arXiv subjects

Luca Izzo

Publications and source records attributed to Luca Izzo.

At least 19 recordsLinked to original sources

arXiv2026

Constraints on the Progenitor of SN 2023bee from its Nebular-Phase Spectra

We present late-time photometry (~100-400 days after peak) and nebular-phase optical/NIR spectroscopy (100-360 days) of the Type Ia supernova (SN Ia) SN 2023bee. This SN Ia was observed TESS immediately before and after explosion and showed a relatively weak early excess light-curve feature detected in multiple UV and optical bands. Possible explanations for this early excess feature include interaction between the SN ejecta and a non-degenerate companion, or the decay of radioactive isotopes synthesized near the ejecta surface in a sub-$\text{M}_\text{Ch}$ double-detonation scenario. However, the degeneracies and incompleteness of existing models have so far prevented a definitive conclusion about its progenitor system. Nebular-phase spectra provide a complementary probe of SN Ia progenitors by constraining stripped H/He-rich material and iron-group abundances sensitive to the progenitor white dwarf density. We report the non-detection of narrow H/He features in nebular phase optical/NIR spectra of SN 2023bee, placing upper limits of $m^{\text{stripped}}_\text{H}\lesssim5.0\times10^{-3}\text{ M}_\odot$ and $m_\text{He}^{\text{stripped}}\lesssim1.2\times10^{-4}\text{ M}_\odot$. These limits strongly disfavor a non-degenerate companion and suggest that companion interaction is unlikely to explain the early excess of SN 2023bee. We also performed line-diagnostic measurements on the 7200 Åcomplex using multi-Gaussian fitting and continuum estimation, with the inferred velocities validated against NIR spectrum at the similar phase. The resulting low Ni-to-Fe mass ratio, $m_\text{Ni}/m_\text{Fe}\lesssim 0.06$, is broadly consistent with a sub-$M_{\text{Ch}}$ double-detonation progenitor system.

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arXiv2026

Gamma-ray Bursts as Cosmological Probes Calibrated with Strongly Lensed Quasars and Neural Networks

Gamma-ray bursts (GRBs) extend distance measurements to redshifts beyond those routinely probed by Type Ia supernovae, but their use in cosmology is limited by calibration circularity and possible redshift-dependent effects. We investigate strongly lensed quasars as external calibrators of the Amati and Combo GRB relations. We reconstruct the luminosity-distance relation using an artificial neural network trained on TDCOSMO strong-lensing distances, with the absolute scale inherited from the lensing-only calibration. To avoid extrapolation, only GRBs within the lens-supported redshift interval are used for calibration. We then infer cosmological parameters through a hierarchical Bayesian analysis of flat $Λ$CDM, with $w_0w_a$CDM examined as a consistency test. For flat $Λ$CDM, Amati yields $H_0=78.26^{+17.87}_{-16.30}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and $Ω_m=0.670^{+0.223}_{-0.276}$, while Combo yields $H_0=78.71^{+13.83}_{-11.29}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and $Ω_m=0.635^{+0.235}_{-0.242}$. Amati is relatively insensitive to the phenomenological redshift term, whereas Combo shows a stronger dependence. Injection--recovery tests indicate that an unmodelled positive Combo trend can bias $H_0$ and $Ω_m$ upward. Strong-lens calibration therefore offers a route to extending the GRB Hubble diagram, but the inferred $H_0$ is not an independent GRB measurement: it retains the TDCOSMO distance scale. The precision is limited by GRB scatter, redshift-dependent systematics, and lens-modelling uncertainties.

astro-ph.CO↗

arXiv2026

Extreme Stellar Death and Galaxy Feedback at z = 2: A Rest-frame Ultraviolet Characterization of the Strongly Lensed Superluminous Supernova 2025wny

We present photometric and spectroscopic follow-up observations of the strongly-lensed H-poor superluminous supernova (SLSN) 2025wny at $z = 2.0155$. We use integral-field Keck-II/KCWI spectroscopy to obtain rest-frame ultraviolet spectra spanning $11$ to $61$ days after maximum light. We use Pan-STARRS photometry to constrain the brightest image's magnification, finding $μ_A=19.4\pm1.6$, a factor of $\sim$2-4 higher than mass-modeling estimates. We fit the Pan-STARRS photometry with a magnetar energy-injection model including $^{56}$Ni decay using the Modular Open Source Fitter for Transients (MOSFiT); the model can reproduce the near-maximum light curve with parameters typical of local universe SLSNe-I, including a magnetar spin period $P_{\rm spin}=4.1^{+0.7}_{-1.0}\,\mathrm{ms}$ and magnetic field $B_{\perp}=2.4^{+1.1}_{-0.8}\times10^{14}\,\mathrm{G}$; however, the late-time plateau suggests an additional power source. Spectroscopically, iron-group element line blanketing is virtually absent in the rest-frame UV spectra, and the absorption features match previously identified SLSN-I features from abundance tomography. Based on narrow ISM line metallicity estimates, SN 2025wny exploded in a sub-solar-metallicity galaxy. From host-galaxy lines in our rest-frame UV-to-optical spectrum we estimate high-velocity outflows of $\sim$575 km s$^{-1}$ and an exceptionally high Ly$α$ escape fraction of $f_{esc}^{Lyα}=0.23\pm0.03$. SN 2025wny matches lower-redshift SLSNe reasonably well. Upcoming surveys like LSST and Roman will discover additional high-redshift SLSNe, further testing whether SLSNe-I explosions remain consistent at the population level in the early universe.

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arXiv2026

EP250302a: violent shell collision in a soft-X-ray-selected GRB-like transient

The Einstein Probe opens a previously unexplored soft X-ray window onto gamma-ray bursts, filling a critical observational gap in the soft X-ray coverage of their prompt emission. In this letter, we present EP250302a, a soft-X-ray-selected, GRB-like transient at $z=1.131$ detected by the Einstein Probe. Follow-up observations from X-ray to radio reveal a narrow X-ray flare at $\sim 1.1$\, ks and subsequent achromatic optical and X-ray rebrightening. These features challenge a standard single-component afterglow model and indicate the need for multiple ejecta components. A violent collision between a late relativistic shell and the decelerated leading blast wave provides a plausible interpretation: the flare arises from internal dissipation of the late ejecta, while the rebrightening is powered by the shocked emission produced in the collision. Quantitative modeling constrains the kinetic energy ratio between the late shell and the initial ejecta to $E_{\rm k,iso,2}/E_{\rm k,iso,1} \sim 5$ (with $E_{\rm k,iso,2} \sim 10^{53}$~erg and $E_{\rm k,iso,1} \sim 2\times10^{52}$~erg), as well as the Lorentz factor contrast to $Γ_{2,0}/Γ_{1,0} \approx 0.98$--$2.27$, required to reproduce the observed flare luminosity and rebrightening amplitude. Such an energetic late shell can be launched in a radiatively inefficient second episode of central-engine activity. Thanks to the well-sampled, early-time multiband coverage facilitated by the EP trigger, EP250302a provides a valuable case to test the physical connection between central-engine activity and shell collisions.

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arXiv2026

Extragalactic Multi-Band Exploration of Red Supergiants (EMBERS): Pipeline, Source Classification, and Bolometric Properties of 101,219 Red Supergiants from James Webb Space Telescope Imaging

The evolution of red supergiants (RSGs), especially in their final stages before exploding as hydrogen-rich Type II supernovae (SNe), remains poorly understood, owing to small samples of well-characterized stars in the Local Group. We present the largest population study of RSGs to date using $\sim10^5$ stars with bolometric parameters inferred from archival James Webb Space Telescope/Near-Infrared Camera (NIRCam) imaging of 15 galaxies within 20 Mpc. We also present jwst123, a newly-developed general-purpose NIRCam reduction and photometry pipeline, and its application to our set of 15 galaxies. We build a simulation-based parameter inference framework to rapidly fit multi-band photometry of $\sim1.5$ million luminous stars in milliseconds per source. Using clustering in the effective temperature ($T_{\rm eff}$)-luminosity ($\log L/L_{\odot}$)-dust optical depth ($τ_V$) phase space, we introduce a novel method to select RSGs with high completeness and purity. Leveraging the scale of our catalog, we find that the dustiest RSGs prefer metal-rich hosts, and RSG luminosities correlate with host specific star formation rate, suggesting that environment shapes RSG evolution. While most selected RSGs agree with theoretical stellar evolution tracks, we identify a new population of 1,669 RSGs that are heavily dust-enshrouded ($τ_V>4$), overluminous ($\log(L/L_{\odot})>5.55$), or anomalously cool ($T_{\rm eff}<3000\,{\rm K}$), including analogs of known SN progenitors. Our catalog provides a population-scale baseline for understanding massive stellar evolution and connecting field RSGs to dusty progenitors of Type II SNe.

astro-ph.SR↗

arXiv2026

GRB 260310A / SN 2026fgk: A Multi-Wavelength Study of a Nearby Underluminous Long GRB and SN with a Complex Afterglow

We present a comprehensive multi-wavelength study of GRB 260310A / SN 2026fgk, a nearby ($z=0.153$), long-duration gamma-ray burst (GRB) with an exceptionally underluminous prompt $γ$-ray emission and a Comptonized spectrum. The burst occurred at the edge of a blue host galaxy at a projected distance of 15 kpc, which is one of the largest offsets reported for a long GRB. The bright optical afterglow, with dense coverage from COLIBRÍ, likely peaked at a few to several hours post-burst, followed by a shallow decay not expected from canonical afterglow models. Both the optical and X-ray light curves show a brief chromatic plateau from $4-7$ days. We show that the subsequent rebrightening observed at $\sim20$ days is best explained by the combined contribution of the associated Type Ic-BL supernova, identified in GTC spectra, and a late-time refreshed shock. The broadband optical to X-ray spectral energy distribution is well described by synchrotron emission from the forward shock, while the radio observations demand an additional emission component. We model the afterglow using (a) an on-axis uniform jet from a dirty fireball with late-time energy injection and (b) a misaligned jet with power-law angular structure, both having material emitting along our line-of-sight (LOS) moving with an initial Lorentz factor of $Γ_0\sim20-35$. We conclude that at more typical GRB distances ($z\gtrsim0.5$) the prompt $γ$-ray emission from this source would likely have escaped detection, whereas its optical afterglow would have remained observable, making the event appear as an orphan afterglow or a gamma-ray quiet fast X-ray transient.

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arXiv2026

From Light to Sound: Spectroscopic Evolution & Sonification of the flaring Nova V612 Scuti

We present photometric and spectroscopic observations of the 2017 Galactic nova V612 Sct, whose optical evolution was marked by multiple unusually large maxima. The eruption included two prominent flares lasting around a month each and reaching amplitudes of about 2.5 mag, followed by a series of smaller flares. Extensive spectroscopic monitoring reveals a striking pattern: with each flare, new absorption systems emerge at progressively higher velocities. This behavior, also seen in other flaring novae, provides evidence for repeated episodes of mass ejection or outflow at increasing velocities. V612 Sct also alternated between Fe II and He/N spectral phases during different stages of the eruption, establishing a clear connection between the photometric flares and major spectral transitions. We present two-dimensional dynamic spectra that directly trace the appearance of new absorption features contemporaneous with the light-curve flares. We also introduce a sonification of the spectroscopic sequence, offering an alternative representation of the temporal evolution of the eruption. These results support a picture in which repeated ejection episodes and shock formation play a central role in powering the multiple maxima observed in flaring novae.

astro-ph.SR↗

arXiv2026

A Multi-Wavelength View of the First Type Ic-BL Supernova with an Einstein Probe X-ray Shock Breakout

In March 2026, the Einstein Probe (EP) discovered its most nearby (z = 0.0343) Fast X-ray Transient (FXT), EP260321a, the first EP FXT to provide a strong match to expectations for X-ray "shock breakout'" (SBO) emission. Here, we present our multi-wavelength follow-up campaign of EP260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN2026gzf. We show that our radio follow-up extending over 5.8 - 54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energy $E_{K} \gtrsim 10^{49}$ erg for circumburst densities $n > 10^{-2}~{\rm cm}^{-3}$ and assuming microphysical parameters $ε_e = ε_B = 0.1$. Our radio data also constrains a median mass-loss rate of $\dot{M} \lesssim 1.2 \times 10^{-5} M_{\odot}~{\rm yr}^{-1}$ for a Wolf-Rayet progenitor. In addition, we derive SN2026gzf's properties, including $^{56}$Ni mass, diffusion timescale, and expansion velocities, from our $\sim$nightly-cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90\% confidence interval across all properties. We further fit SN2026gzf's light curve and determine that combined emission from both interaction with CSM and $^{56}$Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the ZTF Bright Transient Survey and assuming all Type Ic-BL SNe produce EP260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4 - 16 year$^{-1}$. This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP260321a.

astro-ph.HE↗

arXiv2026

Constraining inhomogeneities and asymmetries in SNe, FBOTs, and other high-energy transients from unresolved radio observations

Synchrotron emission in high-energy transients is produced by relativistic electrons accelerated by shocks. As high-energy transients are often unresolved even on angular scales probed by very long baseline interferometry, it is difficult to obtain a full picture of the ejecta and circumstellar medium (CSM) properties that are probed by the radio synchrotron emission. Radio spectra of high-energy transients frequently show optically thick slopes shallower than the standard $F_ν\propto ν^{5/2}$ expected from synchrotron self-absorption (SSA) models, or broader spectra near the self-absorption frequency. Such deviations are often interpreted phenomenologically, without providing clear insights into the structure of the emitting region. We show how information on the homogeneity and symmetry of the emitting region can be directly inferred from SSA spectra, even when the source is unresolved. We discuss the circumstances under which inhomogeneities in the emitting region can change the spectrum below the self-absorption frequency, causing it to follow a different slope. We examine which parameters can be constrained from observations and which remain degenerate. We apply this method to the stripped-envelope supernova (SN) 2016coi and to the fast blue optical transient (FBOT) AT2018cow, showing that SSA spectra constrain the degree of inhomogeneity in these systems, providing strong evidence for inhomogeneities in the emitting region in the SN 2016coi, and asymmetry in the case of AT2018cow, and we infer the characteristics of the emitting region. When well sampled spectra are available, our method can be applied as a general, model-independent, inference method. This approach can be used to constrain inhomogeneities in a variety of unresolved high-energy astrophysical transients, including SNe, FBOTs, tidal disruption events and gamma-ray bursts.

astro-ph.HE↗

arXiv2026

Failed jet breakout in the metal-poor broad-lined type Ic supernova 2026gzf

A long-standing question in the death of massive stars is the role of relativistic jets. While many gamma-ray bursts and some fast X-ray transients seem to be associated with broad-lined type Ic supernovae, the opposite is not true. The lack of observable jet emission in those Ic-BL SNe can be explained by invoking off-axis jets, choked jets that inject all their energy into the stellar envelope, baryon-loaded jets for which the prompt high-energy emission is strongly suppressed, or non-jetted SNe. The lack of exact explosion time in the majority of SNe presents an obstacle to distinguish between these scenarios. Here we report the properties of SN 2026gzf associated with the X-ray thermal Einstein Probe shock-breakout EP260321a at z=0.0343. The absence of compelling shocked cocoon and radio emission up to 54 days, combined with initial expansion velocities of ~30,000 km/s and a circumstellar shell of ~0.07 M$_\odot$, favour a scenario for SN 2026gzf in which a jet was choked in the circumstellar shell. Our high-spatial resolution images of the SN environment show that the progenitor was located between two highly star-forming regions with a metallicity lower than any previously known Ic-BL SN. As the first case of a Ic-BL SN associated with high-energy prompt emission without the signature of a jet, SN 2026gzf provides a unique perspective to understand the successful launch of relativistic jets during the deaths of massive stars.

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arXiv2026

JWST's Dusty Blue BOAT -- GRB 221009A

GRB 221009A, the Brightest Of All Time (BOAT), presents a challenge for afterglow modelling due to its low Galactic latitude and consequent high line-of-sight extinction. This has led to a wide range of conflicting values for the optical spectral index and dust extinction in the literature. We present a re-analysis of the afterglow spectra, using VLT X-Shooter data at 0.5, 4, and 10.5 days, and JWST NIRSpec$/$MIRI data at 13.3 days post-burst. We fit the data with single and smoothly broken power-law models and perform a joint fit with a double smoothly broken power-law (DSBPL) across all epochs. Our analysis reveals a strong degeneracy between the assumed extinction and the inferred intrinsic spectral index, particularly in the optical, explaining the diversity of previous results. The joint DSBPL fit yields a total line-of-sight extinction of $A_{V} = 4.40 \pm 0.01$ and a blue continuum, with an intrinsic spectral index of $β= 0.447 \pm 0.001$. Although marginally preferred by the spectral fits, a wind medium can be rejected by the temporal evolution of the afterglow light curve. The fit spectral index and temporal decline are only consistent with a uniform density medium if an early jet break at $\sim 0.5-1.0$ days is invoked. Our results imply a hard electron distribution index of $p = 1.89 < 2$, challenging standard particle acceleration models and suggesting a narrow, energetic jet core dominates the early optical-to-X-ray emission.

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arXiv2026

GRB 210704A: A Luminous Fast Blue Transient in a GRB Afterglow at $z = 2.34$

We present detailed, multi-wavelength analysis of GRB 210704A: a Fermi Gamma-ray Burst Monitor discovered and Fermi Large Area Telescope (LAT) detected gamma-ray burst (GRB). The burst is dominated by a short ($\approx 2$ s) pulse followed by weaker, softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be $z = 2.34$. This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB's energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around $T_0 + 7$ d ($2$ d in the rest frame). The excess is extremely luminous ($M_{r} = -22.0$ mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi/LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.

astro-ph.HE↗

arXiv2026

No evidence for dynamical dark energy from the Combo correlation of GRBs

Recently, the Dark Energy Spectroscopic Instrument (DESI) collaboration has presented results indicating that dark energy may exhibit dynamical behavior. Calibrated gamma-ray burst (GRB) correlations can be employed to verify or reject a time-evolution of the dark energy (DE) equation of state, $ω(z)$, up to redshifts $z\sim 9$. We use the most updated catalog of GRBs fulfilling the Combo correlation and improve its calibration employing three catalogs of type Ia supernovae at redshifts $z\leq0.075$ and the Bézier interpolation of the Hubble rate, as an alternative to the cosmographic series that fails to be constraining at high redshifts. To test the evolution of $ω(z)$, we adopt a model-independent, redshift-binned DE parametrization. In both the calibration and the DE reconstruction analyses the impact of the spatial curvature on the results is explored. The calibrated Combo correlation yields a Hubble constant $H_0\sim70$ km/s/Mpc which alleviates the existing Hubble tension and is broadly consistent with current measurements, although the uncertainties prevent a high-precision measurement. Regarding the reconstruction of $ω(z)$ of DE, spatially curved scenarios are disfavored and, despite the apparent ''phantom'' behavior at $z\lesssim0.55$ due to the limited statistics caused by the shortage of nearby events, at $z>0.55$ the analysis provides statistically robust evidence in favor of the cosmological constant scenario. The Combo correlation alleviates the Hubble tension and shows no significant evidence in favor of dynamical DE. This suggests that GRBs, as distance indicators, are broadly consistent with the current cosmic distance ladder.

astro-ph.CO↗

arXiv2026

The Hubble Constant from Blue Type Ia Supernovae

There is a persistent tension of about $5σ-6σ$ between the value of the Hubble constant, as derived from the local distance ladder vs. the cosmic microwave background, signaling either unaccounted for systematics in the measurements or `new physics', such as early dark energy. Determining the Hubble constant using Type Ia supernovae requires nontrivial and accurate corrections for dust extinction. To circumvent this obstacle, we here determine the Hubble constant from blue, and hence presumably unextinguished, supernovae. For two different sets of Type Ia supernova data and lightcurve fitting methods, we find that when using blue supernovae only, the derived Hubble constant is consistently lower by $\sim$ 3 km s$^{-1}$ Mpc$^{-1}$ ($70 \pm 2.1$ and $70.3 \pm 3.0$ km s$^{-1}$ Mpc$^{-1}$), and within 1$σ$ of the cosmic microwave background measurement, compared to when using all supernovae. This is consistent with the hypothesis that systematic effects in dust corrections may affect standard supernova cosmology. However, the number of blue calibrating Type Ia supernovae is small (about six), and values of the Hubble constant for a range of different supernova colors are consistent at the 1.2$σ$ level. Upcoming major transient surveys will discover numerous unextinguished SNe~Ia, and thus be able to increase the precision of the Hubble constant measured from blue SNe~Ia, heralding a promising path toward resolving the Hubble constant tension.

astro-ph.CO↗

arXiv2026

Tightening Cosmological Constraints Within and Beyond $Λ$CDM Using Gamma-Ray Bursts Calibrated with Type Ia Supernovae

Context. Gamma-ray bursts (GRBs) reach redshifts beyond Type Ia supernovae (SNe Ia) and can extend distance measurements into the early Universe, but their use as distance indicators is limited by the circularity problem in calibrating empirical luminosity relations. Aims. We present a model-independent methodology to overcome this circularity by combining Pantheon$+$ SNe Ia, a distance reconstruction based on artificial neural networks (ANNs), and two GRB correlations (Amati and Combo) into a distance ladder from low to high redshift, with the goal of constraining cosmological parameters in $Λ\mathrm{CDM}$ and $w_0 w_a \mathrm{CDM}$. Methods. We use the ReFANN to reconstruct the luminosity distance $d_L(z)$ and distance modulus $μ(z)$ from the Pantheon$+$ dataset, with hyperparameters optimized via approximate Bayesian computation rejection and a risk function. This model-independent reconstruction calibrates the Amati and Combo relations using a low-redshift ($z<1$) GRB sample from Fermi GBM and Swift-XRT. The calibrated relations then provide distance estimates for GRBs at $z \geq 1$. Finally, a joint Bayesian analysis simultaneously constrains the cosmological and GRB correlation parameters, ensuring self-consistent uncertainty propagation. Results. We obtain consistent cosmological constraints from two independent GRB correlations. The Hubble constant $H_0$ agrees with SNe Ia values, though potentially influenced by Pantheon$+$ dataset. High-redshift GRBs favour a higher matter density $Ω_m$ than the Pantheon$+$ and hint at possible dark energy evolution.Conclusions. We present a framework that mitigates GRB cosmology's circularity problem, extending the distance ladder to $z \sim 9$ and establishing GRBs as a high-redshift probe.

astro-ph.CO↗

arXiv2026

AT 2024ahzi: A Type IIP Supernova Discovered by the LSST Commissioning Camera

As part of its commissioning, the Vera C. Rubin Observatory observed several fields repeatedly for a month with ComCam, an instrument that uses the same hardware as the LSST camera but covers a smaller field of view. We photometrically classify AT 2024ahzi, a transient discovered by ComCam, as a Type IIP supernova (SN IIP) using both ComCam and DECam photometry. We find that the duration, luminosity, and color of AT 2024ahzi's photometric plateau are all consistent with those from a large sample of SNe II. By comparing its multi-band light curves to SN II models and analytic relations, we place constraints on the SN progenitor, explosion dynamics, and circumstellar environment. We argue that the progenitor has an extended density profile indistinguishable from a slowly accelerating CSM. We discuss how a similar workflow can identify and characterize future Rubin SNe II.

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arXiv2026

Characterising the Standardisation Properties of Type Ia Supernovae in the z band with Hierarchical Bayesian Modelling

Type Ia supernovae (SNe Ia) are standardisable candles: their peak magnitudes can be corrected for correlations between light curve properties and their luminosities to precisely estimate distances. Understanding SN Ia standardisation across wavelength improves methods for correcting SN Ia magnitudes. Using 150 SNe Ia from the Foundation Supernova Survey and Young Supernova Experiment, we present the first study focusing on SN Ia standardisation properties in the z band. Straddling the optical and near-infrared, SN Ia light in the z band is less sensitive to dust extinction and can be collected alongside the optical on CCDs. Pre-standardisation, SNe Ia exhibit less residual scatter in z-band peak magnitudes than in the g and r bands. SNe Ia peak z-band magnitudes still exhibit a significant dependence on light-curve shape. Post-standardisation, the z-band Hubble diagram has a total scatter of RMS $ = 0.195$ mag. We infer a z-band mass step of $γ_{z} = -0.105 \pm 0.031$ mag, which is consistent within $1σ$ of that estimated from gri data, assuming $R_{V} = 2.61$. When assuming different $R_{V}$ values for high and low mass host galaxies, the z-band and optical mass steps remain consistent within $1σ$. Based on current statistical precision, these results suggest dust reddening cannot fully explain the mass step. SNe Ia in the z band exhibit complementary standardisability properties to the optical that can improve distance estimates. Understanding these properties is important for the upcoming Vera Rubin Observatory and Nancy G. Roman Space Telescope, which will probe the rest-frame z band to redshifts 0.1 and 1.8.

astro-ph.CO↗

arXiv2026

What determines the $γ$-ray luminosities of classical novae?

Classical novae in the Milky Way have now been well-established as high-energy GeV $γ$-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the $γ$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $γ$-ray properties ($γ$-ray luminosity and duration) with their outflow velocities, peak $V$-band magnitudes, and the decline times of their optical light curves ($t_2$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $γ$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $10^{34}-10^{37}$ erg s$^{-1}$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $γ$-ray luminosities, but are detectable for a shorter duration. The optical and $γ$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $γ$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $γ$-ray emission. Dusty and non-dusty novae appear to have similar $γ$-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the $γ$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.

astro-ph.HE↗