-12\,{\rm keV})=2.0^{+0.14}_{-0.3}\times 10^{-11}$
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. The column density was
determined using the TBabs command (abundance set to wilms), but was not found
to be well constrained. It was therefore fixed at the Galactic column density
in this direction of N${}_{\rm H}=4.08\times 10^{20}$ cm-2 (HI4PI
Collaboration et al., 2016). Recent 3D dust maps (e.g., Green et al., 2019) do
not provide better constraints due to rather large errors, $E(g-r)=0.03\pm
0.02$. The implied column density is compatible with that inferred from
X-rays. The same model applied to the eRASS1 data with a fixed amount of
interstellar absorption gave a lower temperature $kT=2.0^{+3.4}_{-0.6}$ keV at
a flux of $F_{\rm X}\mbox{(}0.5-2.0\,{\rm keV})=(8.4\pm 1.6)\times 10^{-13}$
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. The bolometric flux of the
best-fitting model during the high state in October 2020 was $F_{\rm
X}=1.6\times 10^{-11}$ $\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$
(eROSITA value).

Figure 3: Finding chart of the new transient ($3\times 3$ arcmin). The
background image is from the DSS, and the eROSITA position (eRASS2) is marked
with a blue cross. All further Gaia objects in the field of view are marked
with red crosses.

###  2.2 Gaia

Gaia EDR3 lists an object (source_ID 2899766827964264192) at a distance of
$2\aas@@fstack{\prime\prime}4$ from the eRASS2 catalog position of the
transient at position $\mathrm{RA}=95\aas@@fstack{\circ}91672681385$,
$\mathrm{DEC}=-26\aas@@fstack{\circ}96405593108$. A finding chart indicating
the eRASS2 X-ray position and all Gaia sources in its vicinity is reproduced
in Fig. 3. It has a parallax of $\pi=1.9749\pm 0.0192\,\mathrm{mas}$ and
photometric properties $G=12.436\pm 0.005$ mag and $Bp-Rp=0.023$. Following
Bailer-Jones et al. (2021), the median geometric distance to SRGt
062340.2$-$265715 is $495.5\pm 4$ pc, which gives an absolute magnitude
$M(G)=3.96$.

Table 1: Archival X-ray observations of SRGt 062340.2$-$265715 as derived from
the ESA upper limit server HILIGT

Mission | Date | Exp time | Fluxa | Fluxa | Fluxa
---|---|---|---|---|---
|  | (s) | 0.2-2 keV | 2-12 keV | 0.2-12 keV
ROSAT-Survey | 1990-09-19 00:47:17 | 661.5 | $2.1\pm 0.2$ |  |
XMM-Newton slew | 2003-04-01 | 5.6 | $2.3\pm 0.8$ | $<28.2$ | $6.4\pm 2.0$
XMM-Newton slew | 2006-10-02 | 9.9 | $5.0\pm 0.8$ | $11.6\pm 3.3$ | $12.4\pm 1.8$
Swift-XRT | 2008-07-17 | 5558.3 | $1.7\pm 0.1$ | $2.3\pm 0.2$ | $3.8\pm 0.2$

a Absorbed flux in units of $10^{-12}$
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. Count rate to flux
conversion performed with a power-law spectral model of slope 2 and absorption
$N_{\rm H}=3\times 10^{20}\,\mathrm{cm}^{-2}$.

###  2.3 Archival X-ray observations

The position of SRGt 062340.2$-$265715 was covered previously by past (ROSAT)
and current (XMM-Newton and Swift/XRT) X-ray missions. The corresponding
information is available via the ESA-supported high-energy light-curve
generator HILIGT (upper limit server)333http://xmmuls.esac.esa.int/hiligt/ and
listed in Tab. 1. Although HILIGT does not offer a thermal spectral model to
convert count rates into fluxes, the given values are at least indicative. The
flux in the soft band, $0.2-2.0$ keV, common to all the missions, was found to
be variable by at least a factor 2.

Figure 4: ANU/WiFeS low-resolution and SALT/HRS high-resolution identification
spectra of SRGt 062340.2$-$265715\. The SALT spectrum only has arbitrary
uncalibrated intensity units. There is high excitation N III 4640Å Bowen
fluorescence and the He II 4540Å and 4686Å lines.

###  2.4 Initial follow-up spectroscopy with WiFeS and SALT

The likely Gaia counterpart was selected for follow-up spectroscopy with the
2.3m telescope of the Australian National University, which is equipped with
the Wide Field Spectrograph (WiFeS; Dopita et al., 2010) and with the 10 m
class Southern African Large Telescope (SALT; Buckley et al., 2006)), which is
equipped with the High Resolution Spectrograph (HRS; Crause et al., 2014).
WiFeS is a double-beam, image-slicing, integral-field spectrograph.
Observations were performed on 2020 October 28.164 with the R3000 grating and
an exposure of 300 s. As the red arm of the instrument was not operating, only
the blue wavelength range between 3500 and 5500 Å is available. The WiFeS
spectrum was reduced using the PyWiFeS reduction pipeline (Childress et al.,
2014), which produced three-dimensional data. We then extracted background-
subtracted spectra from the slitlets that provided significant flux.

HRS is a dual-beam, fibre-fed èchelle spectrograph, covering 3800$-$8900 Å. A
1200 s HRS observation was taken on 2020 October 31.9672 in low-resolution
(LR; R $\sim$15,000) mode. Initial reduction of the HRS spectrum was achieved
using the PySALT package (Crawford et al., 2010), which includes overscan
correction, bias subtraction, and gain correction. The spectrum was extracted
using the HRS pipeline, based on MIDAS routines described in Kniazev et al.
(2016). The ANU/WiFeS and SALT/HRS spectra are displayed in Fig. 4.

The object has a very blue continuum that increases to the short-wavelength
cutoff at 3500 Å. The WiFeS spectrum reveals broad Balmer absorption lines
whose centers are filled with intense emission lines. In addition to H-Balmer
emission lines, the object displays He-emission lines, both neutral and
ionized. Through these early spectra, the object is robustly identified as a
CV. None of the spectra shows any sign of the donor star.

Figure 5: TESS light curve for the SRGt 062340.2-265715. The normalized light
curve was obtained by dividing the background-corrected source flux by its
median value of 513.1 electrons $\mathrm{s}^{-1}$.

The typical full width at half maximum (FWHM) of the absorption lines
corresponds to 3000 $\mathrm{km}\,\mathrm{s}^{-1}$, and those of the Balmer
emission lines correspond to 300-380 $\mathrm{km}\,\mathrm{s}^{-1}$. The
ionized helium line at 4686 Å is detected, but appears to be rather weak. It
has an equivalent width of $\sim-0.9$ Å, an FWHM of $\sim$6 Å ($\sim$380
$\mathrm{km}\,\mathrm{s}^{-1}$), and the flux ratio to the Balmer emission
line H$\beta$ is $F({He\textsc{ii}})/F(H\beta)\simeq 0.26$. H$\beta$ has an
equivalent width of $\sim-4.2$ Å and an FWHM of $\sim$5.5 Å ($\simeq$340
$\mathrm{km}\,\mathrm{s}^{-1}$). The HRS spectrum of the region around He ii
4686 also uncovers the Bowen blend of N iii lines. This feature is often
encountered in bright low-mass X-ray binaries (LMXBs, e.g., Cornelisse et al.,
2008), but also in the two types of magnetic CVs, the polars and the
intermediate polars (e.g., Schachter et al., 1991; Harlaftis & Horne, 1999),
indicating a strong UV/EUV source.

The H$\beta$ emission line has a centroid of 4861.54 Å in the SALT spectrum,
whereas it was found at 4862.59 Å in the WiFeS spectrum while being similarly
broad (5.2 Å vs. 5.5 Å). The difference in wavelength corresponds to a
velocity difference of 65 $\mathrm{km}\,\mathrm{s}^{-1}$, illustrating the
feasibility of a radial velocity study.

###  2.5 TESS

We analyzed data from the Transiting Exoplanet Survey Satellite (TESS, Ricker
et al., 2014). TESS, using four CCD cameras, obtains continuous optical images
of a rectangular field of $24^{\circ}\times 90^{\circ}$ for 27.4 days at a
short cadence (30 minutes to 20 seconds).

The source was observed in sector 6, which was observed at a 30-minute cadence
between MJD 58467.79 to MJD 58489.54 (2018 December 15 to 2019 January 06).
The source was also observed in sector 33 at a faster 2-minute cadence between
MJD 59201.24 (2020 December 18) and MJD 59227.07 (2021 January 13).

We began the analysis with the data obtained in sector 6 (30 min cadence) and
used the Python package Lightkurve (v1) (Lightkurve Collaboration et al.,
2018) to create the light curves from the full-frame images (FFI). From the
FFI from which we extracted our light curves, we selected pixels around the
coordinates of the target, and background or empty pixels with no known Gaia
sources as our background and with a median flux that is lower than a given
threshold. We also used the TESS light curve from the MIT Quick-Look Pipeline
(QLP, Huang et al., 2020) and compared them to our light curves from the full-
frame images using the median background-removal method. The results from the
two programs agreed well. The final extracted light curve (30 min cadence),
normalized to the median value of 513.1 electrons $\mathrm{s}^{-1}$, is
displayed in Fig. 5. The light curve is flat to first order, but displays an
apparent flickering that gives the impression of a possible periodic behavior.
We thus performed a periodogram analysis to search for any underlying
periodicity.

We performed period searches using the Lomb-Scargle (LS, Lomb, 1976; Scargle,
1982) and the phase dispersion minimization technique (PDM, Stellingwerf,
1978) as implemented in the Astropy and the PyAstronomy packages (Astropy
Collaboration et al., 2018; Czesla et al., 2019). The LS-periodogram was then
normalized by the residuals of the data around a constant reference model. The
periodogram (Fig. 6) clearly shows an isolated peak at a period of $3.941\pm
0.010$ h, the error being estimated from the width of the peak ($\sigma\sim
0.01$ h). We also calculated a false-alarm probability of $3.9\times
10^{-158}$ using the method described in Baluev (2008).

In the PDM method, the light curve is divided into different phase bins, and
the cost function, $\Theta=s^{2}/\sigma^{2}$, is minimized to choose the best
period. Here $s$ is the phase bin variance and $\sigma$ is the total data
variance. We divided the data into ten equidistant bins to calculate $\Theta$
and also found a period of 3.941 hours, which is fully consistent with the
results obtained with the Lomb-Scargle periodogram.

Figure 6: Lomb-Scargle periodogram for SRGt 062340.2$-$265715, showing a clear
isolated peak around 3.941 hours. The dotted line shows the periodogram level
corresponding to a maximum peak false-alarm probability of $1\%$ using the
bootstrap method that simulates data at the same observation times to
approximate the true distribution of peak maxima for the case without a
periodic signal. The bootstrap method is performed with periods between 0.1
and 0.5 days and after normalizing by the residuals of the data around a
constant reference model.

Figure 7: TESS light curve for SRGt 062340.2-265715, folded on the 3.941-hour
period. Data are shown with their error bars. The solid lines represent the
best-fit sinusoid. Phase zero was chosen arbitrarily.

The main result of the analysis of the 30-minute cadence data, the presence of
a periodicity at about 3.9 hours, was reported very briefly already in ATEL
14222 by one of us (Pichardo-Marcano, 2020). The more thorough analysis
presented here confirms and refines the initial result. The light curve folded
at the period of 3.941 hours is shown in Fig. 7. It is slightly skewed, with a
more gradual increase and a steeper decrease. There is significant scatter at
any given phase, which indicates further variability on shorter timescales.
This could not be resolved with the low-cadence TESS data of sector 6.

Figure 8: TESS light curve for SRGt 062340.2$-$265715 obtained in sector 33
with a 2 min cadence.

Figure 9: Moving-window periodogram of the TESS 2-minute cadence data for a
sliding window of one-day length and time steps of two hours. The x-axis is
the middle of the one-day window, and the color corresponds to the Lomb-
Scargle power. The broken axis corresponds to the dates around the middle of
the light curve that have some missing data.

For the data obtained with a 2-minute cadence we used the Pre-search Data
Conditioning Simple Aperture Photometry flux (PDCSAP), in which long-term
trends have been removed using so-called cotrending basis vectors (CBVs, Smith
et al., 2012), produced by the TESS Science Processing Operations Center
(SPOC; Jenkins et al., 2016). The light curve of data from sector 33 is shown
in the original time sequence in Fig. 8. On this occasion, the source appeared
fundamentally different compared to the data obtained in sector 6. At the
beginning of the observations, the source was about 1.4 mag brighter than two
years before, and at the end of this one-month interval only by about 0.9 mag.
A Lomb-Scargle period search for the whole dataset from sector 33 no longer
showed the 3.9 h modulation. We then selected shorter time intervals of one or
two days length at the beginning, center, and end of the data train and
searched for periodic behavior between 10 minutes and 5 hours. This revealed
some power at 25 min. To study the stability of this period, we calculated a
dynamic periodogram using a time slice with a length of one day that was moved
forward with a step size of 2 hours. For each slice, an LS periodogram was
computed, and the results were arranged as the two-dimensional periodogram (LS
power as a function of time) that we show in Fig. 9.

Complementary views on the complex timing behavior of the source are given in
Figs. 10 and 11, which show the false-alarm probabilities of the highest peak
in each of the one-day periodograms and the mean periodogram of all the
individual one-day periodograms. We found significant power centered around 25
min (frequency 57.6 d-1), but measured significant frequencies that jumped
between 45 and 65 cycles per day on timescales of about one day. No
significant period was found during the last quarter of the data set. The mean
periodogram has a maximum power at 24.37 min, and the most significant period
in a one-day time slice is observed at 25.201 min with a false-alarm
probability of $6.58\times 10^{-16}$.

Figure 10: For each one-day window we plot the maximum amplitude of the
periodogram and its corresponding period. The color corresponds to the
$\log_{10}$ of the FAP.

Figure 11: Mean periodogram of sector 33 calculated by averaging all one-day
periodograms of Fig. 9.

Figure 12: ASASS-SN and ZTF photometric data for SRGt 062340.2$-$265715 (green
symbols show ASAS-SN $V$, gray symbols show ASAS-SN $g$, and blue symbols
represent ZTF $g$). The magenta triangles mark TESS observations with low and
high cadence (the first and second pair identify TESS sectors 6 and 33,
respectively).

###  2.6 Archival photometry from CRTS, ZTF, ASAS-SN, and ATLAS

Archival photometric information of the new transient is available from the
Catalina Real-Time Transient survey (CRTS, Drake et al., 2009, 196 epochs),
the Zwicky Transient Facility (ZTF, Masci et al., 2019, 55 epochs), the All-
Sky Automated Survey for Supernovae (ASAS-SN, Shappee et al., 2014; Kochanek
et al., 2017, 3350 epochs), and the Asteroid Terrestrial-impact Last Alert
System (ATLAS, Tonry et al., 2018; Heinze et al., 2018, 34 epochs). The
observations of these surveys cover the time intervals 2005 August 28 to 2010
April 19, 2018 November 1 to 2019 November 18, 2014 February 11 to 2020
November 24, and 2021 February 16 to 2021 March 16, respectively, and part of
them are shown in Fig. 12 (omitting the CRTS and ATLAS data). ZTF data were
taken in the $g$ and $r$ filters with only $g$-band data shown, and ASAS-SN
data through $g$ and $V$ filters. All these data show extended phases (years)
with little variability around a mean magnitude of about 12.5, with superposed
scatter with an amplitude of 0.1 mag followed by dimmed phases with a larger
short-term variability amplitude. These excursions toward lower brightness
were observed twice, one at the end of the CRTS data train at MJD 56350, and
the other was covered by both ZTF (partially) and ASAS-SN at MJD 58400. The
minimum brightness of these fainter states is at around 13 mag. TESS
observations in sector 6 revealing the 3.9 hour periodicity were obtained at
the pronounced minimum. No automated survey data (e.g., ASAS-SN or ATLAS) are
available at the time of writing when TESS observed the source again in sector
33.

We searched for periodic behavior of the source in the ASAS-SN data. We
selected $g-$band data obtained between HJD 2458455 and 2458866. During this
400-day interval with 1072 data points, the transient recovered from its faint
state at $g=12.95$ to a bright state at $g=12.3$ with a constant gradient. The
brightness displayed a scatter with a mean amplitude of 0.085 mag around the
trend, while the mean magnitude error was only 0.01 mag. We used the LS period
search method as implemented in ESO-MIDAS and found a period $3.9142\pm
0.0011$ hours, which is consistent with the value found in the TESS data at
2.5$\sigma$.

Figure 13: Differential white-light optical light curve, spanning 3.2 h, of
SRGt 062340.2$-$265715 obtained with _SAAO/SHOC_ at 0.3 s cadence. Time is in
seconds since 00:00 UT on 1 November 2020.

Figure 14: Lomb-Scargle power spectrum of the light curve of SRGt
062340.2$-$265715 (shown in Fig 13) indicating a possible $\sim$2100 s
periodicity.

###  2.7 SAAO high-speed photometry

Following its spectral identification (see subsection 2.4), we performed 3.2 h
of high-speed photometry with the South African Astronomical Observatory 1.0 m
telescope on 2020 November 1, beginning at 23:57:30 UTC. The Sutherland High
speed Optical Camera (SHOC; Coppejans et al., 2013) CCD camera, which uses an
Andor iXon888 frame transfer EM-CCD frame camera (1024 $\times$ 1024 pixels),
was used. The observations were made without a filter (i.e., ‘white light’)
with an exposure time of 0.3 s.

The CCD images were reduced using the $\it{TEAPhot}$ photometry reduction
package (Bowman & Holdsworth, 2019) and included subtraction of median bias
and flat-field correction using median-combined frames from exposures of the
twilight sky. $\it{TEAPhot}$ employs the method of adaptive elliptical
aperture photometry, which was used to create calibrated science images and a
differential light curve using the bright (V = 12.1) reference star, UCAC2
20781392. The differential light curve is shown in Fig. 13.

The light curve shows strong flickering that is typical of CVs and is
reminiscent of some intermediate polars that exhibit spin-related periodic
modulations. We therefore subjected the light curve to a Lomb-Scargle period
analysis, probing frequencies as high as the Nyquist limit (0.6 s). Except for
a strong peak at 0.480 mHz, there were no other significant peaks. The result
is shown in Fig. 14. The strong peak corresponds to a period of $2080\pm 100$
s (34.7 min).

Figure 15: Color-magnitude diagram for eRASS2 sources. Each gray dot is an
eRASS2 source that is matched (within 8 arcsec) with a Gaia source within 500
pc (distances by Bailer-Jones et al. 2018). In dark yellow we plot all CVs
from the final Ritter & Kolb catalogue, in green the nonmagnetic high-
accretion rate UX UMas therein, and in blue the intermediate polars from
Mukai’s compilation. The new CV SRGt 062340.2$-$265715 is identified by the
red symbol.

##  3 Results and discussion

We have analyzed space- and ground-based data of SRGt 062340.2$-$265715, an
object that was initially detected as an X-ray transient with the two X-ray
instruments on board SRG. Follow-up data, in particular the initial
spectroscopy, uniquely identify the object as a CV and thus confirm
Denisenko’s previous classification. At $G=12.5$ mag the object is clearly one
of the brighter examples of its class. An important question is to which, if
any, CV family the object belongs.

The obtained spectra and the location of the object in the color-magnitude
diagram (see Fig. 15) all strongly suggest that it belongs to the novalike
subclass. No dwarf nova outburst was recorded in the multi-year monitoring
observations by CRTS.

The great majority of novalikes are found above the CV period gap. It is thus
tempting to associate the 3.9-hour periodicity found in ASAS-SN and TESS data
(sector 6) with the orbital period of the binary. The disappearance of this
period in TESS sector 33 data is puzzling. It might be premature to associate
the 3.9-hour periodicity with the orbital period , and alternatively, the
photometric phenomenology might be completely changed in the much higher
accretion state of TESS sector 33 data. Time-resolved spectroscopy is needed
to uniquely identify the orbital period by tracing the absorption lines and
the emission cores through an orbital cycle. The blue optical continuum and
the absorption lines are thought to originate from an optically thick
accretion disk. The narrow emission lines might be resolved into two
components, one to be associated with the cooler outer parts of the disk and
the other originating from the irradiated hemisphere of the donor star (see,
e.g., Beuermann & Thomas, 1990; Hernandez et al., 2017, for templates), each
with its own radial velocity curve.

The production sites of X-rays in nonmagnetic novalikes are debated.
Originally, X-rays were assumed to originate from the boundary layer (BL)
between the disk and the white dwarf with a blackbody-like spectral shape, but
the predicted blackbodies were not found (Mauche & Mukai, 2002). The BL is
nevertheless mostly regarded as the X-ray production site (Mukai, 2017), but
other locations such as shocked circumstellar material, the polar caps, a
central corona, a partially obscured boundary layer, or advective hot flows
are discussed (Zemko et al., 2014; Dobrotka et al., 2017; Balman, 2020). The
already observed high degree of X-ray variability on long and short
timescales, that is, between eRASS surveys and between erodays, together with
dedicated spectral observations may shed new light on this old question. If it
belongs to the nonmagnetic novalikes, SRGt 062340.2$-$265715 likely does not
belong to the VY Scl subclass because it does not show the typical anti-dwarf
nova dimmings by $\sim$4 mag.

Accretion in the intermediate polars may occur through streams and disks. The
disks may also be large and extend up to the Roche lobe of the accreting white
dwarf (Hellier et al., 1991). The freely spinning white dwarf in an IP
typically leads to very rich power spectra, in particular at X-ray
wavelengths, in which orbital, spin, and different types of beat periods can
be traced (dependent on the orbital inclination and the accretion mode,
respectively, see, e.g., Norton et al., 1996). The high degree of X-ray
variability through self-eclipses and foreshortening of the accretion spots
and internal absorption by accretion curtains is common to all IPs. The
observed large X-ray variability within eRASS2 and between the two SRG surveys
may thus argue for an IP classification. Moreover, the SAAO-discovered period
of 35 min and the TESS-discovered period at 25 min might support this view
because if these periods are associated with the spin of the white dwarf, they
are typical of IPs. In particular, the TESS-discovered periods at 25 min
indicate an IP nature because many IPs are observed to cluster around period
ratios of $P_{\rm spin}/P_{\rm orb}=0.1$ (Norton et al., 2004, their figure
1), as implied for SRGt 062340.2$-$265715\. To firmly classify the object as
an IP, however, the detection of a stable spin period is required, which is
lacking. The appearance and disappearance of periods at around half an hour is
perhaps difficult to explain in either classification.

Optical power spectra of IPs are, however, affected by reprocessed radiation.
Better insight is possible from a decent uninterrupted X-ray observation to
uncover the intrinsic variability of the source on short timescales. The
existing X-ray observations of SRGt 062340.2$-$265715 with SRG are
insufficient to detect a $\sim$2000 s period. This will require an extensive
future observation (e.g., with XMM-Newton).

The X-ray spectrum of SRGt 062340.2$-$265715 is not too strongly absorbed and
can be modeled with a single purely thermal emission component. No separate
soft X-ray component was found, although the appearance of the Bowen blend
appears to imply the existence of such a component. The eRASS2 bolometric
X-ray flux implies an accretion luminosity of $L_{\rm X,bol}=4.8\times
10^{32}$ erg $\mathrm{s}^{-1}$, which is at the low end for an IP and at the
high end for an UX UMa system.

Figure 16: X-ray to optical color-color diagram. Gray background objects are
all eRASS2 sources that match Gaia within 8 arcsec. Symbol colors are
otherwise the same as in Fig. 15.

In addition to the color-magnitude diagram of Fig. 15, it is instructive to
locate SRGt 062340.2$-$265715 in an X-ray to optical color-color diagram based
on eROSITA and Gaia, which is shown in Fig. 16. The diagrams were constructed
in the following manner. The collaboration-internal preliminary eRASS2 catalog
was matched to within the 8 arcsec FHWM of the point spread functions of the
telescope modules of eROSITA with Gaia DR2. For the matching objects,
distances were determined following Bailer-Jones et al. (2021). We then used
the final edition of the Ritter & Kolb catalog (Ritter & Kolb, 2003) to locate
the CVs. The optically bright novalike CVs of the UX UMa type are identified
with a different color in the diagrams. We also used the compilation of Koji
Mukai444https://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html to highlight
the IPs that were identified in eRASS. The color $Bp-Rp$ was taken directly
from Gaia data. The X-ray to optical color was computed as $\log(F_{\rm
X}(0.6-2.3{\rm keV})+{\rm phot\\_g\\_mean\\_mag}/2.5+4.86$, and the chosen
energy band was the standard band 2 in the current version of the catalog
pipeline (pipeline version 946 Brunner, H. et al., 2021, submitted to A&A).

Our Fig. 15 is to be compared with Fig. 2 of Abril et al. (2020), which
locates the CV sub-families in the Gaia-DR2 HRD. Whether the floor of objects
in Fig. 15 at $G=13$ mag indicates real associations or is due to random
associations needs to be determined. Most CVs are located between the white
dwarf sequence and the main sequence. Their X-ray luminosities are orders of
magnitude higher than those of coronally active stars on the main sequence.
The novalike CVs, in particular the magnetic IPs, overlap in the color-
magnitude diagram with coronally active stars, but they can be discerned
through their much higher X-ray luminosity.

The color-color diagram of Fig. 16 has two main structures. The L-shaped
sequence beginning at $(0.6,-5.5)$ with the knee at $(1,-3)$ delineates active
stars, whereas the cloud around $(1,-0.5)$ is the domain of the active
galactic nuclei. Most of the CVs cannot be distinguished from the numerous
active galactic nuclei in this diagnostic diagram as long as no astrometric
information can be used for a further distinction of galactic and
extragalactic objects (see also Comparat et al., 2020, their Fig. 11).

The transient CV SRGt 062340.2$-$265715 is located in very sparsely populated
areas in both diagrams. It optically belongs to the most luminous known X-ray
detected CVs. It is bluer than most nonmagnetic UX UMa novalike CVs and bluer
than all IPs, although it is not too much separated from the extreme
representatives of these two classes. On the other hand, its X-ray to optical
color is between that of the UX UMas and the IPs. For an IP, SRGt
062340.2$-$265715 is X-ray underluminous by about 1 dex; for a nonmagnetic
novalike, it is also overluminous by about 1 dex.

At a Gaia magnitude of 12.5 mag, SRGt 062340.2$-$265715, if identified as an
IP, would be the IP with the brightest apparent magnitude in eRASS2 by far.
The next brightest IP is TV Col at $\mathrm{phot\\_g\\_mean\\_mag}=13.9$ mag.
For comparison, the first soft X-ray discovered IP in the ROSAT all-sky
survey, PQ Gem, was observed at $\mathrm{phot\\_g\\_mean\\_mag}=14.1$ mag.

Although only a few objects are found in the vicinity of SRGt
062340.2$-$265715 in the diagnostic diagrams of Figs. 15 and 16, several
candidates are located around our new discovery that will be identified in the
next years with the large-scale identification programs of serendipituous
eROSITA sources with the SDSS (Kollmeier et al., 2017), for instance. The
journey has just begun.

###### Acknowledgements.

We thank the referee, John Thorstensen, for constructive criticism. MPM would
like to thank Tom Maccarone for useful discussions. This work is based on data
from eROSITA, the soft instrument aboard SRG, a joint Russian-German science
mission supported by the Russian Space Agency (Roskosmos), in the interests of
the Russian Academy of Sciences represented by its Space Research Institute
(IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG
spacecraft was built by Lavochkin Association (NPOL) and its subcontractors,
and is operated by NPOL with support from the Max Planck Institute for
Extraterrestrial Physics (MPE). The development and construction of the
eROSITA X-ray instrument was led by MPE, with contributions from the Dr. Karl
Remeis Observatory Bamberg & ECAP (FAU Erlangen-Nuernberg), the University of
Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and
the Institute for Astronomy and Astrophysics of the University of Tübingen,
with the support of DLR and the Max Planck Society. The Argelander Institute
for Astronomy of the University of Bonn and the Ludwig Maximilians Universität
Munich also participated in the science preparation for eROSITA. The eROSITA
data shown here were processed using the eSASS/NRTA software system developed
by the German eROSITA consortium. The Mikhail Pavlinsky ART-XC telescope is
the hard X-ray instrument on board the SRG observatory, a flagship
astrophysical project of the Russian Federal Space Program realized by the
Russian Space Agency, in the interests of the Russian Academy of Sciences.
ART-XC was developed by the Space Research Institute (IKI, Moscow) and the
Russian Federal Nuclear Center – All-Russian Scientific Research Institute for
Experimental Physics (RFNC-VNIIEF, Sarov) with the participation of the NASA’s
Marshall Space Flight Center (MSFC). The ART-XC team thanks the Russian Space
Agency, Russian Academy of Sciences and State Corporation Rosatom for the
support of the SRG project and ART-XC telescope. Some of the observations
presented here were obtained with SALT under the transients followup programme
2018-2-LSP-001 (PI: DB), which is supported by Poland under grant no. MNiSW
DIR/WK/2016/07. DB and LT also acknowledge research support from the National
Research Foundation. We acknowledge the use of TESS High Level Science
Products (HLSP) produced by the Quick-Look Pipeline (QLP) at the TESS Science
Office at MIT, which are publicly available from the Mikulski Archive for
Space Telescopes (MAST). Funding for the TESS mission is provided by NASA’s
Science Mission directorate. This work has made use of data from the Asteroid
Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid
Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to
search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284,
and 80NSSC18K1575; byproducts of the NEO search include images and catalogs
from the survey area. This work was partially funded by Kepler/K2 grant
J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and
ST/S006109/1. The ATLAS science products have been made possible through the
contributions of the University of Hawaii Institute for Astronomy, the Queen’s
University Belfast, the Space Telescope Science Institute, the South African
Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS),
Chile. This paper includes data collected with the TESS mission, obtained from
the MAST data archive at the Space Telescope Science Institute (STScI).
Funding for the TESS mission is provided by the NASA Explorer Program. STScI
is operated by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS 5–26555.

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