Axinovae: Axion-Driven Transients

• Axinovae are axion-sector transient phenomena characterized by rapid energy release from heavy-axion instabilities in stars and cosmological settings.
• In massive stars, axion-instability supernovae trigger explosive oxygen burning by softening the core’s pressure support, shifting black-hole mass gaps.
• Recurrent axion-star collapses convert dark matter into relativistic axions, offering novel constraints and detection prospects via helioscopes.

Axinovae are axion-sector explosive or transient phenomena associated with rapid energy release, instability, or detectable burst-like signatures. In the current literature, the most developed realizations are heavy-axion-induced stellar explosions known as Axion Instability Supernovae, recurrent collapse episodes of axion stars inside post-inflationary axion minihalos, and observational scenarios in which axions from a nearby core-collapse supernova are converted into photons in helioscopes such as BabyIAXO or IAXO (Sakstein et al., 2022, Fox et al., 2023, Carenza et al., 26 Feb 2025). This suggests that the term functions less as a single standardized object class than as a family of axion-driven transient phenomena linked by instability, burst emission, or explosive dynamics.

1. Terminology and conceptual scope

The stellar-transient usage is the most explicit. "Axion Instability Supernovae" (AISN) denotes supernova-like explosions of very massive stars in which heavy, strongly coupled axions become abundant and approximately thermalized in late-stage stellar cores, soften the equation of state, and trigger explosive oxygen burning; the pulsational counterpart is denoted PAISN (Sakstein et al., 2022). A distinct cosmological usage appears in "Recurrent Axinovae and their Cosmological Constraints," where an axinova is a violent episode in which an axion star that has grown to its maximum stable mass rapidly contracts and converts an $\mathcal{O}(1)$ fraction of its mass into unbound relativistic axions (Fox et al., 2023). A third usage concerns supernova axion bursts reaching Earth and converting into detectable gamma rays inside BabyIAXO or IAXO, thereby treating a nearby core-collapse supernova as an axion transient source (Carenza et al., 26 Feb 2025).

These usages are related but not identical. AISN and PAISN are powered by thermal axions modifying stellar-core thermodynamics. Recurrent axinovae are dark-sector collapse events in axion minihalos. Helioscope supernova-axion searches are detection scenarios rather than explosion mechanisms. The shared feature is transient behavior generated by axion-sector dynamics.

2. Axion Instability Supernovae in massive stars

AISN arise in the axion-photon “cosmological triangle,” roughly

$$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$

with simulations emphasizing the benchmark $m_a=m_e=511~{\rm keV}$ (Sakstein et al., 2022). The key distinction from ordinary light-axion cooling is that these axions are heavy and trapped or equilibrated, not free-streaming. In late burning stages of a very massive star, axions are produced efficiently by Primakoff conversion and photon coalescence or inverse decay. Because the coupling is strong and the mass lies near the chosen range, the axions are modeled as a thermalized bosonic component of the core plasma.

The instability mechanism parallels pair instability. Once thermal energy is diverted into producing a massive axion population, the effective pressure support is reduced. The core contracts more strongly, heats up further, ignites oxygen explosively, and either ejects mass in pulses or is completely disrupted. For $m_a\lesssim 2m_e$, the axion instability compounds the ordinary pair instability, yielding a smaller $\Gamma_1$ at fixed temperature and density and therefore a stronger inward contraction. The reported helium-core mass regimes for the $m_a=m_e$ benchmark are: black-hole formation below about $38\,M_\odot$, PAISN for roughly $38\lesssim M\lesssim 48\,M_\odot$, and full AISN disruption above about $48\,M_\odot$ (Sakstein et al., 2022).

A principal observable consequence is a downward shift of the upper black-hole mass gap. For the benchmark case, the gap is predicted to lie at

$37 \le M \le 107 \quad (M_\odot),$

instead of the Standard Model pair-instability expectation near $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$0 (Sakstein et al., 2022). The same study states that AISN should be more common than ordinary pair-instability supernovae because their progenitors have lower masses and the stellar IMF has a negative slope, and that they should also be brighter at fixed progenitor mass: a $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$1 AISN is brighter than a $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$2 PISN, while a $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$3 AISN is brighter by more than an order of magnitude. The benchmark $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$4 scenario is nevertheless stated to be disfavored by GWTC-2 population data, which infer a lower edge of the black-hole mass gap beginning at $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$5 (Sakstein et al., 2022).

3. Thermodynamic framework of the stellar instability

The AISN mechanism is formulated as an equation-of-state modification by an equilibrated heavy boson of spin $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$6, degeneracy

$$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$7

and mass $$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$8 (Sakstein et al., 2022). Defining

$$300~{\rm keV} \lesssim m_a \lesssim 2~{\rm MeV}, \qquad g_{a\gamma\gamma}\sim 10^{-5}\ {\rm GeV}^{-1},$$9

the boson pressure, mass density, internal energy density, and specific entropy are

$m_a=m_e=511~{\rm keV}$0

$m_a=m_e=511~{\rm keV}$1

$m_a=m_e=511~{\rm keV}$2

$m_a=m_e=511~{\rm keV}$3

In MESA, these contributions are added to the HELM EOS through

$m_a=m_e=511~{\rm keV}$4

The instability is controlled by the adiabatic response. With

$m_a=m_e=511~{\rm keV}$5

the total adiabatic quantities are recomputed as

$m_a=m_e=511~{\rm keV}$6

The relevant threshold is the usual radiation-supported instability condition,

$m_a=m_e=511~{\rm keV}$7

Adding an axion with $m_a=m_e=511~{\rm keV}$8 enlarges the region in $m_a=m_e=511~{\rm keV}$9 space where this threshold is encountered (Sakstein et al., 2022).

The equilibration assumption is enforced by comparing the axion production rate to the stellar-evolution rate. The paper uses

$m_a\lesssim 2m_e$0

and assumes equilibrium if

$m_a\lesssim 2m_e$1

The decay rate is quoted as

$m_a\lesssim 2m_e$2

A major caveat is that this is an approximation: some Primakoff-produced axions may initially exceed the local escape speed, and a full out-of-equilibrium transport treatment could alter the details, especially through nonlocal energy transport and convection (Sakstein et al., 2022).

4. Recurrent axinovae from axion-star collapse

In the cosmological usage, axinovae are collapse events of axion stars that form in the dense cores of post-inflationary axion minihalos (Fox et al., 2023). The sequence is: post-inflationary PQ breaking generates enhanced small-scale isocurvature fluctuations; dense minihalos or miniclusters form after equality; Bose-enhanced scattering drives condensation into a central axion star; the star grows by accretion or condensation; once it reaches its maximum stable mass, it rapidly contracts and emits relativistic axions; a remnant survives and can regrow, making the process recurrent.

The axion potential is expanded as

$m_a\lesssim 2m_e$3

with

$m_a\lesssim 2m_e$4

Using a Gaussian ansatz, the axion-star energy is

$m_a\lesssim 2m_e$5

with $m_a\lesssim 2m_e$6 and $m_a\lesssim 2m_e$7. The two equilibrium branches satisfy

$m_a\lesssim 2m_e$8

and the maximum stable mass is

$m_a\lesssim 2m_e$9

The collapse criterion is simply $\Gamma_1$0 (Fox et al., 2023).

The growth environment is set by minihalos with characteristic mass

$\Gamma_1$1

and condensation timescale

$\Gamma_1$2

The paper gives explicit gravitational and self-interaction limits,

$\Gamma_1$3

with $\Gamma_1$4 and $\Gamma_1$5. In the constrained parameter region, self-interactions dominate condensation. After collapse, the remnant mass is stated to be $\Gamma_1$6, permitting regrowth and another collapse (Fox et al., 2023).

The cosmological observable is the cumulative conversion of cold dark matter into relativistic axions. The source term is

$\Gamma_1$7

with $\Gamma_1$8 for the benchmark relativistic-energy fraction per burst. The resulting phenomenology is treated as similar, although in principle distinct, from a decaying dark matter fraction. The paper adopts the bound

$\Gamma_1$9

and imposes the analogous requirement

$m_a=m_e$0

A central conclusion is that this yields a new constraint on axion-like dark matter that is independent of couplings to Standard Model particles (Fox et al., 2023).

5. Detection strategies: helioscopes and supernova axion bursts

The observational program most directly connected to transient axion signals is the helioscope program centered on IAXO (Irastorza et al., 2012). The axion-photon interaction is

$m_a=m_e$1

and the conversion probability in vacuum is

$m_a=m_e$2

IAXO is a next-generation axion helioscope designed to improve on CAST by increasing magnetic aperture, instrumenting each bore with x-ray optics, and lowering detector background. The summary paper states a target sensitivity to the axion-photon coupling of a few $m_a=m_e$3, corresponding to about $m_a=m_e$4–$m_a=m_e$5 orders of magnitude beyond CAST, with projected reach into the $m_a=m_e$6–$m_a=m_e$7 decade and the best scenarios approaching $m_a=m_e$8 (Irastorza et al., 2012).

A distinct transient application is the detection of axions from a nearby core-collapse supernova with BabyIAXO or IAXO (Carenza et al., 26 Feb 2025). In this case the source is hot, dense nuclear matter with benchmark conditions

$m_a=m_e$9

and the emitted axion spectrum is parameterized as

$38\,M_\odot$0

The converted photons are MeV gamma rays rather than keV x rays, so a dedicated high-efficiency gamma detector is required. The conversion probability is written as

$38\,M_\odot$1

and the expected signal counts are

$38\,M_\odot$2

The paper states that if a sufficiently nearby core-collapse supernova occurs, with distance $38\,M_\odot$3, and if $38\,M_\odot$4, IAXO could have a realistic chance to detect the burst and constrain pion abundance, the equation of state, and other nuclear processes in extreme environments (Carenza et al., 26 Feb 2025).

Experiment	$38\,M_\odot$5	$38\,M_\odot$6
BabyIAXO	$38\,M_\odot$7	$38\,M_\odot$8
IAXO	$38\,M_\odot$9	$38\lesssim M\lesssim 48\,M_\odot$0
IAXO+	$38\lesssim M\lesssim 48\,M_\odot$1	$38\lesssim M\lesssim 48\,M_\odot$2

The supernova-axion study assumes a gamma-detector efficiency $38\lesssim M\lesssim 48\,M_\odot$3 and threshold $38\lesssim M\lesssim 48\,M_\odot$4, uses a 95% C.L. zero-background criterion $38\lesssim M\lesssim 48\,M_\odot$5, and emphasizes the operational importance of pre-supernova neutrino alerts and rapid repointing (Carenza et al., 26 Feb 2025). It also states that MeV axion energies keep the coherence condition valid up to above $38\lesssim M\lesssim 48\,M_\odot$6, well beyond the usual solar-helioscope coherence range.

6. Relation to axino and ALPino phenomenology

A plausible source of ambiguity is the existence of a large axino and ALPino literature, where the relevant object is not an axion-driven transient but the fermionic superpartner of an axion or axion-like particle. In supersymmetric PQ models, the axion belongs to a chiral superfield

$38\lesssim M\lesssim 48\,M_\odot$7

so the axino $38\lesssim M\lesssim 48\,M_\odot$8 is the spin-$38\lesssim M\lesssim 48\,M_\odot$9 fermionic member of the axion supermultiplet (Choi et al., 2013). The review literature emphasizes that the axino is an EWIMP, that its mass is highly model dependent, and that depending on mass and production history it can behave as hot, warm, or cold dark matter (Choi et al., 2013).

A concrete example is the proposal of a $48\,M_\odot$0 axino warm dark matter particle in a supersymmetric axion model with bilinear $48\,M_\odot$1-parity violation (Choi et al., 2014). In that scenario the decay

$48\,M_\odot$2

can yield a $48\,M_\odot$3 X-ray line, with decay rate

$48\,M_\odot$4

in the limit $48\,M_\odot$5. The same analysis states that fitting the line while respecting $48\,M_\odot$6 favors $48\,M_\odot$7 together with a light Bino, $48\,M_\odot$8 for $48\,M_\odot$9 (Choi et al., 2014). This is axino dark-matter decay phenomenology, not an axion-instability explosion.

An analogous generalization appears for the ALPino, defined as the fermionic supersymmetric partner of an axion-like particle (Choi et al., 2019). The fixed-target study assumes an ALPino LSP and a very light bino-like neutralino NLSP, with the effective interaction

$37 \le M \le 107 \quad (M_\odot),$0

and visible decays

$37 \le M \le 107 \quad (M_\odot),$1

For the two-body mode,

$37 \le M \le 107 \quad (M_\odot),$2

The paper concludes that SHiP can probe roughly $37 \le M \le 107 \quad (M_\odot),$3 in the photon channel and $37 \le M \le 107 \quad (M_\odot),$4 in the $37 \le M \le 107 \quad (M_\odot),$5 channel for sub-GeV neutralino masses (Choi et al., 2019). In this usage, the relevant physics is long-lived neutralino decay and freeze-in dark matter, not axinovae in the stellar or cosmological-collapse sense.

7. Broader significance and open issues

Taken together, the literature presents axinovae as a broader class of axion-driven nonequilibrium phenomena linking stellar structure, compact bosonic condensates, cosmology, and laboratory detection. The stellar version shows that equilibrated heavy bosons can soften the equation of state of massive stars and shift remnant populations (Sakstein et al., 2022). The cosmological version shows that repeated axion-star collapse can convert a non-negligible fraction of dark matter into relativistic axions and thereby be constrained by CMB and large-scale-structure bounds usually associated with decaying dark matter (Fox et al., 2023). The helioscope version shows that a nearby supernova could act as a transient axion source whose measured fluence and spectrum probe pion abundance and dense-matter microphysics (Carenza et al., 26 Feb 2025).

The principal uncertainties are also well defined. For AISN, the dominant caveat is the assumption that heavy axions thermalize and remain trapped; a full out-of-equilibrium transport treatment could alter the instability threshold and convective dynamics (Sakstein et al., 2022). For recurrent axinovae, the main uncertainties lie in the axion-star growth law inside realistic minihalos, the phenomenological treatment of recurrence, and the mapping to effective decaying-dark-matter bounds (Fox et al., 2023). For supernova-axion detection, the uncertainties are dominated by axion emission rates in dense nuclear matter, pion abundance, the use of a single benchmark 1D supernova simulation, and the need for a dedicated MeV gamma detector plus pre-supernova alert capability (Carenza et al., 26 Feb 2025).

In this sense, axinovae occupy a distinctive place in axion physics. They are not simply another manifestation of weakly interacting light particles; they are regimes in which axions or axion-sector condensates materially alter macroscopic dynamics, induce burst-like phenomena, or generate measurable transient signals across astrophysics, cosmology, and experiment.