We discuss the results of the spectroscopic and photometric monitoring of the type IIn supernova (SN) 2023ldh. Survey archive data show that the SN progenitor experienced erratic variability in the years before exploding. Beginning May 2023, the source showed a general slow luminosity rise that lasted for over four months, with some superposed luminosity fluctuations. In analogy to SN 2009ip, we call this brightening ‘Event A’. During Event A, SN 2023ldh reached a maximum absolute magnitude of M_r = −15.52 ± 0.24 mag. The light curves then decreased by about 1 mag in all filters for about two weeks reaching a relative minimum, which was followed by a steep brightening (Event B) to an absolute peak magnitude of M_r = −18.53 ± 0.23 mag, replicating the evolution of SN 2009ip and similar to that of type IIn SNe. The three spectra of SN 2023ldh obtained during Event A show multi-component P Cygni profiles of H I and Fe II lines. During the rise to the Event B peak, the spectrum shows a blue continuum dominated by Balmer lines in emission with Lorentzian profiles, with a full width at half maximum velocity of about 650 km s⁻¹. Later, in the post-peak phase, the spectrum reddens, and broader wings appear in the Hα line profile. Metal lines with P Cygni profiles and velocities of about 2000 km s⁻¹ are clearly visible. Beginning around three months past maximum and until very late phases, the Ca II lines become among the most prominent features, while Hα is dominated by an intermediate-width component with a boxy profile. Although SN 2023ldh mimics the evolution of other SN 2009ip-like transients, it is slightly more luminous and has a slower photometric evolution. The surprisingly homogeneous observational properties of SN 2009ip-like events may indicate similar explosion scenarios and similar progenitor parameters.

We present optical photometric and spectroscopic observations of the peculiar Type Ia supernovae (SNe Ia) ASASSN-20jq/SN 2020qxp. It is a low-luminosity object, with a peak absolute magnitude of M_B = −17.1 ± 0.5 mag, while its post-peak light-curve decline rate of Δm₁₅(B) = 1.35 ± 0.09 mag and color-stretch parameter of s_BV ⪆ 0.82 is similar to that of normal luminosity SNe Ia. That makes it a prevalent outlier in both the SN Ia luminosity-width and the luminosity-color-stretch relations. The analysis of the early light curves indicates a possible “bump” during the first ≈1.4 days of explosion. ASASSN-20jq synthesized a low radioactive ⁵⁶Ni mass of 0.09 ± 0.01 M_⊙. The near-maximum light spectra of the supernova show strong Si II absorption lines, indicating a cooler photosphere than normal SNe Ia; however, it lacks Ti II absorption lines. Additionally, it shows unusually strong absorption features of O Iλ7773 and the Ca II near-infrared triplet. The nebular spectra of ASASSN-20jq show a remarkably strong but narrow forbidden [Ca II] λλ7291, 7324 doublet emission that has not been seen in SNe Ia except for a handful of Type Iax events. There is also a marginal detection of the [O I] λλ6300, 6364 doublet emission in nebular spectra, which is extremely rare. Both the [Ca II] and [O I] lines are redshifted by roughly 2000 km s⁻¹. ASASSN-20jq also exhibits a strong [Fe II] λ7155 emission line with a tilted-top line profile, which is identical to the [Fe II] λ16433 line profile. The asymmetric [Fe II] line profiles, along with the redshifted [Ca II] and emission lines, suggest a high central density white dwarf progenitor that underwent an off-center delayed-detonation explosion mechanism, synthesizing roughly equal amounts of ⁵⁶Ni during the deflagration and detonation burning phases. The equal production of ⁵⁶Ni in both burning phases distinguishes ASASSN-20jq from normal bright and subluminous SNe Ia. Assuming this scenario, we simultaneously modeled the optical and near-infrared nebular spectra, achieving a good agreement with the observations. The light curve and spectroscopic features of ASASSN-20jq do not align with any single sub-class of SNe Ia. However, the significant deviation from the luminosity versus light-curve shape relations (along with several light-curve and spectroscopic features) exhibits similarities to some 2002es-like objects. Therefore, we have identified ASASSN-20jq as an extreme candidate within the broad and heterogeneous parameter space of 2002es-like SNe Ia.

We investigate the optical shock emission from the Large Magellanic Cloud supernova remnant 0540−69.3 (SNR 0540) using Multi Unit Spectroscopic Explorer integral-field-unit data from the Very Large Telescope. The observations cover the spectral range 4650–9300 Å and provide a a 1 × 1  arcmin² field of view, encompassing nearly the entire remnant. We analyse the spatial and spectral properties of shock-related emission lines, and identify clumpy optical shock emission e.g. from [S ii] λλ6716,6731 doublet and the coronal [Fe xiv] λ5303 line (typically at radial velocities |100| and  |170| km s⁻¹⁠, respectively). These features trace the blast-wave shell seen in previous X-ray studies. Post-shock electron density estimates, based on the [S ii]-line ratio, reveal spatial variation, with the highest densities (~ 10⁴⁠ cm⁻³⁠) in the bright knots in the west, and lower densities (⁠~ 3 × 10³ cm⁠⁻³) in the east. The density in the north (southwest) appears significantly lower (higher) but remains unconstrained due to limited signal. We also estimate blast-wave shock velocities using the [Fe xiv] λ5303/[Fe xi] λ7892 ratio, finding low velocities (~ 400⁠ km s⁻¹), consistent with previous studies. All these results support the scenario that the blast wave is interacting with the surrounding interstellar medium, particularly in the western regions. Additionally, we detect four unidentified emission lines, ~2000–3000 km s⁻¹ south from the pulsar in transverse velocity, but their origin remains unclear. Possible explanations, including Fe lines from a high-velocity ejecta clump, all present challenges. Our findings highlight the complex nature of the circum- and interstellar medium surrounding SNR 0540.

We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum images with an angular size of 1 320"x1 260" and PA = 0°. While there is a hint of possible H(Formula presented) and gamma-ray emission, Teleios is exclusively seen at radio-continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of (Formula presented)=-0.6(Formula presented)0.3. Our best estimates from Hi studies and the (Formula presented)-D relation place Teleios as a type Ia SNR at a distance of either (Formula presented)2.2 kpc (near-side) or (Formula presented)7.7 kpc (far-side). This indicates two possible scenarios, either a young (under 1 000 yr) or a somewhat older SNR (over 10 000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance/diameter estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that would point to a much closer distance of (Formula presented)1 kpc and Teleios size of only (Formula presented)3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive Supernova (SN) origin type can be established at this stage. Remarkably, Teleios has retained its symmetrical shape as it aged even to such a diameter, suggesting expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at Teleios’s centre.

We present ultraviolet, optical, and near-infrared photometric and optical spectroscopic observations of the luminous fast blue optical transient (LFBOT) CSS 161010:045834-081803 (CSS 161010). The transient was found in a low-redshift (z = 0.033) dwarf galaxy. The light curves of CSS 161010 are characterized by an extremely fast evolution and blue colors. The V-band light curve shows that CSS 161010 reaches an absolute peak of M V max = − 20.66 ± 0.06 mag in 3.8 days from the start of the outburst. After maximum, CSS 161010 follows a power-law decline ∝t −2.8±0.1 in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf, and AT 2020xnd. However, unlike these objects, the spectra of CSS 161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines with velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS 161010 are unique features not seen in any transient before CSS 161010. The combined observational properties of CSS 161010 and its M * ∼ 108 M ⊙ dwarf galaxy host favor the tidal disruption of a star by an intermediate-mass black hole as its origin.

We present the discovery of possibly the youngest Galactic supernova remnant (SNR) with associated pulsar-wind nebula (PWN), which we name Perun (G329.9−0.5). Perun was serendipitously discovered in the Australian Square Kilometre Array Pathfinder–Evolutionary Map of the Universe survey at 943 MHz, and subsequent follow-up observations were conducted with the Australia Telescope Compact Array observatory at 5500 and 9000 MHz. We combine these with additional radio observations from the MeerKAT, Molonglo Observatory Synthesis Telescope, and Murchison Widefield Array telescopes, infrared (IR) observations from the SpitzerSpace Telescope, and X-ray observations from the Chandra X-ray observatory to perform a multifrequency analysis. The radio morphology shows a small angular size shell (D = 70 arcsec) with a luminous, central PWN. We measure a total spectral index of α = −0.49 ± 0.05, which should be typical for a young, composite SNR. Crucial evidence for Perun’s SNR classification comes from the detection of linear fractional polarization at radio frequencies of ∼7 per cent–10 per cent with both radial and tangential orientations, similar to the young SNR G1.9+0.3. We use data from the Southern Galactic Plane Survey to perform an H I analysis and estimate a favoured distance range of 6–9 kpc, and thus a favoured age range of ∼70–500 yr. We find no high-energy emission in Fermi-Large Area Telescope data. We detect Perun’s outer shell in 24 μm indicating the possible presence of [O IV] and [Fe III] emission, also typical for young SNRs. Overall, these observations and analysis confirm Perun as a young, Galactic SNR with a prominent PWN.

Supernova (SN) 1987A offers a unique opportunity to study how a spatially resolved SN evolves into a young SN remnant. We present and analyze Hubble Space Telescope (HST) imaging observations of SN 1987A obtained in 2022 and compare them with HST observations from 2009 to 2021. These observations allow us to follow the evolution of the equatorial ring (ER), the rapidly expanding ejecta, and emission from the center over a wide range in wavelength from 2000 to 11,000 Å. The ER has continued to fade since it reached its maximum ∼8200 days after the explosion. In contrast, the ejecta brightened until day ∼11,000 before their emission levelled off; the west side brightened more than the east side, which we attribute to the stronger X-ray emission by the ER on that side. The asymmetric ejecta expand homologously in all filters, which are dominated by various emission lines from hydrogen, calcium, and iron. From this overall similarity, we infer the ejecta are chemically well mixed on large scales. The exception is the diffuse morphology observed in the UV filters dominated by emission from the Mg ii resonance lines that get scattered before escaping. The 2022 observations do not show any sign of the compact object that was inferred from highly ionized emission near the remnant’s center observed with JWST. We determine an upper limit on the flux from a compact central source in the [O iii] HST image. The nondetection of this line indicates that the S and Ar lines observed with JWST originate from the O free inner Si-S-Ar-rich zone and/or that the observed [O iii] flux is strongly affected by dust scattering.

We use the light-curve and spectral synthesis code JEKYLL to calculate a set of macroscopically mixed type IIb supernova (SN) models, which are compared to both previously published and new late-phase observations of SN 2020acat. The models differ in the initial mass, in the radial mixing and expansion of the radioactive material, and in the properties of the hydrogen envelope. The best match to the photospheric and nebular spectra and light curves of SN 2020acat is found for a model with an initial mass of 17 M_⊙, strong radial mixing and expansion of the radioactive material, and a 0.1 M_⊙ hydrogen envelope with a low hydrogen mass fraction of 0.27. The most interesting result is that strong expansion of the clumps containing radioactive material seems to be required to fit the observations of SN 2020acat both in the diffusion phase and in the nebular phase. These Ni bubbles are expected to expand due to heating from radioactive decays, but the degree of expansion is poorly constrained. Without strong expansion, there is a tension between the diffusion phase and the subsequent evolution, and models that fit the nebular phase produce a diffusion peak that is too broad. The diffusion-phase light curve is sensitive to the expansion of the Ni bubbles because the resulting Swiss-cheese-like geometry decreases the effective opacity and therefore the diffusion time. This effect has not been taken into account in previous light-curve modelling of stripped-envelope SNe, which may lead to a systematic underestimate of their ejecta masses. In addition to strong expansion, strong mixing of the radioactive material also seems to be required to fit the diffusion peak. It should be emphasized, however, that JEKYLL is limited to a geometry that is spherically symmetric on average, and large-scale asymmetries may also play a role. The relatively high initial mass found for the progenitor of SN 2020acat places it at the upper end of the mass distribution of type IIb SN progenitors, and a single-star origin cannot be excluded.

We present the serendipitous discovery of a new radio-continuum ring-like object nicknamed Kýklos (J1802–3353), with MeerKAT UHF and L-band observations. The radio ring, which resembles the recently discovered odd radio circles (ORCs), has a diameter of ∼80″ and is located just ∼6° from the Galactic plane. However, Kýklos exhibits an atypical thermal radio-continuum spectrum (α = −0.1 ± 0.3), which led us to explore different possible formation scenarios. We concluded that a circumstellar shell around an evolved massive star, possibly a Wolf-Rayet, is the most convincing explanation with the present data.

The type Ia supernova (SN Ia) SN 2020nlb was discovered in the Virgo Cluster galaxy M85 shortly after explosion. Here we present observations that include one of the earliest high-quality spectra and some of the earliest multi-colour photometry of a SN Ia to date. We calculated that SN 2020nlb faded 1.28 ± 0.02 mag in the B band in the first 15 d after maximum brightness. We independently fitted a power-law rise to the early flux in each filter, and found that the optical filters all give a consistent first light date estimate. In contrast to the earliest spectra of SN 2011fe, those of SN 2020nlb show strong absorption features from singly ionised metals, including Fe II and Ti II, indicating lower-excitation ejecta at the earliest times. These earliest spectra show some similarities to maximum-light spectra of 1991bg-like SNe Ia. The spectra of SN 2020nlb then evolve to become hotter and more similar to SN 2011fe as it brightens towards peak. We also obtained a sequence of nebular spectra that extend up to 594 days after maximum light, a phase out to which SNe Ia are rarely followed. The [Fe III]/[Fe II] flux ratio (as measured from emission lines in the optical spectra) begins to fall around 300 days after peak; by the +594 d spectrum, the ionisation balance of the emitting region of the ejecta has shifted dramatically, with [Fe III] by then being completely absent. The final spectrum is almost identical to SN 2011fe at a similar epoch. Comparing our data to other SN Ia nebular spectra, there is a possible trend where SNe that were more luminous at peak tend to have a higher [Fe III]/[Fe II] flux ratio in the nebular phase, but there is a notable outlier in SN 2003hv. Finally, using light-curve fitting on our data, we estimate the distance modulus for M85 to be μ₀ = 30.99 ± 0.19 mag, corresponding to a distance of 15.8^+1.4_-1.3 Mpc.