SN2024bch

General Information

• Name of the source: SN2024bch

• Brief description of the source:
  • Object type: CCSN type IIn-L
  • Distance (Mpc): 16.56
  • Redshift: 0.00387
  • Host galaxy: NGC 3206
  • RA: 10:21:49.740 (hh mm ss), Dec: +56:55:40.51 (dd mm ss)
  • RA, Dec in deg (ICRS): 155.45725, +56.927919

• Other relevant information and data:
  • Date of discovery: 2024-01-29 06:27:21
  • Date of explosion (inferred): 2024-01-28 14:38:24
  • Discovery report: https://www.wis-tns.org/object/2024bch/discovery-cert
  • Optical photometry: https://app.aavso.org/webobs/results/?star=000-BPV-330&num_results=200
  • Optical spectroscopy: https://www.wiserep.org/object/24751
  • Hubble images: here

People involved

Alphabetical order (corresponding authors)

• Arnau Aguasca-Cabot
• Alessandro Carosi
• Alicia López-Oramas
• Andrea Simongini (andrea.simongini@inaf.it)

Paper versions

• SN2024bch_v1: link to CDS
• SN2024bch_v2: link to CDS

Presentations

• 2024-10-25 LST Galactic group Meeting: link to pdf
• 2024-06-21 LST Galactic group Meeting: link to pdf
• 2024-06-10 LST Analysis Call: link to pdf
• 2024-05-21 LST General Meeting, Prague: link to pdf

Data-taking Information

• General information:
  • Start observation date: 2024-02-13
  • Total nights: 6
  • Total hours: 14.6
  • Total runs: 53
• Observation condition: moon and dark
• Observation mode: wobbles
• Joint observations with MAGIC?: yes (16845-16863)
• Joint analysis with MAGIC?: no

SN2024bch observations with LST-1

Run Number	Night	Run Start Time [UTC]	Run Elapsed Time [min]	Mean pointing zenith [deg]	Wobble Position	Used in stacked analysis	Conditions	ELOG
16771	20240213	00:29	20	32.6	W1	True	dark	20240213
16772	20240213	00:49	19	30.5	W2	True	dark	20240213
16773	20240213	01:08	24	29.9	W3	True	dark	20240213
16774	20240213	01:32	17	28.1	W4	True	dark	20240213
16775	20240213	01:49	20	xx	W1	True	dark	20240213
16776	20240213	02:09	20	xx	W2	True	dark	20240213
16777	20240213	02:29	20	28.9	W3	True	dark	20240213
16778	20240213	02:49	20	xx	W4	True	dark	20240213
16779	20240213	03:09	22	30.3	W1	True	dark	20240213
16780	20240213	03:31	19	32.5	W2	True	dark	20240213
16781	20240213	03:50	17	34.3	W3	True	dark	20240213
16803	20240214	01:11	26	29.3	W1	True	dark	20240214
16804	20240214	01:37	18	28	W2	True	dark	20240214
16805	20240214	01:55	15	28.5	W3	True	dark	20240214
16806	20240214	02:10	20	27.8	W4	True	dark	20240214
16807	20240214	02:30	23	28.5	W1	True	dark	20240214
16808	20240214	02:53	18	30	W2	True	dark	20240214
16809	20240214	03:11	20	31.4	W3	True	dark	20240214
16810	20240214	03:31	21	32.3	W4	True	dark	20240214
16811	20240214	03:52	4	34.4	W1	True	dark	20240214
16815	20240215	00:14	16	33.3	W1	False	dark	20240215
16816	20240215	00:30	20	31.4	W2	False	dark	20240215
16817	20240215	00:50	21	30.5	W3	False	dark	20240215
16818	20240215	01:11	21	28.7	W4	True	dark	20240215
16819	20240215	01:32	18	28.3	W1	True	dark	20240215
16820	20240215	01:50	20	28	W2	True	dark	20240215
16821	20240215	02:10	22	28.6	W3	True	dark	20240215
16822	20240215	02:32	19	28.5	W4	True	dark	20240215
16823	20240215	02:51	19	29.7	W1	True	dark	20240215
16824	20240215	03:10	22	31.9	W2	True	dark	20240215
16825	20240215	03:32	18	33.4	W3	True	dark	20240215
16826	20240215	03:50	12	34.6	W4	True	dark	20240215
16845	20240216	01:02	21	29.4	W1	False	dark	20240216
16846	20240216	01:23	20	28.3	W2	False	dark	20240216
16847	20240216	01:43	22	28	W3	False	dark	20240216
16848	20240216	02:05	19	27.8	W4	False	dark	20240216
16849	20240216	02:24	21	28.6	W1	True	dark	20240216
16850	20240216	02:45	20	29.9	W2	True	dark	20240216
16851	20240216	03:05	21	31.4	W3	True	dark	20240216
16852	20240216	03:26	20	32.5	W2	True	dark	20240216
16853	20240216	03:46	14	34.5	W3	True	dark	20240216
16863	20240218	03:10	19	32	W1	False	moon	20240218
16864	20240218	03:29	18	34.7	W2	False	moon	20240218
16866	20240218	03:47	19	36.1	W2	False	moon	20240218
16867	20240218	04:06	28	38.1	W3	False	moon	20240218
16868	20240218	04:25	3	xx	xx	False	moon	20240218
16869	20240218	04:34	19	41.1	W4	True	moon	20240218
16870	20240218	04:53	7	43.4	W1	True	moon	20240218
16980	20240306	00:15	20	28	W1	True	dark	20240306
16981	20240306	00:35	20	28	W2	True	dark	20240306
16982	20240306	00:55	20	28.7	W3	True	dark	20240306
16983	20240306	01:15	20	28.7	W4	True	dark	20240306
16984	20240306	01:35	8	30	W1	True	dark	20240306

Data quality cuts

We performed a standard source independent analysis for this source. The data quality is performed using the data_quality.ipynb notebook from the 2024-LST-School.

• Source selection:
  • zenith_range = [0, 90]
  • min_angle_to_source = 0.35
  • max_angle_to_source = 0.45
• Global cuts:
  • max_diffuse_nsb_std = 2.3
  • max_pointing_dec_std = 0.01
  • min_mean_fit_p = -3.
  • max_LS_periodogram_maxamplitude = 1e-2
  • min_drdi_index = -2.35
  • max_drdi_index = -2.1
  • min_drdi_at_422pe = 1.4
  • min_fraction_around_mode = 0.8
  • max_intensity_at_half_peak_rate = 70
• Data quality plots:

• Relative light yield of the selected nights:

• Results:
  • Good quality runs: 41/53(77%)
  • Total time: 12.1 h
• Extra notes:
  • 16868 is a very short run (3.4939323 min) due to data taking interruption by shifters
  • 16815 16816 16817 16847 16848 16867 were removed due to different NSB level

Data analysis

MC production

According to the data_quality.ipynb, the NSB level in the FoV of SN2024bch is low enough to consider the standard MC.

• MC used:
  • 20240131_allsky_v0.10.5_all_dec_base
• Declination line:
  • dec_6166 (4.8 deg away from SN 2024bch)

Pointing issues

We could not fit 3 OFF positions in several runs due to small offset angular distance of the wobbles.

• Possible solutions (applied in this analysis):
  • Reduce the max_theta_cut to 0.26 deg
  • Use only 1 OFF position

DL3 production

The DL3 are produced using the following standard parameters:

• Intensity cut: [50GeV, infty]
• w1: [0.01, 1]
• r: [0, 1]
• leakage_intensity_width_2: [0, 1]
• event_type: [32, 32]
• theta_containment: 0.7
• gh_efficiency: 0.7
• min_livetime: 300
• max_zenith: 90

DL3 data are stored here:

• /fefs/aswg/workspace/andrea.simongini/Analysis/SN2024bch/DL3

Theta-squared plots

• We produced theta-squared plots with all good quality data:
  • n_wobbles: 4
  • theta2_cut: 0.04
  • energy bounds: [50GeV-100GeV]; [100GeV-1TeV]; [1TeV-10TeV]
  • gammaness_cut: 0.7
  • theta2_cut: 0.07 deg2
• Plots:

• Results:
  • [50GeV-100GeV]: N_on = 287155; N_off = 6220812; Significance = -0.572341
  • [100GeV-1TeV]: N_on = 107335; N_off = 2284060; Significance = -1.255360
  • [1TeV-10TeV]: N_on = 267; N_off = 6189; Significance = -1.701900

No significant excess coming from this source! We go for upper-limits

High-level analysis

For the high-level analysis we set:

• n_off_regions: 1
• safe_mask_method: aeff-max (5%)
• e_reco: [35GeV - 10TeV]
• e_true: [1GeV - 50TeV]
• n_reco_bin_p_dec: 3.5
• n_true_bin_p_dec: 10

Note that the energy treshold (Eth) is ~30GeV. As discussed in the LST analysis call (see presentation from 2024-06-10) the safest and most conservative way to integrate fluxes is to set the lower energy bound of the reconstructed energy to 100GeV.

Spectral Energy Distribution

We are fitting our data with a simple power law distribution with the following parameters:

• Gamma: -2.5
• amplitude: 2e-12 cm−2s−1TeV−1
• bounds: [1e-18, 1e-5] cm−2s−1TeV−1
• ref_energy: 2TeV

We use all good quality data

Light curves

We produced run-wise and night-wise light curves between 100GeV and 10TeV.

Cross-check

Crab check

• General information:
  • we applied the same data-quality cuts as SN2024bch
  • we used the same period of data taking (Feb-Mar 2024)
  • we applied the same max_theta_cut for IRF production
  • we employed a different Monte Carlo production
  • the DL3 and DL4 files are produced with the same specifications as SN2024bch

• Data saved:
  • 29/33 runs (88%)
  • 8.5h

• Relative light yield:

• Monte Carlo production:
  • We used a different production with respect to SN2024bch
  • 20230927_v0.10.4_crab_tuned
  • dec_2276

• Theta-squared plots:

• High-level analysis:

We fitted the spectral energy distribution using:

• • spectral_model = LogParabolaSpectralModel
  • index = 2.5
  • amplitude = 2e-12cm-2 s-1 TeV-1
  • ref = 0.7TeV
  • lambda_ = 0.1TeV-1

Both the spectral energy distribution and the light curves are built between 100GeV and 10TeV.

Progenitor analysis

We performed a 4-steps analysis to investigate the progenitor star of SN2024bch. The data employed are:

• LST-1 data:
  • telescope: LST-1 in mono configuration
  • type: light curves upper limits
  • range: 100GeV-10TeV
  • availability: proprietary data

• Optical data:
  • telescope: many, Cafos, Mistral
  • type: light curves and spectra
  • range: B, V, R, I filters; 4000-9000 A
  • availability: public at WISeREP and AAVSO

• Optical spectrum:
  • telescope: Liverpool Telescope
  • type: spectrum
  • range: 4000-9000 A
  • availability: proprietary data

• Pre-explosion images:
  • telescope: Hubble Space Telescope
  • type: images
  • range: optical
  • availability: public at HLA

The analysis is performed with our own written python codes and with CASTOR (Simongini et al. 2024), an open access software for CCSN data analysis.

Theoretical modeling

VHE photons are thought to be produced from the non-thermal interaction between the fast-moving shock-wave of the supernova ejecta and a dense circumstellar medium (CSM) surrounding a massive progenitor. As the CSM density decreases moving away from the progenitor, the potential gamma-ray signal is expected to peak shortly after the explosion as a dense CSM enhances p-p interaction}. However, during the first tens to hundreds of days after the explosion, the putative VHE signal is significantly attenuated by the gamma-gamma absorption with the optical photons emitted by the supernova photosphere. Several parameters need to be taken into account for a detailed description of the gamma absorption. Among them, the mass-loss rate and the wind velocity of the progenitor star before the onset of the explosion, play a key role is suppressing the VHE signal by up to several orders of magnitude.

To characterize the gamma flux we use the model from Dwarkadas 2013 that relates the gamma flux Fγ(t) to the properties of the CSM, the properties of the supernova event and the pre-explosion properties of the progenitor. To make assumptions on the general parameters of this equation, we followed the same prescriptions as in Abdalla et al. 2019. We use this formula to obtain upper-limits on the mass-loss-rate wind velocity ratio from our gamma-flux upper-limits. Results are shown in the following table:

Upper-limits

Night	Flux [10e-11 cm-2 s-1]	Mdot/uw [10e-4 Msun yr-1 s km-1]	Exsposure [h]
60353	0.68	1.12	3.52
60354	1.05	1.46	2.72
60355	1.14	1.57	2.83
60356	1.66	1.94	1.57
60358	5.16	3.60	0.48
60375	1.26	2.40	1.46

Using the stacked upper-limit on the gamma-flux (Fγ = 0.261e-11 cm-2 s-1), we obtained a stacked mass-loss-rate wind velocity ratio upper-limit of 0.926e-4 Msun yr-1 s km-1.

Optical analysis

Using CASTOR we can create synthetic spectra and synthetic light curves by means of Gaussian Process interpolation techniques applied on real data, as shown in the following figures:

From these synthetic data, CASTOR can reconstruct the parametric map of the event, inferring parameters that belong to the supernova ejecta, to the photosphere, to the progenitor and to the event itself. Here we show the full parametric map:

• Event:
  • Time of explosion: 60337.39 ± 1.9) MJD
  • Time of maximum: 60347.19 ± 0.02 MJD
  • Redshift: 0.00386 ± 0.00016
  • Distance: 16.53 ± 0.67 Mpc
  • Bolometric luminosity: 12.11 ± 0.98 e+41 erg s-1
  • Kinetic energy: 1.03 ± 0.21 e+51 erg
  • Nichel yield: 0.056 ± 0.003 Msun
• Supernova ejecta:
  • Mass: 7.82 ± 1.64 Msun
  • Velocity: 4688 ± 50 km s-1
• Photosphere:
  • Temperature: 10639 ± 792 K
  • Radius: 3380 ± 62 Rsun
• Progenitor:
  • Radius: < 3e+3 Rsun
  • Mass: (9.02, 17.82) ± 1.64 Msun

Pre-explosion images

We collected one image of the host galaxy (NGC 3206) taken with the Hubble Space Telescope on the 14th of May 2001. Some sources are identified within the coordinates of the events:

Candidate progenitor stars

Catalog	RA [deg]	DEC [deg]	MAG	Log (L/L0)
DAOphot	155.45741	56.92795	21.59	3.80
DAOphot	155.45735	56.92793	21.57	3.81
SExtractor	155.45735	56.92800	20.50	4.24
GSCII	155.45735	56.92793	19.76	4.53
HSC	155.45730	56.92801	21.84	3.70
PS1	155.45735	56.92794	19.32	4.71

Thanks to this image we are able to identify the luminosity of the progenitor star, either in a direct or indirect way:

• • if the progenitor star is identified by the catalogs, then we have its luminosity
  • if the progenitor star is not identified by the catalogs, then we have an upper limit to its luminosity.

Classification

The standard scheme for supernovae classification divides type II and type Ib/Ic based on the presence of hydrogen in the optical spectra. Type II can be further divided into II-L and II-P based on the behaviour of the optical light curves. Recently Li et al. 2011 and Faran et al. 2014 defined a Type II-L supernova as one whose R-band (or V-band) light curve decreases by 0.5 magnitudes within the first 50 days following the explosion. A further subtype of core-collapse supernovae (CCSNe) is Type IIn, characterized by strong interaction with circumstellar material (CSI). These supernovae display spectra with prominent, narrow Balmer emission lines on a blue continuum, likely resulting from significant mass loss prior to the explosion.

By analyzing the optical and spectral behavior of SN2024bch, we classify it as a IIn-L, a relatively new subtype of CSI supernovae. This group features Type IIL-like light curves but shows IIn-like spectra in the early days post-explosion. However, the narrow lines weaken and fade quickly, giving way to broader lines. For further details on IIn-L supernovae, see Taddia et al. 2015.

Our claim is supported by the similarity of the optical light curves and spectra with those of SN~1998S, SN~2009kr, SN~2008fq and SN~2013fc: these are all IIL supernovae with circumstellar interaction during the first tens of days. For further details on the analysis, see slides from the GAL-Meeting.

Conclusion

SN~2024bch is likely a IIn-L from a Red Supergiant Progenitor (RSG). There are many independent proofs to identify RSG as the candidate progenitor star:

• • The mass of the progenitor star falls within 10 and 20 solar masses.
  • The radius of the progenitor star is lower than 3000 solar masses.
  • The luminosity of the progenitor star is of the order of Lsun^(4.5)
  • The mass-loss rate wind velocity ratio upper-limit is of the order of 0.926e-4 Msun yr-1 s km-1.
  • The metallicity of the site of explosion of IIn-L is found to be similar to that of sites of II-L supernovae (see Taddia et al. 2015).