Bridging between type IIb and Ib supernovae: SN IIb 2022crv with a very thin Hydrogen envelope

We present optical, near-infrared, and radio observations of supernova (SN) SN IIb 2022crv. We show that it retained a very thin H envelope and transitioned from a SN IIb to a SN Ib; prominent H α seen in the pre-maximum phase diminishes toward the post-maximum phase, while He i lines show increasing strength. SYNAPPS modeling of the early spectra of SN 2022crv suggests that the absorption feature at 6200 ˚A is explained by a substantial contribution of H α together with Si ii , as is also supported by the velocity evolution of H α . The light-curve evolution is consistent with the canonical stripped-envelope supernova subclass but among the slowest. The light curve lacks the initial cooling phase and shows


INTRODUCTION
Core-collapse supernovae (CCSNe) show a great diversity in their observational properties, which reflects the diverse nature of their massive progenitor stars (M ≳ 8 M ⊙ ; Heger et al. 2003;Smartt 2009), especially in the advanced phases of their evolution which is still poorly understood.This is highlighted by the so-called stripped-envelope supernovae (SE-SNe); they constitute a distinct class of CCSNe that strips off some or all of their outer H envelope.The mechanism for the envelope stripping is still controversial, with strong stellar winds (Puls et al. 2008) and/or interaction with a binary companion (Podsiadlowski 1992;Fang et al. 2019) being suggested.Therefore, SE-SNe have been intensively studied to understand the evolution of massive stars in their final phases.One important issue here is where the boundary between SNe IIb and SNe Ib/c (defined by the presence or absence of the H lines in their spectra, respectively) might lie concerning the nature of their progenitors.
The progenitors of SNe IIb and SNe Ib are believed to differ by the amount of the envelope stripping; if the majority of the H envelope is removed before the SN explosion, an H-free SN Ib/c will be the outcome.In some cases, the progenitors of SE-SNe are identified in high-resolution images obtained by the Hubble Space Telescope and/or other facilities (Folatelli et al. 2015;Miles et al. 2018).The methodology has been specially established for the class of SNe IIb, which shows a diverse nature in their progenitors: for SNe IIb 1993J and 2001ig, a supergiant in a binary interacting system is thought to be the progenitor (Maund & Smartt 2009;Ryder et al. 2018); the progenitor of SN IIb 2008ax is likely a highly-stripped star (a low-mass analog of a Wolf-Rayet star) in a binary system with M ZAMS ∼ 10 -14 M ⊙ (Pastorello et al. 2008;Crockett et al. 2008); while yellow supergiant progenitors with M ZAMS ∼ 10 -17 M ⊙ have been reported for SNe IIb 2011dh, 2013df and 2016gkg (Maund et al. 2011;Folatelli et al. 2014;Bersten et al. 2012;Maeda et al. 2015;Kilpatrick et al. 2022;Nayana et al. 2022).On the other hand, the direct detection of the SN Ib/Ic progenitors is far more challenging, so far resulting in only the three cases of iPTF13bvn, SN 2017ein, and SN 2019yvr whose progenitor masses were found between 10 -20 M ⊙ (Groh et al. 2013), 47 -80 M ⊙ (Eldridge & Maund 2016;Kilpatrick et al. 2018), and ∼ 10 M ⊙ (Sun et al. 2022), respectively.
Indeed, the absence of H in spectra of SNe Ib does not necessarily mean that their progenitors are totally H-free.A question then is how much H envelope is required for SE-SNe to be classified as a SN IIb?The presence of only a small amount of the H envelope prior to core collapse can lead to strong H features in the spectra during the photospheric phase, with the Hα and Hβ lines being the most prominent features, along with several strong He lines.The H features in SNe IIb fade over time until the spectra become similar to those of SNe Ib (Filippenko 1988(Filippenko , 2000)).From their synthetic spectra, Hachinger et al. (2012) concluded that even 0.025 − 0.033 M ⊙ of the H mass can produce a strong Hα absorption feature, suggesting that there is a blending between SNe IIb and SNe Ib.Yoon et al. (2017) and Sravan et al. (2019) also showed that if the H mass remains between 0.001 M ⊙ and 0.5 M ⊙ , the features of SNe IIb will arise.Gilkis & Arcavi (2022) recently showed that the minimum mass threshold of H for a SNe IIb is 0.033 M ⊙ similar to Hachinger et al. (2012), but inconsistent with Sravan et al. (2019).Prentice & Mazzali (2017) proposed two additional SE-SNe subcategories: the SNe IIb(I), showing moderately H-rich spectra in which the Hα P-Cygni profile is dominated by the absorption component relative to the emission profile; and the SNe Ib(II), showing weak residual Hα, but no clear appearance of other Balmer lines.The findings by Prentice & Mazzali (2017) along with Hachinger et al. (2012) indicate that SNe IIb and SNe Ib are linked more physically than thought before.
The nature of the SE-SN progenitors can also be obtained from their light curve properties; the 'compact' SNe IIb have a similar bolometric light curve shape to SNe Ib, while 'extended' SNe IIb show a double-peaked light curve owing to a combination of the shock-cooling emission and the radioactive decay of 56 Ni to 56 Co (and then to 56 Fe) (Maund et al. 2004;Morales-Garoffolo et al. 2014).Except for the extended SNe IIb, the first peak due to the shock cooling is not observed in most SE-SNe because the progenitor compactness causes the shock-cooling emission to decay too quickly.The typical rise times of SE-SNe lie in the range of 10-20 d (Prentice et al. 2019) with the average peak absolute magnitudes of M B ∼ −16.99±0.45 and −17.66±0.40mag (Richardson et al. 2006(Richardson et al. , 2014) ) for SNe IIb and Ib, respectively.
Another powerful method to constrain the nature of the progenitors is radio observation.Radio emission from SE-SNe results from an interaction between the SN shock wave and nearby circumstellar medium (CSM) (Chevalier 1982a(Chevalier , 1998)).Radio observations help probe the density structure of the CSM, and thus the massloss history of the progenitor (Weiler et al. 1986;Chevalier 1982aChevalier , 1998;;Maeda et al. 2021).Indeed, the 'extended' and 'compact' SNe IIb classification is linked to the radio property (Chevalier & Soderberg 2010).The extended and compact progenitors naturally arise by the difference in the mass of the H-rich envelope, which also explains the differences in the radio signal through differences in the mass-loss history (e.g., Maeda et al. 2015;Ouchi & Maeda 2017), although this idea has been tested only for a small sample.
In this paper, we study the optical (spectroscopic and photometric), infrared, and radio evolution of SN IIb 2022crv.
The estimated explosion epoch and basic parameters of the SN are noted in subsections 2.1 and 2.2.The optical and near-infrared data reduction and analysis procedures are described in subsection 2.3.A detailed description of the spectral evolution, velocity evolution, and SYNAPPS (Thomas 2013) spectral modeling is given in Section 3. The photometric evolution is elaborated in Section 4, where we discuss the light curve, color evolution, and absolute magnitudes of SN 2022crv compared to other SE-SNe.The bolometric light curve modeling and parameters are described in Section 5.The radio data reduction and analysis are described in Section 6.We discuss the progenitor properties of SN 2022crv in Section 7. Finally, we summarise the results of the study in Section 8.

Estimation of the explosion epoch
The last non-detection of SN 2022crv before discovery was reported to be 2022-02-17 UT 04:42:50 (JD 2459627.70) at a magnitude upper limit of 17.83 with the Prompt5 0.4m telescope by Dong et al. (2022).In addition, Asteroid Terrestrial-impact Last Alert System (ATLAS) data (Tonry et al. 2018;Smith et al. 2020) reveal a deeper non-detection of the source on 2022-02-16 (JD 2459627.46) at a magnitude limit of 18.73 ± 0.40, which is indeed more useful than the later non-detection in constraining the explosion epoch.
We estimated the explosion epoch of SN 2022crv by fitting the very early bolometric evolution using the Valenti and Nagy-Vinko models (see Section 5).The estimated values of the explosion epoch from these two methods are JD 2459627.8±0.5 and JD 2459628.0±0.5, respectively.An average of the above two estimates, combined with the ATLAS non-detection, constrains the explosion time t 0 to be JD = 2459627.75±0.5.With this explosion epoch, the rise time of SN 2022crv bf to reach the bolometric maximum is 15.2 d which is consistent with those found for typical SE-SNe as derived by Prentice et al. (2019) (7.8 -20.7 d to reach bolometric maximum) and Lyman et al. (2016) (∼ 18 d in the R-band).
We see a conspicuous dip of Na I D at 5891.5 Å in the spectra of SN 2022crv taken on 2022-03-12 (JD 2459651.2),2022-03-29 (JD 2459668.0), and 2022-04-08 (JD 2459678.0)UT.For estimating the extinction within the host galaxy, we measure equivalent widths (EW) of the Na I D line in the combined spectra of these three dates to increase the signal-to-noise ratio.Using the formulation by Poznanski et al. (2012), we estimate the host galaxy extinction as A V = 0.467±0.010mag.We thus adopt A V = 0.672±0.010mag as a combination of the extinction within the host galaxy and that within the MW along the line of sight.We use these values of distance and extinction throughout the paper.
Several bias, dark, and twilight flat frames were obtained during the observing runs along with science frames.For the initial pre-processing, several steps, such as bias-subtraction, flat-fielding correction, and cosmic ray removal, were applied to raw images of the SN.We implemented the standard tasks available in the data reduction software IRAF3 for carrying out the preprocessing.Multiple frames were taken on some nights and co-added in respective bands after the geometric alignment of the images to increase the signal-to-noise ratio.
To calibrate the secondary standards in the SN field, we observed a set of Landolt equatorial standards (Landolt et al. 1990): PG 1323, PG 0942, SA 32, and SA 104 on 2022-05-19 (JD 2459718.5)and 2022-05-20 (JD 2459719.5)UT using the 1.3m DFOT and 2m HCT.The observed Landolt field stars with the magnitudes of 10 ≤ V ≤ 13 were observed in a typical seeing of 1. ′′ 5.The average site extinction values in the BVRI bands were taken from Stalin et al. (2008).We calibrated 13 non-variable local standards in the SN field using the transformation equations.These secondary standards were used to convert the instrumental magnitudes into apparent magnitudes.The calibrated BVRI magnitudes of the secondary standards averaged over two nights are listed in Table 5.The Point Spread Function (PSF) photometry for the data from ST, DFOT, and HCT was im-plemented through a reduction pipeline built in Python called RedPipe4 (Singh 2021).SN magnitudes were calibrated using the nightly zero points obtained from the secondary standards.
For the data taken by TriCCS (TriColor CMOS Camera and Spectrograph) attached to the Seimei Telescope, the gri band observations were calibrated using the APASS catalog5 with the same set of secondary standards.The gri band magnitudes from the images taken by the Seimei Telescope were measured using DAOPHOT6 .The ri band magnitudes were then converted to RI.We further added supplemental data from ATLAS, using their forced-photometry archive7 developed by Shingles et al. (2021).In addition, we added g-band data of SN 2022crv from the All Sky Automated Search for SuperNovae (ASAS-SN, Shappee et al. 2014;Kochanek et al. 2017).These additional data points were merged with the light curve from the Seimei Telescope.The final SN magnitudes from all the instrumental setups are tabulated in Table 6.
The near-infrared (NIR) data of SN 2022crv were obtained with the HONIR instrument of KT.The skybackground subtraction was done using a template sky image obtained by dithering individual frames at different positions.We performed PSF photometry and calibrated the SN magnitudes using comparison stars in the 2MASS catalog (Persson et al. 1998).The final NIR magnitudes in the SN field are shown in Table 7.
The spectroscopic observations were carried out using the Himalayan Faint Object Spectrograph and Camera (HFOSC) mounted on the HCT, KOOLS-IFU (Matsubayashi et al. 2019) on the Seimei Telescope, and Aries Faint Object Spectrograph and Camera (AD-FOSC) (Kumar 2016) mounted on the 3.6m Devasthal Optical Telescope (DOT), ARIES, India.Our spectral coverage spans from −10 d to +33 d.We included the publicly available Gemini-N/GMOS spectrum taken on −15 d in our analysis (Andrews et al. 2022).For HCT, we used a 2 ′′ wide slit and Grisms Gr7/Gr8 for taking optical spectra.The DOT spectrum was taken with the 676R grism and similar slit size.The spectra taken with HFOSC and ADFOSC were reduced using the twodspec package in IRAF, followed by wavelength and flux calibration.The slit loss corrections were done by scaling the spectra with respect to the SN photometry.The spectra with KOOLS-IFU were taken through optical 4500 5000 5500 6000 6500 7000 7500 8000 8500  fibers and the VPH-blue grism.The data reduction was performed using the Hydra package in IRAF (Barden 1994) and a reduction software developed for KOOLS-IFU data 8 .For each frame, we performed sky subtraction using a sky spectrum created by combining fibers to which the contributions from the object are negligible.Arc lamps of Hg, Ne, and Xe were used for wavelength calibration.Finally, the spectra were corrected for the heliocentric redshift of the host galaxy.The log of spectroscopic observations is reported in Table 8.

SPECTROSCOPIC EVOLUTION
Figure 1 shows the spectral evolution of SN 2022crv from −15.3 to 33.3 d.Our spectra mainly cover the early photospheric phase, which helps examine the properties of the outermost regions of the expanding ejecta.The very early spectrum (−15.3 d) shows a broad absorption 8 http://www.o.kwasan.kyoto-u.ac.jp/inst/p-kools dip centered at 6170 Å, probably due to Hα with some additional contribution from Si ii 6355 Å, an absorption dip centered around ∼ 5700 Å due to He i 5876 Å, and other features of Fe around 5000 Å.The absorption at 6170 Å corresponds to a high velocity of 18,000 km s −1 , if it is due to Hα.The second spectrum at −9.7 d shows features of the Fe ii triplet (4924, 5018, 5169 Å), the He i 5876 Å feature, and He i 6678 Å superposed with the narrow Hα from the host galaxy.Narrow emission lines due to the host galaxy are also seen for [N ii] 6584 Å.We also see an absorption around 8498 Å due to the Ca ii NIR triplet.The feature around 4500 Å spectra at +0.2 d and +0.4 d look very similar to Mg ii.The feature appearing at 5500 Å is most likely a blend of Fe ii at 5535 Å and Sc ii at 5527 Å.The spectra during this phase show prominent absorption due to a combination of He i 5876 Å and the Na i D absorption.The spectra from +3.5 d to +33.3 d show that He i 5876, He i 6678, and He i 7065 Å grow stronger over time.
The spectra also show a "W"-shaped absorption feature centered around 5000 Å.A similar feature has been previously observed in SN II 2005ap (Quimby et al. 2007), SN Ib 2008D (Modjaz et al. 2009), SN Ib 2009jf (Sahu et al. 2011), SN IIb 2001ig (Silverman et al. 2009) and SN Ib 2015ap (Gangopadhyay et al. 2020).For most of them, this spectral feature is typically seen in the pre-maximum time between −14 d to −10 d.Mazzali et al. (2008) explained the origin of this feature as Fe ii complexes; this is confirmed for SN 2022crv by spectral modeling shown in later sections, but with an additional contribution by Mg ii.

Spectral Comparison
Figure 2 shows the pre-maximum spectral comparison plot of SN 2022crv with other extended SNe IIb (eSNe IIb), compact SNe IIb (cSNe IIb) and SNe Ib.The spectral comparison sample is collected from Wis-eRep (Yaron & Gal-Yam 2012) 9 , and all the relevant papers are cited in Table 1.The spectrum of SN 2022crv shows a distinct dip at 6200 Å.This feature matches well with those seen in cSNe IIb, especially SN 2020acat.In addition to the feature at 6200 Å, the He i features are well-developed in SN 2022crv.This is reminiscent of the behavior seen in SNe IIb, for which the first feature is interpreted as Hα.Assuming this is also Hα in SN 2022crv, the spectrum of SN 2022crv matches with cSNe IIb in terms of H-richness contrary to the shallow Hα seen in eSNe IIb.In addition, the SNe Ib in the comparison sample have a prominent dip of He i compared to SN 2022crv.The inspection here indicates that SN 2022crv had retained a very thin H envelope, showing its similarity with a cSN IIb.
Figure 3 shows the post-maximum spectral comparison plot.The "W" feature is still visible in SN 2022crv, while the feature in the other SNe IIb in the comparison sample has vanished completely.SNe Ib, however, still shows the "W" features.The He i feature at 5876 Å in SN 2022crv is similar to those seen in SN Ib iPTF13bvn and SN Ib 2015ap at this stage.The interesting observation at this stage is the similarity of the 'Hα' feature of SN 2022crv to SNe Ib and some of cSNe IIb, despite its similarity to SNe IIb in the earlier phase; it has substantially diminished over time in SN 2022crv.The spectrum of SN 2022crv in this plot extends to a slightly redder portion of the optical window, covering Ca ii NIR features.It is seen that the Ca ii NIR features are very strong in SN 2022crv.Indeed, with the caveat that the Ca ii NIR features are placed near the edge of the spectra in the other phases, it is possible that Ca ii NIR features are always strong in SN 2022crv, and this behavior is similar to SNe Ib.
Figure 4 shows the late post-maximum spectrum of SN 2022crv as compared to other SE-SNe.SN 2022crv shows very strong He i dips.The plot shows that along with the He i 5876 Å feature, strong absorptions are also noticed at 6678 Å and 7065 Å.The overall He strength in SN 2022crv is among the strongest in SNe IIb and SNe Ib at this phase.
In summary, the overall spectral evolution of SN 2022crv generally traces that of SN Ib, e.g., in He features, the 'Fe II blend' at 5000 Å, and the Ca ii NIR features.However, the feature distinct from SNe Ib is seen at ∼ 6200 Å especially in the pre-maximum phase.The feature is similar to those found in cSNe IIb 2008ax and 2020acat and most likely reflects a substantial contribution by Hα in the early phase; this motivates the 'SN IIb' classification for SN 2022crv.Indeed, the above inspections suggest that SN 2022crv is also similar to these cSNe IIb in the spectral evolution, representing a boundary between SNe Ib and cSNe IIb.
Figure 5 shows the evolution of the Hα+Si,ii and He i EWs.We estimated the EWs of these lines using the absorption profiles of the P-Cygni lines, where the flux In the lower panel of Figure 5, we can see that the EW of the He i 5876 Å line continuously increases in strength as a function of time, for SNe IIb and Ib in general; the EW of the He i 5876 Å line is practically indistinguishable between these subclasses, and SN 2022crv follows this behavior.By +25 d, we see a prominent rise in the He i EW for SN 2022crv relative to the other SNe IIb and SNe Ib in our comparison sample, while it is still within the range expected for SNe IIb and Ib.This behavior is indeed similar to that seen in cSN IIb 2008ax.
In summary, from the evolution of the EWs we conclude that the Hα+Si ii feature initially showed a high  1.
EW consistent with those seen in SNe IIb.As time goes by, in the post-maximum phase the feature has weakened substantially, to a level similar to those found in SNe Ib but much weaker than SNe IIb.Meanwhile, the He i feature has strengthened over time, consistent with the general behavior seen in SNe IIb and Ib.SN 2022crv thus shows a beautiful transition from the IIb class to Ib.
To strengthen the case for the SN IIb classification through line identification, we used the open-source spectral fitting software SYNAPPS (Thomas 2013) to reproduce the pre-peak (-9.7 d), peak (+0.4 d) and two post-peak (+8.3 d and +23.2 d) spectra of SN 2022crv (see Figure 6).SYNAPPS assumes spherical symmetry, homologous expansion, and photosphere emitting blackbody continuum.The emission and absorption lines are formed by resonant scattering, assuming the Sobolev approximation.The velocity at the photosphere, the optical depth of a reference line for each ion, and the minimum and maximum velocities on the distribution of each ion are the fitting parameters for each spectrum.For modeling the SN 2022crv  O i, Mg ii, Ca ii, Fe ii and Sc ii.We obtained a maximum velocity of 30,000 km s −1 for all the ions, though this value is not strongly constrained.In the −9.7 d spectrum, a combination of Hα and Si ii reproduces well the absorption dip at 6200 Å.This further justifies that a non-negligible amount of H is present in SN 2022crv.The inset plot of the figure shows that the absorption dip at 6200 Å for the −15.3 d and the −9.7 d spectra is better reproduced by a combination of Hα and Si ii, rather than using only one of them.The trace of Hα however diminishes rapidly, and He i starts dominating the spectrum later on, as seen in the spectrum on +8.3 d.
The photospheric velocities obtained from the SYNAPPS fit to the −9.7, +0.4,+8.3, and +23.2 d spectra are consistent with the Fe ii velocities we estimate next in Section 3.2.It is derived to be 9400 km s −1 at maximum light.In all the epochs, the 'detach' parameter has been deactivated in the fits, meaning that all the ions are distributed (at least) down to the photosphere rather than concentrated at high velocities.

Velocity evolution
Figure 7 shows the velocity evolution of SN 2022crv compared with other SE-SNe.We estimated the velocities of Fe ii 5169 Å, He i 5876 Å, and Hα 6563 Å by fitting a Gaussian profile to the absorption trough after correcting the spectra for the redshift of the host galaxy.The velocities of SN 2022crv were compared with the sample of SNe IIb and Ib collected from the literature in Table 1.The first panel (1) of Figure 7 shows the velocities of Hα, Fe ii 5169, Si ii 6355, He i 5876, and He i 6678 for SN 2022crv.Assuming that the absorption trough at ∼ 6200 Å arises from Hα, the estimated velocities drop from 18,000 km s −1 to 14,500 km s −1 between −15.3 d and +8.3 d.If instead this absorption feature is attributed to Si ii 6355 Å, the line velocity of Si would be evolving from 8,600 km s −1 to 5,000 km s −1 , which is lower than for Fe ii (Figure 7); assuming the Fe ii traces the photosphere (Dessart & Hillier 2005), the behavior suggests that the feature at ∼ 6200 Å could not be created only by Si ii and the contribution by Hα is likely substantial, as is consistent with the result of the SYNAPPS spectral modeling (Section 3.1 and Figure 6).
Panel 2 in Figure 7 further supports this view.Assuming the feature is created by Hα, the velocity evolution is similar to the other SNe IIb, with one key difference that the Hα feature vanishes quite early for SN 2022crv.This behavior indicates that the SN transitioned early on to a SN Ib.The Hα velocity of SN 2022crv is on the high end among SNe IIb, possibly due to the blend with Si ii, which indicates a thinner H envelope in SN 2022crv than the other SNe IIb.
Panel 3 of Figure 7 shows the velocity evolution of the He i 5876 Å line, which decreases from 12,500 km s −1 to 7,000 km s −1 .Panel 4 of Figure 7 shows the velocities of Fe ii 5169 Å, which decrease from 12,000 km s −1 to 6,500 km s −1 .In general, the velocities of He i 5876 Å and Fe ii 5169 Å (which trances the photosphere) are within the diversity seen for SNe IIb and Ib, suggesting that the key difference between SN 2022crv and SNe IIb, as well as that between SN 2022crv and SN Ib, is mainly in the nature of the H-rich envelope.
To summarise, the velocity evolution supports the idea that the absorption trough around 6200 Å is generated by a combination of Hα and Si ii.The He i and Fe ii velocities are similar to most SNe Ib.The Hα + Si ii and He i velocities thus support the metamorphosis of SN 2022crv from SN IIb to SN Ib.

PHOTOMETRIC EVOLUTION
The multi-band light curve evolution of SN 2022crv is shown in Figure 8.The V -band light curve is compared to other SE-SNe in Figure 9, where the absolute Figure 7.The velocity evolution of several lines seen in SN 2022crv.In panels 2-4, they are compared with the SNe IIb (blue color) and SNe Ib samples (red color).The error in the line velocity measurements can be as large as 500 km s −1 , but this is comparable with the symbol sizes used here.The velocities are estimated using the absorption minima of the P-Cygni profiles for SN 2022crv.The velocities for the comparison sample are adopted from references in Table 1.
magnitude values of the comparison SNe are shifted in the magnitude scale to match that of SN 2022crv.We estimated all the light curve parameters of SN 2022crv by fitting the data to an analytical formulation by Taddia et al. (2018), which is a modified version of Vacca & Leibundgut (1996) to apply to SE-SNe.The shape of the SE-SN light curves can be represented by three components; (i) an initial exponential rise, (ii) a Gaussianlike peak, and (iii) a late linear decay.The maxima of the light curves and the other parameters obtained from this fitting are tabulated in Table 2.We could trace the maxima in all the filters thanks to the pre-maximum discovery.The V -band light curve had a rise-time of ∼15 d.A lag of ∼7 d in the rise time is robustly derived between the B and the I band.Most of the SE-SNe peak earlier in bluer bands, and maxima in other bands follow owing to cooling of the SN ejecta, a trend which is also noticed for SN 2022crv.
The ∆m 15 (V ) estimated from the light curve of SN 2022crv is 0.76±0.04(see Table 2).The ∆m 15 (V ) of SN 2022crv is lower than the average ∆m 15 (V ) quoted by Taddia et al. (2018) indicating the slow-evolving nature of the SN.The relatively slow evolution of SN 2022crv around the peak is also seen in other bands see below.Taddia et al. (2018) found that the late-time decay rates (from ∼ 40 − 100 d) of a sample of SNe IIb and Ib are 1.6 -2.1 mag / 100 d and 1.4 -1.8 mag / 100 d, respectively, in the V band (see also Table 1).As shown in Figure 9, the late-time decay rate of SN 2022crv is consistent with other SNe IIb and Ib.The limited data for SN 2022crv results in a relatively large error in the decay rate (Table 1), and thus it is not clear if the slow evolution as compared to other SE-SNe seen around the peak persists in the late phase or not.In any case, the decay rate is higher than the 56 Co → 56 Fe decay rate, which corresponds to events with higher gamma-ray escape fractions due to higher explosion energy to ejecta mass ratios (Kumar et al. 2022); this is a typical behavior seen in SE-SNe (Maeda et al. 2003).To summarise the light curve parameters, we see that SN 2022crv shows  Barbon et al. (1995) 2018) are also overplotted.The blue-colored points are for SNe IIb, and the red-colored points are for SNe Ib.The data for the comparison sample are taken from the papers cited in Table 1.
for the extinction values tabulated in Table 1 (see Section 2.2).The (B-V) 0 color evolution of SN 2022crv shows an early red-to-blue transition before the V -band maximum.This behavior is shared with single-peaked cSNe IIb like SNe 2008ax and 2010as, but not with double-peaked SNe (or eSNe IIb) like SNe 1993J, 2011fu, and 2013df (Morales-Garoffolo 2016).Up to about +30 d, the color curves become redder again, indicating cooling of the photosphere, and become flatter until ∼ +90 d.A similar trend is also noticed in the (R-I) 0 color.The post-maximum color evolution of SN 2022crv matches reasonably well with the average color evolution templates compiled by Stritzinger et al. (2018) for SE-SNe denoted by shaded regions in the plots, and it is similar to cSNe IIb and SNe Ib in the pre-maximum phase.Figure 12.The BVRI bolometric light curve of SN 2022crv generated using SuperBol.The bolometric light curves of all the comparison objects are also calculated using SuperBol using the values computed in Table 1.
The V -band absolute magnitude of the SE-SN group lies between −16.5 mag and −19.5 mag (Richardson et al. 2006;Drout et al. 2011;Taddia et al. 2018).The peak M V for SN 2022crv is estimated to be −17.82±0.17(−17.40±0.55)and brighter than the average SNe Ib (−17.07±0.56)(Taddia et al. 2018).The peak J and H band magnitudes (see Table 2) of SN 2022crv are brighter than the average NIR magnitudes quoted by Taddia et al. (2018).Figure 9 shows that the light curve shape of SN 2022crv is typical of cSNe IIb and SNe Ib.
Figure 11 compares the absolute magnitudes of SN 2022crv in different bands with the ∆m 15 values, as compared with the statistical sample of Taddia et al. (2018).No strong correlation is seen between these parameters.While SN 2022crv is one of the slowly evolving members among SE-SNe, its properties lie within the scatter in all the bands; it indicates that the light curve properties, and thus the core properties, are typical of SE-SNe.
To summarise, SN 2022crv shows a color evolution consistent with cSNe IIb, indicating the absence of a primary peak.The SN is of average brightness and its lightcurve properties are largely consistent with SE-SNe.

BOLOMETRIC LIGHT CURVE MODELING AND ESTIMATION OF PHYSICAL PARAMETERS
The quasi bolometric light curve of SN 2022crv was constructed using the Python-based code SuperBol (Nicholl 2018).The BVRI magnitudes were corrected for extinction as given in Section 2.2.The blue bands were extrapolated in the late phases using a constant color, as is derived from the multi-band data on an epoch in which such data are available.The flux integration was performed over the optical wavelengths, and the resultant quasi-bolometric light curve of SN 2022crv is plotted with other SNe IIb and Ib in Figure 12.The very early points of SN 2022crv were generated by converting 2022crv The blackbody fits for radius and temperatures using BgVRIJHK magnitudes are plotted in Figure 13.The temperature of the photosphere decreased from 9500 K to 5500 K from -9 d to + 18 d, indicating the cooling of the SN ejecta.During the same period, the radius of the outer envelope increased by 50 AU from 45 AU to 95 AU.On the other hand, using our spectroscopic measurements (an average photospheric velocity of ∼ 8, 000 km s −1 during the period in consideration), the radius must have been increased by ∼ 130 AU during the ∼ 27 d time interval.The spectroscopic and photometric indicators therefore agree within a factor of three, but the difference may be non-negligible.We suspect this might be due to underestimated bolometric luminosity and/or the use of a constant value of photospheric velocity, which is actually decelerating.
The peak properties are mainly determined by the radioactive 56 Ni synthesized in the explosion, the ejecta mass M ej , and the kinetic energy E k of the ejecta.We modeled the early photospheric phase of SN 2022crv using the formulation by Valenti et al. (2008a) which is based on the original formulation by Arnett (1982).The major assumptions are spherical symmetry, homologous expansion, and a constant opacity (κ opt ).The free parameters are M N i (affecting the peak luminosity) and the diffusion time scale τ m (controlling the width of the bolometric light curve).Assuming uniform density, the ejecta kinetic energy E k and τ m are related as: where β ≈ 13.8 is a constant of integration (Arnett 1982) and c is the speed of light.The optical opacity κ opt is adapted to be 0.07 cm 2 g −1 as is frequently adopted for SE-SNe (e.g.Chugai 2000;Taddia et al. 2018).The light curve of SN 2022crv was fitted with this analytical form using least-square optimization up to +36 d postexplosion (Figure 14).With the probable existence of the H-rich envelope attached to SN 2022crv, it is highly interesting to constrain the nature of the envelope using the earliest photometric points.The first point, as reported by AT-LAS, indeed shows a hint of an excessive emission, which might signal the early envelope-cooling emission (Figures 8 and 14).
To constrain the nature of the H-rich envelope, we used the semi-analytical models by Nagy et al. (2014) and Nagy & Vinkó (2016).Here, the bolometric light curve is modeled using a two-component ejecta configuration: an extended, low-mass, H-rich outer envelope and a compact He-rich core.The light curve is thus the combination of radiation from the shock-heated Hejecta and the radioactive decay of 56 Ni to 56 Co.We adopt κ opt = 0.24 cm 2 g −1 for the outer layer (Arnett & Fu 1989) and κ opt = 0.06 cm 2 g −1 for the core.As the photon diffusion time scale is much shorter in the outer shell than in the core, the contributions of the two regions to the overall light curve are well separated.In practice, we first fitted the core properties and then constrained the envelope properties since the strength of the early-cooling emission depends on the underlying 56 Ni-heating light curve.
The best-fit values of 56 Ni, M ej (core and shell), E k (kinetic energy), E Th (thermal energy), and the radii of the core and shell are given in Table 3.Given the uncertainties in the first ATLAS point and the underlying 56 Ni-heating contribution, we regard the envelope radius obtained here as an upper limit for a given envelope mass (see Section 7 for further details).We find that the constraints of R < 1−3 R ⊙ and M ∼ 0.015-0.05M ⊙ can be placed for the properties of the H-rich envelope of SN 2022crv.The detailed interpretation of the progenitor compactness and its correlation with the theoretical models is given in Section 7. The derived core parameters obtained by this analysis (Nagy & Vinkó 2016) are largely consistent with those derived by the ones obtained with the model of Valenti et al. (2008b).This is not surprising as the assumptions are similar between the two formulations.The kinetic energy is overestimated in the latter model, but it likely stems from some difference in details about how the diffusion time scale is converted to the mass and kinetic energy.
Combining all the above analyses of SN 2022crv, we find that the 56 Ni mass is M Ni = 0.12±0.05M ⊙ and ejecta mass is in the range of 3.2-3.9M ⊙ .The ejecta parameters of SN 2022crv are consistent with the range found for SE-SNe but with the ejecta mass on the higher side (Table 4).
6. RADIO OBSERVATIONS SN 2022crv was first observed at radio wavelengths with the Australia Telescope Compact Array (ATCA) on 2022-03-01 UT (Ryder et al. 2022a,b), and radio emission was clearly detected at both 9.0 and 5.5 GHz.Radio monitoring with the ATCA at these 2 frequencies has continued for over a year, and the results are reported in Table 9. Frequent observations of the  nearby source PKS B0919-260 allowed us to monitor and correct for variations in gain and phase for each run.The data for each 2 GHz bandwidth have been edited and calibrated using tasks in the miriad software package (Sault et al. 1995).Robust weighting was employed in imaging the visibilities, and after multifrequency synthesis and deconvolution, the flux densities were obtained from Gaussian fitting to the elliptical beam shape.As the ATCA primary flux calibrator PKS B1934-638 was sometimes not accessible during the observation periods, the relatively stable PKS B0823-500 was observed instead.This source is routinely monitored by ATCA staff, and by comparing its measured interpolated flux density with that measured from an image of the source on the day of observation, the flux densities for SN 2022crv have been placed on a uniform flux scale regardless of the flux calibrator adopted.We also carried out radio observations of SN 2022crv with the upgraded Giant Metrewave Radio Telescope (uGMRT) from 2022-03-31.58 UT to 2022-12-21.04 UT at multiple epochs.The observations were done in band-3 (250−500 MHz), band-4 (550−950 MHz), and band-5 (1050−1450 MHz).The data were recorded in the standard continuum mode with an integration time of 10 seconds.We used 200 MHz bandwidth in band-3 and 400 MHz bandwidth in bands-4 and 5, split into 2048 channels.3C147 was used as the flux density calibrator, and J0837−198 was used as the phase calibrator.We used the Astronomical Image Processing system (AIPS; Greisen 2003) to analyze the uGMRT data and followed standard procedures from Nayana et al. (2017).The calibrated visibilities of the target source were imaged using AIPS task IMAGR.We performed a few rounds of phase-only self-calibration to improve image quality.The flux density was determined by fitting a two-dimensional Gaussian at the SN position using AIPS task JMFIT.We present the details of uGMRT observations and flux densities in Table 9.

Radio light curves and spectral indices
We detect radio emission from SN 2022crv at frequencies from 0.69 to 9.0 GHz during t ∼ 12 − 412 d.The flux densities initially rise at all frequencies, reaching a peak spectral luminosity at 5.5 GHz of 6.51 × 10 27 erg s −1 Hz −1 at t ∼ 100 d.The near-simultaneous spectral index, α (F ∝ ν α ) between 9 GHz and 5.5 GHz is 0.92 ± 0.15 at t ∼ 12 d and approaches a value of − 1.3 by t ∼ 412 d as the light curve transitions from the optically thick to the thin regime.The 1.4/0.7 GHz spectral indices are α = 1.35±0.42,1.41±0.27,and 2.15 ± 0.23 at t ∼ 123, 200, and 308 d, respectively, flatter than the standard optically-thick limit (5/2).This can be attributed to the inhomogeneities in the magnetic field and/or relativistic electron distribution in the emitting region (Björnsson & Keshavarzi 2017;Nayana & Chandra 2021;Chandra et al. 2019)

Radio emission model
In the CSM interaction model of radio SNe, the emission is associated with a forward shock created as the SN ejecta interacts with the wind from the progenitor star before the explosion (Chevalier 1982b).At the shock, electrons are accelerated to relativistic velocities in amplified magnetic fields and emit synchrotron radiation.A fraction of the post-shock energy density is distributed into magnetic fields (ϵ B ) and relativistic electrons (ϵ e ), which are assumed to be constant throughout the evolution of the ejecta.The low-frequency emission is significantly suppressed by an absorption component.The absorption can be either free-free absorption (FFA) due to the ionized wind material along the line of sight (Weiler et al. 1986) or synchrotron self-absorption (SSA) due to the relativistic electrons that generate radio emission (Chevalier 1998).The radio flux density initially rises rapidly and then declines, tracing the transition from the optically thick to the thin regime.
We modeled the radio light curves with a standard SSA model (Chevalier 1998), as the SSA likely dominates for a typical situation found for SE-SNe (Chevalier & Fransson 2006).The spectral and temporal evolution of radio flux densities F (ν, t) is given by In the above equations, K 1 and K 2 are the flux density and optical depth normalization constants, respectively; τ SSA represents the optical depth due to synchrotron self-absorption; a and b denote the temporal indices of radio flux densities in the optically thick and thin regime, respectively; and p is the power-law index of the relativistic electron energy distribution (N (E) ∝ E −p ).We model the radio light curves keeping K 1 , K 2 , a, b, and p as free parameters.We use the Markov Chain Monte Carlo (MCMC) method and choose 32 walkers and 5000 steps to estimate the best-fit values.We execute the fit using the Python package emcee (Foreman-Mackey et al. 2013).The best-fit values of the parameters are K 1 = 0.22 +0.02 −0.02 , K 2 = 93 +22 −18 , a = 1.80 +0.04 −0.04 , b = 0.58 +0.06  −0.06 , and p = 2.80 +0.27 −0.26 .We present the bestfit model and the observed flux densities in Figure 16.The corner plot showing how well the parameters are constrained is shown in Figure 20.
One can derive the shock radius (R s ) and magnetic fields (B) using the peak frequency (ν p ) and peak flux density (F p ) in the SSA scenario (using equations 13 and 14 of Chevalier 1998).We use ν p = 9 and 5.5 GHz and F p from the best-fit modeled light curves to derive R s = (1.07 ± 0.11) × 10 16 cm and (1.79 ± 0.19) × 10 16 cm at ∼ 37 and 75 d, respectively.The corresponding mean shock velocity (R/t) is v ∼ 0.1 c.The post-shock magnetic fields are B = 0.87 ± 0.02 and 0.53 ± 0.01 G, at ∼ 37 and 75 d, respectively.We also estimate the mass-loss rate to be Ṁ ∼ (1.9−2.8)× 10 −5 M ⊙ yr −1 at these epochs from the magnetic field scaling relation (equation 19 of Chevalier 1998) for a wind velocity of v w ∼ 1000 km s −1 (typical of compact WR stars) and ϵ B = 0.33.

PROGENITOR OF SN 2022CRV
In Section 5, we employed the Nagy & Vinkó (2016) bolometric light curve modeling to provide the upper limit for the envelope radius (R env ) as a function of the envelope mass (M env ).The best-fit mass is derived to Figure 17.The plot shows the relation between the radius and mass of the envelope for the SNe IIb progenitor models (blue dots) generated by Ouchi & Maeda (2017).The analytical relation derived by Ouchi & Maeda (2017) that explains the properties of the numerical evolution models is shown by the black line.Also shown here are the possible combinations of the radius and envelope mass of SN 2022crv (red dots) generated from the semi-analytical model of Nagy & Vinkó (2016); note that the radius here is an upper limit for a given mass, and therefore the blue-shaded area is the allowed region for SN 2022crv.
be 0.015-0.05M ⊙ and the radius is constrained to be < 1-3 R ⊙ .The constraint is shown in Figure 17.In this section we check whether the derived range of the envelope properties is consistent with the stellar evolution theory, and then provide a further constraint on the envelope properties assuming that the nature of the progenitor is explained by existing stellar evolution models.
Ouchi & Maeda (2017) (OM17) calculated a grid of binary evolution models for SNe IIb.They further provide a sequence of single-star evolution models that can mimic the binary evolution scenario.In the following, we use this single-star (pseudo-binary-star) model sequence for discussion.Figure 17 shows the relationship between the radius and mass of the envelope for their SN IIb models (blue circles).The key property here is that the radius decreases as the envelope mass decreases below ln(M env [M ⊙ ] ∼ -2), as a result of an equilibrium configuration in the radiative envelope regime.OM17 also showed that this behavior can be approximately described analytically (solid black line), following an argument similar to that presented by Cox & Salpeter (1961).This analytic curve could be used to infer the model behavior in the very compact regime (R env ≤ R ⊙ ) for which the numerical models are unavailable.
Our allowed range of R env and M env (see Figure 17; blue shaded space) marginally overlaps with the model prediction only when the envelope properties are as follows; M env ∼ 0.05 M ⊙ and R env ∼ 3 R ⊙ .Taking these as the most likely nature of the H-rich envelope attached to SN 2022crv, we conclude that the progenitor of SN 2022crv is very compact.The low estimated value of H envelope mass puts SN 2022crv in the category of the cSN IIb class.The radius derived for SN 2022crv is among the smallest so far derived for SNe IIb, and overlaps with SNe Ib within the uncertainties of radii derived for individual objects.As such, SN 2022crv stands as the most compact progenitor for SNe IIb, representing a boundary between SNe IIb and Ib (Gangopadhyay et al. 2018;Sravan et al. 2020;Gilkis & Arcavi 2022).
We plot SN 2022crv (red star symbol) in the peak spectral luminosity vs time to peak (L p − t p ) diagram along with other CCSNe in Figure 18.The dotted lines represent the mean shock velocities in the SSA scenario for p = 3, assuming the equipartition of energy between relativistic electrons and magnetic fields (ϵ e = ϵ B = 0.33).Chevalier & Soderberg (2010) divided the radio-bright SNe IIb into two categories based on their radio properties: SNe cIIb (with compact progenitors), having faster shocks and less dense CSM; and SNe eIIb (with extended progenitors) having slower shocks and denser CSMs (Maeda et al. 2015)  this agrees with the optical behavior of SN 2022crv transitioning from SNe IIb to SNe Ib, as explained in the previous sections.
Figure 19 displays how SN 2022crv is placed in the SN IIb/Ib/Ic progenitor property space.The CSM density is measured in terms of A * using the relation ρ CSM = 5 x 10 11 A * r −2 .The CSM density is estimated from the radio and X-ray combined analyses for SN 1993J (Fransson et al. 1996) and 2011dh (Maeda et al. 2014); from the radio and optical combined analysis for SN 2013df (Maeda et al. 2015); and from the radio data only for SN 2008ax (Chevalier & Soderberg 2010).The estimate of the CSM density for SNe Ib/c is based on radio data alone (Chevalier & Fransson 2006).For the comparison SNe IIb, the radii are taken from progenitor detection (Ouchi & Maeda 2017).On the other hand, the radii of SNe Ib/c are not strongly constrained (see Maeda et al. 2012).As the CSM densities and progenitor radii here do not represent a result of systematic analysis, we note that a substantial uncertainty is involved, and thus it should be taken as a demonstration.Further, additional uncertainty is introduced by the mass-loss wind velocity (assumed to be 20 km s −1 for SNe IIb and 1000 km s −1 for SNe Ib.), which has not been directly obtained for most of the samples.With these caveats in mind, Maeda et al. (2015) formulated a relation between the early-phase cooling emission/progenitor radius and the CSM density/mass-loss rate.For SN 2022crv, we estimated our CSM density and mass-loss rate using the fits to the radio data of 5.5 GHz (t ∼ 75 d), assuming a wind velocity of 20 km s −1 .The progenitor radius for SN 2022crv has been constrained above (see Section 5).
The first panel of Figure 19 shows that the CSM density of SN 2022crv is on the higher end among the sample of SNe Ib/c, similar to cSNe IIb SNe 2008ax and 2011dh.For more extended progenitor objects like SNe 1993J and 2013df, the CSM densities are much higher than for cSNe IIb, and about two orders of magnitude greater than for typical SNe Ib/c.For the shock-cooling luminosity (the value plotted in the x-axis), the eSNe IIb showed signatures of the shock-cooling phase, while for cSNe IIb and Ib/c, only deep limits have been obtained (Cao et al. 2013), including SN 2022crv for which we used the optical-NIR data to limit shock cooling luminosity.
The second panel of Figure 19 relates the CSM densities with the progenitor's radius.The progenitor radius of ∼ 3 R ⊙ obtained for SN 2022crv fits into the relation among eSNe IIb, cSNe IIb, and SNe Ibc, i.e., higher CSM densities for more extended progenitors (Maeda et al. 2015).The progenitor radius of SN 2022crv makes it one of the most compact SNe IIb known so far, penetrating into the regime of SNe Ib/c.
The third panel of Figure 19 shows a more direct (but less certain) relationship between the nature of the progenitor and the mass-loss rates in the final stages of SN evolution.We observe a good overlap between the massloss rates of the SNe IIb and SNe Ib/c; indeed, the relation in the nature of the progenitors is diluted if one uses the mass-loss rate instead of the CSM density.A relatively high mass-loss rate inferred for SN 2022crv, despite its compact nature, further reverses the monotonic relation.Therefore, the relation between the progenitor radius and the mass-loss rate may be more complicated than postulated by Maeda et al. (2015).The binary interaction is believed to play a key role for these SNe IIb (e.g., Benvenuto et al. 2013;Ouchi & Maeda 2017) and thus the mass-loss rate is highly affected by the binary interaction (e.g., Smith 2014, for a review).Depending on the binary separation and mass ratio, the binary evolution could lead to a diversity in the mass-transfer history (e.g., Maeda et al. 2023a).
However, to further quantify the relations, especially the one related to the mass-loss rate, a more systematic approach will be required.The estimated CSM density can vary by orders of magnitudes when only the radio data are used (which is the case for SN 2022crv); the assumption of ϵ e = ϵ B = 0.33 adopted for the radio modeling of SN 2022crv is indeed very simplified and may be considered as an extreme assumption (Maeda et al. 2021(Maeda et al. , 2023b)).Another issue is the wind velocity (v w ), which is generally not well constrained (again, the case for SN 2022crv).

SUMMARY AND CONCLUSION
We present long-term photometric (up to +86 d) and spectroscopic (up to +33 d) observations of SN IIb/Ib 2022crv.The spectral evolution of SN 2022crv implies that it is a SN IIb that retained a thin H-envelope, showing a quick transition to SN Ib.The spectral evolution shows a prominent dip at 6200 Å, which is well reproduced by SYNAPPS spectral modeling as a blend of Si ii and Hα.The evolution of the EW and the wavelength of the absorption minimum of the feature (i.e., the velocity) also supports this identification as well as a quick transition from SN IIb to SN Ib.
The multi-band optical light curve shows the radioactive peak without strong shock-cooling emission, similar to what is seen for cSNe IIb.The absolute magnitude (M V =−17.82±0.17mag) and decay rate (∆m 15 (V )=0.76 ± 0.04) indicate SN 2022crv is a relatively bright and slowly-declining member of the SE-SNe sub-class.The bolometric light curve modeling inferred M Ni = 0.12±0.05M ⊙ and the ejecta mass in the range between 3.2-3.9M ⊙ .With the very early ATLAS data, we could place an upper limit on the radius of the envelope to be 3 R ⊙ and on the H envelope mass to be 0.05 M ⊙ .Our observations of SN 2022crv show that it is one of the most compact SNe IIb progenitors with a very thin H envelope (consistent with the recent reports by Gilkis & Arcavi 2022).Comparison with the SN IIb progenitor evolution models of Ouchi & Maeda (2017) shows that the upper limits, as mentioned above, most likely represent the envelope properties of SN 2022crv; it then represents one of the most compact SN IIb progenitors.This is a new constraint on the division of SN IIb and Ib progenitors.
Radio observations of SN 2022crv spanning 0.44-9.0GHz over a year indicated an interaction with a dense CSM.The SSA light curve modeling provides the bestfit shock radius values to be R s = (1.07 ± 0.11) × 10 16 cm on day 37 and (1.79 ± 0.19) × 10 16 on day 75, in-  (Chevalier & Fransson 2006), though they may contain a systematic uncertainty of up to an order of magnitude (Maeda 2012).
dicating the shock velocity of ∼ 0.1c.The estimated mass-loss rate is in the range of Ṁ ∼ (1.9 − 2.8) × 10 −5 M ⊙ yr −1 , which however involves various uncertainties and thus should be regarded as a rough estimate.In any case, the place of SN 2022crv in the radio luminosity phase diagram also predicts that the progenitor is compact, similar to SN Ib/cSN IIb.SN 2022crv is one of the radio-bright SN, having higher CSM densities than cSNe IIb/Ib, indicating a high CSM density among the group of cSNe IIb and SNe Ib.
The progenitor radius and the CSM density obtained for SN 2022crv fit into the relation among the progenitors of eSNe IIb, cSNe IIb, and SNe Ibc, i.e., higher CSM densities for more extended progenitors.On the other hand, in terms of the mass-loss rate, a high mass-loss rate inferred for SN 2022crv might be viewed as an outlier in the relation suggested so far (i.e., higher mass-loss rates for more extended progenitors).However, further quantifying this will require systematic analyses of the whole sample based on a uniform method, with a need to refine the treatment of the mass-loss wind velocity.Such investigation will be important to further understand the role of binary interaction toward SE-SNe.
While no CSM interaction signatures are seen in the optical spectra of SN 2022crv up to +33 d, we find interaction signatures in the radio waveband.If a CSM is present but not extremely dense, then signatures in the optical become challenging to discern.There is a possibility that it might show up at later stages once the radioactive power decreases, as found for a few SNe IIb so far.Further observations in the years ahead are thus interesting, not only for SN 2022crv but for SE-SNe in general, to reveal whether any interaction signature emerges later.

Figure 1 .
Figure 1.The complete spectral evolution of SN 2022crv beginning at −15.3 d, up to +33.3 d.The earliest spectrum shows a prominent Hα dip which slowly diminishes around the maximum, and prominent lines of He i start developing.

Figure 2 .
Figure 2. The pre-maximum spectrum of SN 2022crv as compared with other members of the SNe IIb and Ib classes.The pink shaded region (top) shows the SN Ib comparison sample, the blue shaded region (middle) shows the eSN IIb, and the green shaded region (bottom) shows the cSNe IIb comparison sample.The labels include the SN name plus the date since maximum light.The data for the comparison sample are taken from WiseRep and the papers given in Table 1.

Figure 3 .
Figure 3.The post-maximum spectrum of SN 2022crv as compared with a group of SE-SNe.The layout and the comparison samples follow the descriptions in Figure 2.

Figure 4 .
Figure 4.The late-time spectrum of SN 2022crv as compared with a group of SE-SNe.The layout and the comparison samples follow the descriptions in Figure 2.

Figure 5 .
Figure5.The EW evolution of the Hα+Si ii (upper) and the He i 5876 Å (lower) lines.For the comparison sample, the EWs of Hα and He i are calculated for those SNe referenced in Table1.

Figure 8 .Figure 9 .Figure 10 .
Figure 8.The apparent magnitude light curves of SN 2022crv in the BgVRIJHK filters.The g-band is adapted from Seimei-TriCCS and ASASSN.

Figure 13 .
Figure 13.The radius and temperature evolution of SN 2022crv up to + 86 d.The radii and temperatures are estimated under the blackbody approximation.

Figure 14 .
Figure14.The bolometric light curve of SN 2022crv is plotted along with the fit with the toy model ofValenti et al. (2008b).The best fit from the two-component semianalytical model proposed byNagy & Vinkó (2016) is also shown along with the contribution from the individual core and shell components.
The 56 Ni mass thus obtained is M Ni = 0.126±0.021M ⊙ and the diffusion time is τ m = 16.10±0.5d.The ejecta mass and kinetic energy thus obtained are M ej = 3.19 M ⊙ and E k = 1.72×10 51 erg, respectively.
. Radio light curves and the evolution of radio spectral indices are shown in Figure 15.

Figure 16 .
Figure 16.Radio light curves of SN 2022crv at frequencies ν = 0.69 − 9 GHz.The solid red curves represent the best-fit SSA model.The filled black circles denote the observed flux densities.

Figure 18 .
Figure 18.The peak spectral luminosities versus time to peak for some well-observed CCSNe from the literature (Bietenholz et al. 2021, and references therein).The position of SN 2022crv is denoted as per the 5.5 GHz light curve.The dotted lines represent the mean velocities of radio-emitting shells in an SSA scenario with p = 3.

Figure 19 .
Figure19.The relation between the pseudo-bolometric optical-NIR luminosity in the 'shock-cooling phase' and the density of the CSM is shown in the first panel.Upper limits for the cooling luminosities are derived for the objects that lacked a primary peak, like SN 2008ax and SN 2022crv.The second panel shows the relation between the CSM density and the progenitor's radius, and the third panel describes the relation between mass-loss rates and the progenitor's radius.The typical CSM densities assumed for SNe Ib/c are indicated by the shaded region(Chevalier & Fransson 2006), though they may contain a systematic uncertainty of up to an order of magnitude(Maeda 2012).

Figure 20 .
Figure 20.The corner plot shows the results of MCMC modeling of the SN 2022crv radio data with the SSA model.The parameters here are according to Eq 3 and 4, respectively.
spectra, we used H i, He i, H SiFigure 6. SYNAPPS modeling of the early (− 9.7 d) up to a few weeks post-maximum (+23.2 d) spectra of SN 2022crv (shown in black), marked with the lines identified from the spectral modeling.The 6200 Å dip is reproduced by a combination of Hα and Si ii, which we can see in the inset showing the spectra at − 15.3 d and − 9.7 d.

Table 2 .
Observed parameters of SN 2022crv

R
Figure 11.Peak absolute magnitude versus ∆m15 of SN 2022crv in comparison with a sample of SNe IIb and SNe Ib from Taddia et al. (2018) and Table1.

Table 3 .
Best fit parameters derived from the bolometric light curve modelling ofNagy and Vinko 2014,2016.
. This figure indicates that SN 2022crv is one of the radio-bright SN IIb/Ib in the comparison sample.The position of SN 2022crv in the L p − t p plane suggests that the SN falls at a boundary between radio-bright SN Ibc and SN IIb;

Table 7 .
Log of Near-Infrared observations of SN 2022crv from HONIR mounted on 1.5m KT.The magnitudes reported are in Vega system.
a Time since V-Band Maximum

Table 8 .
Log of spectroscopic observations of SN 2022crv.The phase is measured with respect to V -band maximum).