EP250702a: Ultra-long Extragalactic Transient

• EP250702a is a unique ultra-long extragalactic transient characterized by day-scale X-ray emission and multiple quasi-periodic gamma-ray bursts.
• The event was observed through a coordinated multiwavelength campaign, providing high-precision localization and detailed spectral diagnostics.
• Its complex temporal structure and fallback-accretion decay offer critical insights into progenitor scenarios such as white dwarf tidal disruptions and collapsar models.

EP250702a is the designation for an unprecedented, ultra-long, repeating extragalactic transient, primarily identified in high-energy astrophysical catalogues and now recognized as a defining event for new classes of gamma-ray and X-ray transients. The event was detected as GRB 250702BDE (alternatively, GRB 250702DBE or simply GRB 250702B) and independently by the Einstein Probe’s Wide-Field X-ray Telescope (EP-WXT) as EP250702a. The transient is notable for its composite day-long X-ray emission, multiple quasi-periodic gamma-ray bursts, and strong multiwavelength afterglow, offering critical empirical constraints on models of ultra-long gamma-ray bursts (ULGRBs), tidal disruption events (TDEs), and compact object interactions (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Li et al., 30 Sep 2025, Song et al., 11 Oct 2025).

1. Discovery and Multiwavelength Observational Campaign

EP250702a was first observed by the Einstein Probe around 2025-07-01 03:00 UT as a bright soft X-ray transient, persisting for approximately 24 hours before any gamma-ray alert (Levan et al., 18 Jul 2025). This early emission was followed by three distinct and exceptionally energetic gamma-ray bursts at 13:09:02, 13:56:06, and 16:21:33 UT on July 2, detected by the Fermi Gamma-ray Burst Monitor (GBM) and corroborated by additional high-energy instruments (Insight-HXMT, GECAM-B) (Zhang et al., 30 Sep 2025, Song et al., 11 Oct 2025). Swift-XRT and Chandra provided high-cadence X-ray follow-up, while HST, Keck, VLT, and MeerKAT enabled host galaxy localization and counterpart detection in the optical, IR, and radio (Levan et al., 18 Jul 2025).

The source was localized with arcminute precision by EP-WXT, and ultimately to sub-arcsecond precision by Swift-XRT, to a non-nuclear position in a host galaxy at $z \sim 0.2$, with complex disk morphology and a prominent dust lane. The afterglow showed pronounced extinction ($A_V \sim 11$ mag), a very red IR counterpart, and a forward/reverse-shock synchrotron spectrum consistent with standard GRB afterglow models (Levan et al., 18 Jul 2025).

2. Temporal Structure: Ultra-Long Duration and Recurrence

A defining feature of EP250702a is its temporal complexity. The high-energy event sequence spans over 7 hours, comprising:

• An initial day-long ($\sim$24 h) soft X-ray episode detected by EP-WXT with peak luminosity $L_X \sim 3 \times 10^{49}$ erg s$^{-1}$ at $z=1.036$ (Li et al., 30 Sep 2025).
• Three separated gamma-ray brightenings (Δ$t_{12} = 2825$ s, Δ$t_{13} \approx 4 \times 2825$ s), suggesting nearly integer-multiplied recurrence, an anomaly among classical GRBs (Levan et al., 18 Jul 2025).
• Each gamma-ray burst displayed substructure with clusters of fast (sub-second) spikes and quasi-periodic separation between main episodes, with intervals of $~4000$ s (Zhang et al., 30 Sep 2025).

X-ray follow-up revealed a broken power-law decay in the afterglow: before 0.1 days, $\alpha_1 \approx 1$, steepening to $A_V \sim 11$0 thereafter (Levan et al., 18 Jul 2025). The soft X-ray decline after the main burst is described by $A_V \sim 11$1, the canonical fallback-accretion scaling (Zhang et al., 30 Sep 2025).

3. High-Energy Spectral Diagnostics and Quasi-Period Oscillations

Spectral fitting of Fermi-GBM data (8 keV – 40 MeV) across all sub-bursts consistently requires broken power-law (Band) models with break energies $A_V \sim 11$2 ranging from 76–403 keV and GRB-typical photon indices:

• $A_V \sim 11$3 $A_V \sim 11$40.9 to 0.3 (low-energy)
• $A_V \sim 11$5 $A_V \sim 11$61.6 to $A_V \sim 11$72.1 (high-energy)

The individual gamma-ray episodes are characterized by a lack of monotonicity in fluence, unusual for GRB prompt emission (Song et al., 11 Oct 2025). Periodogram, wavelet, and AR(2) time-series analyses detected possible quasi-period oscillations (QPOs):

• No significant QPO in the first episode (D).
• Candidate QPO at 0.046 Hz (21.7 s) in episode B (FAP $A_V \sim 11$89% global after correction).
• Stronger candidate QPO at 0.024 Hz (41.7 s) in episode E (FAP $A_V \sim 11$95%).

While these do not constitute strict periodicity, the evolution and persistence of QPO structure support jet precession or periodic accretion modulation (Song et al., 11 Oct 2025).

4. Progenitor Scenarios: Collapsar Versus White Dwarf Tidal Disruption

Three hypotheses are considered for EP250702a's nature:

Atypical Collapsar ULGRB: The hour-long $\sim$0/X-ray emission, multi-episode burst pattern, and precise $\sim$1 post-burst decay could be attributed to collapse of an extended blue/red supergiant (envelope $\sim$2 cm) into a black hole, with initial jet breakout followed by fallback accretion driving continued jet power (Zhang et al., 30 Sep 2025). The day-long precursor, quasi-periodicity, and delayed onset of $\sim$3 decay support this model, provided a self-regulated, episodic jet-choking engine.

WD-IMBH Tidal Disruption Event: A carbon–oxygen white dwarf (WD) on an eccentric orbit about an intermediate-mass black hole ($\sim$4–$\sim$5) can be partially stripped at periastron, producing recurrent, relativistic jet-powered gamma-ray flares with orbital period $\sim$6 s (for $\sim$7 cm) (Levan et al., 18 Jul 2025, Li et al., 30 Sep 2025). The observed recurrence, host offset, $\sim$8 fallback, and beaming-corrected energetics match this scenario, making it compelling, especially given the super-Eddington X-ray emission emerging post-shutdown (Li et al., 30 Sep 2025).

Precessing/Magnetar Jet or Micro-TDE: Less favored interpretations include intermittent jet formation due to rotational instabilities or partial disruption-driven mini-AGN disks. The clear periodic substructure and wavelet-detected QPOs are consistent with jet precession or periodic fallback in a micro-TDE (Song et al., 11 Oct 2025).

5. Host Galaxy and Environment

The host galaxy is morphologically complex, with significant dust extinction and an off-nuclear transient position (offset $\sim$92.3 kpc). HST imaging yields $L_X \sim 3 \times 10^{49}$0, a stellar mass $L_X \sim 3 \times 10^{49}$1, and no nuclear association. No SN features were detected to date, despite deep optical/NIR observations, plausibly due to extinction or suppression by jet activity (Levan et al., 18 Jul 2025).

6. Physical Interpretation and Theoretical Implications

Key derived quantities and physical models include:

• Fallback timescale for disrupted WD debris: $L_X \sim 3 \times 10^{49}$2 s, with accretion rate $L_X \sim 3 \times 10^{49}$3.
• Peak X-ray luminosity $L_X \sim 3 \times 10^{49}$4 erg s$L_X \sim 3 \times 10^{49}$5.
• Jet Lorentz factor $L_X \sim 3 \times 10^{49}$6, inferred from high-energy cutoff non-detection.
• Disk blackbody fits at late times yield $L_X \sim 3 \times 10^{49}$7, consistent with slim-disc accretion or WD–TDE thermonuclear models (Li et al., 30 Sep 2025).

Beaming-corrected energetics, recurrence, and super-Eddington late-time emission all favor partial tidal stripping of a WD by an IMBH as the physical origin, but a self-regulating collapsar cannot be excluded a priori, especially if SN emission is undetectable.

7. Classification, Significance, and Future Prospects

EP250702a defines a new boundary in high-energy time-domain astronomy:

• Ultra-long duration (day-scale), ultra-energetic, repeating high-energy transient.
• Clear host galaxy offset, recurrence, and fallback-powered decay.
• Candidate quasi-periodicity in prompt emission.

The event framework encompasses the yet-rare white dwarf–IMBH TDE, opening access to compact object demographics, fallback accretion physics, relativistic jet properties, and the dense interior structure of degenerate stars (Levan et al., 18 Jul 2025, Zhang et al., 30 Sep 2025, Li et al., 30 Sep 2025, Song et al., 11 Oct 2025). Ongoing late-time monitoring (radio, IR, X-ray), improved localization, and further population studies will determine if EP250702a establishes a distinct class of quasi-periodic, repeating ultra-long transients.

Parameter	Value/Description	Source
Peak X-ray luminosity	$L_X \sim 3 \times 10^{49}$8 erg s$L_X \sim 3 \times 10^{49}$9 (0.3–10 keV)	(Li et al., 30 Sep 2025)
Recurrence interval	2825 s, 4$^{-1}$02825 s	(Levan et al., 18 Jul 2025)
Jet Lorentz factor	$^{-1}$1	(Li et al., 30 Sep 2025)
QPOs detected	$^{-1}$2 Hz, $^{-1}$3 Hz ($^{-1}$422, 42 s)	(Song et al., 11 Oct 2025)
Host offset	$^{-1}$5 kpc	(Levan et al., 18 Jul 2025)

The synthesis of temporal, spectral, and host-galaxy data thus positions EP250702a as both a prototype and an outlier, illuminating the endpoints of stellar evolution, jet launch mechanisms, and potential new extragalactic transient classes.