Supernova 2018aoq and a distance to Seyfert galaxy NGC 4151

ABSTRACT

We present optical photometric observations of SN 2018aoq from 2 to 100 d after explosion, and 7 spectra at epochs from 11 to 71 d. The light curves and spectra are typical for SNe II-P. As previously reported, SN 2018aoq appears to be of intermediate brightness between subluminous and normal SNe II-P. SN 2018aoq was discovered in Seyfert galaxy NGC 4151, for which the distance is uncertain. We utilized the expanding photosphere method using three sets of filter combinations and velocities derived from the absorption minima of Fe ii lines and obtained a distance of 20.0 ± 1.6 Mpc. The standard candle method applied to SN 2018aoq yields a distance of 16.6 ± 1.1 Mpc. Both values are consistent with the distance measurements for NGC 4151 based on geometric method.

supernovae: individual: SN 2018aoq, galaxies: individual: NGC4151

1 INTRODUCTION

NGC 4151 is well-known Seyfert 1 galaxy, one of the nearest galaxies with active nucleus.

NGC 4151 is one of the rare objects for which there exist two independent dynamical measurements for the mass of the central black hole. To first order, the black hole mass derived by the stellar dynamical modelling depends linearly on the assumed distance to the galaxy (Onken et al. 2014).

However, the actual distance to the galaxy is rather uncertain.

The Extragalactic Distance Data base (Tully et al. 2009) presents distance measurements based on the Tully–Fisher relation: the individual estimate for NGC 4151 is 3.9 ± 0.4 Mpc, and the group-average distance is 11.2 ± 1.1 Mpc. The reability of these distance estimates is doubtful, as discussed by Onken et al. (2014).

The reprocessing of the emission of the active nucleus by the accretion disc provides another method for measuring distance to NGC 4151. The correlation of wavelength-dependent time-delays with the luminosity of the central engine yielded estimates of 19 Mpc (Cackett, Horne & Winkler 2007) and 29 Mpc (Yoshii et al. 2014). Hönig et al. (2014) applied a geometric method, measuring the size of region of hot dust emission as determined from time-delays and infrared interferometry, which yielded 19.0 ± 2.5 Mpc.

The discovery of a type II-P supernova (SN) 2018aoq in NGC 4151 presents a new possibility to obtain an independent estimate of the distance to the galaxy.

The optical transient Kait-18P = 2018aoq was discovered on 2018-04-01.4316 by the Lick Observatory Supernova Search at the unfiltered magnitude of 15.3 at a distance of 73 arcsec from the centre of NGC 4151. Spectroscopic observations with the 1.5-m Kanata telescope classified the transient as a Type II SN.¹

Observations of type II-P SNe can be used to determine distances to their host galaxies using the expanding photosphere method (EPM), which was first developed by Kirshner & Kwan (1974). The method is based on measuring the angular radius of the photosphere from photometric data and comparing the resulting expansion rate to the velocity extracted from the spectral data. The EPM provides estimates of distance independent of extragalactic distance ladder. The method requires high-quality spectroscopic and photometric monitoring of SNe and was applied mostly to nearby objects (e.g. Hamuy et al. 2001; Takáts & Vinkó 2006; Jones et al. 2009; Bose & Kumar 2014), although recently it became possible to perform the EPM on SNe at cosmologically significant redshifts (e.g. Gall et al. 2016, 2018). The other method for distance determinations using SNe II-P is the standardized candle method (SCM) (Hamuy & Pinto 2002), based on a correlation between the brightness and the expansion velocity of SNe during the plateau phase. This method relies on the local distance calibrators and yields distances that are in reasonable agreement with the EPM (e.g. Nugent et al. 2006; Poznanski et al. 2009; Olivares et al. 2010; Gall et al. 2018).

2 OBSERVATIONS

Photometric UBVRI CCD observations of SN 2018aoq were carried out at the 60-cm and 50-cm telescopes of Crimean Observatory of Sternberg Astronomical Institute (SAI), the 70-cm and 20-cm telescopes of Moscow Observatory of SAI, the 1-m telescope of Institute of Astronomy of Russian Academy of Science (INASAN) at Simeiz Observatory, the 60-cm telescope of Stará Lesná Observatory of the Astronomical Institute of Slovak Academy of Science, and the 60-cm telescope of Shamakhy Astrophysical Observatory.

The standard image reductions and photometry were made using the iraf.²The magnitudes of the SN were derived by a PSF-fitting relatively to a sequence of local standard stars, which were calibrated by Lyutyi (1973), Doroshenko et al. (2005), and Roberts & Rumstay (2012). The photometry was transformed to standard Johnson–Cousins UBVRI magnitudes by means of instrumental colour terms.

The surface brightness of the host galaxy at the location of the SN is low, nevertheless we checked if the galaxy background affects the photometry. We used the images obtained before SN outburst at the Shamakhy Observatory for galaxy subtraction. We found that for most of the images the effect of galaxy background does not exceed the errors of magnitudes, but for the 50-cm telescope of SAI it may amount to 0.05–0.1 mag. We applied galaxy subtraction for all images obtained with this telescope.

The magnitudes of standard stars are presented in Table A1, the photometric data are presented in Table A2.

Prediscovery observations were reported by Nazarov et al. (2018). We carried out photometry on their images, using our local standard stars and applying galaxy subtraction, and obtained new magnitude estimates, which are also reported in Table A2. The light curves are shown in Fig. 1.

Figure 1.

The light curves of SN 2018aoq. The phase is in reference to the explosion date JD 2458 208. The error bars are shown only if they exceed the size of a symbol.

The shape of the light curves is typical for SNe II-P. The first observations were obtained on the rising part of the light curves, and we can determine the epoch when the SN reached the plateau phase as JD 2458 215 ± 1 (April 6). Yamanaka et al. (2018) obtained images of NGC 4151 on JD 245 8209.0 (March 31.5) and derived an upper limit of 17.5 mag in the R band, O’Neill et al. (2019) reported that the SN was fainter than 18.89 mag in the ‘orange’ ATLAS filter on JD 245 8206.97 (March 29). We used a polynomial fit to the R-band magnitudes on the rise and found that the best estimate for the epoch of explosion is JD 2458 208 ± 1, 7 d before start of the plateau. This value of rise time is in agreement with the average rise time for SNe II-P reported by Gall et al. (2015).

The colours of SN 2018aoq at maximum light (JD 2458215) were quite blue: U − B = −0.76 mag, B − V = −0.12 mag. The blue colours and the absence of detectable interstellar lines in the spectra allow to conclude that the absorption in the host galaxy was negligible. The galactic extinction is small E(B − V)_gal = 0.02 mag (Schlafly & Finkbeiner 2011). O’Neill et al. (2019) compared the colour curves of SN 2018aoq to those of type II-P SNe for which the extinction is well-known, and derived the total extinction for SN 2018aoq E(B − V)_tot = 0.04 mag, only slightly larger than E(B − V)_gal. We used E(B − V)_tot = 0.04 for all further calculations.

Spectroscopic observations were obtained at the 2-m telescope of Shamakhy Astrophysical Observatory. The modified Universal Astronomical Grating Spectrograph provided the wavelength range of 3900–7000 Å with a dispersion of 115 Å mm⁻¹, which corresponds to 4.1 or 8.2 Å pixel⁻¹ for different CCD binning. The journal of spectroscopic observations is presented in Table A3, the spectra are shown in Fig. 2.

Figure 2.

The spectra of SN 2018aoq. The ages are relative to the date of explosion (JD 2458 208). The vertical dashed lines indicate absorption minima of Fe ii lines used for the EPM. These lines are not detected in the first spectrum, which was not used for the EPM.

We continue the observations of SN 2018aoq, the complete set of data and its analysis will be presented in a separate paper.

3 THE EPM DISTANCE

The EPM (Kirshner & Kwan 1974) determines a distance D to the SN from the relation θ = R/D, where θ is the angular radius of photosphere, R is its linear radius.

The method can be applied if the ejecta is spherically symmetric, the envelope undergoes free expansion, so that the velocity of matter v and the radial distance r are connected by v = r/(t − t₀), where t₀ is the zero-point time, which might be offset from the true moment of explosion.

The photospheric flux of SN is described by a modified Planck spectrum F_ν(R) = ζ²πB_ν(T_col), where ζ is the correction factor, T_col is the colour temperature, and B_ν(T_col) is the Planck function.

The description of the EPM is presented in a number of papers, (e.g. Hamuy et al. 2001; Takáts & Vinkó 2006; Jones et al. 2009; Gall et al. 2018). We applied the EPM following the prescriptions of Hamuy et al. (2001).

The correction factor ζ cannot be determined from observations. The empirical relations between ζ and T_col were established by Eastman, Schmidt & Kirshner (1996) and Dessart & Hillier (2005). We used the relation by Dessart & Hillier (2005), which is confirmed by our research (Baklanov, in preparation) and by Vogl et al. (2019).

We used three sets of filter combinations to derive the temperature and angular radius of SN photosphere. The errors in quantities θ and T_col were estimated using Monte Carlo technique. Samples of data points were drawn from normal distributions of uncertainty in the photometric fluxes.

The velocity of matter at the photosphere v_ph can be measured by the blueshift of weak absorption lines, the lines of Fe ii λ5018 Å and λ5169 Å are used more often (Takáts & Vinkó 2012). The observed spectra were corrected for the redshift of the galaxy z = 0.003 32³ and continuum subtracted, using polynome fitting with the snid package (Blondin & Tonry 2007). The spectra were smoothed with a Savitzky–Golay filter (Savitzky & Golay 1964) and wavelengths of absorption minima were determined. The uncertainties of velocity measurements were estimated to be in the range of 4–6 per cent, depending on the spectral resolution and photon statistics of the detector.

The computations were carried out for three sets of filter combinations: BVI, BV, and VI, for the velocities derived from the lines Fe iiλ5018, λ5169 and for the average velocities.

Table 1 presents the basic EPM quantities. T_col, ζ, and θ are given only for the BVI filter set, for other sets they are similar.

Table 1.

The EPM quantities derived for SN 2018aoq. The uncertainties are in parentheses.

JD	Phase,	v(Fe iiλ5018)	v(Fe iiλ5169)	Tcol(BVI)	ζ	θ
2458 200 +	d	km s−1	km s−1	K		10−11 rad
34.36	26.4	3529 (140)	3893 (154)	7022 (225)	0.662	1.62 (0.11)
35.33	27.3	3529 (234)	3893 (231)	6968 (212)	0.667	1.63 (0.10)
46.35	38.4	3125 (185)	3083 (183)	6245 (181)	0.747	1.80 (0.12)
52.45	45.5	2707 (108)	2680 (106)	5975 (184)	0.786	1.88 (0.14)
56.40	48.4	2706 (108)	2666 (106)	5703 (187)	0.833	1.95 (0.17)
79.39	71.4	1903 (75)	1862 (90)	5365 (135)	0.905	1.94 (0.13)

The ratio θ/v as a function of time for the filter sets BVI, BV, and VI is presented in Fig. 3.

Figure 3.

The ratio θ/v as a function of time for three filter sets, for average velocity.

We determined t₀ and D using the Markov Chain Monte Carlo method in the emcee software package (Foreman-Mackey et al. 2013).

The results are presented in Table 2.

Table 2.

The EPM distances for SN 2018aoq. The uncertainties are in parentheses.

Filter set	Fe ii line	D, Mpc	t0, JD 2458 000 +
BVI	5018	21.4 (2.6)	200.3 (5.5)
BVI	5169	19.4 (1.7)	205.8 (3.2)
BVI	Average	20.2 (2.1)	203.5 (4.3)
BV	5018	19.9 (3.9)	203.3 (7.7)
BV	5169	19.7 (3.6)	204.3 (6.5)
BV	Average	20.0 (3.9)	203.3 (7.6)
VI	5018	21.0 (3.3)	199.2 (7.0)
VI	5169	19.1 (2.2)	204.6 (4.4)
VI	Average	19.8 (2.7)	202.4 (5.6)

4 THE SCM DISTANCE

The SCM (Hamuy & Pinto 2002) is based on a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe ii P-Cygni feature observed during the plateau phase.

We used our estimates of expansion velocity from the shift of Fe iiλ5169 line and photometry in the VI bands and applied the SCM using the calibration by Polshaw et al. (2015), based on the Cepheid distances to well-observed SNe II-P. We obtain distance estimates D_V = 16.7 ± 1.4 Mpc, D_I = 16.5 ± 1.3 Mpc, and the average D = 16.6 ± 1.1 Mpc.

5 DISCUSSION

All distance estimates presented in Table 2 are consistent with each other, and we may accept the average value D = 20.0 ± 1.6 Mpc as the EPM distance for SN 2018aoq and NGC 4151, which is in good agreement with the result of Hönig et al. (2014).

The estimates of t₀ are earlier than the explosion epoch derived from photometry, but for most of the data the difference does not exceed the uncertainties. We should note that the epoch t₀ from the EPM fit may be offset from the explosion date (Takáts & Vinkó 2006).

Recently O’Neill et al. (2019) utilized the SCM method for SN 2018aoq as calibrated by Polshaw et al. (2015) to obtain a distance of 18.2 ± 1.2 Mpc. Our SCM distance is about 9 per cent shorter than the result of O’Neill et al. (2019), because of small differences in the observational data.

The expansion velocity of SN 2018aoq is low, about 2600 km s⁻¹ at 50 d past explosion. The luminosity at the plateau is M_I = −16.4 mag for the EPM distance, and M_I = −16.0 mag for the SCM distance. SN 2018aoq appears to be an intermediate object between subluminous and normal SNe II-P, as was suggested by O’Neill et al. (2019).

The distance measurements by the EPM and SCM may have systematic errors, for the EPM they result from the adopted values of dilution factor ζ, which may also be a function of chemical composition and density structure of the envelope. Other reasons for uncertainty are the difference of photospheric velocity from that derived from the Fe ii lines and absence of spherical symmetry of the ejecta. The major sources of errors for the SCM are the calibration process and the diversity of properties for SNe II-P.

In the case of SN 2018aoq the SCM distance is about 18 per cent shorter than the EPM distance, but both values are consistent with the most reliable distance estimate for the host galaxy D = 19.0 ± 2.5 Mpc, based on geometric technique (Hönig et al. 2014).

We may conclude that these results confirm the applicability of SNe II-P for distance measurements.

ACKNOWLEDGEMENTS

The work of DYT and PVB was partly supported by the Russian Science Foundation Grant No. 16-12-10519. The work of SS was partially supported by Grants VEGA 2/0008/17 and APVV-15-0458. The work of IV was supported by the scholarship of the Slovak Academic Information Agency (SAIA), by the Russian Science Foundation Grant No. 14-12-00146 and Russian Foundation for Basic Research Grant No. 18-502-12025. The work on photospheric velocity determination was done by MSP and was supported by the Russian Science Foundation Grant No.19-12-00229. We thank the anonymous referee for constructive suggestions which helped to improve the paper.

Footnotes

1

https://wis-tns.weizmann.ac.il/object/2018aoq

2

iraf is distributed by the National Optical Astronomy Observatory, which is operated by AURA under cooperative agreement with the National Science Foundation.

3

https://ned.ipac.caltech.edu/

APPENDIX A: DATA TABLES

Table A1.

UBVRI magnitudes of local standard stars.

α(J2000)	δ(J2000)	U	B	V	R	I	References
12:09:53.538	39:26:00.15	11.68	9.76	8.15			1
12:10:31.442	39:26:41.26	13.24	12.47	11.45	10.92	10.44	1,2,3
12:10:25.255	39:23:55.45		15.08	14.38	13.99	13.66	3
12:10:38.529	39:20:29.56	15.61	14.01	12.77	12.10	11.50	1,2

Note. (1) Lyutyi (1973), (2) Doroshenko et al. (2005), (3) Roberts & Rumstay (2012).

Table A2.

UBVRI magnitudes of SN2018aoq.

JD	Phase,	U	σU	B	σB	V	σV	R	σR	I	σI	Telescope
2458 000 +	d
209.46	1.5			15.59	0.04	15.73	0.04	15.49	0.06	15.82	0.09	K70
213.30	5.3			15.51	0.06	15.44	0.08	15.28	0.05			M20
213.39	5.4			15.44	0.04	15.47	0.03	15.29	0.03	15.16	0.04	C50
213.42	5.4			15.30	0.04	15.43	0.04	15.23	0.08	15.22	0.05	C60
214.39	6.4			15.32	0.04	15.43	0.05	15.20	0.04	15.16	0.04	C60
215.32	7.3	14.52	0.05	15.28	0.04	15.40	0.04	15.19	0.04	15.15	0.04	C60
216.33	8.3	14.57	0.05	15.33	0.04	15.42	0.03	15.21	0.04	15.16	0.04	C60
217.32	9.3	14.58	0.07	15.45	0.05			15.26	0.03	15.21	0.04	C60
217.52	9.5	14.77	0.05	15.54	0.04	15.46	0.03	15.24	0.04	15.23	0.04	T60
218.36	10.4	14.79	0.05	15.44	0.04	15.46	0.04	15.25	0.04	15.20	0.04	C60
219.35	11.4	14.84	0.05	15.50	0.04	15.50	0.04	15.24	0.04	15.20	0.04	C60
220.27	12.3							15.26	0.05	15.25	0.09	M20
221.33	13.3	15.14	0.14	15.50	0.06	15.51	0.03	15.22	0.03	15.14	0.04	C60
222.34	14.3	15.20	0.13	15.55	0.05	15.53	0.03	15.16	0.05	15.17	0.03	C60
223.28	15.3					15.52	0.05	15.24	0.05	15.13	0.09	M20
224.31	16.3	15.36	0.05	15.67	0.04	15.52	0.03	15.22	0.03	15.15	0.03	C60
230.36	22.4	16.18	0.06	16.06	0.04	15.63	0.03	15.29	0.03	15.15	0.03	C60
231.36	23.4			16.17	0.04	15.68	0.03	15.32	0.04	15.17	0.03	C50
232.30	24.3			16.22	0.04	15.70	0.04	15.33	0.04	15.17	0.04	M70
232.34	24.3			16.25	0.04	15.68	0.03	15.33	0.04	15.18	0.03	C50
234.28	26.3					15.66	0.03			15.21	0.03	A60
235.25	27.2			16.34	0.07	15.67	0.03	15.37	0.03			A60
235.35	27.4			16.33	0.05	15.69	0.04	15.34	0.03	15.19	0.04	C50
236.29	28.3	16.70	0.06	16.32	0.04	15.67	0.04	15.33	0.03	15.19	0.04	T60
236.30	28.3			16.38	0.06	15.70	0.04	15.34	0.04	15.15	0.04	C50
236.47	28.5			16.35	0.04	15.70	0.03	15.35	0.03	15.13	0.04	M70
237.30	29.3			16.42	0.04	15.67	0.03	15.34	0.03	15.16	0.04	C50
240.31	32.3			16.42	0.05	15.72	0.04	15.29	0.05	15.14	0.05	C50
241.26	33.3					15.76	0.04	15.39	0.03	15.17	0.03	A60
241.34	33.3			16.49	0.04	15.70	0.03	15.35	0.03	15.13	0.03	C50
242.28	34.3			16.51	0.04	15.71	0.04	15.35	0.04	15.09	0.04	M70
242.32	34.3			16.50	0.04	15.72	0.03	15.33	0.03	15.13	0.03	C50
243.32	35.3			16.51	0.04	15.71	0.03	15.32	0.03	15.11	0.03	C50
245.34	37.3	17.05	0.06	16.56	0.04	15.69	0.04	15.32	0.04	15.10	0.05	C60
246.28	38.3			16.58	0.04	15.70	0.03	15.33	0.04	15.08	0.04	M70
246.41	38.4			16.66	0.06	15.73	0.04	15.29	0.04	15.11	0.04	A60
249.31	41.3			16.58	0.05	15.71	0.03	15.30	0.04	15.11	0.04	M70
249.38	41.4			16.61	0.04	15.70	0.04	15.29	0.03	15.10	0.04	T60
250.32	42.3			16.60	0.04	15.68	0.03	15.32	0.04	15.08	0.05	M70
253.37	45.4			16.66	0.04	15.74	0.06	15.32	0.05	15.02	0.04	A60
254.35	46.4			16.64	0.05	15.68	0.04	15.28	0.03	15.02	0.04	M70
255.42	47.4					15.72	0.05	15.33	0.05	15.11	0.03	A60
256.35	48.4			16.60	0.04	15.74	0.04	15.33	0.05	15.05	0.04	A60
257.36	49.4			16.68	0.03	15.78	0.04	15.36	0.04	15.03	0.04	A60
260.34	52.3			16.77	0.04	15.73	0.03	15.31	0.04	15.04	0.05	M70
262.30	54.3			16.86	0.05	15.73	0.04	15.31	0.04	15.05	0.04	M70
263.33	55.3			16.87	0.05	15.74	0.04	15.26	0.04	14.98	0.05	M70
264.32	56.3			16.79	0.05	15.74	0.03	15.30	0.03	15.05	0.04	M70
267.34	59.3			16.85	0.05	15.77	0.04	15.33	0.03	15.05	0.05	M70
271.31	63.3			16.98	0.05	15.78	0.04	15.32	0.03	15.07	0.05	M70
271.38	63.4			16.87	0.04	15.77	0.03	15.31	0.03	15.01	0.03	S100
272.31	64.3			16.93	0.04	15.79	0.05	15.32	0.05	15.05	0.04	S100
273.35	65.4			16.91	0.05	15.79	0.06	15.34	0.05	15.05	0.05	S100
275.27	67.3			16.88	0.05	15.81	0.07			15.02	0.03	A60
277.27	69.3			16.95	0.05	15.82	0.04	15.34	0.03	15.06	0.04	S100
278.30	70.3			16.97	0.05	15.85	0.05	15.37	0.05	15.09	0.04	S100
282.31	74.3			17.05	0.06	15.87	0.08	15.42	0.05	15.06	0.03	A60
283.29	75.3			17.02	0.04	15.83	0.04	15.36	0.04	15.05	0.04	S100
283.33	75.3					15.84	0.04	15.32	0.04	15.02	0.04	M70
284.29	76.3			16.94	0.05	15.84	0.04	15.37	0.04	15.07	0.05	S100
285.35	77.4			17.10	0.07	15.86	0.07	15.42	0.05	15.00	0.05	A60
285.37	77.4			17.05	0.06	15.80	0.03	15.32	0.03			M70
288.32	80.3					15.85	0.05	15.32	0.07	15.05	0.11	M70
290.32	82.3					15.87	0.06	15.36	0.06			M70
299.30	91.3			17.15	0.05	15.98	0.04	15.48	0.04	15.18	0.04	S100
308.44	100.4					15.97	0.12	15.50	0.05	15.11	0.11	S100

Note: A60 = 60-cm reflector of Shamakhy Observatory, C60 = 60-cm reflector of Crimean Observatory of SAI, C50 = 50-cm meniscus telescope of Crimean Observatory of SAI, T60 = 60-cm reflector of Stará Lesná Observatory, S100 = 1-m reflector at Simeiz Observatory, M70 = 70-cm reflector of Moscow Observatory of SAI, M20 = 20-cm meniscus telescope of Moscow Observatory of SAI, K70  = 70-cm reflector of Crimean Astrophysical Observatory (reprocessing of data by Nazarov et al. 2018)

Table A3.

Journal of spectroscopic observations of SN 2018aoq.

JD 2458 200 +	Phase	Exp. time (s)	CCD binning
19.34	11.3	900	8×8
34.36	26.4	3600	4×4
35.33	27.3	1200	8×8
46.35	38.4	1800	8×8
52.45	44.5	3600	4×4
56.40	48.4	3600	4×4
79.39	71.4	3600	4×4