X-Ray-luminous Supernovae: Threats to Terrestrial Biospheres

Each SN will have its own unique characteristics and evolutionary behavior, with variables resulting from both the internal and external environment of the progenitor star (Alsabti & Murdin 2017). Most important to the effects imposed on a nearby planet in the interstellar neighborhood, however, are the flux, spectrum, and duration of the ionizing radiation emitted as a result of the event. The exact perturbations that are then imposed on the planetary system will be dependent on the characteristics of the planet itself. We are primarily concerned here with the threats to terrestrial biospheres. As such, with Earth still being the sole confirmed substrate for any sort of viable life-form, we typically situate our analysis of a nearby SN with respect to the deleterious effects that would pertain to Earth's modern environment.

In general, for a terrestrial planet that harbors a robust atmosphere, the most direct effects will be triggered by the radiative alteration of the planet's atmospheric chemistry. The multifarious physical processes governing such interactions can make it difficult to parameterize the exact implications. But, early research as far back as Krassovskij & Šklovskij (1958) speculated on the general implications that nearby SNe may have on life, and numerous others have researched the habitability consequences of high influxes of radiation from a variety of interstellar events (Martín et al. 2011; Chen et al. 2018; Louca et al. 2022; Ambrifi et al. 2022, to name a very select few).

With such a complex system as Earth's atmosphere, any large, energetic perturbation would induce a wide range of responses. Moreover, merely assessing the threat imposed on an Earth-like biosphere must take into account the multiple different phases of Earth's atmosphere and even the nature of life throughout its 4.5 Gyr history. The effects on the biosphere, say, during the Archean Earth (>2.5 Gya), prior to the oxygenation of the atmosphere or land-based life-forms, would be entirely different from effects imposed on the oxidizing atmosphere and complex organisms of the current Phanerozoic Eon (<540 Mya).

Consequences related to atmospheric heating and escape are a focus of much habitability work related to other energetic events, such as outflows from active galactic nuclei (AGNs; Ambrifi et al. 2022) and extreme ultraviolet radiation from quasars (Chen et al. 2018). But while the X-ray emission from SNe are indeed high-energy phenomena, the total, time-integrated energy output of radiation pales in comparison to those more sustained events, and the SN would need to be at such close distances to impose any significant heating or escape that other factors would dominate at that point. Notably, however, Smith et al. (2004) calculated that for a hypothetical biosphere located on a planet with a far thinner atmosphere than found on Earth, even atmospheric heating and escape effects may be significant for an SN. Speculations about hypothetical biospheres are beyond the scope of this paper.

Smith et al. (2004) also analyzed the propagation of ionizing radiation through model atmospheres of terrestrial exoplanets. They found that while even the thinnest of atmospheres will often block all X-ray radiation from reaching the surface, there may be a significant amount of incident X-ray energy that will redistribute into diffuse, but "biologically effective," UV at the surface. More generally, the incident X-ray energy can produce substantial transient fluctuations in atmospheric ionization levels—an effect that would be particularly relevant for advanced, technological civilizations (e.g., radio communication). These sudden ionospheric disturbances are certain to occur in the event of an influx of X-ray radiation, and are often a major focus of research regarding the influx of X-rays from large solar flares (Mitra 1974; Hayes et al. 2017, 2021; Siskind et al. 2022).

While these cited studies have examined the atmospheric influences of ionizing radiation, a notable barrier to extrapolating directly from these previous data (and many others, such as Segura et al. 2010; Chen et al. 2021) is that none yet have resolved the particular influences of the X-ray phase of SN emission. For example, the Smith et al. (2004) study utilized spectral models most relevant for solar flares, gamma-ray bursts (GRBs), and the non-X-ray-specific emission of SNe. The emission profiles of the X-ray-luminous SNe are far more energetic, enduring, and of harder photon spectra than those of solar flares, but particularly less energetic and of softer photon spectra than GRBs. This is a commonality among all the modeled results we reference herein since this is the first paper we are aware of that is specific to the distinct, X-ray phase of SN emission. We intend to more accurately model the X-ray emission profile against a climate model in a future study.

Since our research is the first to focus solely on the X-ray phase of SN emission, we elect to provide a broad assessment of the lethal effects that sits within the context of most prior research into SN threats to habitability. As such, we focus the majority of our analysis only on the deleterious mechanism of ozone depletion. Much previous literature on the habitable influence of SNe often addresses the loss of stratospheric ozone as the most notable of the general consequences to Earth's modern-day atmosphere, as it is the ozone that currently serves as the biosphere's most formidable line of defense against external radiation (Whitten et al. 1976; Ellis & Schramm 1995; Gehrels et al. 2003; Thomas et al. 2005a, 2005b; Ejzak et al. 2007; Melott & Thomas 2011). Justification for this approach with respect to hard X-rays is provided further in Section 2.

Ruderman (1974) was likely the first to orient focus on the catalytic cycle of ozone depletion that would result from photoionization delivered by the SN explosion. Subsequent research into the effects of nearby SNe on habitability has thus similarly used ozone loss as a proxy for biological damage, and hence how "lethal" an SN will be (see Melott & Thomas 2011 for a detailed review).

This prior research often pertains to the two distinct phases of ionizing radiation that are present in all SNe regardless of environment: (1) the prompt arrival of energetic photons—predominantly gamma rays—associated with the massive outbursts of energy, and (2) the later influx of charged cosmic rays that propagate outwards with the SN blast.

Despite the magnificence of the initial outburst, the accompanying gamma rays radiate E_γ ∼ 2 × 10⁴⁷ erg (Gehrels et al. 2003), a small fraction of the ∼10⁵¹ erg of the blast energy. As we will see below, for likely SN distances, this emission is unable to trigger a notable rate of ozone depletion unless further enhanced by subsequent X-ray emission and, therefore, does not represent a significant threat to life on its own. The exception to this occurs in the unique circumstances of an SN accompanied by a GRB, where relativistic jets lead to beamed gamma-ray emission. Though astronomically rare, GRBs from SNe and compact object mergers are hypothesized to have also directly influenced Earth's geological past (Scalo & Wheeler 2002; Melott et al. 2004; Thomas et al. 2005a, 2005b; Piran & Jimenez 2014). In fact, the recent GRB 2210009A, which was over 700 Mpc away from Earth (de Ugarte Postigo et al. 2022; Williams et al. 2023), actually caused a slight, but measurable, perturbation in the D-region of the ionosphere (Hayes & Gallagher 2022); fortunately, this delivered far below any type of lethal dosage.

A later, more destructive phase of SN ionizing radiation occurs hundreds to tens of thousands of years after the initial arrival of photons, in which a planet's atmosphere is bathed with an influx of cosmic rays freshly accelerated by the SN. Cosmic rays carry a much larger fraction of the SN blast energy (≳10%) and so present a greater threat. The time history of SN cosmic rays experienced on Earth depends on their distribution in and around the SN remnant that accelerates them. The escape of cosmic rays from SN remnants remains a topic of active research and depends on the cosmic-ray energy and remnant age. For example, the highest-energy cosmic rays can escape early to act as precursors to the SN blast, while lower-energy particles that are the bulk of cosmic rays are advected into the remnant and remain there until its end stages due to self-confining magnetohydrodynamic instabilities excited by the cosmic rays themselves (Drury 2011; Nava et al. 2016). Broadly speaking, for an observer on Earth, the cosmic flux should be the most intense around the time the forward shock arrives, which is thousands of years after the explosion.

The spectrum of the newly accelerated cosmic rays should have more high-energy particles than the equilibrium propagated spectrum throughout most of the galaxy (Aharonian & Atoyan 1996; Telezhinsky et al. 2012; Bell et al. 2013; Brose et al. 2020). And with the bulk of the cosmic rays trapped in the remnant, the duration of the SN cosmic-ray exposure will be that of the blast passage—many thousands of years.

If the SN is close enough that the biosphere is exposed to a dose of cosmic rays, this can represent the most harmful stage of a nearby SN on a terrestrial environment, simply due to the extremely high energy and prolonged influential presence of the cosmic rays. As these cosmic rays penetrate deep into the atmosphere, lingering for hundreds to thousands of years, they alter the atmospheric chemistry of the planet. It is this phase that has typically been the focus of prior research on the terrestrial consequences of SNe. In said research, it is generally assumed that a rate of 30%–50% globally averaged ozone depletion (in reference to modern-day Earth levels) harbors the potential to impose an extinction-level event on Earth (Melott & Thomas 2011). The distance the SN would have to be to impose such consequences is often labeled the "lethal distance." In their review of astrophysical ionizing radiation and Earth, Melott & Thomas (2011) have surmised a typical lethal distance for an SN to be around 10 pc. However, this value is largely variable due to a variety of factors, and in a more recent detailed study, Melott & Thomas (2017) have also estimated that this distance may be upwards of 50 pc for certain interstellar conditions. These general parameters serve as the foundation for our assessment of the threat imposed by X-ray-luminous SNe.

In this paper, we now turn the analytical focus to the persistent high-energy X-ray emission that characterizes certain SNe. The timing of the X-ray emission is situated in between the two previously mentioned general stages: months and/or years after the initial outburst and hundreds to thousands of years before the arrival of cosmic rays. Thus, the associated damage from the X-ray influx would occur as a separate phase of ionizing radiation, altering our notions of the timeline by which an SN influences a nearby planet.

Figure 1 illustrates a hypothetical, but realistic, timeline by which an X-ray-luminous SN's radiation would interact with a nearby planet (note that the timeline uses a log scale). The three phases from left to right are (1) the initial arrival of photons (green) in the SN outburst, (2) the X-ray phase of emission (yellow) delayed by months to years after the outburst, followed by (3) the arrival of cosmic rays (blue) with the SN remnant thousands of years later.

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The X-ray emission is thought to arise from interactions with a dense circumstellar medium (CSM) carved out during the star's lifetime (Smith 2014; Chandra 2018; Dwarkadas 2019). It has long been theorized (Ruderman 1974) that some SNe may have high-enough X-ray emission to trigger the catalytic cycle of ozone depletion similar to that of the later cosmic rays—albeit at different magnitudes and shorter timescales. And while X-ray significance has been acknowledged from the beginning, the absence of empirical X-ray SN observations to parameterize such discussion has correlated with their absence from discussions on planetary influence.

In the last few decades, the field of X-ray astronomy has come of age (Wilkes et al. 2022), most notably due to innovations in telescopic sensitivity employed through Chandra, Swift-XRT, XMM-Newton, and NuSTAR. The combined capabilities of these telescopes have allowed us to gain a keen understanding of the X-ray evolution from all types of SNe, and these observations have served to both confirm and upend certain notions of stellar processes (Dwarkadas 2014; Wilkes et al. 2022).

For this study, we have scoured the literature for X-ray SNe to compare the evolutionary characteristics of 31 X-ray-luminous SNe (occasionally called "interacting SNe," due to the necessary interaction with a CSM). We use these data to conduct a threat assessment that serves as an initial parameterization for the impact that SN X-ray emission can have on terrestrial planets. In Section 2, we briefly summarize the general threat to terrestrial planets that is imposed by ionizing photon events, explaining the key value of critical fluence and determining an appropriate fluence value for the associated X-ray data. This discussion outlines the process by which we can then parameterize our subsequent assessment of the X-ray emission.

In Section 3 we then present the collection of light curves for the 31 SNe analyzed. We identify the key characteristics of each spectral type and identify the emission trends relevant to terrestrial atmospheres. Throughout our analysis of the data, we refrain from including values based on speculation of any unconfirmed X-ray emission, i.e., we only display confirmed X-ray observations recorded in the literature. However, as will be discussed in Section 3, there are clear indications that each SN actually has a larger amount of total X-ray emission than has been observed. Therefore, by limiting our data analysis to the confirmed observations only, we are providing a conservative, lower-end estimate of their overall X-ray energy output. Nevertheless, we maintain this restriction, because an overarching purpose of this paper is to provide empirical evidence for the threat that SN X-ray emission can impose on terrestrial planets. This subsequently enables us to provide a conservative estimate for the ranges at which the X-ray-luminous SNe would be threatening to a nearby biosphere, and thus, when we present our threat assessment in Section 4, we will have gone to great lengths to show that the inferences we make are appropriate, and the generalizing rate of lethality for X-ray-luminous SNe is significant.

We discuss our results in Section 5 and conclude with the summation of our findings in Section 6, most notably, that SN X-ray emission—a distinct phase of an SN's evolution—can certainly impose effects on terrestrial atmospheres and biospheres at formidable distances. These results have implications for planetary habitability, the Galactic habitable zone, and even Earth's own evolutionary history.

In general, the amount of ozone depletion induced by an astrophysical ionizing event is mainly dependent on the spectrum and total amount of radiation incident on Earth—not the rate or duration of the event (Ejzak et al. 2007; Melott & Thomas 2011)—and will be a function of the fluence, ${ \mathcal F }$, which is the energy deposited per unit area of the atmosphere. This is simply the integrated flux of radiation arriving at the top of the planet's atmosphere. An explosion at distance d with an X-ray flux F_X has an X-ray fluence, ${{ \mathcal F }}_{{\rm{X}}}$, of

$$\begin{eqnarray}&&{{ \mathcal F }}_{{\rm{X}}}={\int }_{{t}_{i}}^{{t}_{f}}{F}_{{\rm{X}}}(t)\ {dt}=\displaystyle \frac{{\int }_{{t}_{i}}^{{t}_{f}}{L}_{{\rm{X}}}(t)\ {dt}}{4\pi {d}^{2}}=\displaystyle \frac{{E}_{{\rm{X}}}}{4\pi {d}^{2}}\ .\end{eqnarray} \tag{ 1 }$$

We see that the fluence depends on the total X-ray energy output in the observed window:

$$\begin{eqnarray}&&{E}_{{\rm{X}}}={\int }_{{t}_{i}}^{{t}_{f}}{L}_{{\rm{X}}}\ {dt},\end{eqnarray} \tag{ 2 }$$

which is the X-ray luminosity integrated over time.

In principle, the X-ray fluence in Equation (1) could be diminished by absorbing material along the sight line to the SN. However, for the ≲50 pc distances of interest, this is likely a negligible effect, and we can thus appropriately adopt the unabsorbed luminosity when given.

To compare the magnitude of the threat associated with the X-ray emission of each SN, we will calculate the furthest distances at which they could be from an Earth-like planet to impose lethal effects on the biosphere. A simple comparison point is the "lethal distance," which is the terminology used throughout Thomas et al. (2005a, 2005b) and Melott & Thomas (2011, 2017) and which we define as the approximate distance at which an SN would impose severe lethality on a terrestrial biosphere. We will thus characterize the degree of damage by a critical fluence, ${{ \mathcal F }}_{{\rm{X}}}^{\mathrm{crit}}$, in the X-ray band.

Demanding that the fluence in Equation (1) be equal to the critical fluence at which the X-ray emission would impose lethal consequences, we solve to find the associated distance as

$$\begin{eqnarray}&&{D}_{{\rm{X}}}^{\mathrm{leth}}={\left(\displaystyle \frac{{E}_{{\rm{X}}}}{4\pi {{ \mathcal F }}_{{\rm{X}}}^{\mathrm{crit}}}\right)}^{1/2}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&=14\ \mathrm{pc}{\left(\displaystyle \frac{{E}_{{\rm{X}}}}{{10}^{49}\ \mathrm{erg}}\right)}^{1/2}{\left(\displaystyle \frac{400\,\mathrm{kJ}\,{{\rm{m}}}^{-2}}{{{ \mathcal F }}_{{\rm{X}}}^{\mathrm{crit}}}\right)}^{1/2}.\end{eqnarray} \tag{ 4 }$$

To then evaluate the range of influence for X-ray SNe, we must (1) appropriately parameterize the critical fluence value for the soft X-rays of the SNe data and (2) find each SN's total observed X-ray energy output, E_X. The remainder of this section is devoted to the discussion of critical fluence and its appropriate value for X-ray emission. Total SN X-ray energy outputs will be calculated and discussed in Section 3.

Table 1, column 3, displays the observed energy band for the SN X-ray observations in each paper. We note that these observations are mainly limited to the soft X-ray energy band (≤ 10 keV). Soft X-ray photons each carry significantly lower energy than the gamma rays and cosmic rays that most prior studies relevant to nearby SNe have focused on. Moreover, the duration of the X-ray radiation (months to years) is longer than that of a GRB but significantly shorter than that of the cosmic rays that arrive later (hundreds to thousands of years). These factors must be accounted for to appropriately assign a critical fluence value to the X-ray lethal distance.

Importantly, Earth's thick atmosphere is more or less opaque to soft X-rays, meaning that the photons arriving at the top of the atmosphere will not reach the surface, and it is unlikely that the majority of these photons make it down to the stratospheric level of the ozone layer. Nonetheless, due to the sheer magnitude of energy associated with SNe, even the softer X-rays can make it to an altitude adequate to initiate the NO_x -induced catalytic cycle of ozone depletion—a process further dependent on meteorological conditions that facilitate the downward transport of NO_x into the stratosphere (Solomon et al. 1982). This is an important caveat and will be addressed in more detail in the next subsection.

Further complicating the matter, a number of planetary conditions can play a role in determining the impact of astrophysical ionizing radiation on the atmosphere. Time of year, angle of incidence, geomagnetic activity, and meteorological conditions are just a few of the many influences determining the Earth system's response (Tartaglione et al. 2020). Previously published work has addressed these factors through the use of atmospheric chemistry models that can calculate globally averaged ozone depletion.

The most useful of these studies with respect to ozone-related effects and astrophysical events are Thomas et al. (2005a, 2005b) and Ejzak et al. (2007). These papers each utilized an atmospheric model of modern-day Earth (the Goddard Space Flight Center two-dimensional atmospheric model) to explore the atmospheric effects associated with ionizing radiation from astrophysical sources. Since they both provide insight into the fluence ranges and sensitivity to photon energy, their results prove vital for our analysis.

Thomas et al. (2005a, 2005b) studied the relation between ozone depletion and critical fluence. They compared the response to ionizing radiation with three different fluence values (10 kJ m⁻², 100 kJ m⁻², 1000 kJ m⁻²), finding that the percent change in the globally averaged column density of ozone scales with fluence as a power law with an index ∼0.3. This less than linear relationship occurs due to the ozone depletion becoming saturated at higher fluence values. While this work specifically used timescales associated with GRBs, its general implications for ionizing radiation remain the same, and it provides necessary context for scaling our critical fluence value.

Ejzak et al. (2007) then utilized the same atmospheric model to examine the terrestrial consequences of spectral and temporal variability for ionizing photon events. In this study, they varied the burst duration (10⁻¹–10⁸ s) and average photon energies (1.875 keV–187.5 MeV), while holding fluence constant at 100 kJ m⁻². They found that at a given fluence, higher-energy photons were more damaging to ozone because they penetrate deeper into the atmosphere, creating a significant increase in NO_x at stratospheric altitudes.

Nonetheless, at the fluence used in the study, gamma rays with energies 187.5 keV and above destroy ≳33% of the ozone. This is sufficient to represent a lethal dose, particularly for the most vulnerable biota. For this reason, ${{ \mathcal F }}_{\gamma }^{\mathrm{crit}}=100\ \mathrm{kJ}\,{{\rm{m}}}^{-2}$ is often assigned to be the critical fluence for gamma-ray exposure (Melott & Thomas 2011). Turning to X-rays, Ejzak et al. (2007) found that a photon energy of 1.875 keV (which most closely corresponds to the soft X-ray bands cited here) induces a globally averaged ozone depletion of ∼22% (at the constant 100 kJ m⁻² fluence). This depletion level would persist for a couple of years, begin to noticeably recover after about five years, and effectively complete recovery in a bit over a decade—well before the later arrival of the cosmic rays. While this may seem relatively short on geological timescales, it is nevertheless many generations for the UV-transparent single-celled organisms at the base of the marine food chain and could potentially leave measurable traces in the paleontological record (Cockell 1999). For reference, the peak of anthropogenic-related ozone destruction was around ∼5% globally averaged depletion in the 1990s (Salawitch et al. 2019), so we are discussing average depletion levels significantly higher than any recent phenomena.

Still, the globally averaged ozone depletion of ∼22% at an X-ray fluence of 100 kJ m⁻² would be very unlikely to induce severe lethality or serve as a lone trigger for an extinction-level event and thus would not fit the parameterization for a lethal distance. Therefore, a fluence value of ∼100 kJ m⁻² is too small for the softer X-ray band. A higher critical fluence value is instead required for the given photon energies and burst durations of X-ray-luminous SNe. To account for this, we combine the modeled results from both Ejzak et al. (2007) and Thomas et al. (2005a, 2005b) (ozone depletion and fluence scales less than linearly) and adopt a higher critical fluence value of

$$\begin{eqnarray}&&{{ \mathcal F }}_{{\rm{X}}}^{\mathrm{crit}}\approx 400\ \mathrm{kJ}\,{{\rm{m}}}^{-2}=3.8\times {10}^{45}\ \mathrm{erg}\,{\mathrm{pc}}^{-2}\end{eqnarray} \tag{ 5 }$$

to induce ≳30% prolonged ozone depletion. With this value, an X-ray-luminous SN located at the typical SN lethal distance cited in Melott & Thomas (2011) (D = 10 pc) would require a total X-ray energy output of E_X = 4.8 × 10⁴⁸ erg.

The percentages estimating biological damage are not precise, as a variety of conditions within the Earth system could alter the exact effects of ionizing radiation. The upshot of this is that the critical fluence value of 400 kJ m⁻² is also somewhat of a crude approximation, and the definition of a "lethal distance" is somewhat loose. However, these definitions and values remain sufficient to provide us with appropriate approximations for our threat assessment conducted in this paper, the purpose being to identify whether X-rays from interacting SNe exhibit a notable range of influence. A more extensive analysis of the associated atmospheric effects and variations in biological damage can be found throughout the aforementioned literature.

To put the fluence value of 400 kJ m⁻² into the context of an X-ray influx that has impacted Earth before, we can reference the largest solar flare on record, the 1859 Carrington Event. This event is typically classified as an X-45 solar flare for its peak flux of ∼45 × 10⁻⁴ W m⁻² in soft X-rays (Cliver & Dietrich 2013). Following the calculations from Equations (1) and (2), this peak emission would have to persist from the Sun at that magnitude for approximately 2.8 yr to induce a fluence of 400 kJ m⁻². Moreover, this peak is in the soft X-ray band, without substantial evidence of significant emission in the 10+ keV range that characterizes interacting SN emission (see Section 3). Nonetheless, solar flares can provide us with concrete empirical evidence of influxes of soft X-ray emission perturbing Earth's atmosphere.

Further limitations in our approximation for the critical fluence and associated effects stem from the fact that no agreed-upon spectral model for X-ray-luminous SNe exists, and thus, we are extrapolating from modeled effects of events such as GRBs, which would have notably harder spectra than would be characteristic of the X-ray phase of emission from an SN.

The most notable discrepancy in using these spectral models would be with respect to the altitude of energy deposition in the atmosphere. In short, this complicates any direct extrapolation of the Ejzak et al. (2007) 1.875 keV results for our assessment. Ejzak et al. (2007) used a GRB spectrum, which they modeled as a broken power law in photon energy, with an adjustable peak energy in which the spectral index changes. A consequence of this power-law spectrum is that there are always photons with energy >10 keV. A corollary of this is that when Ejzak et al. (2007) calculate the ozone depletion at a peak photon energy of 1.875 keV, there is still a significant proportion of higher photon energies (10+ keV) being inputted as well. This peripheral energy falls outside the spectral range of the confirmed SN observations. However, a good portion of the energy is within the range of all of the X-ray data we use. Moreover, if only considering the <10 keV photons, this would be a quite conservative approach, because it is very likely that substantial X-ray emission exists for each of these SNe outside their cited energy ranges, i.e., in the harder X-ray band that most X-ray telescopes cannot detect (>10 keV). NuSTAR, the newest X-ray telescope that actually has the capability of sensing the more energetic photons (∼3–79 keV), has already observed significant X-ray emission in these harder spectra for some SNe (Chandra et al. 2015; Brethauer et al. 2022; Thomas et al. 2022), and there is nothing to indicate this is abnormal. Ejzak et al. (2007) confirmed that harder spectra would result in more ozone depletion and would thus require a lower critical fluence to induce lethal effects on the biosphere. We therefore aimed to ensure that our approximation is conservative so as to not overestimate the SN influence.

We turn now to the atmospheric deposition of X-rays and the resulting ozone damage. X-rays are less penetrating than gamma rays generally, and X-ray absorption is a strong function of photon energy, with opacities dropping as ${\kappa }_{X}\propto {\varepsilon }_{{\rm{X}}}^{-3}$. Thus, low-energy (soft) X-rays are absorbed high in the atmosphere, while harder X-rays penetrate more deeply. We therefore expect soft X-rays to be less effective ozone depletion agents than harder X-rays and gamma rays.

Figure 2 illustrates this behavior, showing atmospheric transmission for a range of X-ray energies ε_X ∈ [1, 100] keV, compared to an indication of ozone layer. Plotted is the flux F(z) at height z relative to the incident flux F₀ at the top of the atmosphere. We have $F(z)/{F}_{0}\,=\,{e}^{-\tau ({\varepsilon }_{{\rm{X}}})}$, with the optical depth τ(ε_X) = κ(ε_X) Σ(z). Here the opacity κ is from the dry air tabulation of mass absorption coefficients by the NIST database (Hubbell 2004), ⁷ and we use a simple exponential model for the atmosphere to evaluate the mass column density ${\rm{\Sigma }}(z)={\int }_{z}\rho ({z}^{{\prime} })\,{{dz}}^{{\prime} }$. We see that in all cases the flux ratio starts at unity at high altitudes then drops rapidly at a characteristic height that is energy dependent. In particular, the flux of ε_X = 1 keV and 3 keV photons is cut off below ∼70 km, substantially above the majority of the ozone layer, roughly plotted by location in dashed gray. Photons with 10 keV and above overlap progressively more with the ozone.

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The justification for considering ozone depletion, however, can be verified with empirical evidence of the photochemical coupling between the lower thermosphere and the upper stratosphere of Earth's atmosphere (Solomon et al. 1982; Randall et al. 2006). Essentially, most ionization of N₂ induced by the photons, as well as the subsequent creation of NO_x , will occur above the stratosphere—primarily in the lower thermosphere at altitudes above ∼90 km. In fact, it is the variability in the flux of the Sun's soft X-rays that serve as the primary control for nitric oxide density in the lower thermosphere of the tropics (Barth et al. 1999). However, Solomon et al. (1982) first resolved that substantial amounts of NO_x produced in the thermosphere can reach the stratosphere and trigger the catalytic cycle of ozone depletion. This descent of NO_x from the thermosphere to the stratosphere has since been both modeled and consistently observed in a magnified response to solar flares (Rohen et al. 2005; Randall et al. 2006; Maliniemi et al. 2021; Bailey et al. 2022; Siskind et al. 2022). The process is particularly prevalent at high latitudes during polar winter, with the vertical transport of NO_x being further facilitated by various meteorological conditions, such as polar vortices. We therefore would expect that much of the global ozone depletion resulting specifically from SN X-rays would actually be contingent upon this photochemical coupling and would often be concentrated over the polar regions. The specifics of the depletion timeline and duration would be further dependent on the varying seasonality and geophysical conditions throughout the months and years of prolonged X-ray emission, with some months of the event being more/less lethal than others.

All considered, these variables are why the Ejzak et al. (2007) results are so useful, as they provide a general assessment, and their inputs closely align with the X-ray emission observed. One small caveat noted by the authors is that the direct relation between fluence and ozone depletion begins to weaken for burst durations longer than 10⁸ s (≈3 yr), as meteorological conditions (e.g., rainout) begin to remove NO_x compounds from the atmosphere and thus dampen any prolonged increase in ozone depletion. Many of the SN X-ray emission profiles we analyzed have longer durations than 10⁸ s, which would seem to indicate that more consideration is needed than simply finding their E_X value. However, for each SN analyzed, either all or the vast majority of their total energy output (>95%) occurs well within this timeframe of 10⁸ s and, therefore, effects such as rainout will not significantly alter our calculation of SN influence. In the future, an atmospheric model simulation that is specifically attuned to the associated energy inputs for SN X-ray light curves would be needed to refine speculation any further. For our threat assessment conducted here, this approximation suffices, and we adopt the critical flux in Equation (5) as a rough indication of the effect of SN X-ray irradiation.

With an analytical method established for comparing X-ray emission effects, we now turn to the empirical data for SN X-ray emission collected in the last half-century.

Figure 3 displays the X-ray light curves for 30 of the 31 SNe that we identified in Table 1 (SN 1987A has been removed for clarity; see caption). These curves illustrate the evolution of each SN's X-ray emission over time in the cited observed band, with each point representing a confirmed luminosity value taken from the sources cited in Table 1. Both axes are logged. The figure further illuminates the often-limited range of available observations for each SN in the epoch they are observed.

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To our knowledge, the most comprehensive depictions of such SN X-ray emission prior to this study were the SN X-ray database ⁸ (Ross & Dwarkadas 2017; Nisenoff et al. 2020) and the Dwarkadas & Gruszko (2012) X-ray SN light-curve compilation (and extended in Dwarkadas 2019). We utilized these resources as an initial guide for our data collection and analysis, but then scoured the original papers and observational reports (cited in Table 1) to compile our luminosity data and expand the scope of the analysis.

Often the original papers directly provided luminosity calculations for the observed X-ray emission, from which we would directly adopt their derived values. When only an X-ray flux was given, we conducted the simple luminosity calculation using the adopted distance cited in the paper (L_X = 4π d² F_X), ignoring any effects of absorption due to the relatively short (astronomically) distances we are concerned with here.

From these data, we can calculate each SN's total X-ray energy emitted, E_X, through an integration of their respective X-ray light curves as seen in Equation (2). To do this, we take a simple empirical approach: we interpolate linearly between the observed epochs. We have tried other methods including using simple fitting functions and find that these give similar results. As described in Section 2 (see Equation (3)), we are using this total energy output to characterize the general threat imposed by each SN.

Figure 4 illustrates how these cumulative X-ray energy outputs evolve over time. For clarity, we limit the displayed SNe to those containing >5 confirmed observations (with SN 2008D omitted for clarity) and only use measured data points that result in abrupt onset. We linearly interpolate the luminosity between the points, leading to the rise of the curves, which terminate at the final observation. The endpoint of each curve gives the total observed E_X. The shape of the rise from onset to the final point indicates the time history of the radiation dose delivery.

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The range of widths of the curves in Figure 4 gives a general sense of the range of timescales associated with the measured SNe. The curves generally rise rapidly and then taper off, corresponding to a high initial X-ray luminosity that diminishes over time. In some examples, we have SNe measured at both early and late times (SN 1993J), which provide us with the understanding that these emissions are present for long periods of time after they first appear. Note that the time axis is logarithmic so the durations are not uniform widths across the plot.

A primary motive of our study is to work explicitly with confirmed SN data. Accordingly, two notable parameters are confining our analysis of the SN data to provide restrictive limitations in our overall assessment: (1) We are only considering the emission of these SNe from the given data points; i.e., we do not plot emission that may occur outside the epochs observed, and (2) we are only considering the emission of these SNe within the cited X-ray band reported by the original papers.

The likelihood that X-ray activity occurred outside the observational windows and outside the typical range of X-ray telescopic sensitivity (≤10 keV) is very high, particularly for SNe with scant observational data. For example, in researching the surprising X-ray emission of Type Ib/c SNe, Margutti et al. (2017) have proposed that as much as ∼40% of Ib/C SNe could be X-ray luminous at t ≳1000 days. Brethauer et al. (2022) reported on seven years of Chandra–NuSTAR observations of SN 2014C, which they note is a late interacting SN best modeled and observed at thermal emission with T ≈ 20 keV. The implications of this continually growing evidence are that a significant percentage of SN X-ray emission is simply being unobserved, particularly in the shorter wavelengths. However, no agreed-upon spectral model for SN X-ray emission exists within the literature, and we make no attempt to adopt one here.

Therefore, with the limitations of (1) only observed epochs and (2) only observed energy ranges, all considered, our calculations will thus yield conservative values for the total X-ray energy outputs. We accept this given that here we are merely analyzing the empirical evidence to determine if these emissions can induce a notable impact with the confirmed energy alone and will extrapolate further in a future study. As the astronomical community gains further insight into the harder spectrum of X-ray emission and the X-ray observational cadence increases, we will be able to expand our assessment.

Figure 5 is the same as Figure 3, but with the light-curve coloring now grouped by spectral classification. This serves to highlight the X-ray emission disparities of the different spectral types. There are three key aspects emphasized here that are relevant to our threat assessment:

• 1.  
  The X-ray emission spectra from Type IIn SNe are significantly higher on average than all other types of SNe. Their X-ray emission is most readily seen months and/or years after the explosion. This is a characteristic of a progenitor star with a high rate of mass loss creating a dense CSM, with which the SN shock wave collides (Dwarkadas 2019). The rather late interaction reflects the travel time for the SN blast to reach the CSM. Chandra et al. (2020). Note however that this trend is partially attributable to observational biases, as some SNe IIn were not discovered or observed until well after their explosion dates (e.g., SN 1986J, 1988Z, and 1978K). Note also that most X-ray SN observations are sensitive to soft photons. The only Type IIn that has been observed by NuSTAR is SN 2010jl, which exhibited significant hard X-ray emission (10–80 keV) over two years after the explosion (we have not included this lone data point at that energy range since we are using only data sets with multiple confirmed observations). Regardless, the high, prolonged X-ray energetics imply that SNe IIn typically have the largest range of influence on terrestrial atmospheres during this phase of the SN.
• 2.  
  There are thus far three observed SNe that have been initially classified as non-IIn SNe yet have X-ray emission spectra comparable to those of the IIn class. These are SN 2012ca (Type Ia), SN 2001em (Ib/c), and SN 2014C (Type Ib). The evolution of their light curves calls into question their initial classifications, as well as the traditional spectral classification system as a whole (Margutti et al. 2017). Chandra et al. (2020) propose that these SNe have essentially "metamorphosed" into Type IIn SNe, transitioning from noninteracting to interacting SNe. Their high X-ray energetics potentially make them as lethal to nearby biospheres as the IIn class. Furthermore, SN 2014C was observed for several years with NuSTAR (range 3–80 keV) and showed significant luminosity output in the hard X-ray spectrum throughout (Thomas et al. 2022; Brethauer et al. 2022). The evidence for these harder emissions elsewhere would have implications for the critical fluence value discussed in Section 2, as the amount of ozone depletion scales with photon energy and thus would ultimately serve to increase their range of influence. Whether the characteristics of these non-IIn spectra are typical for those of their class could also have significant consequences for the habitable zone, as a higher rate of occurrence for X-ray emission could greatly increase the statistical threat that SNe pose for planetary habitability (see Section 4).
• 3.  
  The X-rays originating from other types of SNe (IIL, IIP, etc.) are relatively small in comparison. These SNe are "noninteracting" in the absence of a dense CSM, and the X-ray luminosities peak upon shock breakout at the considerably lower durations required to make their X-ray emission a threat to terrestrial biospheres. Though the luminosities in this shock breakout can be relatively large, they do not persist long enough to be of any significance regarding the lethal effects evaluated here but could nevertheless impose ionospheric disturbances.

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This last point is best encapsulated by the observed data from SN 2008D (whose entire observed light curve is only partially displayed for clarity in Figures 3 and 5). SN 2008D is quite a unique event in X-ray astronomy, in that it was a serendipitous discovery of an SN upon initial outburst (Soderberg et al. 2008). This offered a view of the SN breakout immediately in the X-ray band. Its peak observed X-ray luminosity of ∼3.8 × 10⁴³ erg s⁻¹ (Modjaz et al. 2009) is actually the most luminous observation in our data—nearly two orders of magnitude larger than the most energetic observations of Type IIn SNe. However, this burst of energy immediately dissipates within a matter of minutes by a few orders of magnitude, resulting in a relatively low total X-ray energy.

This bolsters a key notion underlying our analysis: For non-GRB SNe, the initial influx of photons from the SN outburst is nonthreatening to terrestrial biospheres at formidable distances. Instead, the prolonged X-ray emission that arises from CSM interaction (months/years after outburst) provides an additional threat and alters the timeline by which a nearby SN influences a terrestrial biosphere. In general, noninteracting SNe would not have much further lethal influence beyond the hazardous effects associated with their cosmic rays discussed in previous nearby SN studies.

Figure 6 illustrates the approximate ranges of influence for the top 17 SNe analyzed (those with ${D}_{{\rm{X}}}^{\mathrm{leth}}\gt 1$ pc). Here, we display three distinct values for a range of lethal influence, with each value derived from Equation (3), using three separate critical X-ray fluences (displayed on plot, left to right): ${{ \mathcal F }}_{X}^{\mathrm{crit}}=400\ \mathrm{kJ}\,{{\rm{m}}}^{-2}$ (square), 200 kJ m⁻² (triangle), and 100 kJ m⁻² (circle). This serves to illustrate the distances at which these SNe would maintain significant influence on an Earth-like biosphere.

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We have gone to great lengths to justify that the leftmost value (400 kJ m⁻²), which comes from Equation (5), is the most appropriate to calculate the lethal distance if the entirety of the X-ray emission was captured by confirmed observations. And therefore, as discussed extensively in Section 2, the square on each SN's respective line corresponds to the range at which it would likely induce 30%+ ozone depletion from merely the observed X-ray emission alone.

But we have also shown throughout our discussion that it is very unlikely that the total X-ray emission from these SNe is encapsulated by the limited epochs and energy ranges thus far reported. So here, we now provide some appropriate—but still conservative—estimations for the potential ranges at which interacting SNe could be lethal.

The rightmost value displayed along the lines for each SN in Figure 6 corresponds to a fluence of 100 kJ m⁻², which is often used in previous studies that utilize atmospheric chemistry models to analyze radiative effects on Earth-like atmospheres (Thomas et al. 2005a, 2005b; Ejzak et al. 2007). In particular, Ejzak et al. (2007) specifically used this value with an average photon energy of 1.875 keV and a burst duration of ∼10⁸ s, which are quantities that align most accurately with our given X-ray data. In these simulations, such an event induced globally averaged ozone depletion rates of approximately 22%, followed by recovery timescales of over a decade. So at the marked distances of the circle on each SN's line, these modeled effects would likely occur at the distance from merely the observed X-ray emission alone. Depletion of such scale is unlikely to trigger any type of extinction-level event and would be unlikely to fit the parameter for our lethal distance. Nevertheless, even at these levels, the event would be a substantial forcing on a terrestrial planet.

Of particular note is that if all X-ray-luminous SNe do indeed emit in the harder X-ray band (>10 keV) undetectable by most X-ray telescopes and do indeed emit outside the limited windows of our observations, then it is entirely appropriate to assign a critical fluence of 200 kJ m⁻² or even 100 kJ m⁻² for the lethal distance calculation of Equation (3). Figure 6 shows that this adjustment would significantly expand the distances from which these SNe could impose lethal effects. And importantly, it is very likely the case, given (1) the confirmed evidence of hard X-ray emission that is seen by NuSTAR in SN 2010jl (Chandra et al. 2015) and SN 2014C (Brethauer et al. 2022; Thomas et al. 2022), and (2) the likelihood that all X-ray-luminous SNe emit X-rays beyond the limited observed epochs (see Section 3).

We have therefore shown that the most X-ray-luminous SNe can yield lethal distances of

$$\begin{eqnarray}&&{D}_{{\rm{X}}}^{\mathrm{leth}}\ \sim \ 20-50\ \mathrm{pc},\end{eqnarray} \tag{ 6 }$$

and we have remained conservative in estimating their X-ray output. This range is larger than the canonical 8–10 pc value, which has significant consequences, as we will now see.

Having shown that X-ray-luminous SNe can affect habitability at formidable distances, we now infer the implications that this will have for the overall Galactic habitable zone. The variables of most importance here are the rates of occurrence for interacting SNe, along with the geometry and dispersion of the galaxy itself. The large distances we found above correspond to a larger volume of influence, making the stage of SN X-ray emission all the more significant.

We wish to compute a rate Γ_i (r) of SNe of Type i within a distance r (D. Sovgut et al. 2023, in preparation). This depends on the global Milky Way rate ${f}_{X}{{ \mathcal R }}_{\mathrm{cc}}$ of X-ray-luminous SNe, which we write as a product of the CCSN rate ${{ \mathcal R }}_{\mathrm{cc}}={{dN}}_{\mathrm{cc}}/{dt}$ and the fraction f_X of SNe that are X-ray bright. The distances of interest are smaller than the scale height, h_cc, for core-collapse progenitors, i.e., we have r ≲ h_cc ∼ 100 pc. In this limit, to a good approximation, we have

$$\begin{eqnarray}&&\begin{array}{l}{\rm{\Gamma }}(r)\ \approx \ {f}_{X}{{ \mathcal R }}_{\mathrm{cc}}{\rho }_{\mathrm{cc}}({R}_{\odot },{z}_{\odot })\displaystyle \frac{4\pi {r}^{3}}{3}\\ =\,\displaystyle \frac{{r}^{3}}{3{R}_{\mathrm{cc}}^{2}{h}_{\mathrm{cc}}}\ {e}^{-{z}_{\odot }/{h}_{\mathrm{cc}}}{e}^{-{R}_{\odot }/{R}_{\mathrm{cc}}}\ {f}_{X}{{ \mathcal R }}_{\mathrm{cc}},\end{array}\end{eqnarray} \tag{ 7 }$$

where the observer's position is that of the Sun, namely, (R_⊙, z_⊙) in Galactocentric coordinates. We have assumed all core-collapse events follow the same "double exponential" distribution with probability density ${\rho }_{\mathrm{cc}}(R,z)={e}^{-| z| /{h}_{\mathrm{cc}}}{e}^{-R/{R}_{\mathrm{cc}}}/4\pi {R}_{\mathrm{cc}}^{2}{z}_{\mathrm{cc}}$, normalized to ∫ρ dV = ∫ρ R dR dz d ϕ = 1.

For numerical values, we follow Murphey et al. (2021). We adopt a solar distance, R_⊙ = 8.7 kpc, and height, z_⊙ = 20 pc. We assume CCSN-like within a thin disk with scale radius and height, R_thin, h_thin = 2.9 kpc, 95 pc, and we adopt the present Galactic core-collapse rates of ${{ \mathcal R }}_{\mathrm{cc}}={3.2}_{-2.6}^{+7.3}\ \mathrm{events}/\mathrm{century}$. With these parameters, the "lethal rate" for non-IIn CCSN is

$$\begin{eqnarray}&&{{\rm{\Gamma }}}_{\mathrm{CC}}({r}_{\mathrm{CC}})={0.5}_{-0.4}^{+1.2}\ \mathrm{events}/\mathrm{Gyr}\ {\left(\displaystyle \frac{{r}_{\mathrm{CC}}}{10\ \mathrm{pc}}\right)}^{3},\end{eqnarray} \tag{ 8 }$$

where we adopt the fiducial lethal distance r_CC = 10 pc suggested recently by Melott & Thomas (2011). This gives a mean recurrence time ${{\rm{\Gamma }}}_{\mathrm{CC}}^{-1}=2\ \mathrm{Gyr}$, which gives about a 50% chance that a lethal event has occurred in the ∼1 Gyr history of complex life on Earth. Note the sensitivity to the adopted distance: If we instead use r_CC = 20 pc as suggested by Thomas & Yelland (2023), the rate jumps to ${{\rm{\Gamma }}}_{\mathrm{CC}}={4.0}_{-3.6}^{+9.2}\ \mathrm{Gyr}$, and the recurrence timescale is only ${{\rm{\Gamma }}}_{\mathrm{CC}}^{-1}\simeq 250\ \mathrm{Myr}$.

Now we assume Type IIn SNe have a lethal distance of r_IIn = 30 pc and are a fraction f_IIn = 0.07 of all core-collapse events (Li et al. 2011; also see Cold & Hjorth 2023). Then, the "lethal rate" from these events is

$$\begin{eqnarray}&&\begin{array}{l}{{\rm{\Gamma }}}_{\mathrm{IIn}}({r}_{\mathrm{IIn}})={f}_{\mathrm{IIn}}{{\rm{\Gamma }}}_{\mathrm{CC}}({r}_{\mathrm{IIn}})={1}_{-0.8}^{+2}\ \mathrm{events}/\mathrm{Gyr}\ \left(\displaystyle \frac{f}{0.07}\right)\\ \times \,{\left(\displaystyle \frac{{r}_{\mathrm{IIn}}}{30\ \mathrm{pc}}\right)}^{3}.\end{array}\end{eqnarray} \tag{ 9 }$$

This is twice the global average rate in Equation (8)! This illustrates that Type IIn events pose an outsized threat despite their relatively modest rates of occurrence. The local core-collapse rate for all lethal core-collapse events is Γ_tot = Γ_CC + Γ_IIn = (690 Myr)⁻¹, assuming the fiducial lethal distances in Equations (8) and (9).

As discussed in Section 3, there is continuing evidence for non-IIn SNe evolving to show characteristics of interactions that result in high X-ray luminosities at late times (Margutti et al. 2017; Chandra et al. 2020; Thomas et al. 2022). This sort of evolution may be common for Type Ib/c SNe but simply missed by X-ray telescopes due to a lack of late-time observations. Margutti et al. (2017) estimate that as high as ∼40% of Ib/c may evolve to be interacting. If these continue to have X-ray luminosities comparable to IIn explosions, this will have profound effects on the rates of lethality discussed here, since Type Ib/c events comprise about 19% of all SNe and about 25% of all CCSNe. This could then roughly triple the rate shown in Equation (9).

We also note that the result in Equation (9) depends sensitively on the Type IIn fraction and especially on their typical X-ray lethal distance. In particular, we see that the relative threat is set by the ratio ${{\rm{\Gamma }}}_{\mathrm{IIn}}/{{\rm{\Gamma }}}_{\mathrm{CC}}={f}_{\mathrm{IIn}}{({r}_{\mathrm{IIn}}/{r}_{\mathrm{CC}})}^{3}$. Clearly, further observations—particularly in the X-ray—will be critical to firming up this estimate and determining the true impact of X-ray-luminous SNe for Galactic habitability.

The empirical evidence for near-Earth SNe in the geologically recent past comes most readily from detections of ⁶⁰Fe in the geological record. The recorded abundances have allowed for estimates of the distance from Earth at which the SN(e) likely occurred. These estimates range from 20 to 150 pc (Fields & Ellis 1999; Fields et al. 2005; Fry et al. 2015), and candidate star clusters have been proposed at distances around 50 pc and 100 pc (Benítez et al. 2002; Mamajek 2007; Hyde & Pecaut 2018). Remarkably, the lower end of this distance range includes the X-ray lethality distance in Equation (6). It is uncertain whether the SN Plio event ∼3 Myr ago was a strong X-ray emitter. But if this event or any one of the ≳10 other SNe needed to form the Local Bubble were X-ray luminous, there could have been significant consequences. It is thus quite possible that SN X-ray emission imposed lethal effects on Earth organisms or, at minimum, once altered the Earth's atmospheric ozone levels.

For further comparison, previous assessments of SN ozone damage found a lethal distance ranging from D^leth ∼ 8pc (Gehrels et al. 2003) to ∼10pc (Melott & Thomas 2011), indicated by the dashed line (red) in Figure 6. These do not include the effects of high-energy cosmic-ray muons, which could extend this range, In a recent re-evaluation that considered both ozone and muon effects, Thomas & Yelland (2023) revised the lethal distance up to 20 pc. We note that in these estimates, the ozone damage due to cosmic rays dominates here because the gamma-ray energy E_γ ∼ 2 × 10⁴⁷ erg and associated critical fluence ${{ \mathcal F }}_{\gamma }^{\mathrm{crit}}=100\ \mathrm{kJ}\,{{\rm{m}}}^{-2}$ give only ${D}_{\gamma }^{\mathrm{leth}}\approx 4\ \mathrm{pc}$. We further note, however, that the gamma-ray emission mostly arises from the radioactive decays of ${}^{56}\mathrm{Ni}\mathop{\longrightarrow }\limits^{9\ \mathrm{days}}{}^{56}\mathrm{Co}\mathop{\longrightarrow }\limits^{111\ \mathrm{days}}{}^{56}\mathrm{Fe}$ and thus is proportional to the ⁵⁶Ni yield. SNe with a large ⁵⁶Ni production, such as Type Ia events, will have a larger gamma-ray fluence and thus present a greater threat during this phase.

We must also again draw attention to the fact that the later cosmic rays would linger for substantially longer than the duration of X-ray emission (see Figure 1). So while it seems readily apparent that the X-ray-luminous SNe would impose their lethal effects at larger distances from their X-ray emission alone, it remains an open question for further research as to how exactly the specifics of this lethality compare to that associated with cosmic rays alone.

To this point, we have been primarily considering lethal effects related to ozone depletion, as this is a common point of comparison among the previous literature discussing the lethality of SNe. Importantly, however, there will be other effects that would pose interesting consequences for a terrestrial atmosphere or a planetary system in general. We reserve a more detailed assessment of these effects for a later study but can make general comments about the likely scenarios here.

Numerous studies have observed and modeled the middle atmosphere effects with respect to X-class solar flare events, which are characterized by an influx of solar soft X-ray emission (Veronig et al. 2002; Pettit et al. 2018; Siskind et al. 2022). In addition to the already mentioned increase in odd nitrogen production in the mesosphere and lower thermosphere, the enhanced photoionization resulting solely from the soft X-ray enhancement of a flare can cause a sudden ionospheric disturbance, which would primarily deliver a "dramatic increase in electron density" and an "altitude-dependent temperature increase" (Mitra 1974; Hayes et al. 2017; Pettit et al. 2018). This ionization is sensitive to even small-scale changes in X-ray activity and would be most influential on the D and E regions of the ionosphere. The consequences of this on Earth are not necessarily relevant to biological lethality; however, the enhanced ionization would be measurable by modern instrumentation and likely have particular relevance for technologically advanced civilizations as these enhancements can have a considerable impact on radio communications, astronaut health, satellite degradation, and more.

A relevant point of comparison for these sudden ionospheric disturbances is with respect to solar flares. Cliver & Dietrich (2013) offered a detailed analysis of the largest solar flare on record, the 1859 Carrington Event. Here they used the data from Veronig et al. (2002) to determine the total soft X-ray fluence of this event as 6.4 J m⁻²—the largest in recorded history that some speculate was matched by the large X-class solar flare of 2003 (Curto et al. 2016). An interesting thought arises here that contextualizes the shear magnitude of X-ray energy we have been considering in these SNe. Let us take the median of the SN IIn, SN 2005kd, which had a total observed X-ray energy of 1600 × 10³⁹ J (Table 2). If we now demand that the fluence in Equation (3) match only that of the Carrington Event's X-ray fluence, we derive a distance of 4600 pc. Now, utilizing the same parameters in Equation (9) and demanding the distance be the 4600 pc, we calculate a recurrence interval of ∼4 events per millennium. At these distances, however, dust extinction would now have to be taken into account—particularly for an SN within the galactic plane—serving to lessen the magnitude of X-rays arriving at the planet. Thus, further absorption effects would need to be considered, and in all, the comparison is not exact, but this nevertheless offers a simplified clarification for the distances at which the X-ray emission could perturb the atmosphere.

At this low of a fluence, no lethal effects would occur on a Phanerozoic Earth environment, but instead, a similar transient disturbance to the upper levels of the atmosphere as that imposed by only the electromagnetic radiation of solar flares or the recent galactic-scale GRB 221009A. These disturbances can be readily detected on Earth by very-low-frequency (VLF) amplitude measurements of the ionosphere, as well as the Geostationary Operational Environmental Satellite (GOES) X-ray sensor utilized for solar flares (Hayes et al. 2017, 2021). Relatedly, the disturbances may offer interesting observational prospects for the examination of exoplanetary atmospheric compositions and processes (Chen et al. 2021). Therefore, should an X-ray-luminous SN occur in the Milky Way, multiple pathways exist for modern instrumentation to measure these atmospheric disturbances.

A Milky Way SN has not been convincingly observed by the naked eye since Kepler's SN 1604 and thus has not been detected by modern astronomical instruments. Interestingly however, there have been five SNe recorded in the historical record (seen on Earth by the naked eye) within the last millennium, with distance estimates ranging between 1.4 and 10 kpc. If but one of these were characteristic of an interacting SN, the possibility remains that their X-ray phase of emission caused a significant sudden ionospheric disturbance in Earth's recent past.