KSP-OT-201611a: A Distant Population II Dwarf Nova Candidate Discovered by the KMTNet Supernova Program

TL;DR

This study reports the discovery and detailed photometric analysis of a distant Population II dwarf nova, KSP-OT-201611a, revealing its outburst behavior, color variations, and potential classification as an SU UMa-type dwarf nova, located far from the Galactic plane.

Contribution

It provides the first detailed multi-color photometric characterization of a distant Population II dwarf nova, enhancing understanding of accretion processes in such rare objects.

Findings

Detected two outbursts with distinct characteristics.

Indications that the object is an SU UMa-type dwarf nova.

Located at a large Galactocentric distance (~13.8 kpc).

Abstract

We present a multi-color, high-cadence photometric study of a distant dwarf nova KSP-OT-201611a discovered by the Korea Microlensing Telescope Network Supernova Program. From October 2016 to May 2017, two outbursts, which comprises a super/long outburst followed by a normal/short outburst separated by $\sim$91 days, were detected in the $BVI$ bands. The shapes and amplitudes of the outbursts reveal the nature of KSP-OT-201611a to be an SU UMa- or U Gem-type dwarf nova. Color variations of periodic humps in the super/long outburst possibly indicate that KSP-OT-201611a is an SU UMa-type dwarf nova. The super and normal outbursts show distinctively different color evolutions during the outbursts due most likely to the difference of time when the cooling wave is formed in the accretion disk. The outburst peak magnitudes and the orbital period of the dwarf nova indicate that it is at a large Galactocentric distance ($\sim$13.8 kpc) and height ($\sim$1.7 kpc) from the Galactic plane. KSP-OT-201611a, therefore, may provide a rare opportunity to study the accretion disk process of Population II dwarf novae.

Tables

Table 1. Table 1: Photometric catalog for KSP-OT-201611a

$B$			$V$			$I$
MJD	Mag	Error	MJD	Mag	Error	MJD	Mag	Error
	[mag]	[mag]	[days]	[mag]	[mag]	[days]	[mag]	[mag]
57721.16	19.27	0.02	57721.16	19.18	0.02	57721.12	19.44	0.07
57721.27	19.34	0.02	57721.17	19.16	0.02	57721.17	19.25	0.04
57721.31	19.35	0.02	57721.22	19.19	0.02	57721.17	19.30	0.04
57722.22	19.22	0.01	57721.27	19.25	0.02	57721.22	19.30	0.04
57722.31	19.00	0.01	57721.32	19.29	0.02	57721.27	19.25	0.04
57723.18	18.99	0.01	57722.17	18.95	0.02	57721.32	19.31	0.03
57723.22	19.09	0.01	57722.22	19.04	0.01	57722.17	19.05	0.04
57723.27	19.00	0.01	57722.27	19.06	0.02	57722.22	19.09	0.03
57723.32	19.05	0.01	57722.32	18.93	0.01	57722.27	19.11	0.04
57723.59	19.16	0.03	57723.18	18.79	0.01	57722.32	19.06	0.03
57723.64	19.14	0.03	57723.23	18.99	0.01	57723.18	18.91	0.03
57724.18	19.18	0.01	57723.27	18.95	0.02	57723.23	19.07	0.03
57724.23	19.15	0.01	57723.32	18.95	0.01	57723.27	19.03	0.03
57724.28	19.04	0.01	57723.59	19.04	0.02	57723.32	19.03	0.03
57724.33	19.21	0.01	57723.64	18.98	0.03	57723.59	19.00	0.03
57724.67	19.22	0.03	57723.69	18.93	0.02	57723.64	18.98	0.04
57725.18	19.25	0.02	57724.18	19.07	0.02	57723.69	19.04	0.04
57725.23	19.17	0.01	57724.23	19.07	0.02	57724.18	19.16	0.03
57725.28	19.29	0.01	57724.28	18.91	0.01	57724.23	19.14	0.03
57725.32	19.23	0.01	57724.33	19.09	0.02	57724.28	18.96	0.03
57727.18	19.50	0.02	57724.67	19.10	0.03	57724.33	19.22	0.03
57727.23	19.50	0.02	57725.18	19.20	0.02	57724.67	19.14	0.04
57727.28	19.55	0.02	57725.23	19.05	0.02	57724.72	19.05	0.04
57727.33	19.53	0.02	57725.28	19.18	0.02	57725.19	19.22	0.04
57728.19	19.67	0.02	57725.32	19.14	0.02	57725.23	19.16	0.03
57728.24	19.64	0.02	57727.18	19.45	0.03	57725.28	19.22	0.04
57728.29	19.59	0.02	57727.23	19.41	0.02	57725.32	19.24	0.03
57728.34	19.69	0.02	57727.28	19.36	0.02	57727.18	19.43	0.05
57729.11	19.84	0.04	57727.33	19.36	0.02	57727.24	19.46	0.04
57729.16	19.64	0.02	57728.19	19.54	0.02	57727.29	19.46	0.04
57729.21	19.81	0.02	57728.24	19.53	0.02	57727.34	19.41	0.04
57729.26	19.70	0.02	57728.29	19.43	0.02	57728.19	19.62	0.05
57729.31	19.72	0.01	57728.34	19.50	0.02	57728.24	19.58	0.04
57730.21	19.84	0.04	57729.11	19.61	0.04	57728.29	19.49	0.04
57734.28	20.68	0.11	57729.17	19.43	0.03	57728.34	19.58	0.04
57813.79	20.36	0.04	57729.21	19.60	0.02	57729.12	19.66	0.06
57813.83	20.06	0.03	57729.26	19.59	0.02	57729.17	19.53	0.04
57815.05	20.33	0.04	57729.31	19.57	0.02	57729.21	19.74	0.06
57815.10	20.35	0.04	57729.71	19.49	0.05	57729.26	19.69	0.05
57815.79	21.02	0.10	57730.26	19.58	0.06	57729.31	19.70	0.05
57815.84	20.90	0.09	57734.28	20.55	0.11	57729.71	19.61	0.06
			57734.33	20.61	0.08	57730.26	19.59	0.06
			57813.79	20.11	0.04	57730.30	19.68	0.07
			57813.83	20.02	0.03	57734.29	20.53	0.11
			57815.05	20.28	0.04	57734.33	20.32	0.10
			57815.10	20.21	0.05	57813.79	20.21	0.06
			57815.15	20.33	0.05	57813.84	20.08	0.05
			57815.79	21.02	0.11	57815.05	20.21	0.07
			57815.84	20.90	0.11	57815.10	20.26	0.07
			57815.89	20.79	0.11	57815.14	20.26	0.08
						57815.79	20.68	0.09
						57815.84	20.90	0.14
NOTE : Measured magnitudes are extiction-corrected.

Table 2. Table 2: Properties of the first outburst

Parameter		Values
	$B$	$V$	$I$
Peak Magnitude ([mag])	19.05 $\pm$ 0.06	18.94 $\pm$ 0.06	19.02 $\pm$ 0.06
Epoch of Peak brightness ($t_{p_1}$ [day])	57723.06 $\pm$ 0.12	57723.01 $\pm$ 0.08	57723.14 $\pm$ 0.07
Rising rate (${\tau }_{r_1}$ [day mag-1])	0.66 $\pm$ 0.07	0.62 $\pm$ 0.07	0.85 $\pm$ 0.05
Decay Rate of Plateau (${\tau }_p$ [day mag-1])	8.45 $\pm$ 1.93	9.36 $\pm$ 0.69	10.22 $\pm$ 0.80
Decline Rate (${\tau }_{d_1}$ [day mag-1])	1.00 $\pm$ 0.38	0.78 $\pm$ 0.15	1.33 $\pm$ 0.18
Amplitude of Outburst ($A_{p_1}$ [mag])	4.46 $\pm$ 0.13	4.51 $\pm$ 0.13	3.66 $\pm$ 0.11
Duration of Outburst ($D_{p_1}$ [day])	19.09 $\pm$ 1.65	18.43 $\pm$ 0.72	19.28 $\pm$ 0.63
NOTE : All uncertainties are measured from the bootstrap re-sampling method,
but uncertainties of amplitudes are propagated from the peak and quiescence errors.

Table 3. Table 3: Properties of the second outburst

Parameter		Values
	$B$	$V$	$I$
Peak Magnitude ([mag])	19.44 $\pm$ 0.31	19.40 $\pm$ 0.14	19.50 $\pm$ 0.09
Epoch of Peak brightness ($t_{p_2}$ [day])	57814.00 $\pm$ 0.13	57814.03 $\pm$ 0.06	57814.04 $\pm$ 0.03
Rising Rate (${\tau }_{r_2}$ [day mag-1]	0.23 $\pm$ 0.02	0.32 $\pm$ 0.05	0.34 $\pm$ 0.02
Decline Rate (${\tau }_{d_2}$ [day mag-1])	1.21 $\pm$ 0.12	1.22 $\pm$ 0.17	1.41 $\pm$ 0.24
Amplitude of Outburst ($A_{p_2}$ [mag])	4.07 $\pm$ 0.25	4.05 $\pm$ 0.18	3.18 $\pm$ 0.13
Duration of Outburst ($D_{p_2}$ [day])	5.90 $\pm$ 1.41	6.23 $\pm$ 0.90	5.60 $\pm$ 0.51
NOTE.– All uncertainties are measured from the bootstrap re-sampling method,
but uncertainties of amplitudes are propagated from peak and quiescence errors.

Table 4. Table 4: Parameters of KSP-OT-201611a

Parameter	Values (for max. freq. / local second max. freq.)
Expected type of dwarf nova	SU UMa / U Gem
Frequency of humps ($f_s$)	13.72 $\pm$ 0.01 / 7.18 $\pm$ 0.01 cycle day-1
Period of humps ($P_s$)	1.750 $\pm$ 0.001 / 3.343 $\pm$ 0.004 hour
Orbital perioda ($P_o$)	1.69 $\pm$ 0.01 / 3.343g $\pm$ 0.004 hour
Mass of primary starb ($M_1$)	0.75 / 0.75 $M_{\odot }$
Mass of secondary starc ($M_2$)	0.11 $\pm$ 0.01 / 0.21 $\pm$ 0.01 $M_{\odot }$
$I$-band absolute magnitude of secondary ($M_{I2}$)	11.56 $\pm$ 0.01 / 9.33 $\pm$ 0.01 mag
Effective temperature of secondary stard ($T_{eff2}$)	2976 $\pm$ 10 / 3319 $\pm$ 10 K
Absolute magnitude of superoutburst peake ($M_p$)	4.61 $\pm$ 0.41 / 4.29 $\pm$ 0.41 mag
Distance modulusf ($m-M$)	14.33 $\pm$ 0.41 / 14.66 $\pm$ 0.41 mag
Distance from the Sun ($d_S$)	7.3${}_{-1.3}{}^{}{}_{}{}^{+1.5}$ / 8.5${}_{-1.5}{}^{}{}_{}{}^{+1.8}$ kpc
Distance from the Galactic center ($d_{GC}$)	13.8${}_{-1.1}{}^{}{}_{}{}^{+1.3}$ / 14.9${}_{-1.3}{}^{}{}_{}{}^{+1.6}$ kpc
Scale height from the Galactic plane (Z)	$-$1.7${}_{-0.4}{}^{}{}_{}{}^{+0.3}$ / $-$2.0${}_{-0.4}{}^{}{}_{}{}^{+0.3}$ kpc
a Equation (7) of Mennickent et al. (1999)
b Table 8 of Knigge et al. (2011) only provides $M_1=0.75$  $M_{\odot }$.
c,d Table 6 of Knigge et al. (2011); Uncertainties are propagated from that of $P_o$
e Equation (4) of Patterson (2011)
f for super/long outburst
g In the case of U Gem types, $P_o$ is the same with period of humps.
∗ It is noted that the study of Knigge et al. (2011) was stemed from dwarf novae with solar metallicities.

Full text

stars: dwarf novae — surveys — techniques: photometric

††software: DAOPHOT (Stetson, 1987), astropy (Astropy Collaboration et al., 2013, 2018)

1 Introduction

Dwarf novae as a subset of cataclysmic variables are known as close binary systems comprising a white dwarf primary with an accretion disk and a main sequence secondary (Warner, 1995). Dwarf novae, in general, have three major types: Z Cam, U Gem, and SU UMa (Osaki, 1996) depending on unique characteristics of their light curves. Z Cam types have intervals of constant brightness called standstills, whereas U Gem types are featured by regular quasi-periodic outbursts. SU UMa types, which are more frequently found than the other types (Otulakowska-Hypka et al., 2016), show a combination of normal and superoutbursts. The superoutbursts of SU UMa types typically have longer outburst duration ($\sim$12–20 days) than normal outbursts ($\lesssim$8 days), and the former are usually brighter than the latter by $\sim$0.8 mag in the $V$ band (Patterson, 2011; Otulakowska-Hypka et al., 2016). Their peak brightness is known to have a tight correlation with the orbital period (Warner, 1987; Patterson, 2011).

Studies of dwarf novae have been focused on the analysis of observed light curves since they can provide important information about the nature of the observed dwarf novae. For eclipsing dwarf novae, the origin of the accretion disk instability leading to outbursts can be investigated based on the shape of the light curves (Vogt, 1983; Ioannou et al., 1999; Webb et al., 1999; Baptista et al., 2000). In this case, outside-in outbursts show symmetric light curves, whereas inside-out outbursts produce asymmetric light curves. Also, if dwarf novae show a delay between the UV and visible light, large and small UV delays mean outside-in and inside-out outbursts, respectively (Cannizzo et al., 1986; Cannizzo & Kenyon, 1987). The accretion rate can also be investigated using the time interval between outbursts (Smak, 1984; Schreiber et al., 2003). In addition, if multi-color information is available, more detailed analysis of accretion process and related physical parameters can be conducted, including size, temperature, surface density, and viscosity (Mayo et al., 1980; Cannizzo & Kenyon, 1987). Unfortunately, however, multi-color observations have been very sparse in dwarf nova observations, limiting our understanding of their origin (Brown et al., 2018; Shugarov et al., 2018).

The typical $V$-band absolute magnitudes of dwarf novae are within a range of 7–9 mag (Warner, 1987), and they have been mostly found in the solar neighborhood (\lesssim$$1 kpc) (Downes et al., 2001; Özdönmez et al., 2015). These nearby dwarf novae are part of the thin disk of the Milky Way (Ak et al., 2013). Their mass donors, i.e., the main sequence secondaries, have relatively high metallicity and low velocity (Ak et al., 2013; Harrison & Hamilton, 2015; Harrison, 2016). Most of the dwarf novae that have been studied so far, therefore, belong to Population I (PopI) group. The properties of low metallicity Population II (PopII) dwarf novae have been poorly studied, mostly due to the lack of observed samples (Hawkins & Veron, 1987; Howell & Szkody, 1990; Edmonds et al., 2003). Handful of PopII dwarf novae that have been studied so far, to the best of our knowledge, are those in the globular cluster 47 Tucanae (Edmonds et al., 2003) at 4.5 kpc from the Sun (Zoccali et al., 2001) with ${\rm[Fe/H]}$ $\simeq$ $-$0.78 (Thygesen et al., 2014). Furthermore, few dwarf novae are spectroscopically classified as thick disk components (Ak et al., 2013). The only spectroscopically confirmed PopII dwarf nova is SDSS J1507+52 of [Fe/H] $=-1.2$ and $d=250$ pc (Uthas et al., 2011). Due to the low metallicity, the accretion disk luminosity of PopII dwarf novae is expected to be higher than those of PopI (Stehle et al., 1997). Surprisingly, however, the dwarf novae in 47 Tucanae have been observed with much smaller accretion disk luminosity than theoretically expected (Edmonds et al., 2003). The origin of this discrepancy is not well understood, and it is imperative to obtain more observational samples of PopII dwarf novae.

In this paper, we present the discovery and high-cadence, multi-color monitoring of a new dwarf nova KSP-OT-201611a most likely belong to PopII group at the distance of $\sim$13.8 kpc from the Galactic center and the height of $\sim$1.7 kpc from the Galactic plane. Section 2 and 3 provide our discovery and photometry of the source and analysis of its light curves and colors, respectively. We determine the nature of KSP-OT-201611a to be an SU UMa-type dwarf nova and measure orbital period and related parameters in Section 4. We discuss the properties of KSP-OT-201611a in Section 5 and provide summary and conclusion in Section 6.

2 KSP-OT-201611a : Discovery and Photometry

As part of the Korea Microlensing Telescope Network (KMTNet; Kim et al., 2016) Supernova Program (KSP; Moon et al., 2016), we have monitored a 4-deg2 area toward the lenticular galaxy NGC 2292 since October 2016. In the program, we have obtained 60-s exposure of $BVI$ bands with typical of $\sim$8-hour cadence. We discovered a new transient source, which we name KSP-OT-201611a, at $(\alpha,\delta)_{\rm J2000}=(06^{h}\,42^{m}\,02.1^{s},-26^{\circ}\,00^{\prime}\,21.6^{\prime\prime})$ or $(l,b)=(235.51135^{\circ},-13.45849^{\circ})$ on November 29, 2016 (${\rm MJD}=57721.17$ day) at a $V$-band magnitude of $19.19\pm 0.02$ mag (see below for the photometric calibration).

Figure 1 compares $V$-band images centered on KSP-OT-201611a obtained before the first detection and when the source is at its peak brightness. It also shows a deep image made by stacking 170 individual exposures of 60 seconds obtained when the source was fainter than the detection limit in each image. As seen in the right panel of Figure 1, KSP-OT-201611a appears as a very faint source of $V=23.45\pm 0.11$ mag on the deep stack image (see Section 3.2).

We conducted PSF photometry using DAOPHOT (Stetson, 1987) and used the AAVSO Photometric All-Sky Survey database (APASS)111https://www.aavso.org/apass $BVi^{\prime}$ standard stars for photometric calibration. We used about 300 stars near the source to obtain Penny function based PSF of each image. The typical limiting magnitude of 60-s exposure image obtained in this way is \sim$$21.5 mag at $S/N=5$ under 1.2$\arcsec$ seeing in the $V$ band. Given the discrepancy between our $I$-band observations and $i^{\prime}$-band standard photometric system used in APASS, we adopt the relation $I=i^{\prime}-0.4$ known between the KMTNet $I$ band and the APASS $i^{\prime}$ band (Park et al. in preparation; Park et al., 2017). We use the extinction of $A_{B}=0.41$ mag, $A_{V}=0.32$ mag, and $A_{I}=0.20$ mag using the extinction $E(B-V)=0.101$ mag obtained toward KSP-OT-201611a in the Galactic extinction model of Schlegel et al. (1998) with $R_{V}=3.1$. The extinction-corrected photometric results are shown in Table 2. In this paper, we use extinction-corrected magnitudes and colors unless otherwise specified.