arXiv:1805.02188v1 [astro-ph.SR] 6 May 2018 yee sn L 2018 using 8, Typeset May version Draft eutn nms ae sasproa(N xlso.Terhetero Their explosion. (SN) supernova a as cases most in resulting [email protected] orsodn uhr ac Elias-Rosa Nancy author: Corresponding 7 2 10 6 1 9 5 15 8 4 3 14 13 12 11 ot Africa. South USA. Breoa,Spain (Barcelona), Spain 09Lso,Portugal. Lisboa, 1049 ilrSno elw ilrIsiuefrBscRsac i Research Basic for Institute Miller Fellow, Senior Miller ntttd iece elEpi(SCIE) apsUB C UAB, Campus (CSIC-IEEC), l’Espai Ci`encies de de Institut eateto srnm,Uiest fClfri,Berkel California, of University Astronomy, of Department NF-Osraoi srnmc iPdv,vcl dell’Oss vicolo Padova, di Astronomico Osservatorio - INAF ilnimIsiueo srpyis(A) ucoMonse˜ Nuncio (MAS), Astrophysics of Institute Millennium srpyisRsac ete colo ahmtc n Ph and Mathematics of School Centre, Research Astrophysics eatmnod inisFıia,UiesddAde Bel Andres F´ısicas, Universidad Ciencias de Departamento twr bevtr,Uiest fAioa usn Z857 AZ Tucson, Arizona, of USA. University 91125, Observatory, Steward CA Pasadena, 100-22, Mailcode Caltech/IPAC, ti elkonta asv tr ( stars massive that known well is It pc eecp cec nttt,30 a atnDie B Drive, Martin San 3700 Institute, Science Telescope Space h sa li ete eateto srnm,AlbaNova, Astronomy, of Department Centre, Klein Oskar The likroOsraoy etrfrBcyr srnm Kle Astronomy Backyard for Center Observatory, Kleinkaroo uoenSaeAtooyCnr EA) uoenSaeAge Space European (ESAC), Centre Astronomy Space European ETA eatmnod ´sc,IsiuoSpro T´ecn Superior F´ısica, Instituto de Departamento CENTRA, ITPC,Dprmn fPyisadAtooy University Astronomy, and Physics of Department PACC, PITT ac Elias-Rosa, Nancy H YEINSPROA21B:TEEPOINO TRI OUTB IN STAR A OF EXPLOSION THE 2010BT: SUPERNOVA IIN TYPE THE ilaoPignata, Giuliano ahnSmith, Nathan A T Rcie uy1 07 eie eebr2,21;Accepted 2017; 27, December Revised 2017; 1, July (Received E X atoMiluzio, Matteo manuscript ,2 1, 4 ,9 8, uiaKotak, Rubina culrD a Dyk, Van D. Schuyler r .Fox, D. Ori tl nAASTeX61 in style 13 umte oASJournals AAS to Submitted .A .Monard, G. A. L. > M 10 5 ABSTRACT li Galbany, Lluis asm Turatto, Massimo M 8 cec,Uiest fClfri,Bree,C 94720, CA Berkeley, California, of University Science, n ⊙ 3 y A97031,USA. 94720-3411, CA ey, o va e´bia22 atao 300,Chile. 8320000, Santiago, Rep´ublica 252, Avda. lo, vleu oteclas fteselrcore, stellar the of collapse the to up evolve ) o ´tr az10 rvdni,Snig,Chile. Santiago, Providencia, 100, S´otero Sanz nor raoi ,Pdv -52,Italy I-35122, Padova 5, ervatorio tfn Benetti, Stefano c,Uiesdd eLso,AeiaRvsoPi 1, Pais Rovisco Avenida Lisboa, de Universidade ico, 0 USA. 20, sc,QensUiest efs,BlatB71N UK. 1NN, BT7 Belfast Belfast, University Queen’s ysics, lioe D228 USA. 21218, MD altimore, nao,Sn eea1,P o 8,Cltdr 6660, Calitzdorp 281, Box PO 1B, Helena Sint inkaroo, mıd a arn /,013Cerdanyola 08193 S/N, Magrans Can am´ı de 14 tchl nvriy 09 tchl,Sweden. Stockholm, 10691 University, Stockholm c,Vlauv el and,Mdi,E-28692, Ca˜nada, Madrid, la de Villanueva ncy, fPtsug,Ptsug,P 56,USA. 15260, PA Pittsburgh, Pittsburgh, of n ata Ergon Mattias and 11 ataoGonz Santiago 1 lxiV Filippenko, V. Alexei eet srltdmil to mainly related is geneity 1 a ,2018) 8, May nioCappellaro, Enrico alez-Gait ´ 15 ,7 6, an, URST ´ 12 1 2 Elias-Rosa et al. different configurations of the progenitor star at the moment of the explosion, and to their immediate environments. We present photometry and spectroscopy of SN 2010bt, which was classified as a Type IIn SN from a spectrum obtained soon after discovery and was observed extensively for about two months. After the seasonal interruption owing to its proximity to the Sun, the SN was below the detection threshold, indicative of a rapid luminosity decline. We can identify the likely progenitor with a very luminous star (log L/L⊙ ≈ 7) through comparison of Hubble Space Telescope images of the host galaxy prior to explosion with those of the SN obtained after maximum light. Such a luminosity is not expected for a quiescent star, but rather for a massive star in an active phase. This progenitor candidate was later confirmed via images taken in 2015 (∼ 5 yr post-discovery), in which no bright point source was detected at the SN position. Given these results and the SN behavior, we conclude that SN 2010bt was likely a Type IIn SN and that its progenitor was a massive star that experienced an outburst shortly before the final explosion, leading to a dense H-rich circumstellar environment around the SN progenitor.
Keywords: galaxies: individual (NGC 7130) — stars: evolution — supernovae: general — supernovae: individual (SN 2010bt) SN 2010bt 3
1. INTRODUCTION
Type II “narrow” (IIn) supernovae (SNe) are a heterogeneous subset of core-collapse (CC) SNe
(Schlegel 1990). According to Li et al. (2011); Smith et al. (2011a), they represent 9% of all CC-SNe and seem to preferentially occur in small and late-type spiral galaxies. This subclass of objects can be distinguished from other types of CC-SNe by their spectral ap- pearance. On the other hand, they can also be confused with nonterminal outbursts of very massive stars. The broad absorption lines typical of many SNe are weak or absent in SNe IIn throughout their evolution. Instead, they show strong, narrow Balmer emission components (with full width at half-maximum intensity [FWHM] ranging from a few tens to a few hundreds of km s−1) atop broader emission (which can have intermediate-velocity components with FWHM ≈ 1000 kms−1, as well as broad components with FWHM of a few thousands of km s−1; see, e.g., Filippenko 1997). The narrow lines are thought to arise from the surrounding circumstellar material (CSM) ionized by the shock-interaction emission (e.g., Chugai & Danziger 1994). Their light curves, however, exhibit a wide range of properties (see, e.g., Smith 2017). This diversity is related to the mass loss history during the evolution of massive stars. It was suggested that the progenitors of a fraction of these interacting SNe are massive stars in a luminous blue variable (LBV) phase. These stars are among the most luminous (Mbol < −9.6 mag) and massive (> 50 M⊙) stars in late-type galaxies (e.g., Humphreys & Davidson 1994). The evidence arises from the identification of the progenitor stars of few SNe IIn, such as SN 2005gl (Gal-Yam et al. 2007; Gal-Yam & Leonard 2009), SN 2009ip (Smith et al. 2010; Foley et al. 2011; Smith et al. 2014), and SN 2015bh (Elias-Rosa et al. 2016; Th¨one et al. 2017). A luminous, blue point-like source, originating from either a young cluster or a single star, was first proposed as the progenitor of SN 2010jl by Smith et al. (2011c). Subsequently, Fox et al. (2017) argued that this source was most likely a massive young cluster, although they did not rule out the possibility of a very luminous progenitor star obscured by dust (see also Dwek et al. 2017). However, the nature of these progenitors is not fully clear, and other precursor channels have been proposed (see, e.g.,
Mauerhan & Smith 2012 in the case of the SN 1998S, or Smith 2014 for a larger review of this topic). 4 Elias-Rosa et al.
As mentioned above, the powerful nonterminal outbursts of very massive stars can mimic gen- uine SN explosions, in terms of energetics and spectral appearance (both show incipient narrow lines of hydrogen in emission); thus, they are usually referred to as “SN impostors” (e.g., SN 1997bs,
Van Dyk et al. 2000; SN 2000ch, Pastorello et al. 2010; see also the general discussions in Smith et al. 2011b and Van Dyk & Matheson 2012). Discriminating between SN impostors and SNe IIn is chal- lenging, and for several objects both possibilities remain viable (see, e.g., the case of SN 2007sv; Tartaglia et al. 2015).
SN 2010bt was discovered in NGC 7130 on 2010 April 17.11 (UT dates are used throughout this paper) at an unfiltered magnitude of 15.9, and confirmed on 2010 April 18.14 at a magnitude of 15.8 (Monard 2010; Figure 1). The presence of Balmer emission with multicomponent profiles in the spectrum from 2010 April 18.39 led to the classification of SN 2010bt as a SN IIn, a few days after discovery (Turatto et al. 2010). NGC 7130 (or IC 5135; z = 0.0161; morphological type Sa pec) was classified as a Seyfert 2 galaxy by Phillips et al. (1983) — i.e., an active galactic nucleus (AGN) with obscuring material that precludes a direct view of its nuclear region. Throughout the paper, we adopt a distance to NGC 7130 of 65.4 ± 4.6 Mpc (µ = 34.08 ± 0.15 mag), resulting from the recession velocity of the galaxy (de Vaucouleurs et al. 1976) corrected for Local Group infall into the Virgo cluster
−1 −1 −1 (Mould et al. 2000), vVir = 4771 ± 17 km s (z = 0.016), assuming H0 = 73 km s Mpc (values taken from NED).
This paper presents and discusses our photometric and spectroscopic observations of SN 2010bt in Section 2. In Section 3, we compare pixel-by-pixel four sets of Hubble Space Telescope (HST) data, and we discuss the nature of the progenitor star in Section 4. We conclude in Section 6.
2. THE NATURE OF SN 2010BT
2.1. Photometry
1 NED, NASA/IPAC Extragalactic Database; http://nedwww.ipac.caltech.edu/. SN 2010bt 5
Figure 1. V -band image of SN 2010bt in NGC 7130 obtained with the 1.3 m SMARTS tele- scope+ANDICAM at CTIO on 2010 May 18 (field of view ∼ 6′ × 6′). The SN and local photometric sequence stars are indicated.
Optical BV RI images of SN 2010bt were obtained with the 1.3 m Small & Moderate Aperture Research Telescope System (SMARTS)+ANDICAM at Cerro Tololo Inter-American Observatory (CTIO), the 6.5 m Magellan Clay Telescope+LDSS-3 at Las Campanas Observatory, and the 3.58 m
New Technology Telescope (NTT)+EFOSC2 at the European Southern Observatory (ESO) of La Silla Observatory, all located in Chile. The data were obtained thanks to a collaboration between American and European researchers. We also include in our dataset unfiltered data from the Bronberg Observatory (South Africa) and archival images from the 1.0 m Jacobus Kapteyn Telescope at Roque 6 Elias-Rosa et al. de Los Muchachos Observatory (Spain). More information about the telescopes and instruments used in this follow-up campaign can be found in Table 1. The photometric observations were processed following the standard recipe in iraf for CCD images
(trimming, overscan, bias, and flat-field corrections). Due to the location of SN 2010bt in NGC 7130, contamination of the SN photometry by the host-galaxy light was a serious problem. We there- fore used the template-subtraction technique to remove this background and hence to measure more accurately the SN magnitudes. The template images of NGC 7130 were obtained with the 1.3 m
SMARTS telescope+ANDICAM at CTIO on 2011 August 03. Each SN image was registered geomet- rically and photometrically with their corresponding template using a dedicated pipeline (SNOoPY). This consists of a collection of python scripts calling standard iraf tasks (through pyraf), and other specific analysis tools, in particular sextractor, for source extraction and star/galaxy sepa- ration. The instrumental magnitudes of the SN and the reference stars in the SN field were measured in the subtracted images (produced with hotpants) using the point-spread-function (PSF) fitting technique with the daophot package. In order to calibrate the instrumental magnitudes to the stan- dard photometric vegamag system, we used the magnitudes and colors of 15 local sequence stars in the SN field (Figure 1 and Table 2). The unfiltered data were transformed to the Johnson-Cousins R band, for which the effective wavelength is similar to the natural instrumental band defined by the CCD quantum efficiency of the detector that was used. Near-infrared (NIR) observations were also obtained with the 3.6 m NTT+SOFI at the ESO Ob- servatory of La Silla. We include in this work the data obtained at the 8.2 m unit telescope UT4 of the Very Large Telescope + HAWK-I at the ESO Observatory of Cerro Paranal (PI E. Cappellaro, 083.D-0259(A)), presented by Miluzio et al. (2013). The data were reduced using standard proce- dures for the VLT+HAWK-I data (see Miluzio et al. 2013 for details on the data reduction). For the NIR images we used two set of images of the field as templates to subtract from the SN images, depending on the instrumentation, e.g., images from ESO NTT+SOFI taken on 2004 October 04 (PI P. Lira, 074.B-0375(A)) and images from ESO VLT UT4+HAWK-I on 2011 April 24 (PI E. SN 2010bt 7
Cappellaro, 083.D-0259(A)). The instrumental JHKs photometry was calibrated using Two Micron All Sky Survey (2MASS2) stars in the field. When the transient was not detected, upper limits were adopted corresponding to 2.5 times the standard deviation in the background. Uncertainty estimates were obtained through an artificial star experiment, combined (in quadrature) with the PSF fit error returned by daophot and the propagated uncertainties from the photometric calibration. The final vegamag calibrated magnitudes of SN 2010bt are listed in Tables 3 and 4. The single epoch with magnitudes in the Sloan system were transformed to the vegamag system and included in Table 3 by employing the relations given by Blanton & Roweis (2007). Space-based optical and ultraviolet (UV) data taken with the Ultraviolet/Optical Telescope (UVOT) onboard the Neil Gehrels Swift Observatory (Swift) complement our ground-based pho- tometry. The calibrated SN images in the UVOT-Vega system were obtained from SOUSA (Swift’s Optical/Ultraviolet Supernova Archive; Brown et al. 2014). Upper limits corresponding to 3 times the standard deviation in the background were estimated when the transient was not detected. The Swift photometry for SN 2010bt is reported in Table 5.
Finally, HST observed the SN 2010bt field at four epochs: 1994, 2003, 2010, and 2015 (see Sections 3 and 4 for more details). The vegamag magnitudes of the progenitor candidate and transient were obtained using Dolphot (see Table 9).
SN 2010bt was observed during the first ∼ 50 (optical) to 80 (NIR) d after discovery. Subsequently it came too close to the Sun’s direction and was lost. The telescopes could point to the field again around two months after the last observation in the NIR; however, at that time the SN was no longer visible (at least with ground-based telescopes). Instead, it was detected in a F606W (∼ V )-band
HST exposure obtained ∼ 175 d after discovery. The UBV RIJHK light curves, including the UBV - UVOT data, are shown in Figure 2. In the figure, phase is relative to the discovery date on 2010
2 http://www.ipac.caltech.edu/2mass/. 8 Elias-Rosa et al.
April 17.10, or MJD 55303.1. However, in the following, and in order to better compare SN 2010bt with other SNe, we will refer to the V maximum computed as the discovery date minus 10 d (see Section 2.3 for more details).
As one can see in Figure 2, SN 2010bt was discovered after maximum light. The light curves show a decline similar to those of the rapidly declining SNe II studied by Anderson et al. 2014 (with an average V -band light-curve decline > 1.3 mag in the first 50 d after peak). In fact, the UBV light curves of SN 2010bt show constant decline rates of 4.3, 4.0, and 3.3 mag/50 d3, respectively, while the redder light curves (RI) slightly flatten out in brightness around day 30 after SN discovery. The K-band light curve was also slowly declining (2.0 mag/50 d, considering the interval from ∼ 38 d to 80 d after the discovery date). SN 2010bt was only detected in two epochs in the Swift UV W 1 band, with a similar decline to that of the bluer light curves (∼ 3.4 mag/50 d).
Figure 3 shows a comparison between the evolution of the absolute V magnitude of SN 2010bt, compared with the SNe IIn 1996al (Benetti et al. 2016), 1998S (Liu et al. 2000; Fassia et al. 2000; Pozzo et al. 2004), and the SNe IIn/SN impostors 2009ip (Pastorello et al. 2013; Fraser et al. 2013; Mauerhan et al. 2013; Margutti et al. 2014) and 2015bh (Elias-Rosa et al. 2016). The comparison
SNe have been corrected for extinction using published estimates and assuming the Cardelli et al. (1989) extinction law (see also Table 6). SN 2010bt exhibits a rapid decline at early times, similar to the decline from the “b” event of SN 2009ip and slightly faster than SN 1996al. This decline rate is also reminiscent of rapidly declining SNe II or even of SNe IIb (see, e.g., Li et al. 2011; Anderson et al.
2014). The absolute V magnitude at maximum of SN 2010bt was −19 mag or brighter, consistent with both the typical V -band peak magnitudes of SNe IIn (MV = −18.4 ± 1.0 mag; Kiewe et al.
2012) and of SNe II-P/L (MV = −16.89 ± 0.98 mag; Galbany et al. 2016).
The early-time (B − V )0, (V − R)0, and (R − I)0 color curves of SN 2010bt (see Figure 4) exhibit rapid evolution from blue to red, similar to the color evolution of the other SNe IIn, yet different from SN 1996al.
3 Considering the interval from ∼ 1 d to 50 d after the discovery date. SN 2010bt 9
We have computed the pseudobolometric light curve of SN 2010bt by integrating the flux at differ- ent wavelengths derived from the extinction-corrected optical apparent magnitudes over the sparse observations in the UV W 1 through K bands. Fluxes were measured considering only the epochs when V -band observations were available. When photometric measurements in one band at given epochs were not available, the flux was estimated by interpolating magnitudes from epochs close in time or, when necessary, by extrapolating the missing photometry assuming a constant color. We estimated the pseudobolometric flux at each epoch by integrating the spectral energy distribution
(SED) using the trapezoidal rule and assuming zero flux outside the integration boundaries. Finally, the effective fluxes were converted to luminosities using the adopted distance to the SN (see Sec- tion 1). The errors in the bolometric luminosity include the uncertainties in the distance estimate, the extinction, and the apparent magnitudes. Note that the UV W 1 band provides about 13% of the luminosity. In Figure 5 we present the pseudobolometric light curve of SN 2010bt as well as those of SNe 1996al, 1998S, 2009ip, and 2015bh computed with the same prescriptions (for SNe 1996al and 1998S we have no observations in UV bands). As shown earlier, the luminosity decline of SN 2010bt is similar to that of SN 2009ip, while it was more luminous than all other SNe in the sample, with the possible exception of SN 1998S. Considering the first epoch in the V band for SN 2010bt, its peak may have reached a luminosity > 1.3 × 1043 erg s−1 (SN 1998S had a luminosity at peak of 1.6 × 1043 erg s−1). Assuming that the tail of SN 2010bt followed the radioactive 56Co decay with full trapping, and considering the explosion date to be between 15 and 50 d before the discovery date (see Section 2.3) and the HST detection in F606W (∼ V ) at 175.3 d (from discovery), we can roughly estimate
56 ≤ 0.03 ± 0.01 M⊙ for the Ni mass, using the formula given by Hamuy (2003). This value is at the
56 low end in comparison with typical values for CC-SNe, 0.001–0.3 M⊙ (Hamuy 2003). The Ni mass of SN 2010bt is also in agreement to what was estimated for the interacting SNe 1996al, 2009ip, and
2015bh (MNi ≤ 0.02, ≤ 0.08, and ≤ 0.04 M⊙, respectively; Benetti et al. 2016; Fraser et al. 2013; Margutti et al. 2014; Smith et al. 2014; Elias-Rosa et al. 2016). 10 Elias-Rosa et al.
K -5 12 H -4 J -3 I -2 14 R -1 V B +1 U +2 16
18
20 Apparent magnitude Apparent
22
0 50 100 150 200 Phase from discovery [d]
Figure 2. Optical and NIR light curves of SN 2010bt. Upper limits are indicated by symbols with arrows. The solid marks on the abscissa indicate the phases at which spectra were obtained. The dashed lines show the slopes of the light curves during the first 20 days from discovery. The light curves have been shifted for clarity by the amounts indicated in the legend. The uncertainties for most of the data points are smaller than the plotted symbols. SN 2010bt 11 hile Chile ). 1 − g Observatory, South Africa , Chile , La Palma, Spain c , La Silla Obs., Chile g d pixel ′′ ··· ··· ··· ··· ··· ··· 05 . ) 1 − e pixel ′′ ( 0.50 0.33 ORM e Planetary Camera (0 f ANDICAM 0.37 CTIO IRACSwift 0.60 WFPC2ACS/HRCACS/WFC 0.05 WFC3/UVIS 0.03 0.05 0.04 Basic Information about Telescopes and Instruments Used Telescope Instrument Pixel Scale Location b Table 1. Spitzer Space Telescope HST HST HST HST . 5 , and 4 , a 3 WFPC2 2.4 m ACS/HRC 2.4 m ACS/WFC 2.4 m WFC3 2.4 m Table key See Tables Small & Moderate Aperture Research TelescopeCerro System. Tololo Inter-American Observatory. European Southern Observatory. WFPC2 contains four chips. The SN 2010bt field was observed with th JKT Acquisition and Guidance Unit. Observatorio del Roque de los Muchachos. EFOSC2HAWK-I 3.6mNewTechnologyTelescopeHST 8.2 m Very Large Telescope-UT4 EFOSC2 HAWK-I 0.12 0.11 ESO ESO, La Silla Obs., BO 0.3mMeadeRCX400f/8telescope SBIGST8-XMECCD1.37 Bronber ANDICAM 1.3 m SMARTS a b c d e Swift 0.3 m Ritchey-Chretien UV/optical Telesc. LDSS-3SOFI 6.5 mSpitzer Magellan Clay Telescope 3.6 m New Technology Telescope 0.8 m LDSS-3 SOFI 0.19 0.29 Las Campanas Obs., C ESO, La Silla Obs., Chile HST HST JAG 1.0mJacobusKapteynTelescope JAG HST f g 12 Elias-Rosa et al.
Table 2. Magnitudes of the Local Comparison Starsa
Star BVRIJHK
(mag) (mag) (mag) (mag) (mag) (mag) (mag)
1 19.86 (0.07) 19.04 (0.05) 18.51 (0.03) 17.96 (0.04) ·········
2 20.24 (0.07) 18.70 (0.05) 17.60 (0.07) 16.19 (0.04) 14.81 (0.03) 14.14 (0.02) 13.85 (0.04)
3 21.28 (0.07) 19.66 (0.04) 18.30 (0.04) 16.51 (0.05) 14.84 (0.03) 14.20 (0.04) 13.92 (0.05)
4 18.68 (0.04) 17.80 (0.04) 17.20 (0.03) 16.62 (0.03) 15.91 (0.07) 15.40 (0.09) 15.38 (0.13)
5 18.13 (0.04) 17.40 (0.03) 16.92 (0.02) 16.51 (0.03) 15.90 (0.07) 15.54 (0.08) 15.43 (0.14)
6 19.23 (0.04) 18.28 (0.04) 17.65 (0.04) 17.10 (0.03) 16.56 (0.11) 15.96 (0.11) 15.52 (0.17)
7 17.31 (0.03) 16.49 (0.03) 16.00 (0.03) 15.54 (0.03) 14.86 (0.03) 14.27 (0.04) 14.29 (0.05)
8 19.61 (0.05) 19.15 (0.06) 18.89 (0.04) 18.51 (0.04) ·········
9 20.63 (0.04) 19.19 (0.04) 18.35 (0.04) 17.18 (0.04) 15.94 (0.06) 15.32 (0.07) 15.33 (0.13)
10 19.97 (0.06) 18.68 (0.03) 17.87 (0.03) 17.05 (0.02) 15.96 (0.08) 15.40 (0.07) 15.29 (0.13)
12 17.08 (0.03) 16.46 (0.02) 16.12 (0.04) 15.73 (0.02) 15.26 (0.04) 14.84 (0.05) 14.76 (0.09)
12 20.12 (0.04) 19.18 (0.05) 18.62 (0.05) 18.12 (0.07) ·········
13 20.97 (0.03) 19.58 (0.01) 18.76 (0.02) 17.79 (0.04) 16.65 (0.11) 16.20 (0.15) 15.38 (0.15)
14 18.99 (0.04) 18.47 (0.04) 18.11 (0.04) 17.80 (0.03) ·········
15 15.99 (0.04) 14.98 (0.03) 14.38 (0.06) 13.83 (0.05) 12.93 (0.03) 12.41 (0.02) 12.33 (0.03) aQuoted uncertainties are 1σ. SN 2010bt 13
Table 3. Optical Johnson-Cousins Photometry of SN 2010bt (vegamag)a
Date MJD Phaseb U B V R I Instrument key
(days) (mag) (mag) (mag) (mag) (mag)
20010813 52134.0 -3169.1 ········· > 18.0 ··· JAG
20100417 55303.1 0.0 ········· 16.00 (0.41) ··· BO
20100418 55304.1 1.0 ········· 16.08 (0.18) ··· BO
20100418 55304.4 1.3 ··· 17.00 (0.08) 16.33 (0.08) 16.05 (0.10) 15.79 (0.12) EFOSC2
20100420 55306.4 3.3 ··· 17.11 (0.03) 16.54 (0.03) 16.26 (0.04) 15.88 (0.04) ANDICAM
20100420 55306.9 3.8 16.88 (0.14) 17.33 (0.15) 16.58 (0.17) ······ Swift
20100422 55308.1 5.0 17.09 (0.17) 17.35 (0.16) 16.42 (0.15) ······ Swift
20100423 55309.4 6.2 ··· 17.42 (0.08) 16.81 (0.07) 16.50 (0.05) 16.17 (0.04) ANDICAM
20100424 55310.2 7.0 17.26 (0.19) 17.58 (0.18) 17.09 (0.24) ······ Swift
20100426 55312.8 9.7 17.71 (0.26) 17.75 (0.22) ········· Swift
20100427 55313.3 10.2 ··· 17.89 (0.18) 17.16 (0.16) 16.73 (0.09) 16.39 (0.14) ANDICAM
20100428 55314.7 11.6 17.71 (0.26) 17.77 (0.20) 17.25 (0.26) ······ Swift
20100430 55316.0 12.9 > 18.3 ············ Swift
20100430 55316.0 12.9 ··· 18.23 (0.28) 17.46 (0.30) ······ Swift
20100430 55316.4 13.3 ··· 18.11 (0.15) 17.36 (0.13) 16.89 (0.10) 16.66 (0.09) ANDICAM
20100501 55317.9 14.7 ······ > 17.8 ······ Swift
20100502 55318.9 15.7 18.15 (0.35) 18.16 (0.27) ········· Swift
20100503 55319.4 16.3 ··· 18.24 (0.14) 17.52 (0.12) 16.95 (0.09) 16.70 (0.10) ANDICAM
20100504 55320.9 17.8 18.27 (0.39) 18.46 (0.34) 17.69 (0.38) ······ Swift
20100506 55322.1 19.0 ········· 17.18 (0.14) ··· BO
Table 3 continued on next page 14 Elias-Rosa et al. Table 3 (continued)
Date MJD Phaseb U B V R I Instrument key
(days) (mag) (mag) (mag) (mag) (mag)
20100506 55322.2 19.1 18.10 (0.35) 18.27 (0.30) ········· Swift
20100506 55322.2 19.1 ······ > 17.7 ······ Swift
20100507 55323.4 20.2 ··· 18.56 (0.06) 17.79 (0.05) 17.30 (0.04) 16.96 (0.06) ANDICAM
20100508 55324.5 21.4 > 18.3 > 18.6 ········· Swift
20100510 55326.3 23.2 ··· 18.98 (0.13) 17.98 (0.11) 17.52 (0.06) 17.22 (0.06) ANDICAM
20100511 55327.1 24.0 ········· > 16.7 ··· BO
20100518 55334.4 31.3 ··· 19.58 (0.11) 18.59 (0.09) 17.98 (0.06) 17.69 (0.07) ANDICAM
20100519 55335.4 32.3 ··· 19.60 (0.08) 18.62 (0.07) 18.01 (0.06) 17.68 (0.06) ANDICAM
20100523 55339.3 36.2 ··· 19.84 (0.14) 18.84 (0.12) 18.19 (0.08) 17.91 (0.12) ANDICAM
20100524 55340.1 37.0 ········· > 16.5 ··· BO
20100524 55340.3 37.2 ··· 19.89 (0.14) 18.74 (0.12) 18.23 (0.11) 17.95 (0.12) ANDICAM
20100526 55342.3 39.2 ··· 20.10 (0.12) 19.04 (0.11) 18.34 (0.07) 18.03 (0.08) ANDICAM
20100531 55347.4 44.2 ··· 20.47 (0.13) 19.22 (0.12) 18.48 (0.07) 18.16 (0.09) ANDICAM
20100604 55351.3 48.2 ··· 20.79 (0.13) 19.48 (0.11) 18.63 (0.10) 18.24 (0.07) ANDICAM
20100613 55360.1 57.0 ········· > 16.6 ··· BO
20100915 55454.2 151.1 ··· > 19.8 > 19.8 > 18.9 > 19.7 EFOSC2
20101004 55473.1 170.0 ··· > 20.4 > 20.0 > 19.3 > 18.7 EFOSC2
20101009 55478.4 175.3 ······ 22.50 (0.05) ······ HST ACS/WFC
20101028 55497.2 194.1 ··· > 21.3 > 21.0 > 20.2 > 19.7 EFOSC2
20150917 57283.0 1979.9 ··· > 18.6 ··· > 18.5 ··· LDSS-3
20151014 57309.0 2005.9 ······ > 24.4 ······ HST WFC3
aQuoted uncertainties are 1σ. bPhases are relative to the discovery, MJD = 55303.1. SN 2010bt 15
Table 4. Near-Infrared Photometry of SN 2010bt (vegamag)a
Date MJD Phaseb JHK Instrument key
(days) (mag) (mag) (mag)
20100525 55341.0 37.9 ······ 16.69 (0.23) HAWK-I
20100606 55353.0 49.9 ······ 17.51 (0.31) HAWK-I
20100707 55384.0 80.9 ······ 18.49 (0.22) HAWK-I
20100917 55456.1 153.0 > 18.7 > 17.4 > 18.9 SOFI
20101029 55498.1 195.0 > 17.2 > 15.4 ··· SOFI aQuoted uncertainties are 1σ. bPhases are relative to the discovery, MJD = 55303.1.
2.2. Spectroscopy
Four low-resolution optical spectra of SN 2010bt were obtained on 2010 April 18 (classification spectrum), September 15 and 16, and October 28 with the 3.58 m NTT+EFOSC2 at ESO of La Silla
(Chile). Only the first spectrum was taken with the slit along the parallactic angle to avoid differential flux losses (Filippenko 1982). For the other spectra it was necessary to point to a nearby bright star, and then rotate the slit to position the SN 2010bt site inside the aperture. Basic information on our spectra is reported in Table 7.
All spectra were reduced following standard procedures with iraf routines. The two-dimensional (2D) spectroscopic frames were debiased and flat-fielded, before the optimized extraction (Horne 1986) of the 1D spectra. Wavelength calibration was accomplished with the help of arc-lamp ex- posures obtained the same night. Small adjustments estimated from night-sky lines in the object frames were applied. The spectra were flux calibrated using the well-exposed continua of spectropho- tometric standard stars (Oke 1990; Hamuy et al. 1992, 1994). An atmospheric extinction correction was applied using tabulated extinction coefficients for the ESO-La Silla Observatory. The strongest 16 Elias-Rosa et al.
Table 5. Swift UV Photometry of SN 2010bt (vegamag)a
Date MJD Phaseb UVW1 UVM2 UVW2
(days) (mag) (mag) (mag)
20100420 55306.9 3.8 > 18.5 > 18.8 > 18.9
20100422 55308.1 5.0 18.20 (0.34) ······
20100422 55308.2 5.0 ··· > 18.8 > 18.9
20100424 55310.2 7.0 18.48 (0.42) ······
20100424 55310.2 7.0 ··· > 18.7 > 18.9
20100426 55312.8 9.7 > 18.5 ··· > 18.4
20100428 55314.7 11.6 > 18.5 > 18.3 > 18.9
20100430 55316.0 12.9 > 18.6 > 18.9 > 19.0
20100502 55318.9 15.7 > 18.6 > 18.9 > 19.0
20100504 55320.9 17.8 > 18.5 > 18.8 > 18.9
20100506 55322.2 19.1 > 18.5 > 18.9 > 18.9
20100508 55324.5 21.4 > 18.5 > 18.9 > 18.9 aQuoted uncertainties are 1σ. bPhases are relative to the discovery, MJD = 55303.1.
telluric absorption bands were removed using the standard-star spectra. Finally, the flux of each spectrum was cross-checked against the photometry. Figure 6 shows the sequence of optical spectra of SN 2010bt4, and in Figures 7 and 8 we com- pare some of these spectra with those of SNe IIn 1996al (Benetti et al. 2016), 1998S (Leonard et al.
2000), and the transients SN 2009ip (Pastorello et al. 2013; Fraser et al. 2013) and SN 2015bh
4 Our spectra are available on WiseREP (Yaron & Gal-Yam 2012). SN 2010bt 17
Table 6. Properties of the Comparison Supernovae
† SN Host galaxy Redshift Distance E(B − V )tot Vmax Date Source
(Mpc) (mag) (MJD)
1996al NGC 7689 0.007 22.9 0.110 50265.0 a
1998S NGC 3877 0.002 15.7 0.219 50890.5 b
2009ip NGC 7259 0.006 25.0 0.019 56203.5 c
2015bh NGC 2770 0.007 29.3 0.208 57167.0 d
2010bt NGC 7130 0.016 65.4 (4.6) 0.40 (0.14) 55293.1‡ This work † −1 −1 Distances have been scaled to H0 = 73 kms Mpc . ‡We have assumed V maximum date = discovery date (MJD 55303.1) −10 d. Sources: a, Benetti et al. (2016); b, Fassia et al. (2000), Leonard et al. (2000), Pozzo et al. (2004), NED; c, Pastorello et al. (2013), Fraser et al. (2013), Mauerhan et al. (2013), Margutti et al. (2014); d, Elias-Rosa et al. (2016).
(Elias-Rosa et al. 2016) at similar epochs. All of the spectra have been corrected for extinction and deredshifted using values from the literature (see also Table 6). The early-time spectrum (Figure 6, top panel) exhibits a blue continuum, with relatively weak spectral features, except for the strong Balmer emission lines with complex, yet relatively narrow profiles. SN 2010bt does not show signs of the blue pseudocontinuum that characterizes some of the most energetic SNe IIn such as SN 1988Z (Turatto et al. 1993; Kiewe et al. 2012). The pseudocon- tinuum is a “blue excess” arising from the overlap of emission lines when the expanding ejecta of the SN interact with the surrounding circumstellar material. The lack of a pseudocontinuum indicates that for SN 2010bt, the continuum is thermal with a blackbody-like behavior. Balmer emission lines show P-Cygni absorption components with expansion velocities of 4000–3500 km s−1, estimated from their absorption minima. A blend of He i λ5876 and Na i D is also visible in the spectrum, with the broad emission centered at ∼ 5950 A˚ and with a very broad FWHM
≈ 290 A.˚ The photospheric temperature at this epoch, estimated by fitting the SED of SN 2010bt 18 Elias-Rosa et al.