Discovery of a Rapid, Luminous Nova in NGC 300 by the KMTNet Supernova Program

Over the past few years, wide-field variability surveys have significantly advanced our understanding of high-energy transients, from thermonuclear runaways and various types of supernovae (SNe) (e.g., Rau et al. 2009, and references therein), to γ-ray (Ackermann et al. 2013) and fast radio bursts (Thornton et al. 2013).

In the optical regime, contemporary experiments are typically sensitive to two types of explosive phenomena: "local" optical transients (OTs) with small peak luminosities ($-10\lesssim {M}_{{\rm{b}}}\lesssim 5$ mag), such as classical and dwarf novae, and luminous OTs with ${M}_{{\rm{b}}}\lesssim -15$ mag (e.g., Law et al. 2009; Rau et al. 2009; Chornock et al. 2013, and references therein). The intermediate luminosity regime, which is as of yet poorly explored, is thought to be populated by rare cosmic explosions, such as rapid under-luminous SNe (Drout et al. 2013), the accretion-induced collapse of white dwarfs (WDs; Nomoto & Kondo 1991; Metzger et al. 2009), fallback SNe (Dexter & Kasen 2013), electromagnetic counterparts to compact-object mergers (Berger et al. 2013), and orphan short-GRB afterglows (Totani & Panaitescu 2002).

Exploring this parameter space is challenging for two reasons: first, our limited understanding of the underlying physical mechanisms makes it difficult to predict characteristic observational signatures and the extent to which these OTs blend with novae and SNe populations. Second, because these events are expected to be both rapid and rare, identification requires sampling of a sufficiently large volume with high temporal resolution, thereby driving the need for deep high-cadence surveys.

High-cadence experiments are additionally motivated by outstanding questions in longstanding astrophysical problems. For instance, our understanding of "infant" thermonuclear runaways and SNe is limited, with questions regarding trigger mechanisms, shock break-out emission, ejecta masses, progenitor structure, and asymmetries still remaining (Bode & Evans 2008; Smartt 2009).

Motivated by those questions, we have secured ∼20% of the Korea Microlensing Telescope Network (KMTNet; Kim et al. 2016) observing time through 2020 for a dedicated survey focused on infant and/or rapidly evolving OTs, which we call the KMTNet Supernova Program (KSP; see Moon et al. 2016). KMTNet is a network of three identical 1.6-m optical telescopes located at the Cerro Tololo Inter-American Observatory in Chile, the South African Astronomical Observatory, and the South Spring Observatory in Australia. Each telescope is equipped with a 2° × 2° wide-field detector, comprised of four e2v CCDs that offer 04 pixel⁻¹ sampling (Kim et al. 2016). KSP is capable of providing deep (≲21.5 mag⁶ ), high-cadence continuous monitoring in B, V, and I bands (Moon et al. 2016).

In this paper we present the discovery of KSP-OT-201509a, a rapidly evolving OT found toward a spiral arm of the Sculptor galaxy NGC 300. The transient stands out for its rapid decay rate and showcases the potential of KSP for providing well-sampled multi-color light curves of rapidly evolving eruptions. The paper is structured as follows. In Section 2, we provide a brief overview of the KSP data, and present the discovery of KSP-OT-201509a, alongside its multi-color evolution. In Section 3 we discuss the nature of the transient based on the light-curve characteristics, and in Section 4 we use Hubble Space Telescope (HST) archival images to place constraints on the progenitor of KSP-OT-201509a and its environment. Finally, we explore the ramifications of our result and conclude with a brief discussion on the prospects of the KSP in nova search in Section 5.

Between 2015 July 1 and 2016 January 10, KSP monitored a 15 deg² area toward the Sculptor group, including a 4 deg² field around NGC 300 (d = 1.86(7) Mpc⁷ ; M − m = 26.35; Rizzi et al. 2006). We collected ∼1300 frames per BV I band, with a mean cadence of ∼3.5 hr and an intra-day cadence ranging from ∼10 to 40 minutes in each filter.

The data were processed using our custom KSP pipeline, which relies mostly on public software. More specifically, after acquisition flat-fielding and correction of the CCD cross talk, the data were automatically transferred to the KASI KMTNet data center for further processing. The data were then reduced using SExtractor⁸ for source extraction, SCAMP⁹ for astrometry and absolute photometry, and HOTPANTS¹⁰ for image subtraction. The photometric calibration is based on four AAVSO All-Sky Survey standard stars in the field (see Moon et al. 2016 for details). No color correction is applied. We use 60-s exposures which typically yield a limiting magnitude of ∼21.5 mag at S/N = 5 under 12 seeing for point sources. The astrometric solution, which is derived using ∼10,000 unsaturated stars with counterparts in the second Hubble guide star catalog (Lasker et al. 2008), accounts for scale, offset, rotation, and distortion and is better than 012. The photon-limited astrometric precision is generally better than ∼05 under 12 seeing.

KSP-OT-201509a was discovered in the KSP data toward an NGC 300 spiral arm (Figure 1) as a faint, rapidly evolving OT. The source first appeared on an I-band image recorded on UT 270.76¹¹ with an apparent magnitude of ${m}_{{\rm{I}}}=20.7(3)$ mag, at (α,δ)_J2000 = (0:55:09.422, −37:42:16.5), 3373 (≃2 kpc) away from the center of the galaxy.

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The original light curve produced by our automated pipeline was contaminated by systematics, evident by the large-scale scatter (∼0.4 mag) around the dominant decay trend. For this reason, we re-analyzed the data using the photometry of nearby stars. More specifically, we determined the local PSF by fitting a Moffat function plus a first-degree polynomial for the background to four bright unsaturated stars within 35 of the transient. Apparent magnitudes were calculated by integrating the PSF over the Kron radius. Figure 2 shows the BV I light curves of KSP-OT-201509a from first detection to its disappearance below the detection limit ∼22 days later.

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2.1. Rising Phase

2.2. Maximum and Decline Phase

KSP-OT-201509a was detected again at UT 272.33 at B = 18.39(1), shortly after the onset of its decay phase (Figure 2). An extrapolation of our best-fit models between the rising and early-decline phases suggests that the peak luminosity was not missed by more than ∼0.6 days. We place the maximum light between 17.9–17.6 mag, 17.1–16.7 mag, and 17.5–17.2 mag in B, V, and I band, respectively.

Assuming that the transient is indeed associated with NGC 300, the former corresponds to peak absolute magnitudes between −8.45 and −9.65 mag. We do not account for the negligible foreground extinction (${A}_{{\rm{V}}}^{{\rm{f}}}=0.03$ mag; Schlegel et al. 1998), nor for any reddening from NGC 300, which should be of the same order since the galaxy is viewed nearly edge-on.

The post-maximum evolution is characterized by a rapid decay (Figure 2). To quantify the decay rate, we adopt a decline law of the form ${F}_{{\rm{v}}}\propto {t}^{-\alpha }$, where t is the time since maximum. For B and V we infer α = 1.98(6) and 1.84(6), respectively. In the I band, the decay rate evolves from α = 1.41(6) at the onset of the decay phase to α = 2.00(7) after ∼UT 279. From the best-fit light curves and the times of maximum light derived above we infer ${t}_{2}^{V}=1.7(6)$ and ${t}_{3}^{V}=3.8(7)$ days for the time required for the V-band light curve to fade by 2 and 3 magnitudes, respectively.

The color evolution of KSP-OT-201509a is shown in Figure 3. The decay phase starts with B − V ≃ 0.5 mag and V − I ≃ −0.3 mag. The color indexes then rapidly evolve to B − V ≃ 0.0 mag and V − I ≃ 1.0 mag in less than one and four days, respectively. Subsequently, the excess in the I band grows up to V − I ≃  1.5 mag within a few days, while B − V reverts to negative values.

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In Section 2 we presented the light-curve properties of KSP-OT-201509a. The peak absolute brightness inferred assuming association with NGC 300 and the rapid rising phase exclude a SN explosion or a SN impostor as the origin of the transient. Similarly, the fast post-maximum evolution disfavors an outburst on a non-degenerate star and/or a luminous red nova, since those generally evolve on longer timescales (see Pastorello et al. 2007; Williams et al. 2015, and references therein). Those features, together with the presence of a multi-color pre-maximum halt and the smooth post-maximum evolution, suggest that KSP-OT-201509a is a rapidly evolving classical nova. Indeed, the light curve shown in Figure 2 exhibits similarities to other well-sampled fast galactic novae such as V5589 Sgr (Eyres et al. 2017, see Section 5 for a more detailed discussion). Finally, the observed peak luminosity/decline rate ratio is in good agreement with established empirical relations. For instance, the tangent maximum-magnitude/rate of decline (MMRD) relation of della Valle & Livio (1995) predicts ${M}_{{\rm{V}}}-9.0(4)$ mag (here the error accounts for the uncertainty in t₂ and the internal scatter of the MMRD relation). For those reasons, we believe KSP-OT-201509a is indeed a fast classical nova. We hereafter adopt this classification.

We analyzed a set of archival HST/ACS frames toward NGC 300 obtained using the f606w and f814w filters (see Binder et al. 2015 for the original work). The images were taken on 2014 July 2 with 850-s and 611-s exposure times, respectively.

We measured instrumental magnitudes and performed absolute calibration using DOLPHOT (Dolphin 2000). Pre-determined PSF models were used to extract instrumental magnitudes which were then corrected to infinite apertures using 12 bright isolated stars within 15 from KSP-OT-201509a. Our absolute flux calibration is based on the most recent infinite aperture and zeropoint values for the ACS CCDs (see Bohlin 2016, and references therein). We determine the 5σ photometric limit to be ${m}_{{\rm{f}}606{\rm{w}}}\leqslant 27.6$ $({M}_{{\rm{f}}606{\rm{w}}}\leqslant 0.84)$ and ${m}_{{\rm{f}}814{\rm{w}}}\leqslant 26.5$ $({M}_{{\rm{f}}814{\rm{w}}}\leqslant 0.14)$ mag. We used the default ACS astrometric calibration (see Anderson & King 2006), which provides a distortion-free system to a level of 5 mas and then fitted for position offsets using the four common GSC sources nearest to KSP-OT-201509a.

Figure 4 shows the f606w image around KSP-OT-201509a, with 05 and 10 error circles that correspond roughly to the 68% and 95% KSP position uncertainty for a source close to the detection limit. The star nearest to the nominal KMTNet position of the source is located at (α,δ)_J2000 = (${0}^{{\rm{h}}}{55}^{{\rm{m}}}{09.422}^{{\rm{s}}}$, −37°42''1650) and has ${m}_{{\rm{f}}606{\rm{w}}}=26.11(9)$ and ${m}_{{\rm{f}}814{\rm{w}}}=25.34(10)$ mag. The brightest point-like source within the 95% error circle has ${m}_{{\rm{f}}606{\rm{w}}}=22.348(7)$ and ${m}_{{\rm{f}}814{\rm{w}}}=21.344(8)$ mag, consistent with what one would expect for a super-giant at the distance of NGC 300.

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It is unlikely that either of the HST sources inside the KMTNet error box (Figure 4) is the progenitor of KSP-OT-201509a in quiescence, as one would expect 4 ≤ M ≤ 8 for a typical main-sequence companion, which is well below the sensitivity of the HST data. The broader region, which is part of an NGC 300 spiral arm, is characterized by a large number of bright stars (with ${M}_{{\rm{f}}606{\rm{w}}}\simeq -5$). This indicates that KSP-OT-201509a is likely associated with a region with high star-forming activity.

Nova outbursts result from thermonuclear runaway eruptions on a white dwarf (WD). They occur in binaries of the cataclysmic-variable type, in which matter is accreted from a non-degenerate companion (e.g., Darnley et al. 2012). Material accumulating on the WD surface eventually causes the envelope to become electron degenerate, leading to a runaway thermonuclear flash which ultimately gives rise to the nova phenomenon.

It is well established, both theoretically and observationally, that nova timescales, amplitudes, and repentance rates depend sensitively on the WD mass, mass-accretion rate, envelope mass, companion type, and wind power (Yaron et al. 2005). With few exceptions, bright and fast novae (hereafter FNe) occur in systems with massive WDs and high mass-accretion rates between ${\dot{M}}_{\mathrm{acc}}\simeq {10}^{-8}$ and 10⁻⁴ M_⊙ yr⁻¹ (Prialnik & Kovetz 1995; Yaron et al. 2005).

FNe rising phases last up to few days (e.g., Kato et al. 2009; Strope et al. 2010). During this time, the effective temperature increases dramatically (initially at constant radius) causing the surface brightness to rise by 10–20 mag. Recent studies find no strong correlation between the ascent rate and peak brightness (e.g., Cao et al. 2012), although no safe conclusions can be drawn from existing data. In addition, because of the rapid evolution timescales, very few infant FNe have been sampled with sufficient temporal cadence in multiple bands to probe the underlying eruption mechanism in detail.

The work presented here provides an unprecedented multi-color view of an early FN eruption phase. We find that the brightness of KSP-OT-201509a increases at −3.7 mag day⁻¹ in V, indicating a total rising-phase duration between ∼2.5 and 5 days.

In addition, our data provide evidence for a short-lived pre-maximum halt (PMH) after UT 270.67 (Figure 2). While PMHs have been observed in both slow and fast novae (see Hounsell et al. 2010, 2016, and references therein), it is yet unclear if they reflect an intrinsic change of the WD.

For instance, some early studies suggest that they may be triggered by an external condition, such as a sudden enhancement of mass loss from the donor. More recent theoretical work based on detailed 1D simulations (Hillman et al. 2014) found that PMHs are explained naturally by a decrease in the convection-transport efficiency. The rise to peak brightness continues after the opacity decreases to allow for radiation transport to take over at the onset of the mass-loss phase. In KSP-OT-201509a, one sees that the color index remains constant during the PMH and later evolves rapidly between the late-rise and early-decline phases (see Figures 2 and 3). This is consistent with a transition in the emission mechanism expected in the latter scenario (see below).

In the early-decline phase it is expected that the continuum spectrum is dominated by free-free scattering above the optically thick photosphere. For the idealized case of pure optically thin thermal Bremsstrahlung, Kato et al. (2009) found a universal decay law, F_ν ∝ t^−1.75, that matches well the observed light curve of KSP-OT-201509a (Section 2). This in turn suggests that the transient evolution depends strongly on the wind rate and velocity, and less so on the WD mass (Kato et al. 2009, and references therein). At later phases, especially after UT 276.9, the transition to a steeper I-band decline rate suggests the presence of an additional thermal emission component (Hachisu & Kato 2016), which may indicate the presence of additional emission components such as the formation of a dust shell.

Considering that no spectroscopic information is available for KSP-OT-201509a, we resort to other historical FNe and theoretical studies to draw further conclusions on the WD mass and the nature of the donor star as below. Examples of well-studied bright ($-8.5\leqslant {M}_{V}\leqslant -10.5$) FNe in the Galaxy include (Strope et al. 2010) V838 Her (${t}_{2}\simeq 1$ day), V1500 Cyg (${t}_{2}\simeq 2$ days), V2275 Cyg (${t}_{2}\simeq 3$ days), and more recently V2491 Cyg (${t}_{2}\simeq 2$ days; Hounsell et al. 2016) and V5589 Sgr (${t}_{2}\simeq 2.5$ days Eyres et al. 2017). Spectra from the early eruption phases for these FNe indicate terminal wind velocities of ∼1000 to 3000 km s⁻¹. In almost all cases, FNe are associated with massive (≥1 ${M}_{\odot }$) WDs and a total wind mass-loss of few times 10⁻⁶ M${}_{\odot }$. Given the observational similarities between KSP-OT-201509a and these FNe, we therefore conclude that the former most likely also hosts a massive WD of $\geqslant 1$ ${M}_{\odot }$.

The discovery of KSP-OT-201509a in an early phase and dense multi-color monitoring of its light curve through its entire eruption demonstrates the potential for KSP to provide an unprecedented views of novae and related phenomena. Based on the KSP early performance and sensitivity so far, we expect to detect several tens of classical novae, as well as other transients, including infant and nearby SNe (Moon et al. 2016).

This research has made use of the KMTNet system operated by the Korea Astronomy and Space Science Institute (KASI) and the data were obtained at three host sites of CTIO in Chile, SAAO in South Africa, and SSO in Australia. KSP is partly supported by an NSERC Discovery Grant to D.S.M. J.A. is a Dunlap Fellow at the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto. The Dunlap Institute is funded by an endowment established by the David Dunlap family and the University of Toronto. We have made extensive use of NASA's Astrophysics Data System. This research made use of Astropy, a community-developed core Python package for Astronomy.

Facilities: HST(WFPC2) - .

Software: Astropy.