A fast powerful X-ray transient from possible tidal disruption of a white dwarf

TL;DR

The paper presents EP250702a, a fast, luminous X-ray transient interpreted as a tidal disruption event where a white dwarf is disrupted by an intermediate-mass black hole, launching a relativistic jet. Multiwavelength data from Einstein Probe, Fermi/GBM, Chandra, JWST, and radio/IR facilities reveal a one-day, highly beamed X-ray/gamma-ray outburst with isotropic energy $\gtrsim5\times10^{53}$ erg, followed by a soft thermal late-time component and an afterglow consistent with jet–ISM interaction. Timing and spectral analyses constrain a jet bulk Lorentz factor $\Gamma_j\gtrsim56$ and BH spin $a_\bullet$ in the range $\sim0.6$–$0.99$, with a non-negligible beaming factor $f_b$ and an off-nuclear host at $z=1.036$. The theoretical WD-IMBH TDE model successfully reproduces the light curve, including rapid early variability and a slow late-time decay, and implies extremely high accretion rates with radiative efficiency $\eta\approx0.02$, offering a new window on IMBH demographics and potential gravitational-wave counterparts for future space-based detectors.

Abstract

Stars captured by black holes (BHs) can be torn apart by strong tidal forces, producing electromagnetic flares. To date, more than 100 tidal disruption events (TDEs) have been observed, each involving invariably normal gaseous stars whose debris falls onto the BH, sustaining the flares over years. White dwarfs (WDs), which are the most prevalent compact stars and a million times denser--and therefore tougher--than gaseous stars, can only be disrupted by intermediate-mass black holes (IMBHs) of 10^2--10^5 solar masses. WD-TDEs are considered to generate more powerful and short-lived flares, but their evidence has been lacking. Here we report observations of a fast and luminous X-ray transient EP250702a detected by Einstein Probe. Its one-day-long X-ray peak as luminous as 10^(47-49) erg/s showed strong recurrent flares with hard spectra extending to several tens of MeV gamma-rays, as detected by Fermi/GBM and Konus-Wind, indicating relativistic jet emission. The jet's X-ray dropped sharply from 3 x 10^49 erg/s to around 10^44 erg/s within 20 days (10 days in the source rest frame). These characteristics are inconsistent with any known transient phenomena other than a jetted-TDE evolving over an unprecedentedly short timescale, indicating the disruption of a WD by an IMBH. At late times, a new soft component progressively dominates the X-ray spectrum, exhibiting an extreme super-Eddington luminosity, which possibly originates from an accretion disc. WD-TDEs open a new window for investigating the elusive IMBHs and their surrounding stellar environments, and they are prime sources of gravitational waves in the band of space-based interferometers.

Figures (12)

• Figure 1: The field of EP250702a in EP, Chandra, and HST imaging. (a)--(c), X-ray images of EP250702a taken by EP-WXT, EP-FXT, and Chandra, respectively. The position of EP250702a is indicated with red circles, and the blue contour shown in (a) gives the 1$\sigma$ position uncertainty of GRB 250702B. (d), Optical image of EP250702a observed by HST. The red, orange, and blue ellipses indicate the positions of the X-ray, near-infrared (NIR), and radio counterparts of EP250702a, respectively. The NIR position and its uncertainties are obtained from VLT observationsLevan2025. The radio position and its uncertainties are obtained from MeerKAT observations (Supplementary material, observations and data reduction).
• Figure 2: Long-term X-ray light curve and spectral evolution of EP250702a. (a), Comparison of EP250702a with other X-ray transients, including jetted TDEs (Sw J1644+57, Swift J1112.2-8238, AT2022cmc), an ultra-long GRB (GRB 211024B) and a jetted TDE candidate EP241021aShu2025, on their X-ray light curves. The trigger time of EP250702a is set to be the start time of the first WXT observation when EP250702a begins to emerge (Supplementary material, observations and data reduction), about one day before the first Fermi/GBM trigger. The data points of EP250702a with red borders and red error bars represent measurement from the Chandra observation. The overlaid gray dashed line represents the single powerlaw decay fit, and the overlaid red dashed curve represents the two-segment power-law decay fit to the long-term X-ray light curve of EP250702a. The vertical dashed black lines denote the trigger times of the Fermi/GBM flares, and the vertical dashed gray one gives the time of the untriggered weak Fermi/GBM flare (Supplementary material, observations and data reduction). (b), The photon indices for EP250702a derived from absorbed power-law spectral fitting to the X-ray spectra. All error bars represent 1$\sigma$ uncertainties.
• Figure 3: Light curves and spectral energy distributions (SEDs) during the flaring episodes of EP250702a. (a), Light curves observed during the flaring episodes by the BGO detector of Fermi/GBM (top panel), the NaI detector of Fermi/GBM (middle panel), and EP-WXT (bottom panel). Six distinct flaring episodes are indicated by grey shaded regions with color-coded boundaries. (b), SEDs corresponding to the six flaring episodes. The SEDs are derived from the joint spectral fits over the time intervals indicated by the labels. Solid and dashed lines represent the best-fit unabsorbed and absorbed models, respectively. Error bars on the data points denote the 1$\sigma$ confidence level, and the shaded regions around the best-fit lines indicate the corresponding 1$\sigma$ confidence bands.
• Figure 4: X-ray spectra of EP250702a detected by EP and Chandra. (a), Evolution of X-ray spectra for EP250702a showing significant spectral softening trend in later observations. The spectrum taken from the first FXT observation on 2 July is shown in black points, and the late-time stacked spectrum (with the data taken from 12 July to 15 July, when the averaged spectral photon index is greater than 2) is shown in red. The late-time stacked spectrum has been scaled by a factor of 10 for visual clarity. Both spectra were fitted with an absorbed power-law model, and the fitting residuals were shown in the lower panel. (b), Joint X-ray spectral fitting of simultaneous Chandra and FXT observations, taken on 18 July, with the best-fit absorbed blackbody (dashed line) and power-law (dotted line) model components, as well as their combined model const*tbabs*ztbabs*(bbody+powerlaw) (solid line) (Supplementary material, spectral analysis). The fitting residuals are shown in the lower panel. The black and red points represent the Chandra and FXT observations, respectively. All error bars represent 1$\sigma$ uncertainties.
• Figure S1: Localization results of GRB 250702B derived from Fermi/GBM data using the BALROG method. The two-dimensional and marginalized one-dimensional localization probability distribution plots for Flares A (brown), D (red), B (blue), and E (orange) were generated using GetDistLewis2025JCAP. The filled contours in the two-dimensional plot represent the 1$\sigma$, and 2$\sigma$ and 3$\sigma$ statistical localization regions for each flare. The black star and dashed lines indicate the soft X-ray position of EP250702a, as determined by Chandra.