SN 2023emq: A Flash-ionized Ibn Supernova with Possible C iii Emission

SN 2023emq is a fast-evolving transient initially classified as a rare Type Icn supernova (SN), interacting with a H- and He-free circumstellar medium (CSM) around maximum light. Subsequent spectroscopy revealed the unambiguous emergence of narrow He lines, confidently placing SN 2023emq in the more common Type Ibn class. Photometrically, SN 2023emq has several uncommon properties regardless of its class, including its extreme initial decay (faster than >90% of Type Ibn/Icn SNe) and sharp transition in the decline rate from 0.20 to 0.07 mag day−1 at +20 days. The bolometric light curve can be modeled as CSM interaction with 0.32M ⊙ of ejecta and 0.12M ⊙ of CSM, with 0.006M ⊙ of nickel, as expected of fast, interacting SNe. Furthermore, broadband polarimetry at +8.7 days (P = 0.55% ± 0.30%) is consistent with spherical symmetry. A discovery of a transitional Type Icn/Ibn SN would be unprecedented and would give valuable insights into the nature of mass loss suffered by the progenitor just before death, but we favor an interpretation that SN 2023emq is a Type Ibn SN that exhibited flash-ionized features in the earliest spectrum, as the features are not an exact match with other Type Icn SNe to date. However, the feature at 5700 Å, in the region of C iii and N ii emission, is significantly stronger in SN 2023emq than in the few other flash-ionized Type Ibn SNe, and if it is related to C iii, it possibly implies a continuum of properties between the two classes.

1. INTRODUCTION The study of stellar explosions entered a new era in the wake of untargeted, high-cadence transient sur-veys allowing the discovery of transient phenomena that evolve on much faster timescales than typically expected of supernovae (SNe).Such rapidly evolving transients (RETs) were first discovered in archival searches in surveys such as Pan-STARRS1 Medium Deep Survey (Drout et al. 2014) and Dark Energy Survey (Pursiainen et al. 2018;Wiseman et al. 2020), resulting in large samples of spectroscopically unclassified events.Even without classifications these events expanded our understanding of stellar deaths as their light curves evolved too fast to be explained by the decay of radioactive 56 Ni -the canonical power source of many typical SNe -and alternative power sources had to be considered.
In recent years an increasing number of fast events have been discovered in real time allowing intense followup campaigns.A significant number of them appear to be interacting H-poor SNe (e.g.Ho et al. 2023), either Type Ibn SNe characterised by a plethora of narrow helium emission lines (e.g.Foley et al. 2007;Pastorello et al. 2008), or Type Icn with emission lines of carbon and oxygen (e.g.Gal-Yam et al. 2022;Perley et al. 2022).While tens of Type Ibn SNe have been discovered over the last 15 years, so far only five Type Icn SNe have been identified -all within the last four years.These are SN 2019hgp (Gal-Yam et al. 2022), SN 2019jc (Pellegrino et al. 2022a), SN 2021csp (Fraser et al. 2021;Perley et al. 2022), SN 2021ckj (Pellegrino et al. 2022a;Nagao et al. 2023) and SN 2022ann (Davis et al. 2023).Although not all Type Ibn are considered to be fast evolving (e.g.Inserra 2019), a significant subpopulation of them evolve on similar timescales to RETs, whilst all Icn SNe fall into this bright-and-fast parameter space.
The existence of transitional events that change type between Ibn and IIn supernovae over time (e.g.Pastorello et al. 2015a;Reguitti et al. 2022), suggests a continuum in the circumstellar material (CSM) properties of these events and consequently in the mass loss history of their progenitor stars.It may therefore be reasonable to expect that transitional events between non-hydrogen dominated CSM events (i.e.Type Icn to Ibn) also exist, although such events have not been identified to date.The discovery of such potential transitional events would be extremely valuable for our understanding of how massive stars lose their mass and create their CSM during their final stages giving insight into the chemical structure of the progenitor and the continuous or episodic nature of their mass loss.
Here we present an analysis of SN 2023emq, a SN initially identified as a Icn SN (Pellegrino et al. 2023), but which evolved to resemble a Ibn SN ∼ 10 d later (Pursiainen & Leloudas 2023), possibly implying transitional nature.This paper is structured as follows: in Section 2 we present the observations and the data reduction procedures, in Section 3 we present the analysis of the data and in Section 4 we conclude our findings.The spectroscopy and photometry presented in this paper have been corrected for Milky Way extinction E B−V = 0.105 ) when the SN was already undetectable (see Table 1).
The location of the SN (red circle), is on top of a bright region of the host galaxy.(Schlafly & Finkbeiner 2011).Throughout the paper, we assume a flat ΛCDM cosmology with Ω M = 0.3 and H 0 = 70 km s −1 Mpc −1 and adopt redshift z = 0.0338.
2. OBSERVATIONS AND DATA REDUCTION SN 2023emq was discovered by Asteroid Terrestrialimpact Last Alert System (ATLAS; Tonry et al. 2018;Smith et al. 2020) on 2023 Apr 01 under the name ATLAS23ftq, with a previous non-detection two days prior (Tonry et al. 2023).The SN is associated with LEDA797708 (Makarov et al. 2014), a visibly blue galaxy in Pan-STARRS1 3π images, with an offset of ∼ 3.7 ′′ from the brightest core region (Flewelling et al. 2020) Smartt et al. 2015) survey and VLT/X-Shooter (Vernet et al. 2011) under the DDT programme.The NOT spectrum was reduced using the PyNOT-redux reduction pipeline, 2 the NTT spectrum with the PESSTO pipeline (Smartt et al. 2015) and the X-Shooter spectrum as described in Selsing et al. (2019).We also analysed the Global SN Project classification spectrum (Pellegrino et al. 2023) available in WISeREP 3 (Yaron & Gal-Yam 2012).The spectral log is provided in Table 2. Finally, we obtained one epoch of NOT/ALFOSC V -band imaging polarimetry on 2023 Apr 14 (+8.7 d).The data was reduced following Pursiainen et al. (2023).

ANALYSIS 3.1. Photometry
The multi-band light curve of SN 2023emq is shown in Figure 2 (top).While the decline is well covered, only the ATLAS o-band has pre-peak data.To estimate the peak MJD and magnitude we use Gaussian process (GP) interpolation using the setup presented in Pursiainen et al. (2020) and find the peak to be at MJD ∼ 60040 and −18.7±0.1 mag.To highlight the extreme evolution timescale of SN 2023emq, we compare its light curve to the enigmatic AT 2018cow and to example Icn/Ibn SNe chosen for their fast light curve evolution in Figure 2 (left).While the rise of the SN is not exceptional, its initial decline is fast regardless of its type, and by the time of the last spectrum at ∼ +20 d, the SN had already declined by 3.5 mag in brightness.In fact, the decline rate in the first 15 days post-peak (∼ 0.20 mag/d) is at the extreme end of Ibn/Icn SNe and comparable to the events similar to AT 2018cow (e.g.Prentice et al. 2018;Perley et al. 2019;Ho et al. 2020;Perley et al. 2021;Yao et al. 2022), as demonstrated in Figure 2 (middle).However, based on our NOT R-band data we can determine that after the observation at +20 d the decline 1 fallingstar-data.com/forcedphot/ 2 github.com/jkrogager/PyNOT/ 3 www.wiserep.orgslowed to 0.07 mag/d -three times slower than the early decline.While the decline rates of Ibn SNe are expected to decrease in time and similar breaks have been seen before (e.g.SN 2015G; Shivvers et al. 2017), such extreme transitions are rare in the population (see e.g.Hosseinzadeh et al. 2017).Out of the Type Icn SNe, only SN 2022ann has exhibited a similar transition where the decline rate decreased from a maximum of ∼ 0.14 mag/d to ∼ 0.03 mag/d (Davis et al. 2023).
The late-time decline of SN 2023emq is faster than that expected from the 56 Co to 56 Fe decay (0.0098 mag/d), but the decay likely contributes at these epochs.Using the semi-analytical light curve models of Chatzopoulos et al. (2012Chatzopoulos et al. ( , 2013)), we fit the bolometric light curve of SN 2023emq (for details see Appendix A) with a combined CSM-interaction and nickel decay model.We assumed shell-like CSM with a setup presented by Pellegrino et al. (2022a) for type Icn SNe except for the opacity for which we used κ = 0.2 cm 2 s −1 as appropriate for H-poor CSM (Chatzopoulos et al. 2013).As shown in Figure 2 (right), we find a good fit with M CSM = 0.12 +0.01 −0.01 M ⊙ and M ejecta = 0.32 +0.04 −0.04 M ⊙ (1σ) with M Ni = 0.006 +0.001 −0.001 M ⊙ .The recovered values of M CSM and M ejecta are smaller than found in the literature for Type Ibn SNe (e.g.Pellegrino et al. 2022b), as expected given the fast photometric evolution.The M Ni is similar to the lowest estimates for Type Ibn/Icn SNe (e.g.Pellegrino et al. 2022a).However, based on our late-time non-detections the decline continued furiously.The SN was no longer detected in host-subtracted NOT R band photometry taken at +78 d with a 5σ upper limit of 22.7 mag.A deeper limit of 25.1 mag was obtained at +95 d with VLT.As the decline continued faster than expected of 56 Co decay, the amount of 56 Ni synthesised in the explosion has to be truly marginal.

Spectroscopy
The spectral time series of SN 2023emq is compared to example Type Ibn and Icn SNe, chosen based on their spectral similarity to SN 2023emq, in Figure 3.We adopt a redshift of z = 0.0338, determined based on host galaxy emission lines present in the 2D X-Shooter spectrum.In the classification spectrum, the most notable emission lines at 5700 Å and 4690 Å are identified as C III λ5696 and a blend of C III λ4650 and He II λ4686 (Pellegrino et al. 2023).These lines are common in Type Icn SNe, as demonstrated by well-studied SN 2019hgp (Gal-Yam et al. 2022), but emission at similar wavelengths has also been seen in a few Type Ibn SNe (e.g.SN 2015U).In the subsequent spectra the SN has evolved into a typical Type Ibn SN characterised by strong He I emission lines.The spectra are virtually identical to that of Ibn SN 2014av shown in the figure.
The spectral evolution of the SN suggests that it could be a transitional Type Icn/Ibn SN, but the question relies on whether the early spectroscopic signatures are also seen in Type Ibn SNe, and important SNe to com- emission is reported.This is also the case for H-rich flash-ionised spectra, as the line is typically extremely faint if seen at all (e.g.SN 1998S; Fassia et al. 2001).Furthermore, Shivvers et al. (2016) showed that a similar but stronger emission line at 5700 Å was also seen in an early spectrum of Type Ibn SN 2015U in addition to the blended emission line at 4650 Å (see Figure 3).The authors suggested that the line is N II λ5680, supported by the presence of several N II absorption lines in the later spectra (Pastorello et al. 2015b;Shivvers et al. 2016).Regardless of the exact identification of the line in SN 2023emq, emission has been detected at the two wavelengths in both Ibn and Icn SNe, and the presence of emission alone does not warrant a Icn classification.
In Figure 4 (left), we investigate what emission lines are present in the first spectrum in comparison to Ibn and Icn SNe.The line that best matches the feature at 4690 Å is He II, with a fainter contribution from N III/C III on the blue side of the feature.The presence of He II is indicative of Type Ibn SN, as in Type Icn the line is not prominent (e.g.Pellegrino et al. 2022a).Furthermore, the feature of Ibn SN 2015U appears to have the same shape as in SN 2023emq if the He II contribution is ignored while in Icn SNe C III λ4650 typically shows a P Cygni profile not seen in SN 2023emq.At 5700 Å the line is well-matched with both C III λ5696 and N II at 5680 Å and cannot be clearly distinguished.In comparison to Ibn SNe, only SN 2015U exhibits any clear emission in the interval, and in this regard SN 2023emq appears more similar to Icn SNe, where the emission is prominent.The trend is also visible in Figure 5  Given the presence of the prominent He II emission in the first spectrum, the early spectral similarity to SN 2015U and the flash-ionised Type Ibn SNe as well as the unambiguous Ibn nature after peak brightness, we favour the interpretation that SN 2023emq is a flashionised Type Ibn SN.The unusually prominent emission line at 5700 Å might imply some kind of continuum between Icn and Ibn SNe and could indicate that the SN 2023emq is between the two populations if it is related to C III.However, due to the unclear nature of the line, it is possible that it is related to N II like suggested for SN 2015U (Shivvers et al. 2016).In this case, SN 2023emq is the second example of Ibn SN with N II emission with no obvious connection to Type Icn SNe.
We also used the X-Shooter spectrum to investigate the effect of host galaxy extinction.The Na ID doublet, which is typically used to estimate extinction in a galaxy, is not clearly visible in our high-resolution X-Shooter spectrum, and we can estimate the upper limit for E B−V of the host by generating Gaussian absorption profiles over the spectrum.Assuming that the full width at half maximum of the line is the resolution of the spectrograph (35 km/s at 5890 Å), we find a 3σ limit for the total equivalent width of the doublet is EW ∼ 0.146 Å.Using the empirical equation of Poznanski et al. (2012), this equates to E B−V ≲ 0.02, indicating that the effect of the host galaxy extinction should be negligible.

Polarimetry
The polarimetry taken at +8.7 d shows that SN 2023emq is polarised as demonstrated in Figure 6 (right).We measure the dimensionless Stokes parameters to be Q = −0.43 ± 0.27% and U = 0.94 ± 0.27% corresponding to P = 1.03 ± 0.27%.In addition, it is possible to obtain measurements for two stars found in the ALFOSC field of view shown in Figure 6 (left).Both stars are bright, isolated targets, and based on the astrometric solutions from Gaia Data Release 3 (Gaia Collaboration et al. 2022) they also lie 150 pc above the Milky Way plane and should therefore be sufficiently far to probe the Milky Way dust content (Tran 1995).As such, they should be reliable estimators of the Galactic interstellar polarisation (ISP).The SN is clearly offset from the stars on the Q -U plane (Figure 6), which is an indication of intrinsic polarisation.Assuming the Galactic ISP can be obtained by averaging the two stars, we find Galactic ISP-corrected values for SN 2023emq of Q = −0.24± 0.30% and U = 0.37 ± 0.30%, resulting in P = 0.55 ± 0.30% (corrected for polarisation bias following Plaszczynski et al. 2014).For Ibn SNe, the emission feature is clearly seen in only SN 2015U (see Figure 4), and for the other Ibn spectra we show 3σ upper limits (open triangles) determined assuming a Gaussian profile with FWHM measured for SN 2023emq (∼ 3000 km/s).SN 2023emq does not exhibit as prominent emission as seen in most Icn spectra, but the line is also far more significant than seen in most flash-ionised Ibn SNe, possibly implying a continuum of properties between the two classes.The EWs were estimated with Gaussian fits over a linear background.For Ibn SNe we used the spectra of SN 2010al, SN 2015U, SN 2019uo and SN 2019wep that exhibited the strong blended emission at 4650 Å -indicative of the flash-ionised phase.For the Icn SN 2019hgp and SN 2021csp we used the spectra that showed C III λ5696 emission, excluding the lower resolution LT/SPRAT spectra (for details see Gal-Yam et al. 2022;Perley et al. 2022).
While we have no means to directly probe the host galaxy ISP, its maximum value should correlate with the host extinction following P ISP < 9 × E B−V (Serkowski et al. 1975).Using the derived limit on host extinction (E B−V ≲ 0.02) we find that the effect of host galaxy dust content should be P ISP,Host ≲ 0.19%.As the derived polarisation degree (P ∼ 0.55%) is more significant, it is likely intrinsic to the SN.Given the spectrum at the time does not exhibit strong absorption lines that could induce polarisation (e.g.Wang & Wheeler 2008), the polarisation is likely that of the continuum.If the photosphere is an oblate ellipsoid, P ∼ 0.55% corresponds to a lower limit of the physical minor over major axial ratio is b/a ≲ 0.9 (Höflich 1991), but as shown by Pursiainen et al. (2022) the CSM can also be found in a disk/torus, and the CSM does not need to be in a uniform ellipsoidal shape.However, given the detection is not very strong, we conclude that the CSM shows high degree of spherical symmetry, in perhaps marginally aspherical configuration.
Polarimetry of Type Ibn/Icn SNe is very scarce in the literature, and polarimetric data that probes the intrinsic properties of the SNe has been presented only for one Ibn and one Icn SN.The spectral polarimetry obtained for Type Ibn SN 2015G 5 d after discovery showed P ∼ 2.7%, indicative of high asymmetry (Shivvers et al. 2017), while the one epoch for Type Icn SN 2021csp at +3.5 d shows low polarisation, implying high spherical symmetry (Perley et al. 2022).Additionally, Shivvers et al. (2016) presented three epochs of spectropolarimetry of the highly reddened Type Ibn SN 2015U at ∼ +5 d and conclude that the observed polarisation signal was dominated by the contribution of dust in the host galaxy, and no firm conclusions on the polarisation of the SN were made.As such, our broad-band polarimetry for SN 2023emq is only the third observation that constrains the photospheric geometry of Ibn/Icn SNe.The H-poor CSM appears to often show high degree of spherical symmetry, but more events need to be observed to investigate the distribution of their photospheric shape.

CONCLUSIONS
We have presented an analysis of the photometric, spectroscopic and polarimetric properties of the fast evolving H-poor interacting SN 2023emq.While the rise of the SN is not particularly fast in the context of Type Ibn and Icn SNe, its initial decline (∼ 0.20 mag/d) is remarkable and even comparable to AT 2018cow.After +20 d the decline rate slowed significantly to (∼ 0.07 mag/d).Such a large transition in the decline rate is extreme and to our knowledge has not been seen before in interacting H-poor SNe.We modelled the bolometric light curve with a combined CSM interaction and nickel decay model and find good matches with M ejecta ∼ 0.32M ⊙ and M CSM ∼ 0.12M ⊙ , with ∼ 0.006M ⊙ of 56 Ni.While the late-time light curve is still faster than expected of 56 Co to 56 Fe decay, it is likely that the radioactive decay is playing a part.
The SN was initially classified as Type Icn based on the prominent emission features at 4650 Å and 5700 Å identified as C III as typical in Icn SNe (e.g.Gal-Yam et al. 2022;Perley et al. 2022), but by +8 d the SN had evolved into a typical Type Ibn characterised by prominent, narrow He I emission features, possibly implying a transitional nature.We, however, conclude that SN 2023emq is flash-ionised Ibn SN instead.As emission at 4650 Å and 5700 Å has been seen in a few Type Ibn SNe, just the presence of emission is not yet enough to classify the SN as Type Icn.The emission at 4650 Å seen in the first spectrum of SN 2023emq is dominated by He II with contribution from C III/N III, indicative of Ibn nature.Additionally, the C III λ4650 has typically a prominent P Cygni profile in Type Icn SNe, but only emission is present in SN 2023emq.However, while emission at 5700 Å has been reported in some Type Ibn SNe, it is remarkably strong in SN 2023emq and in this regard the SN is more similar to Type Icn SNe.In conclusion, based on our analysis we consider SN 2023emq a flashionised Ibn SN, but the strong emission at 5700 Å could indicate that the SN is between the Ibn and Icn populations if it is related to C III.More spectra obtained immediately after discovery are needed to investigate the importance of the emission at 5700 Å and further investigate the diversity of the flash-ionised features in core-collapse SNe.
We also obtained V -band polarimetry for SN 2023emq, making it only the third Type Ibn/Icn SN with optical polarimetry that probed the intrinsic emission from the SN.We find polarisation degree P = 0.55 ± 0.30% after correction for Galactic ISP and polarisation bias.Based on the upper limit on host extinction, the polarisation is likely not caused by the host dust content, and we conclude that the polarisation is intrinsic to the SN.Given the spectrum is dominated by interaction lines at the time, the result implies that the CSM shows high spherical symmetry in possibly marginally aspherical configuration.More polarimetric observations of these SN classes are required to characterise their geometric diversity and thus gain crucial insight to how the CSM is produced.(Becker 2015), Matplotlib (Hunter 2007), Numpy (Harris et al. 2020), SciPy (Virtanen et al. 2020), Source Extractor (Bertin & Arnouts 1996), spalipy (Lyman 2021) LMFIT (Newville et al. 2014)

APPENDIX
A. BOLOMETRIC LIGHT CURVE The bolometric light curve of SN 2023emq was constructed using blackbody fits to the multi-band epochs.The five epochs with UVOT data at +3 -8 d and the NOT BV RI epoch at +20 d were fit with a blackbody model to determine the evolution of temperature and radius shown in Figure 7.For these six epochs, we generated the bolometric luminosity assuming the spectral energy distribution is described by the blackbody fits.During the rise with only o-band data, we used the linearly declining temperature curve to estimate the temperature at the time of each data point and estimated the bolometric luminosity by scaling the resulting blackbody to the o-band magnitude.We also tested the assumption of constant temperature during the rise by adopting the temperature measured at +3 d, but the bolometric light curve did not change significantly due to the small change in the temperature.After +20 d, we adopted the temperature found at +20 d (∼ 7500 K) and scaled the blackbody to the late-time R-band data.As such, our bolometric luminosity is well constrained only between +3 and +20 days.Outside this range the bolometric luminosities are uncertain and we have assumed 25% errors.

Figure 1 .
Figure 1.The host environment of SN 2023emq in the VLT/FORS2 R band observation taken at +95 d (2023-07-12) when the SN was already undetectable (see Table1).The location of the SN (red circle), is on top of a bright region of the host galaxy.
. Late-time image of the environment obtained with FOcal Reducer and low dispersion Spectrograph (FORS2; Appenzeller et al. 1998) on the Very Large Telescope (VLT) at European Southern Observatory (ESO), Paranal, Chile, is shown in Figure 1.The photometry of SN 2023emq was collected from the public ATLAS and Zwicky Transient Facility (ZTF; Bellm et al. 2019) surveys, and with Target-ofopportunity observations with the Neil Gehrels Swift Observatory UV-Optical telescope (UVOT, PIs: Brown and Pellegrino).The late-time (≳ +20 d) evolution was monitored with aperture photometry from Alhambra Faint Object Spectrograph and Camera (ALFOSC) mounted on the Nordic Optical Telescope at La Palma, Spain (PI: Pursiainen) and from FORS2/VLT with

Figure 4 .
Figure 4. Left: The classification spectrum of SN 2023emq with the possible line identifications.The feature at 4690 Å is dominated by He II λ4686 with faint blue excess possibly caused by either N III λ4640 or C III λ4650.At 5700 Å the emission could be either N II λ5680 or C III λ5696.The spectrum of SN 2015U from Figure 3 as well as early spectra of the Type Icn SN 2019hgp (Gal-Yam et al. 2022) and the flash-ionised Ibn SNe2010al, 2019uo, and 2019wep (Pastorello et al. 2015aGangopadhyay et al. 2019Gangopadhyay et al. , 2022) ) are shown for comparison.Emission is seen in all SNe around 4650 Å.At 5700 Å the emission feature is either faint (SN 2015U) or not clearly detected(SNe 2010al, 2019uo, 2019wep)  in the Ibn SNe, but it is prominent in the Type Icn SN 2019hgp.Right: The only notable features in the X-Shooter NIR spectrum.He I λ10830 is clear, but the strong C I emission line λ10690 seen in Type Icn SN 2021csp(Fraser et al. 2021) is not present.He I λ20587 is also tentatively identified.butcomparable to the ones measured at later phases just before C III emission faded away (e.g.Perley et al. 2022).On the other hand, SN 2023emq is similar to SN 2015U, but clearly above the 3σ upper limits (open triangles) estimated for the other Ibn SNe.Finally, we searched the later spectra of SN 2023emq for signatures of C II or C I as seen in Icn SNe, but found no clear features.In the X-Shooter NIR spectrum, taken at +21.4 d, we identify a clear He I λ10830 emission line (4, right), but do not see the strong C I λ10690, detected in Icn SN 2021csp at a similar epoch (+20.5 d;Fraser et al. 2021).Given the presence of the prominent He II emission in the first spectrum, the early spectral similarity to SN 2015U and the flash-ionised Type Ibn SNe as well as the unambiguous Ibn nature after peak brightness, we favour the interpretation that SN 2023emq is a flashionised Type Ibn SN.The unusually prominent emission line at 5700 Å might imply some kind of continuum between Icn and Ibn SNe and could indicate that the SN 2023emq is between the two populations if it is related to C III.However, due to the unclear nature of the line, it is possible that it is related to N II like suggested for SN 2015U(Shivvers et al. 2016).In this case, SN 2023emq is the second example of Ibn SN with N II emission with no obvious connection to Type Icn SNe.We also used the X-Shooter spectrum to investigate the effect of host galaxy extinction.The Na ID doublet, which is typically used to estimate extinction in a galaxy, is not clearly visible in our high-resolution X-Shooter spectrum, and we can estimate the upper limit

Figure 5 .
Figure5.Equivalent width (EW) of the emission at 5700 Å as function of rest frame phase for SN 2023emq, Icn SNe 2019hgp and 2021csp and flash-ionised Type Ibn SNe.For Ibn SNe, the emission feature is clearly seen in only SN 2015U (see Figure4), and for the other Ibn spectra we show 3σ upper limits (open triangles) determined assuming a Gaussian profile with FWHM measured for SN 2023emq (∼ 3000 km/s).SN 2023emq does not exhibit as prominent emission as seen in most Icn spectra, but the line is also far more significant than seen in most flash-ionised Ibn SNe, possibly implying a continuum of properties between the two classes.The EWs were estimated with Gaussian fits over a linear background.For Ibn SNe we used the spectra of SN 2010al, SN 2015U, SN 2019uo and SN 2019wep that exhibited the strong blended emission at 4650 Å -indicative of the flash-ionised phase.For the Icn SN 2019hgp and SN 2021csp we used the spectra that showed C III λ5696 emission, excluding the lower resolution LT/SPRAT spectra (for details seeGal-Yam et al. 2022;Perley et al. 2022).

Figure 6 .
Figure 6.The NOT V -band polarimetry of SN 2023emq taken at +8.7 d post-peak.Left: The field of view of the observation.In the polarimetric mode, the extraordinary (e) and ordinary (o) beams are overlaid 15 ′′ apart.The SN and the two bright stars are marked.Other sources in the image were either too faint or too close to the edge for reliable estimation of the Stokes parameters.Right: The Stokes Q -U plane.Both the SN and the two stars found in the image are shown.The SN is clearly offset from the stars.After correcting for the Galactic ISP and polarisation bias, we find intrinsic polarisation of P = 0.55 ± 0.30%.The dashed lines mark the location of Q = 0% and P = 1%.

Figure 7 .
Figure7.The blackbody modelling of SN 2023emq.Left: The blackbody fits to the six multi-band epochs.Note the break on the y-axis.Right: The temperature and radius evolution of SN 2023emq derived from blackbody fits hexagons).The temperature is linearly declining, and the slope was used to determine the temperature at the time of o-band data points on the rise (diamonds).
Horizon 2020 research and innovation programme (grant agreement No. 948381) and by UK Space Agency Grant No. ST/Y000692/1.S.J.S. acknowledges funding from STFC Grant ST/X006506/1 and ST/T000198/1.P.W. acknowledges support from the Science and Technology Facilities Council (STFC) grant ST/R000506/.This work is based (in part) on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO DDT programme 2111.D-5006 (PI: Pursiainen) and as part of ePESSTO+ under ESO program ID 108.220C (PI: Inserra) and on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias under NOT programmes 67-009.The NOT data presented here were obtained with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT.This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; by products of the NEO search include images and catalogs from the survey area.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, and the South African Astronomical Observatory.Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.NOIRLab is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.LBNL is managed by the Regents of the University of California under contract to the U.S. Department of Energy.This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration.Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo, Financiadora de Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the Ministerio da Ciencia, Tecnologia e Inovacao, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy Survey.The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH) Zurich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciencies de l'Espai (IEEC/CSIC), the Institut de Fisica d'Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig Maximilians Universitat Munchen and the associated Excellence Cluster Universe, the University of Michigan, NSF's NOIRLab, the University of Nottingham, the Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University.

Table 1 .
Optical photometry of SN 2023emq.Note-The magnitudes are corrected for Galactic extinction.NOT/ALFOSC and VLT/FORS2 data are host-subtracted using the last epoch of VLT/FORS2 as template for R band and Legacy Survey data in g, r and i bands (Dey et al. 2019) for B, V and I data, respectively.Only the first few entires are shown here and the complete machine-readable table is available online.(This table is available in its entirety in machine-readable form.)