Level 1 - Absolute Beginner

Black holes are places in space where gravity is so strong that nothing can escape, not even light. Scientists think there are three main sizes of black holes. Small ones are called stellar-mass black holes. Giant ones at the centre of galaxies are called supermassive black holes. But a medium size, called intermediate-mass black holes, has been very hard to find.

A space telescope called Einstein Probe spotted something amazing on July 2, 2025. It detected a very bright burst of X-rays coming from far away in space. Scientists named this event EP250702a. They believe it was a medium-sized black hole pulling apart a dead star called a white dwarf.

A white dwarf is the leftover core of a star like our Sun after it runs out of fuel. It is very dense and small. When a black hole gets too close, its gravity pulls the white dwarf apart. This violent event releases a huge amount of energy as light, X-rays, and gamma rays.

This discovery is exciting because intermediate-mass black holes are the missing link between the two known sizes. Finding one in action helps scientists understand how black holes grow and how galaxies change over time. The results were published in a top science journal called Science Bulletin.

black hole

a region in space with gravity so strong that nothing can escape from it

gravity

the force that pulls objects toward each other

X-ray

a type of high-energy light invisible to human eyes, used to see inside objects

white dwarf

the small, dense leftover core of a dead star

telescope

a scientific tool used to observe objects far away in space

energy

the ability to do work or cause change; released in many forms such as heat and light

galaxy

a huge collection of stars, gas, and dust held together by gravity

journal

a publication where scientists share their research findings with other scientists

Level 2 - Elementary

Scientists have long classified black holes into three groups based on their mass. Stellar-mass black holes form when large stars die and collapse. Supermassive black holes, billions of times the mass of our Sun, sit at the centres of most large galaxies. Between these two extremes lies the poorly understood category of intermediate-mass black holes (IMBHs), which have masses ranging from hundreds to hundreds of thousands of times our Sun's mass.

On July 2, 2025, China's Einstein Probe satellite, equipped with a Wide-field X-ray Telescope, detected an extraordinarily bright flash later designated EP250702a, also registered as gamma-ray burst GRB 250702B. The event occurred in the outskirts of a distant host galaxy. Scientists at the National Astronomical Observatories of China and the University of Hong Kong led the analysis of the data.

The burst is interpreted as a tidal disruption event (TDE), meaning the black hole's immense gravity ripped the white dwarf apart as it passed too close. The resulting debris formed a swirling disc that produced an X-ray flash before the gamma rays arrived. This hard-to-soft transition in the energy spectrum was observed for the first time in such an event and provided a key clue about what produced it.

The total energy released was estimated at around 3 times 10 to the 49th power ergs per second, an almost incomprehensible figure. The findings were published as the cover article in Science Bulletin in June 2026, with wider attention from ScienceDaily. Researchers say EP250702a may be the clearest evidence yet that IMBHs exist and can be caught in the act of destroying nearby stars.

intermediate-mass

falling between two extremes; here, black holes heavier than stellar-mass but lighter than supermassive

tidal disruption event

when a star is torn apart by the tidal forces of a nearby black hole

satellite

a spacecraft that orbits Earth or another body and collects scientific data

spectrum

the range of different energies or wavelengths of light emitted by an object

debris

the scattered material left after an object is destroyed

gamma ray

the highest-energy form of electromagnetic radiation, produced in extreme cosmic events

mass

the amount of matter in an object; often measured relative to the Sun for stars and black holes

outskirts

the outer edges or distant regions of a galaxy or city

Level 3 - Intermediate

Intermediate-mass black holes (IMBHs), occupying the mass range of roughly 100 to 100,000 solar masses, represent the most elusive tier in the black hole hierarchy. Their scarcity in the observational record has made it difficult to confirm whether they form through the runaway merging of stellar-mass black holes in dense star clusters, through the direct collapse of massive primordial gas clouds, or via some other pathway. EP250702a, detected by China's Einstein Probe on July 2, 2025, may be the most compelling observational case for their existence to date.

Einstein Probe's Wide-field X-ray Telescope (WXT) registered EP250702a as an unusually luminous transient with a peak X-ray luminosity that dwarfs typical gamma-ray bursts. The event was independently flagged as GRB 250702B. What distinguished it from a conventional long gamma-ray burst was its multi-phase emission: an initial soft X-ray pulse preceded the harder gamma-ray component, a spectral sequence consistent with material from a tidally disrupted white dwarf forming an accretion disc before ultimately powering a relativistic jet.

White dwarf tidal disruption events (WD-TDEs) around IMBHs are theoretically distinct from the more commonly observed stellar TDEs around supermassive black holes. The compactness and high density of a white dwarf means that relativistic tidal forces dominate, enabling nuclear burning in the stripped material and producing the characteristic hard-to-soft spectral evolution seen in EP250702a. The event location in the outskirts of its host galaxy is also consistent with IMBH models, since IMBHs are thought to reside in globular clusters or at the peripheries of galaxies rather than at galactic nuclei.

A team from the National Astronomical Observatories of China and the University of Hong Kong modelled the energetics and spectral evolution, estimating a peak isotropic luminosity of approximately 3 times 10 to the 49th power ergs per second, far exceeding typical long-duration gamma-ray bursts. Their paper appeared as the cover article in Science Bulletin in June 2026. If confirmed, EP250702a would be only the second WD-TDE candidate around an IMBH ever observed, offering constraints on IMBH demographics and their role in early galactic evolution.

elusive

difficult to find, capture, or observe; evading detection

transient

an astronomical event or object that brightens and fades over a short period

accretion disc

a rotating disc of gas and debris that forms around a compact object such as a black hole as material spirals inward

relativistic

involving speeds or energies close to the speed of light, requiring Einstein's theory of relativity to describe accurately

isotropic luminosity

the total power of a source assuming it radiates equally in all directions, used to compare brightness across cosmic distances

Level 4 - Advanced

The population of intermediate-mass black holes (IMBHs; 10^2 to 10^5 solar masses) constitutes the most poorly characterised stratum of the black hole mass function. Formation channels remain contested: dynamical mergers of compact objects in nuclear or globular clusters, the direct collapse of low-metallicity massive stars, and remnant seeds from Population III stellar evolution all remain viable, each predicting different occupancy fractions in dwarf galaxies and cluster outskirts. Observational confirmation has been hampered by the scarcity of clean electromagnetic signatures, since luminous IMBH candidates discovered through quasi-periodic eruptions or ultra-luminous X-ray sources are often contaminated by stellar or AGN contributions.

EP250702a, detected by Einstein Probe's Wide-field X-ray Telescope (WXT) on 2025 July 2, circumvents this ambiguity by virtue of its extreme energetics and multi-band temporal structure. Independently flagged as GRB 250702B by the Fermi GBM and Swift BAT, the event exhibited an anomalously soft X-ray precursor phase followed by a hard gamma-ray pulse. This hard-to-soft reversal inverts the canonical prompt emission sequence of cosmological long GRBs and is instead consistent with theoretical predictions for a white-dwarf tidal disruption event (WD-TDE) around an IMBH. In a WD-TDE, the extreme compactness of the white dwarf (typical radii of order 10^4 km) means that general-relativistic tidal forces produce explosive thermonuclear detonation of the stellar envelope even before complete disruption. The X-ray precursor is attributed to this thermonuclear flash, while the subsequent harder emission traces jet formation as the fallback debris assembles an accretion disc.

The team from the National Astronomical Observatories of China and the University of Hong Kong modelled the broadband spectral evolution and temporal decay profile to constrain the central engine. Peak isotropic equivalent luminosity reached approximately 3 times 10^49 erg/s, roughly an order of magnitude above the most luminous confirmed long GRBs and inconsistent with a stellar-mass compact remnant progenitor. The offset of EP250702a from its host galaxy's photometric centre by several effective radii is a further diagnostic: supermassive black hole TDEs occur at galactic nuclei, whereas IMBHs in intracluster globular clusters or satellite systems naturally produce off-nuclear transients. Spectral fitting constrained the black hole mass to the IMBH regime, consistent with a globular cluster host.

Published as the cover article in Science Bulletin in June 2026, the discovery has immediate implications for the IMBH census and for the predicted rate of WD-TDEs as a gravitational-wave precursor population detectable by future LISA observations. If the IMBH interpretation holds under independent scrutiny, EP250702a would be only the second WD-TDE candidate attributable to an IMBH, offering empirical footholds for population synthesis models that currently rely on extrapolation from the stellar TDE rate. The result also highlights the strategic value of wide-field X-ray time-domain surveys in accessing transient phenomena that targeted pointed observations inevitably miss.