an interactive field guide
Searching for
Exploding Black Holes
Half a century ago Stephen Hawking predicted that black holes evaporate — and that the smallest ones end their lives in an explosion brighter than anything gravity has built since. Somewhere in the data of our γ-ray telescopes, that flash may already be waiting.
begin ↓Black holes are not quite black
In 1974, Hawking showed that quantum effects near the event horizon force black holes to radiate like thermal bodies . The temperature is set by nothing but the mass:
The rule is merciless: smaller means hotter. A stellar-mass black hole radiates at a hundred-millionth of a kelvin — utterly invisible. But shrink one to the mass of a mountain and it glows in γ-rays. And because radiating mass makes it smaller still, evaporation runs away. The final moments are an explosion.
scale: ≈ a large mountain — evaporating today← M_U = 5.1×10¹⁴ g: lifetime = age of the universe
What does the light of a dying black hole look like? Two components : direct photons radiated straight off the horizon, and a fragmentation component — quarks and gluons that hadronize into pions, whose decays flood the spectrum with γ-rays. For any hole hotter than the QCD scale, fragmentation dominates.
T = 106 MeV · remaining lifetime ≈ 1.3×10⁷ yr. Parameterization from Ukwatta et al. (2016) Eqs. 31–34, as implemented in Analytical_Modelling.ipynb.
The mass-loss rate integrates all of this over every available species:
This produces the strangest lightcurve in astrophysics. Every other transient rises and fades. A black hole explosion only ever brightens, following a universal curve fixed by known physics — right up to the instant the hole ceases to exist.
Forged in the first second
No star can make a small black hole — stellar collapse bottoms out near three solar masses. But the infant universe could. In its first fraction of a second, space was filled with dense plasma; any patch with density contrast would collapse the moment it entered the causal horizon . The mass swallowed is the mass inside the horizon at that instant:
So the cosmic clock doubles as a mass dial: when a primordial black hole (PBH) formed sets how big it was born — and therefore when it dies.
An overdense patch collapsing when the universe is 10⁻²³ s old traps roughly a horizon mass: ~10¹⁵ g. These are the holes whose explosions we could witness today.
Nature would not mint a single mass but a distribution ψ(M), whose shape encodes the formation mechanism. The explosion rate we could hope to see today is set by how much of that distribution sits at the critical mass — holes whose lifetime equals the age of the universe :
Expected from a smooth, symmetric peak in the inflationary power spectrum. Width σ controls how much of the distribution leaks into constrained regions.
The explosion rate today is set by how much of ψ sits at M_U: ṅ = ρ_DM ψ(M_U) / 3t_U (Boluna et al. Eq. 3.8). For every allowed mass function it stays far below the HAWC bound of 3400 pc⁻³ yr⁻¹ — unless the distribution is spiked almost exactly at M_U.
A collider built by gravity
Here is the property that makes exploding black holes more than a curiosity: Hawking radiation is democratic. Gravity couples to everything, so a black hole radiates every particle species lighter than its temperature — quarks, neutrinos, gravitons, and anything else that exists, including particles that never touch our detectors .
The evaporation coefficient α(M) literally counts the particle content of nature. As the hole shrinks and heats, each new species unlocks like a threshold in a collider energy scan:
Each step: the shrinking hole gets hot enough to radiate a new species (T = 1.058×10¹³ GeV·g / M). dM/dt = −α(M)/M², so more channels → faster evaporation. Dark degrees of freedom leave gravity no place to hide — they must be radiated too, whether or not they couple to light. Thesis §2.8.
In its final seconds a PBH reaches temperatures beyond any accelerator — a cosmic-scale collider whose luminosity, spectrum, and duration all depend on the full particle spectrum of nature. A dark sector with N copies of the Standard Model would shorten the final TeV burst by ~N and dim its photons by the same factor . Watching one explosion is a census of everything that can exist.
Even without new physics, evaporation carries a unique fingerprint. The measured power-law slope of the spectrum evolves as the hole shrinks, converging to a universal value that no astrophysical source reproduces:
As the hole approaches its final moments, the measured power-law slope at any energy converges to γ = 1.5 — a fingerprint no astrophysical source shares. This is the primary template used to screen the Fermi transient catalog. (Boluna et al. Eq. 4.8, Fig. 6.)
Six eyes on the γ-ray sky
No single telescope covers the evaporation story. The spectrum spans nine decades of energy as the hole heats from MeV to TeV, so the search is inherently multi-mission : space telescopes (Fermi's GBM and LAT) own the keV–GeV band with huge fields of view; ground arrays (VERITAS, HAWC, LHAASO) own the TeV band with vast effective areas.
Combining instruments honestly is its own craft. The analysis behind this site uses threeML , the multi-mission maximum-likelihood framework: every instrument keeps its own response and background model, while a single physical source model is fit jointly across all of them .
Raw time-tagged photon events from each GBM scintillator and LAT tracker are pulled straight from the Fermi archives. Every detector has its own response, background, and quirks.
Only the nearest explosions count
Here is the sobering part. A PBH explosion is intrinsically identical every time — same mass, same lightcurve, same luminosity. That makes the detection criterion pure geometry :
Run that requirement through every instrument and you get a sensitivity frontier: the farthest distance each telescope could see a hole of a given mass.
Anything below a curve is detectable by that instrument (N_S ≥ 10 photons or 5σ over background, 1-yr observation). Regenerated from the repo detectability pipeline (Analytical_Modelling.ipynb; cf. Boluna et al. Fig. 4). Above the dashed contours the source would visibly streak across the sky — a smoking-gun signature, but also a challenge for catalog association.
The peak reach is a fraction of a parsec — thousands of times closer than the nearest star. And nearness cuts both ways: anything that close should visibly move. At galactic speeds (~220 km/s), a source at 0.01 pc drifts about a degree per year — a smoking-gun streak, but also a reason catalogs might discard the very sources we want.
Detection horizons (◆) are the peak reach from the sensitivity frontier above. The tension in one picture: γ-ray telescopes only see explosions within a bubble much smaller than the distance to the nearest star, while abundance limits make such nearby events rare.
Fitting the flash
Suppose a short burst trips the GBM. Is it a neutron-star merger at a billion parsecs, or a black hole dying at a thousandth of one? The lightcurve holds the answer. The PBH template rises as toward the explosion epoch τ — time-reversed compared to every conventional burst — optionally followed by an afterglow from the ejecta shell .
These are the actual light curves fitted in the thesis pipeline (Lightcurve_Fitting/Fitting.ipynb). In the real analysis, ultranest explores this exact parameter space against every GBM detector simultaneously via threeML — you are doing by hand what nested sampling does with 400 live points.
In the production pipeline these fits run through threeML with ultranest's nested sampling — hundreds of live points exploring normalization, decay index, and afterglow timing against every GBM detector simultaneously, returning posterior distributions and Bayesian evidence for template comparison.
The candidate hunt
The Fermi GBM has logged thousands of bursts since 2008. Almost all are ordinary. The search strategy applies three cuts inherited from the physics: the burst must be short (T90 within 0.2–5 s), hard (more high-energy fluence than low), and local — no measured redshift, since a real PBH burst comes from inside our stellar neighborhood.
The idea traces back to Cline's BATSE analyses in the 1990s , which found a curious subpopulation of very short, anomalously hard bursts. The repo's Fermi catalog search modernizes this on Fermi's much deeper catalog.
Real sources from the repo's Fermi catalog search (H>1_T90[0.2-5]_RS=0.csv): each passed hardness > 1, T90 within 0.2–5 s in GBM or LAT-LLE, and no measured redshift. Sources marked ∞ had zero GBM-band fluence — the hardest events in the sample. Click a point to inspect it.
Where do they point?
Geometry offers a free hypothesis test. Neutron stars, magnetars, X-ray binaries — everything born of stars traces the Milky Way's disk. But local PBH explosions sample only our tiny corner of the halo, so their sky map should be isotropic, indifferent to the galactic plane .
Galactic coordinates, galactic center at the middle, plane along the equator. If these sources were neutron stars or other stellar remnants they would trace the disk; local PBH explosions should be isotropic. Sky positions from the candidate CSV and the 1FLT fitted-parameter table (cf. BoresightSelection.py, thesis Fig. 3.5).
Cline additionally reported an anomalous cluster of very short bursts in one octant of the sky — never confirmed, never fully refuted. A larger candidate sample with Fermi-era statistics is exactly what could settle it.
Echoes weeks after the flash
The γ-ray flash may not be the last word. The explosion dumps ~10²⁵ J of electron-positron pairs into the interstellar medium; Rees noted in 1977 that this conducting fireball plows the ambient magnetic field into a coherent low-frequency radio pulse — potentially detectable across the entire galaxy, far beyond the γ-ray horizon.
And blazar physics adds a slower channel: adiabatically expanding ejecta re-radiate at progressively lower frequencies, with radio peaking 40–140 days after the γ-ray flare in observed jets. Would a spherical PBH shell do the same? That is an open question the repo's afterglow notes flag explicitly .
The last ~10⁹ g evaporate in about a microsecond–millisecond of TeV-scale emission. This is the 'burst' a GBM-like detector triggers on.
The observational strategy: catalog the position of every candidate burst, then watch those coordinates for a late-arriving radio transient. A match would be extraordinary — no astrophysical short GRB should produce this particular γ→radio sequence from a stationary point at parsec distance. Radio non-detections (ETA: < 2.3×10⁻⁷ pc⁻³ yr⁻¹, Cutchin 2015) already constrain the Rees–Blandford channel.
Sources that only brighten
There's a second way to catch a dying black hole: before the explosion. For months to years, a nearby PBH would appear as a faint GeV point source with a steadily rising flux :
The Fermi LAT Transient Catalog lists 35 sources with no known counterpart at any wavelength. The thesis transient-fitting pipeline fit every one of them with this two-parameter model .
Notice the degeneracy: raising τ and shrinking d can produce nearly identical decade-long light curves — exactly why the 33 fitted 1FLT sources carry wide error bars (TransientSources_fitted_params.csv). The tell that finally breaks the degeneracy is proper motion: at milliparsec distances the source should drift degrees per year, which the LAT catalog does not observe for these sources.
The verdict so far: the fits succeed — two clusters of solutions, one near 10¹² g at milliparsec distances, one near 10¹⁴⁻¹⁵ g further out — but both imply proper motion of degrees per year that the LAT catalog does not observe. The simplest reading is that these transients are something else. The method, though, now exists and sharpens with every year of data.
Where the evidence stands
No exploding black hole has been found. This site would be dishonest to imply otherwise — and dishonest to imply the search is hopeless. Here is the current ledger, claim by claim. Click to expand.
The deeper reason to keep looking: a single confirmed detection would simultaneously prove black holes evaporate (quantum gravity's only accessible prediction), demonstrate dark matter physics beyond WIMPs, and census every particle degree of freedom in nature — visible or dark. Few observations in physics carry that much payload.
The tools are improving on schedule: CTA will push the sensitivity frontier past every current instrument, LAT keeps accumulating exposure, and the template methods built in this research — universal lightcurves, spectral-index screening, transient fitting, proper-motion vetoes — are ready for the data.