A Search for Relativistic Ejecta in a Sample of ZTF Broad-lined Type Ic Supernovae

All photometric observations presented in this work were conducted with the Palomar Schmidt 48-inch (P48) Samuel Oschin telescope as part of the ZTF survey (Bellm et al. 2019; Graham et al. 2019), using the ZTF camera (Dekany et al. 2020). In default observing mode, ZTF uses 30 s exposures, and survey observations are carried out in r and g band, down to a typical limiting magnitude of ≈20.5 mag. P48 light curves were derived using the ZTF pipeline (Masci et al. 2019) and forced photometry (see Yao et al. 2019). Hereafter, all reported magnitudes are in the AB system. The P48 light curves are shown in Figures 1 and 2. All the light curves presented in this work will be made public on the Weizmann Interactive Supernova Data Repository ²⁴ (WISeREP; Yaron & Gal-Yam 2012).

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Preliminary spectral type classifications of several of the SNe in our sample were obtained with the Spectral Energy Distribution Machine (SEDM) mounted on the Palomar 60-inch telescope (P60), and quickly reported to the Transient Name Server (TNS). SEDM is a very-low-resolution (R ∼ 100) integral field unit spectrograph optimized for transient classification with high observing efficiency (Blagorodnova et al. 2018; Rigault et al. 2019).

After initial classification, typically further spectroscopic observations are carried out as part of the ZTF transient follow-up programs to confirm and/or improve classification, and to characterize the time-evolving spectral properties of interesting events. Among the series of spectra obtained for each of the SNe presented in this work, we select one good quality photospheric phase spectrum (Figures 3 and 4; gray) on which we run SNID (Blondin & Tonry 2007) to obtain the best match to an SN Ic-BL template (black), after clipping the host emission lines and fixing the redshift to that derived either from the Sloan Digital Sky Survey (SDSS) host galaxy or from spectral line fitting (Hα; see the Appendix for further details). Hence, in addition to SEDM, in this work we also made use of the following instruments: the Double Spectrograph (DBSP; Oke et al. 1995), a low-to-medium-resolution grating instrument for the Palomar 200-inch telescope Cassegrain focus that uses a dichroic to split light into separate red and blue channels observed simultaneously; the Low Resolution Imaging Spectrometer (LRIS; Oke & Gunn 1982; Oke et al. 1995), a visible-wavelength imaging and spectroscopy instrument operating at the Cassegrain focus of Keck I; and the Alhambra Faint Object Spectrograph and Camera (ALFOSC), a CCD camera and spectrograph installed at the Nordic Optical Telescope (NOT; Djupvik & Andersen 2010). All spectra presented in this work will be made public on WISeREP.

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For eight of the 16 SNe presented in this work we carried out follow-up observations in X-rays using the X-Ray Telescope (XRT; Burrows et al. 2005) on the Neil Gehrels Swift Observatory (Gehrels et al. 2004). We selected these SNe based on a combination of opportunistic availability of observing time via Swift Guest Investigator programs led by our team (for SNe 2020jqm, 2020lao, 2020rph, and 2021ywf; see also column 7 in Table 3), and the relevance of some events for specific ZTF experiments, such as the census of the local universe and the redshift completeness fraction ones (for SNe 2018etk, 2018hom, and 2019hsx; see also Graham et al. 2019). For SN 2021hyz, Swift observations were carried out serendipitously via an independent program (PI: Paolo Giommi).

We analyzed these observations using the online XRT tools, ²⁵ as described in Evans et al. (2009). We correct for Galactic absorption, and adopt a power-law spectrum with photon index Γ = 2 for count rate to flux conversion for nondetections, and for detections (two out of nine events) where the number of photons collected is too small to enable a meaningful spectral fit (one out of two detections). Table 4 presents the results from coadding all observations of each source.

Table 3. Properties of the Radio Ejecta of the SNe in Our Sample for Which We Detect a Radio Counterpart

SN	βs	$\dot{M}$	Er	Er /Ek
		(M⊙ yr−1)	(erg)
2018hom	0.35	1.1 × 10−6	3.6 × 1047	<0.04%
2020jqm	0.33(0.048)	3.0 × 10−6(2.7 × 10−4)	2.0(5.7) × 1048	0.04%(0.1%)
2020tkx	0.14	1.7 × 10−5	1.1 × 1048	<0.07%
2021ywf	0.19	2.2 × 10−6	2.3 × 1047	0.02%

Note. We report the SN name, the estimated SN shock speed normalized to the speed of light (β_s ), the mass-loss rate of the pre-SN progenitor ($\dot{M}$), the energy coupled to the fastest (radio-emitting) ejecta (E_r ), and the ratio between the last and the kinetic energy of the explosion (estimated from the optical light curve modeling, E_k ). See Section 3.5 for a discussion of these estimates and their uncertainties.

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Table 4. Swift/XRT Observations of Nine of the 16 SNe Ic-BL in Our Sample

SN	TXRT	TXRT – T ${}_{\exp }$	Exp.	F0.3–10 keV
	(MJD)	(d)	(ks)	(10−14 erg cm−2 s−1)
2018etk	58377.85	49 ± 1	4.8	<4.2
2018hom	58426.02	${9.0}_{-0.7}^{+0.4}$	4.3	<6.4
2019hsx	58684.15	${53}_{-0.4}^{+0.5}$	15	${6.2}_{-1.8}^{+2.3}$
2020jqm	59002.09	23 ± 1	7.4	<3.3
2020lao	59007.40	14 ± 1	14	<2.9
2020rph	59088.89	16.43 ± 0.02	7.5	<3.6
2020tkx	59125.38	22 ± 4	8.1	<3.3
2021hyz	59373.09	66.9 ± 0.9	4.7	<3.5
2021ywf	59487.60	19.7 ± 0.5	7.2	${5.3}_{-3.3}^{+4.9}$

Note. We provide the MJD of the Swift observations, the epoch of the XRT observations since the estimated explosion time (Table 4) in the oberver's frame, the XRT exposure time, and the 0.3–10 keV unabsorbed flux measurements (or 3σ upper limits for nondetections).

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We observed the fields of the SNe Ic-BL in our sample with the VLA via several of our programs using various array configurations and receiver settings (Table 2).

The VLA raw data were calibrated in CASA (McMullin et al. 2007) using the automated VLA calibration pipeline. After manual inspection for further flagging, the calibrated data were imaged using the tclean task. Peak fluxes were measured from the cleaned images using imstat and circular regions centered on the optical positions of the SNe, with radius equal to the nominal width (FWHM) of the VLA synthesized beam (see Table 2). The rms noise values were estimated with imstat from the residual images. Errors on the measured peak flux densities in Table 2 are calculated adding a 5% error in quadrature to the measured rms values. This accounts for systematic uncertainties on the absolute flux calibration.

We checked all of our detections (peak flux density above 3σ) for the source morphology (extended versus point-like), and ratio between the integrated and peak fluxes using the CASA task imfit. All sources for which these checks provided evidence for extended or marginally resolved emission are marked accordingly in Table 2. For nondetections, upper limits on the radio flux densities are given at the 3σ level unless otherwise noted.

We confirm the SN Type Ic-BL classification of each object in our sample by measuring the photospheric velocities (v_ph). SNe Ic-BL are characterized by high expansion velocities evident in the broadness of their spectral lines. A good proxy for the photospheric velocity is that derived from the maximum absorption position of Fe ii λ5169 (e.g., Modjaz et al. 2016). We caution, however, that estimating this velocity is not easy given the strong line blending. We first preprocessed one high-quality spectrum per object using the IDL routine WOMBAT, then smoothed the spectrum using the IDL routine SNspecFFTsmooth (Liu et al. 2016), and finally ran SESNSpectraLib (Liu et al. 2016; Modjaz et al. 2016) to obtain final velocity estimates.

In Figure 5 we show a comparison of the photospheric velocities estimated for the SNe in our sample with those derived from spectroscopic modeling for a number of SNe Ib/c. The velocities measured for our sample are compatible, within the measurement errors, with what was observed for the PTF/iPTF samples. Measured values for the photospheric velocities with the corresponding rest-frame phase in days since maximum r-band light of the spectra that were used to measure them are also reported in Table 5.

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Table 5. Optical Properties of the 16 SNe Ic-BL in Our Sample

SN	${T}_{r,\max }$	${M}_{r}^{\mathrm{peak}}$	${M}_{g}^{\mathrm{peak}}$	${{\rm{T}}}_{\exp }$ – T ${}_{r,\max }$	MNi	τm	vph( a )	Mej	Ek
	(MJD)	(AB mag)	(AB mag)	(d)	(M⊙)	(d)	(104 km s−1)	(M⊙)	(1051 erg)
2018etk	58337.40	−18.31 ± 0.03	−18.30 ± 0.02	−9 ± 1	${0.13}_{-0.02}^{+0.01}$	${5.0}_{-2}^{+2}$	2.6 ± 0.2 (5)	0.7 ± 0.5	3 ± 2
2018hom	58426.31	−19.30 ± 0.11	−18.91 ± 0.01	$-{9.3}_{-0.4}^{+0.7}$	0.4 ± 0.1	6.9 ± 0.2	1.7 ± 0.2 (27)	>0.7	>1
2018hxo	58403.76	−18.68 ± 0.06	−18.4 ± 0.1	$-{28.6}_{-0.3}^{+0.2}$	0.4 ± 0.2	6 ± 2	0.6 ± 0.1 (48)	>0.1	>0.02
2018jex	58457.01	−19.06 ± 0.02	−18.61 ± 0.04	−18.49 ± 0.04	${0.53}_{-0.06}^{+0.07}$	${13}_{-3}^{+2}$	1.8 ± 0.3 (8)	3 ± 1	7 ± 3
2019hsx	58647.07	−17.08 ± 0.02	−16.14 ± 0.04	$-{15.6}_{-0.5}^{+0.4}$	${0.07}_{-0.01}^{+0.01}$	12 ± 1	1.0 ± 0.2 (-0.2)	1.6 ± 0.4	1.0 ± 0.5
2019xcc	58844.59	−16.58 ± 0.06	−15.6 ± 0.2	−11 ± 2	0.04 ± 0.01	${5.0}_{-0.9}^{+1.4}$	2.4 ± 0.2 (6)	0.7 ± 0.3	2 ± 1
2020jqm	58996.21	−18.26 ± 0.02	−17.39 ± 0.04	−17 ± 1	${0.29}_{-0.04}^{+0.05}$	18 ± 2	1.3 ± 0.3 (-0.5)	5 ± 1	5 ± 3
2020lao	59003.92	−18.66 ± 0.02	−18.55 ± 0.02	−11 ± 1	0.23 ± 0.01	7.7 ± 0.2	1.8 ± 0.2 (9)	1.2 ± 0.2	2.5 ± 0.7
2020rph	59092.34	−17.48 ± 0.02	−16.94 ± 0.03	−19.88 ± 0.02	0.07 ± 0.01	${17.23}_{-0.9}^{+1.2}$	1.2 ± 0.5 (-1)	4 ± 2	3 ± 3
2020tkx	59116.50	−18.49 ± 0.05	−18.19 ± 0.03	−13 ± 4	0.22 ± 0.01	${10.9}_{-0.8}^{+0.7}$	1.32 ± 0.09 (53)	>1.5	>1.5
2021xv	59235.56	−18.92 ± 0.07	−18.99 ± 0.05	$-{12.8}_{-0.3}^{+0.2}$	${0.30}_{-0.02}^{+0.01}$	${7.7}_{-0.5}^{+0.7}$	1.3 ± 0.1 (3)	0.9 ± 0.1	1.0 ± 0.2
2021aug	59251.98	−19.42 ± 0.01	−19.32 ± 0.06	−24 ± 2	0.7 ± 0.1	17 ± 7	0.8 ± 0.3 (1)	3 ± 2	1 ± 1
2021epp	59291.83	−17.49 ± 0.03	−17.12 ± 0.09	−15 ± 1	0.12 ± 0.02	${17}_{-3}^{+4}$	1.4 ± 0.5 (-4)	5 ± 2	6 ± 5
2021htb	59321.56	−16.55 ± 0.03	−15.66 ± 0.07	−19.38 ± 0.02	0.04 ± 0.01	13 ± 2	1.0 ± 0.5 (-6)	1.8 ± 0.9	1 ± 1
2021hyz	59319.10	−18.83 ± 0.05	−18.81 ± 0.01	−12.9 ± 0.9	${0.29}_{-0.02}^{+0.01}$	${7.7}_{-0.4}^{+0.5}$	2.3 ± 0.3 (16)	>1.3	>4
2021ywf	59478.64	−17.10 ± 0.05	−16.5 ± 0.1	−10.7 ± 0.5	0.06 ± 0.01	8.9 ± 0.8	1.2 ± 0.1 (0.5)	1.1 ± 0.2	0.9 ± 0.3

Note. We list the SN name; the MJD of maximum light in r band; the absolute magnitude at r-band peak; the absolute magnitude at g-band peak; the explosion time estimated as days since r-band maximum (in the source's rest frame); the estimated nickel mass; the characteristic timescale of the bolometric light curve; the photospheric velocity; the ejecta mass; and the kinetic energy of the explosion. See Sections 3.1 and 3.2 for discussion.

^a Rest-frame phase days of the spectrum that was used to measure the velocity.

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We note that the spectra used to estimate the photospheric velocities of SN 2019xcc, SN 2020lao, and SN 2020jqm are different from those used for the classifications of those events as SNe Ic-BL (see the Appendix and Figure 3). This is because for spectral classification we prefer later-time but higher-resolution spectra, while for velocity measurements we prefer earlier-time spectra even if taken with the lower-resolution SEDM.

In our analysis we correct all ZTF photometry for Galactic extinction, using the MW color excess E(B − V)_MW toward the positions of the SNe. These are all obtained from Schlafly & Finkbeiner (2011). All reddening corrections are applied using the Cardelli et al. 1989 extinction law with R_V = 3.1. After correcting for MW extinction, we interpolate our P48 forced-photometry light curves using a Gaussian process via the GEORGE ²⁶ package with a stationary Matern32 kernel and the analytic functions of Bazin et al. (2009) as the mean for the flux form. As shown in Figure 1, the color evolution of the SNe in our sample are not too dissimilar with one another, which implies that the amount of additional host extinction is small. Hence, we set the host extinction to zero. Next, we derive bolometric light curves calculating bolometric corrections from the g- and r-band data following the empirical relations of Lyman et al. (2014, 2016). For SN 2018hxo, since there is only one g-band detection, we assume a constant bolometric correction to estimate its bolometric light curve. These bolometric light curves are shown in the bottom panel of Figure 2.

We estimate the explosion times ${{\rm{T}}}_{\exp }$ of the SNe in our sample as follows. For SN 2021aug, we fit the early ZTF light curve data following the method presented in Miller et al. (2020), where we fix the power-law index of the rising early-time temporal evolution to α = 2, and derive an estimate of the explosion time from the fit. For most of the other SNe in our sample, the ZTF r- and g-band light curves lack enough early-time data to determine an estimate of the explosion time following the formulation of Miller et al. (2020). For all these SNe we instead set the explosion time to the midpoint between the last nondetection prior to discovery, and the first detection. Results on ${{\rm{T}}}_{\exp }$ are reported in Table 5.

We fit the bolometric light curves around peak (−20 to 60 rest-frame days relative to peak) to a model using the Arnett formalism (Arnett 1982), with the nickel mass (M_Ni) and characteristic timescale τ_m as free parameters (see ,e.g., Equation A1 in Valenti et al. 2008). The derived values of M_Ni (Table 5) have a median of ≈0.22 M_⊙, compatible with the median value found for SNe Ic-BL in the PTF sample by Taddia et al. (2019), somewhat lower than for SN 1998bw for which the estimated nickel mass values are in the range (0.4–0.7) M_⊙, but comparable to the M_Ni ≈ 0.19–0.25 M_⊙ estimated for SN 2009bb (see, e.g., Lyman et al. 2016; Afsariardchi et al. 2021). We note that events such as SN 2019xcc and SN 2021htb have relatively low values of M_Ni, which are however compatible with the range of 0.02–0.05 M_⊙ expected for the nickel mass of magnetar-powered SNe Ic-BL (Nishimura et al. 2015; Suwa & Tominaga 2015; Chen et al. 2017). We also note that the median value of M_Ni derived for the sample of Ic-BL SNe presented here is compatible with the median value of ≈0.23 M_⊙ we find for the larger sample of ZTF Ic-BL presented in G. P. Srinivasaragavan et al. (2023, in preparation).

Next, from the measured characteristic timescale τ_m of the bolometric light curve, and the photospheric velocities estimated via spectral fitting (see the previous section) we derive the ejecta mass (M_ej) and the kinetic energy (E_k) via the following relations (see, e.g., Equations (1) and (2) in Lyman et al. 2016):

$$\begin{eqnarray}&&{\tau }_{m}^{2}{v}_{{\rm{p}}{\rm{h}},max}=\displaystyle \frac{2\kappa }{13.8c}{M}_{{\rm{e}}{\rm{j}}},\,\,\,\,\,\,{v}_{{\rm{p}}{\rm{h}},\,max}^{2}=\displaystyle \frac{5}{3}\displaystyle \frac{2{E}_{{\rm{k}}}}{{M}_{{\rm{e}}{\rm{j}}}},\end{eqnarray} \tag{ 1 }$$

where we assume a constant opacity of κ = 0.07 g cm⁻².

We note that to derive M_ej and E_k as described above we assume the photospheric velocity evolution is negligible within 15 days relative to the peak epoch, and use the spectral velocities measured within this time frame to estimate ${v}_{\mathrm{ph},\ \max }$ in Equation (1). However, there are four objects in our sample (SN 2018hom, SN 2018hxo, SN 2020tkx, and SN 2021hyz) for which the spectroscopic analysis constrained the photospheric velocity only after day 15 relative to the peak epoch. For these events, we only provide lower limits on the ejecta mass and kinetic energy (see Table 5).

Considering only the SNe in our sample for which we are able to measure the photospheric velocity within 15 days since the peak epoch, we derive median values for the ejecta masses and kinetic energies of 1.7 M_⊙ and 2.2 × 10⁵¹ erg, respectively. These are both a factor of ≈2 smaller than the median values derived for the PTF/iPTF sample of SNe Ic-BL (Taddia et al. 2019). This could be due to either an intrinsic effect, or to uncertainties in the measured photospheric velocities. In fact, we note that the photospheric velocity is expected to decrease very quickly after maximum light (see, e.g., Figure 5). Since the photospheric velocity in Equation (1) of the Arnett formulation is the one at peak, our estimates of ${v}_{\mathrm{ph},\max }$ could easily underestimate that velocity by a factor of ≈2 for many of the SNe in our sample. This would in turn yield an underestimate of M_ej by a factor of ≈2 (though the kinetic energy would be reduced by a larger factor). A more in-depth analysis of these trends and uncertainties will be presented in G. P. Srinivasaragavan et al. (2023, in preparation).

Based on the explosion dates derived for each object in Section 3.2 (Table 5), we searched for potential GRB coincidences in several online archives. No potential counterparts were identified in both spatial and temporal coincidence with either the Burst Alert Telescope (BAT; Barthelmy et al. 2005) on the Neil Gehrels Swift Observatory ²⁷ or the Gamma-ray Burst Monitor (GBM; Meegan et al. 2009) on Fermi. ²⁸

Several candidate counterparts were found with temporal coincidence in the online catalog from the KONUS instrument on the Wind satellite (SN 2018etk, SN 2018hom, SN 2019xcc, SN 2020jqm, SN 2020lao, SN 2020tkx, and SN 2021aug). However, given the relatively imprecise explosion date constraints for several of the events in our sample (see Table 5), and the coarse localization information from the KONUS instrument, we cannot firmly associate any of these GRBs with the SNe Ic-BL. In fact, given the rate of GRB detections by KONUS (∼0.42 d⁻¹) and the time window over which we searched for counterparts (30 days in total; derived from the explosion date constraints), the observed number of coincidences (13) is consistent with random fluctuations. Finally, none of the possible coincidences were identified in events with the explosion date constraints more precise than 1 day.

Seven of the nine SNe Ic-BL observed with Swift-XRT did not result in a significant detection. In Table 4 we report the derived 90% confidence flux upper limits in the 0.3–10 keV band after correcting for Galactic absorption (Willingale et al. 2013).

While observations of SN 2021ywf resulted in a ≈3.2σ detection significance (Gaussian equivalent), the limited number of photons (eight) precluded a meaningful spectral fit. Thus, a Γ = 2 power-law spectrum was adopted for the flux conversion for this source as well. We note that because of the relatively poor spatial resolution of Swift-XRT (estimated positional uncertainty of 91 radius at 90% confidence), we cannot entirely rule out unrelated emission from the host galaxy of SN 2021ywf (e.g., active galactic nuclei, X-ray binaries, diffuse host emission, etc.; see Figure 6).

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For SN 2019hsx we detected enough photons to perform a spectral fit for count rate to flux conversion. The spectrum is found to be relatively soft, with a best-fit power-law index of ${\rm{\Gamma }}={3.9}_{-2.1}^{+3.0}$. Our Swift observations of SN 2019hsx do not show significant evidence for variability of the source X-ray flux over the timescales of our follow up. While the lack of temporal variability is not particularly constraining given the low signal-to-noise ratios of individual epochs, we caution that also in this case the relatively poor spatial resolution of Swift-XRT (66 radius position uncertainty at 90% confidence) implies that unrelated emission from the host galaxy cannot be excluded (see Figure 6).

The constraints derived from the Swift-XRT observations can be compared with the X-ray light curves of low-luminosity GRBs, or models of GRB afterglows observed slightly off-axis. For the latter, we use the numerical model by van Eerten & MacFadyen (2011) and van Eerten et al. (2012). We assume equal energies in the electrons and magnetic fields (_B = _e = 0.1), and an interstellar medium (ISM) of density n = 1–10 cm⁻³. We note that a constant-density ISM (rather than a wind profile) has been shown to fit the majority of GRB afterglow light curves, implying that most GRB progenitors might have relatively small wind termination-shock radii (Schulze et al. 2011). We generate the model light curves for a nominal redshift of z = 0.05 and then convert the predicted flux densities into X-ray luminosities by integrating over the 0.3–10 keV energy range and neglecting the small redshift corrections. We plot the model light curves in Figure 7, for various energies, different power-law indices p of the electron energy distribution, and various off-axis angles (relative to a jet opening angle, set to θ_j = 0.2). In the same figure we also plot the X-ray light curves of low-luminosity GRBs for comparison (neglecting redshift corrections). As evident from this figure, our Swift/XRT upper limits (downward-pointing triangles) exclude X-ray afterglows associated with higher-energy GRBs observed slightly off-axis. However, X-ray emission as faint as the afterglow of the low-luminosity GRB 980425 cannot be excluded. As we discuss in the next section, radio data collected with the VLA enable us to exclude GRB 980425/SN 1998bw-like emission for most of the SNe in our sample.

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As evident from Table 2, we have obtained at least one radio detection for 11 of the 16 SNe in our sample. None of these 11 radio sources were found to be coincident with known radio sources in the VLA FIRST (Becker et al. 1995) catalog (using a search radius of 30'' around the optical SN positions). This is not surprising since the FIRST survey had a typical rms sensitivity of ≈0.15 mJy at 1.4 GHz, much shallower than the deep VLA follow-up observations carried out within this follow-up program. We also checked the quick look images from the VLA Sky Survey (VLASS), which reach a typical rms sensitivity of ≈0.12 mJy at 3 GHz (Law et al. 2018; Villarreal Hernández & Andernach 2018). We could find images for all but one (SN 2021epp) of the fields containing the 16 SNe BL-Ic in our sample. The VLASS images did not provide any radio detection at the locations of the SNe in our sample.

Five of the 11 SNe Ic-BL with radio detections are associated with extended or marginally resolved radio emission. Two other radio-detected events (SN 2020rph and SN 2021hyz) appear point-like in our images, but show no evidence for significant variability of the detected radio flux densities over the timescales of our observations. Thus, for a total of seven out of 11 SNe Ic-BL with radio detections (see Figure 8), we consider the measured flux densities as upper limits corresponding to the brightnesses of their host galaxies, similarly to what was done in, e.g., Soderberg et al. (2006a) and Corsi et al. (2016). The remaining four SNe Ic-BL with radio detections (Figure 9) are compatible with point sources (SN 2018hom, SN 2020jqm, SN 2020tkx, and SN 2021ywf), and all but one (SN 2018hom) had more than one observation in the radio via which we were able to establish the presence of substantial variability of the radio flux density. Hereafter we consider these four detections as genuine radio SN counterparts, though we stress that with only one observation of SN 2018hom we cannot rule out a contribution from host-galaxy emission, especially given that the radio follow up of this event was carried out with the VLA in its most compact (D) configuration with poorer angular resolution.

In summary, our radio follow-up campaign of 16 SNe Ic-BL resulted in four radio counterpart detections, and 12 deep upper limits on associated radio counterparts.

3.5.1. Fraction of SN 1998bw-like SNe Ic-BL

The local rate of SNe Ic-BL is estimated to be ≈5% of the core-collapse SN rate (Li et al. 2011; Shivvers et al. 2017) or ≈5 × 10³ Gpc⁻³ yr⁻¹ assuming a core-collapse SN rate of ≈10⁵ Gpc⁻³ yr⁻¹ (Perley et al. 2020). Observationally, we know that cosmological long GRBs are characterized by ultrarelativistic jets observed on-axis, and have an intrinsic (corrected for beaming angle) local volumetric rate of ${79}_{-33}^{+57}$ Gpc⁻³ yr⁻¹ (e.g., Ghirlanda & Salvaterra 2022, and references therein). Hence, only ${ \mathcal O }(1) \%$ of SNe Ic-BL can make long GRBs. For low-luminosity GRBs, the observed local rate is affected by large errors, ${230}_{-190}^{+490}$ Gpc⁻³ yr⁻¹ (see Bromberg et al. 2011, and references therein), and their typical beaming angles are largely unconstrained. Hence, the question of what fraction of SNe Ic-BL can make low-luminosity GRBs remains to be answered.

Radio observations of SNe Ic-BL are a powerful way to constrain this fraction independently of relativistic beaming effects that preclude observations of jets in X-rays and γ-rays for off-axis observers. However, observational efforts aimed at constraining the fraction of SNe Ic-BL harboring low-luminosity GRBs independently of γ-ray observations have long been challenged by the rarity of the SN Ic-BL optical detections (compared to other core-collapse events), coupled with the small number of these rare SNe for which the community has been able to collect deep radio follow-up observations within 1 yr since explosion (see, e.g., Soderberg et al. 2006b). Progress in this respect has been made since the advent of the PTF, and more generally with synoptic optical surveys that have greatly boosted the rate of stripped-envelope core-collapse SN discoveries (e.g., Shappee et al. 2014; Tonry et al. 2018d; Sand et al. 2018).

In our previous work (Corsi et al. 2016), we presented one of the most extensive samples of SNe Ic-BL with deep VLA observations, largely composed of events detected by PTF/iPTF. Combining our sample with the SN Ic-BL 2002ap (Gal-Yam et al. 2002; Mazzali et al. 2002) and SN 2002bl (Armstrong 2002; Berger et al. 2003), and the circumstellar medium (CSM)-interacting SN Ic-BL 2007bg (Salas et al. 2013), we had overall 16 SNe Ic-BL for which radio emission observationally similar to SN 1998bw was excluded, constraining the rate of SNe Ic-BL observationally similar to SN 1998bw to <6.61/16 ≈ 41%, where we have used the fact that the Poisson 99.865% confidence (or 3σ Gaussian equivalent for a single-sided distribution) upper limit on zero SNe compatible with SN 1998bw is ≈6.61.

With the addition of the 16 ZTF SNe Ic-BL presented in this work, we now have doubled the sample of SNe Ic-BL with deep VLA observations presented in Corsi et al. (2016), providing evidence for additional 15 SNe Ic-BL (all but SN 2021epp; see Figure 10) that are observationally different from SN 1998bw in the radio. Adding to our sample also SN 2018bvw (Ho et al. 2020a), AT 2018gep (Ho et al. 2019), and SN 2020bvc (Ho et al. 2020b), whose radio observations exclude SN 1998bw-like emission, we are now at 34 SNe Ic-BL that are observationally different from SN 1998bw. Hence, we can tighten our constraint on the fraction of 1998bw-like SNe Ic-BL to <6.61/34 ≈ 19% (99.865% confidence). This upper limit implies that the intrinsic rate of 1998bw-like GRBs is ≲950 Gpc⁻³ yr⁻¹. Combining this constraint with the rate of low-luminosity GRBs derived from their high-energy emission, we conclude that low-luminosity GRBs have inverse beaming factors $2/{\theta }^{2}\lesssim {4}_{-3}^{+20}$, corresponding to jet half-opening angles $\theta \gtrsim {40}_{-24}^{+40}$ deg.

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For 10 of the SNe in the sample presented here we also exclude relativistic ejecta with radio luminosity densities in between ≈5 × 10²⁷ erg s⁻¹ Hz⁻¹ and ≈10²⁹ erg s⁻¹ Hz⁻¹ at t ≳ 20 days, pointing to the fact that SNe Ic-BL similar to those associated with low-luminosity GRBs, such as SN 1998bw (Kulkarni et al. 1998), SN 2003lw (Soderberg et al. 2004), SN 2010bh (Margutti et al. 2013), or to the relativistic SN 2009bb (Soderberg et al. 2010) and iPTF17cw (Corsi et al. 2017), are intrinsically rare. We note that these constraints could be greatly improved in the future with more systematic VLA follow-up campaigns (e.g., via large VLA programs). Indeed, the sample of SNe Ic-BL discussed here is only about ≈39% of the total sample of ZTF Ic-BL events presented in G. P. Srinivasaragavan et al. (2023, in preparation). Moreover, we note that none of our observations exclude radio emission similar to that of SN 2006aj. This is not surprising since the afterglow of this low-luminosity GRB faded on timescales much faster than the 20–30 days since explosion that our VLA monitoring campaign allowed us to target. To enable progress, obtaining prompt (≲5 days since explosion) and accurate spectral classification paired with deep radio follow-up observations of SNe Ic-BL should be a major focus of future studies. At the same time, as discussed in Ho et al. (2020b), high-cadence optical surveys can provide an alternative way to measure the rate of SNe Ic-BL that are similar to SN 2006aj independently of γ-ray and radio observations, by catching potential optical signatures of shock-cooling emission at early times. Based on an analysis of ZTF SNe with early high-cadence light curves, Ho et al. (2020b) concluded that it appears that SN 2006aj-like events are uncommon, but more events will be needed to measure a robust rate.

3.5.2. Properties of the Radio-emitting Ejecta

While none of the SNe in our sample for which we obtained a radio detection shows evidence for ejecta as relativistic as that of SN 1998bw, hereafter we aim to better constrain their radio properties within the synchrotron self-absorption (SSA) model for radio SNe (Chevalier 1998). Within this model, the measured radio peak frequency and peak flux can provide information on the size of the radio-emitting material (and hence its speed), as well as on the mass-loss rate of the SN progenitor. We start from Equations (11) and (13) of Chevalier (1998):

$$\begin{eqnarray}\begin{array}{rcl}{R}_{p} & \approx & 8.8\times {10}^{15}\,\mathrm{cm}{\left(\displaystyle \frac{\eta }{2\alpha }\right)}^{1/(2p+13)}{\left(\displaystyle \frac{{F}_{p}}{\mathrm{Jy}}\right)}^{(p+6)/(2p+13)}\\ & & \times \,{\left(\displaystyle \frac{{d}_{L}}{\mathrm{Mpc}}\right)}^{(2p+12)/(2p+13)}{\left(\displaystyle \frac{{\nu }_{p}}{5\,\mathrm{GHz}}\right)}^{-1},\,\,\,\end{array}\end{eqnarray} \tag{ 2 }$$

where α ≈ 1 is the ratio of relativistic electron energy density to magnetic energy density, F_p is the flux density at the time of SSA peak, ν_p is the SSA frequency, and where R/η is the thickness of the radiating electron shell. The normalization of the above Equation has a small dependence on p and in the above we assume p ≈ 3 for the power-law index of the electron energy distribution. Setting R_p ≈ v_s t_p in Equation (2), and considering that ${L}_{p}\approx 4\pi {d}_{L}^{2}{F}_{p}$ (neglecting redshift effects), we get:

$$\begin{eqnarray}\begin{array}{rcl}\left(\displaystyle \frac{{L}_{p}}{{\rm{erg}}\,{{\rm{s}}}^{-1}\,{{\rm{Hz}}}^{-1}}\right) & \approx & 1.2\times {10}^{27}{\left(\displaystyle \frac{{\beta }_{s}}{3.4}\right)}^{(2p+13)/(p+6)}\\ & & \times \,{\left(\displaystyle \frac{\eta }{2\alpha }\right)}^{-1/(p+6)}\\ & & {\left(\displaystyle \frac{{\nu }_{p}}{5{\rm{GHz}}}\displaystyle \frac{{t}_{p}}{1{\rm{day}}}\right)}^{(2p+13)/(p+6)},\end{array}\end{eqnarray} \tag{ 3 }$$

where we have set β_s = v_s /c. We plot in Figure 11 with blue dotted lines the relationship above for various values of β_s (and for p = 3, η = 2, and α = 1). As evident from this figure, relativistic events such as SN 1998bw (for which the nonrelativistic approximation used in the above equations breaks down) are located at β_s ≳ 1.

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To estimate the speed of the radio-emitting ejecta of the four SNe for which we have a radio detection, we use Equation (3) above and our radio measurements to place each of these four SNe in Figure 11. In doing so, the following considerations are relevant. For SN 2018hom, we only have one radio observation. Hence, contamination from the host galaxy cannot be excluded, and we cannot really constrain the time of the peak and peak luminosity at the observed frequency. However, if the luminosity we measure at the time of our observation is close to the peak and dominated by SN emission, then the implied ejecta speed is ≈0.3c (Figure 11). For SN 2020tkx, we have three observations that indicate a rise followed by a decay in the observed flux at 10 GHz over the corresponding epochs (the third observation is at 5 GHz, but for optically thin emission we expect the flux at 10 GHz to be less than that measured at 5 GHz at the same epoch; see Table 3). Hence, we constrain the emission peak to be at 10 GHz at ≈33–71 days after explosion (see Tables 5 and 3). Moreover, our two 10 GHz observations indicate a relatively flat rise of the luminosity density (L ∝ t^a with a ≈ 0.85) when compared with the theoretical expectations for the optically thick prepeak evolution of L ∝ t^a with a ≈ 1.7–2.5 (see the discussion for model 1 and model 4 in Section 2 of Chevalier 1998). Hence, it is reasonable to assume that the light curve at 10 GHz is already flattening at ≈33 days since explosion, indicating that ν_p is likely passing in band around that epoch. In Figure 11, we show the constraint derived assuming a peak epoch of ≈33 days. In the case of SN 2020jqm, its double-peaked radio light curve yields two possible estimates of the ejecta speed, potentially associated with two different ejecta shells interacting with the CSM (similarly to the case of PTF11qcj; see e.g., Corsi et al. 2014; Palliyaguru et al. 2019). We report in Figure 11 both these estimates. We note that the second radio peak is assumed to occur at the time of our last observation of SN 2020jqm, and yields an ejecta speed that suggests radio-loud CSM interaction similar to PTF 11qcj. Finally, in the case of SN 2021ywf, our observations show a decreasing radio light curve, L ∝ t^−b with b ≈ 0.65, indicating that the peak occurred at ≲19 days since explosion. Considering the fastest-evolving radio-emitting Ic-BL SN we know of (SN 2006aj), it is reasonable to assume 5 days ≲ t_p ≲ 19 days at 5 GHz. In Figure 11, we show the constraint derived on the ejecta speed of SN 2021ywf setting t_p ≈ 19 d. For p = 3, an earlier peak time of 5 days would imply a radio ejecta speed ≈ (5/19)^−1.3× higher than the value of ≈0.19c shown in Figure 11. Hence, for this event we cannot exclude relativistic radio ejecta with speed comparable to that of iPTF17cw.

Our radio measurements can also be used to get an estimate of the SN progenitor mass-loss rate. To this end, we note that in the SSA model the magnetic field can be expressed as (see Equations (12) and (14) in Chevalier 1998):

$$\begin{eqnarray}\begin{array}{rcl}{B}_{p} & \approx & 0.58\,{\rm{G}}{\left(\displaystyle \frac{\eta }{2\alpha }\right)}^{4/(2p+13)}{\left(\displaystyle \frac{{{\rm{F}}}_{p}}{\mathrm{Jy}}\right)}^{-2/(2p+13)}\\ & & \times \,{\left(\displaystyle \frac{{d}_{L}}{\mathrm{Mpc}}\right)}^{-4/(2p+13)}\left(\displaystyle \frac{{\nu }_{p}}{5\,\mathrm{GHz}}\right).\end{array}\end{eqnarray} \tag{ 4 }$$

Now consider an SN shock expanding in a CSM of density:

$$\begin{eqnarray}&&\rho \approx 5\times {10}^{11}\,{\rm{g}}\,{\mathrm{cm}}^{-1}{A}_{* }{R}^{-2},\end{eqnarray} \tag{ 5 }$$

where:

$$\begin{eqnarray}&&{A}_{* }=\displaystyle \frac{\dot{M}/({10}^{-5}{M}_{\odot }\,{\mathrm{yr}}^{-1})}{4\pi {{\rm{v}}}_{w}/({10}^{3}\mathrm{km}\,{{\rm{s}}}^{-1})}.\end{eqnarray} \tag{ 6 }$$

Assuming that a fraction _B of the energy density $\rho {{\rm{v}}}_{s}^{2}$ goes into magnetic fields:

$$\begin{eqnarray}&&\displaystyle \frac{{B}_{p}^{2}}{8\pi }={\epsilon }_{B}\rho {{v}_{s}}^{2}={\epsilon }_{B}\rho {R}_{p}^{2}{t}_{p}^{-2},\end{eqnarray} \tag{ 7 }$$

one can write:

$$\begin{eqnarray}\begin{array}{rcl}\left(\displaystyle \frac{{L}_{p}}{\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{Hz}}^{-1}}\right) & \approx & 1.2\times {10}^{27}{\left(\displaystyle \frac{\eta }{2\alpha }\right)}^{2}{\left(\displaystyle \frac{{\nu }_{p}}{5\,\mathrm{GHz}}\displaystyle \frac{{t}_{p}}{1\,\mathrm{day}}\right)}^{(2p+13)/2}\\ & & \times \,{\left(5\times {10}^{3}{\epsilon }_{B}{A}_{* }\right)}^{-(2p+13)/4},\,\,\,\,\,\,\end{array}\end{eqnarray} \tag{ 8 }$$

where we have used Equations (4), (6), and (7). We plot in Figure 11 with yellow dashed lines the relationship above for various values of $\dot{M}$ (and for p = 3, η = 2, α = 1, _B = 0.33, and v_w = 1000 km s⁻¹). As evident from this figure, relativistic events such as SN 1998bw show a preference for smaller mass-loss rates. We note that while the above relationship depends strongly on the assumed values of η, _B , and v_w , this trend for $\dot{M}$ remains true regardless of the specific values of these (uncertain) parameters. We also note that the above analysis assumes mass loss in the form of a steady wind. While this is generally considered to be the case for relativistic SNe Ic-BL, binary interaction or eruptive mass loss in core-collapse SNe can produce denser CSMs with more complex profiles (e.g., Montes et al. 1998; Soderberg et al. 2006a; Salas et al. 2013; Corsi et al. 2014; Margutti et al. 2017; Balasubramanian et al. 2021; Maeda et al. 2021; Stroh et al. 2021).

Finally, the total energy coupled to the fastest (radio-emitting) ejecta can be expressed as (e.g., Soderberg et al. 2006a):

$$\begin{eqnarray}&&{E}_{r}\approx \displaystyle \frac{4\pi {R}_{p}^{3}}{\eta }\displaystyle \frac{{B}_{p}^{2}}{8\pi {\epsilon }_{B}}=\displaystyle \frac{{R}_{p}^{3}}{\eta }\displaystyle \frac{{B}_{p}^{2}}{2{\epsilon }_{B}}.\end{eqnarray} \tag{ 9 }$$

In Table 3 we summarize the properties of the radio ejecta derived for the four SNe for which we detect a radio counterpart. These values can be compared with $\dot{M}\approx 2.5\times {10}^{-7}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ and E_r ≈ (1−10) × 10⁴⁹ erg estimated for SN 1998bw by Li & Chevalier (1999), with $\dot{M}\approx 2\times {10}^{-6}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ and E_r ≈ 1.3 × 10⁴⁹ erg estimated for SN 2009bb by Soderberg et al. (2010), and with with $\dot{M}\approx (0.4-1)\times {10}^{-5}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ and E_r ≈ (0.3−4) × 10⁴⁹ erg estimated for GRB 100316D by Margutti et al. (2013).

3.5.3. Off-axis GRB Afterglow Constraints

The two X-ray detections of SN 2019hsx and SN 2021ywf shown in Figure 7 are consistent with several GRB off-axis light curve models and, in the case of SN 2021ywf, also with GRB 980425-like emission within the large errors. However, for this interpretation of their X-ray emission to be compatible with our radio observations (see Table 2), one needs to invoke a flattening of the radio-to-X-ray spectrum, similar to what has been invoked for other stripped-envelope SNe in the context of cosmic-ray–dominated shocks (Ellison et al. 2000; Chevalier & Fransson 2006).

Hereafter, we finally consider what type of constraints our radio observations put on a scenario where the SNe Ic-BL in our sample could be accompanied by relativistic ejecta from a largely (close to 90°) off-axis GRB afterglow that would become visible in the radio band when the relativistic fireball enters the subrelativistic phase and approaches spherical symmetry. Because our radio observations do not extend past 100–200 days since explosion, we can put only limited constraints on this scenario. Hence, hereafter we present some general order-of-magnitude considerations rather than a detailed event-by-event modeling.

Following Corsi et al. (2016), we can model approximately the late-time radio emission from an off-axis GRB based on the results by Livio & Waxman (2000), Waxman (2004), Zhang & MacFadyen (2009), and van Eerten et al. (2012). For fireballs expanding in an ISM of constant density n (in units of cm⁻³), at timescales t such that:

$$\begin{eqnarray}&&t\gtrsim (1+z)\times {t}_{\mathrm{SNT}}/2,\end{eqnarray} \tag{ 10 }$$

where the transition time to the spherical Sedov–Neumann–Taylor (SNT) blast wave, t_SNT, reads:

$$\begin{eqnarray}&&{t}_{\mathrm{SNT}}\approx 92\,{\rm{d}}{\left({E}_{51}/n\right)}^{1/3},\end{eqnarray} \tag{ 11 }$$

the luminosity density can be approximated analytically via the following formula (see Equation (23) in Zhang & MacFadyen 2009, where we neglect redshift corrections and assume p = 2):

$$\begin{eqnarray}&&\begin{array}{l}{L}_{\nu }({\rm{t}})\approx 4\pi {d}_{{\rm{L}}}^{2}{F}_{\nu }({\rm{t}})\approx 2\times {10}^{30}\left(\displaystyle \frac{{\epsilon }_{e}}{0.1}\right){\left(\displaystyle \frac{{\epsilon }_{B}}{0.1}\right)}^{3/4}{n}^{9/20}\\ \ \ \times \,{E}_{51}^{13/10}{\left(\displaystyle \frac{\nu }{1\,\mathrm{GHz}}\right)}^{-1/2}{\left(\displaystyle \frac{t}{92\,{\rm{d}}}\right)}^{-9/10}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{Hz}}^{-1}.\,\,\,\,\,\,\,\end{array}\end{eqnarray} \tag{ 12 }$$

In the above equations, E₅₁ is the beaming-corrected ejecta energy in units of 10⁵¹ erg. We note that here we assume again a constant-density ISM in agreement with the majority of GRB afterglow observations (e.g., Schulze et al. 2011).

We plot the above luminosity in Figure 12 together with our radio observations and upper limits, assuming _e = 0.1, _B = 0.1, and for representative values of low-luminosity GRB energies and typical values of long GRB ISM densities n. As evident from this figure, our observations exclude fireballs with energies E ≳ 10⁵⁰ erg expanding in ISMs with densities ≳1 cm⁻³. However, our observations become less constraining for smaller energies and ISM density values.

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Our first ZTF photometry of SN 2018etk (ZTF18abklarx) was obtained on 2018 August 1 (MJD 58331.16) with the P48. This first ZTF detection was in the r band, with a host-subtracted magnitude of 19.21 ± 0.12 mag (Figure 1), at α = 15^h17^m0253, $\delta =+03^\circ 56^{\prime} 38\buildrel{\prime\prime}\over{.} \,7$ (J2000). The object was reported to the TNS by ATLAS on 2018 August 8, who discovered it on 2018 August 6 (Tonry et al. 2018a). The last ZTF nondetection prior to ZTF discovery was on 2018 July 16, and the last shallow ATLAS nondetection was on 2018 August 2, at 18.75 mag. The transient was classified as a Type Ic SN by Fremling et al. (2018a) based on a spectrum obtained on 2018 August 13 with SEDM. We reclassify this transient as an SN Type Ic-BL most similar to SN 2006aj based on a P200 DBSP spectrum obtained on 2018 August 21 (see Figure 3). SN 2018etk exploded in a star-forming galaxy with a known redshift of z = 0.044 derived from SDSS data.

Our first ZTF photometry of SN 2018hom (ZTF18acbwxcc) was obtained on 2018 November 1 (MJD 58423.54) with the P48. This first ZTF detection was in the r band, with a host-subtracted magnitude of 16.60 ± 0.04 mag (Figure 1), at α = 22^h59^m2296, $\delta =+08^\circ 45^{\prime} 04\buildrel{\prime\prime}\over{.} 6$ (J2000). The object was reported to the TNS by ATLAS on 2018 October 26, and discovered by ATLAS on 2018 October 24 at o ≈ 17.3 mag (Tonry et al. 2018b). The last ZTF nondetection prior to ZTF discovery was on 2018 October 9 at g > 20.35 mag, and the last ATLAS nondetection was on 2018 October 22 at o > 18.25 mag. The transient was classified as an SN Type Ic-BL by Fremling et al. (2018b) based on a spectrum obtained on 2018 November 2 with SEDM. SN 2018etk exploded in a galaxy with an unknown redshift. We measure a redshift of z = 0.030 from star-forming emission lines in a Keck I LRIS spectrum obtained on 2018 November 30. We plot this spectrum in Figure 3, along with its SNID template match to the Type Ic-BL SN 1997ef. We note that this SN was also reported in the recently released ASAS-SN bright SN catalog (Neumann et al. 2023).

Our first ZTF photometry of SN 2018hxo (ZTF18acaimrb) was obtained on 2018 October 9 (MJD 58400.14) with the P48. This first detection was in the g band, with a host-subtracted magnitude of 18.89 ± 0.09 mag (Figure 1), at α = 21^h09^m0580, $\delta =+14^\circ 32^{\prime} 27\buildrel{\prime\prime}\over{.} 8$ (J2000). The object was first reported to the TNS by ATLAS on 2018 November 6, and first detected by ATLAS on 2018 September 25 at o = 18.36 mag (Tonry et al. 2018c). The last ZTF nondetection prior to discovery was on 2018 September 27 at r > 20.12 mag, and the last ATLAS nondetection was on 2018 September 24 at o > 18.52 mag. The transient was classified as an SN Type Ic-BL by Dahiwale & Fremling (2020a) based on a spectrum obtained on 2018 December 1 with Keck I LRIS. In Figure 3 we plot this spectrum along with its SNID match to the Type Ic-BL SN 2002ap. SN 2018etk exploded in a galaxy with an unknown redshift. We measure a redshift of z = 0.048 from star-forming emission lines in the Keck spectrum.

Our first ZTF photometry of SN 2018jex (ZTF18acpeekw) was obtained on 2018 November 16 (MJD 58438.56) with the P48. This first detection was in the r band, with a host-subtracted magnitude of 20.07 ± 0.29 mag, at α = 11^h54^m1387, $\delta =+20^\circ 44^{\prime} 02\buildrel{\prime\prime}\over{.} 4$ (J2000). The object was reported to the TNS by AMPEL on November 28 (Nordin et al. 2018). The last ZTF last nondetection prior to ZTF discovery was on 2018 November 16 at r > 19.85 mag. The transient was classified as an SN Type Ic-BL based on a spectrum obtained on 2018 December 4 with Keck I LRIS. In Figure 3 we show this spectrum plotted against the SNID template of the Type Ic-BL SN 1997ef. AT2018jex exploded in a galaxy with an unknown redshift. We measure a redshift of z = 0.094 from star-forming emission lines in the Keck spectrum.

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 3, respectively. We note that this SN was also reported in the recently released ASAS-SN bright SN catalog (Neumann et al. 2023).

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 3, respectively.

Our first ZTF photometry of SN 2020jqm (ZTF20aazkjfv) was obtained on 2020 May 11 (MJD 58980.27) with the P48. This first detection was in the r band, with a host-subtracted magnitude of 19.42 ± 0.13 mag, at α = 13^h49^m1857, $\delta =-03^\circ 46^{\prime} 10\buildrel{\prime\prime}\over{.} 4$ (J2000). The object was reported to the TNS by ALeRCE on May 11 (Forster et al. 2020). The last ZTF nondetection prior to ZTF discovery was on 2020 May 08 at g > 17.63 mag. The transient was classified as an SN Type Ic-BL based on a spectrum obtained on 2020 May 26 with SEDM (Dahiwale & Fremling 2020b). SN 2020jqm exploded in a galaxy with an unknown redshift. We measure a redshift of z = 0.037 from host-galaxy emission lines in a NOT ALFOSC spectrum obtained on 2020 June 6. We plot the ALFOSC spectrum along with its SNID match to the Type Ic-BL SN 1998bw in Figure 3.

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 3, respectively. We note that this SN was also reported in the recently released ASAS-SN bright SN catalog (Neumann et al. 2023).

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 4, respectively.

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 4, respectively.

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 4, respectively.

Our first ZTF photometry of SN 2021aug (ZTF21aafnunh) was obtained on 2021 January 18 (MJD 59232.11) with the P48. This first detection was in the g band, with a host-subtracted magnitude of 18.73 ± 0.08 mag, at α = 01^h14^m0481, $\delta =+19^\circ 25^{\prime} 04\buildrel{\prime\prime}\over{.} 7$ (J2000). The last ZTF nondetection prior to ZTF discovery was on 2021 January 16 at g > 20.12 mag. The transient was publicly reported to the TNS by ALeRCE on 2021 January 18 (Munoz-Arancibia et al. 2021a), and classified as an SN Type Ic-BL based on a spectrum obtained on 2021 February 09 with SEDM (Dahiwale & Fremling 2021). SN 2020jqm exploded in a galaxy with an unknown redshift. We measure a redshift of z = 0.041 from star-forming emission lines in a P200 DBSP spectrum obtained on 2021 February 08. This spectrum is shown in Figure 4 along with its template match to the Type Ic-BL SN 1997ef.

Our first ZTF photometry of SN 2021epp (ZTF21aaocrlm) was obtained on 2021 March 5 (MJD 59278.19) with the P48. This first ZTF detection was in the r band, with a host-subtracted magnitude of 19.61 ± 0.15 mag (Figure 1), at α = 08^h10^m5527, $\delta =-06^\circ 02^{\prime} 49\buildrel{\prime\prime}\over{.} 3$ (J2000). The transient was publicly reported to the TNS by ALeRCE on 2021 March 5 (Munoz-Arancibia et al. 2021b), and classified as an SN Type Ic-BL based on a spectrum obtained on 2021 March 13 by ePESSTO+ with the ESO Faint Object Spectrograph and Camera (Kankare et al. 2021). The last ZTF nondetection prior to discovery was on 2021 March 2 at r > 19.72 mag. In Figure 4 we show the classification spectrum plotted against the SNID template of the Type Ic-BL SN 2002ap. SN 2021epp exploded in a galaxy with a known redshift of z = 0.038.

Our first ZTF photometry of SN 2021htb (ZTF21aardvol) was obtained on 2021 March 31 (MJD 59304.164) with the P48. This first ZTF detection was in the r band, with a host-subtracted magnitude of 20.13 ± 0.21 mag (Figure 1), at α = 07^h45^m3119, $\delta =46^\circ 40^{\prime} 01\buildrel{\prime\prime}\over{.} 4$ (J2000). The transient was publicly reported to the TNS by SGLF on 2021 April 2 (Poidevin et al. 2021). The last ZTF nondetection prior to ZTF discovery was on 2021 March 2, at r > 19.88 mag. In Figure 4 we show a P200 DBSP spectrum taken on 2021 April 09 plotted against the SNID template of the Type Ic-BL SN 2002ap. SN 2021htb exploded in an SDSS galaxy with redshift z = 0.035.

Our first ZTF photometry of SN 2021hyz (ZTF21aartgiv) was obtained on 2021 April 03 (MJD 59307.155) with the P48. This first ZTF detection was in the g band, with a host-subtracted magnitude of 20.29 ± 0.17 mag (Figure 1), at α = 09^h27^m3651, $\delta =04^\circ 27^{\prime} 11^{\prime\prime}$ (J2000). The transient was publicly reported to the TNS by ALeRCE on 2021 April 3 (Forster et al. 2021). The last ZTF nondetection prior to ZTF discovery was on 2021 April 1, at g > 19.15 mag. In Figure 4 we show a P60 SEDM spectrum taken on 2021 April 30 plotted against the SNID template of the Type Ic-BL SN 1997ef. SN 2021hyz exploded in a galaxy with redshift z = 0.046.

We refer the reader to Anand et al. (2023) for details about this SN Ic-BL. Its P48 light curves and the spectrum used for classification are shown in Figures 1 and 4, respectively.