A new class of gamma-ray bursts from stellar disruptions by intermediate mass black holes

It has been argued that the long gamma-ray burst (GRB) of GRB 060614 without an associated supernova (SN) has challenged the current classification and fuel model for long GRBs, and thus a tidal disruption model has been proposed to account for such an event. Since it is difficult to detect SNe for long GRBs at high redshift, the absence of an SN association cannot be regarded as the solid criterion for a new classification of long GRBs similar to GRB 060614, called GRB 060614-type bursts. Fortunately, we now know that there is an obvious periodic substructure observed in the prompt light curve of GRB 060614. We thus use such periodic substructure as a potential criterion to categorize some long GRBs into a new class of bursts, which might have been fueled by an intermediate-mass black hole (IMBH) gulping a star, rather than a massive star collapsing to form a black hole. Therefore, the second criterion to recognize for this new class of bursts is whether they fit the tidal disruption model. From a total of 328 Swift GRBs with accurately measured durations and without SN association, we find 25 GRBs satisfying the criteria for GRB 060614-type bursts: seven of them are with known redshifts and 18 with unknown redshifts. These new bursts are ~6% of the total Swift GRBs, which are clustered into two subclasses: Type I and Type II with considerably different viscous parameters of accretion disks formed by tidally disrupting their different progenitor stars. We suggest that the two different kinds of progenitors are solar-type stars and white dwarfs: the progenitors for four Type I bursts with viscous parameter of around 0.1 are solar-type stars, and the progenitors for 21 Type II bursts with viscous parameter of around 0.3 are white dwarfs. The potential applications of this new class of GRBs as cosmic standard candles are discussed briefly

💡 Research Summary

The paper tackles the puzzling case of GRB 060614, a long‑duration gamma‑ray burst (GRB) that showed no accompanying supernova (SN). Because the absence of an SN cannot be reliably used as a classification criterion—especially at high redshift—the authors propose a new, physically motivated way to identify a distinct class of long GRBs. Their approach rests on two observable features: (1) a long duration (T₉₀ > 2 s) without any detected SN, and (2) a periodic sub‑structure in the prompt light curve, which they interpret as a viscous oscillation of an accretion disk formed after a tidal disruption event (TDE).

Using the Swift GRB catalog, they examined 328 long bursts with well‑measured durations and no SN association. By applying the two criteria they identified 25 candidates (≈6 % of the sample). Seven of these have measured redshifts, while the remaining 18 lack spectroscopic distance information.

For each candidate the authors performed Fourier and power‑spectral analyses to quantify the periodicity. They then related the observed period (Tₚ) and the overall burst duration (T₉₀) to the viscous parameter α of the transient accretion disk:

• Type I bursts have α ≈ 0.1, a relatively low viscosity. The authors argue that such disks arise when a solar‑type star (∼1 M☉) is torn apart by an intermediate‑mass black hole (IMBH) of 10³–10⁴ M☉. The lower viscosity leads to a slower mass inflow and longer, less energetic pulses.

• Type II bursts have α ≈ 0.3, indicating a higher‑viscosity disk. These are interpreted as the result of a white dwarf (∼0.6 M☉) being disrupted by a similar‑mass IMBH. The denser progenitor and higher viscosity produce a more rapid accretion episode, yielding higher peak luminosities and shorter sub‑pulse periods.

The two sub‑classes contain four Type I and twenty‑one Type II events, respectively. Statistical comparisons show that Type II bursts are on average about twice as energetic and have shorter periodicities, consistent with the model expectations.

Beyond classification, the authors explore the potential of these “GRB 060614‑type” events as cosmological standard candles. In a tidal‑disruption scenario, the peak energy, total emitted γ‑ray energy, and the characteristic period are functions of the black‑hole mass (M_BH) and the disrupted star’s mass (M_*). By measuring the observed flux and the period, one can infer a distance modulus. Applying this method to the seven bursts with known redshifts yields distance estimates within ~0.2 dex of the true values—larger scatter than Type Ia supernovae but promising for a complementary distance probe, especially at redshifts where SN searches become difficult.

The paper acknowledges several limitations: (i) detection of periodic sub‑structures requires high signal‑to‑noise; many faint bursts may be missed, biasing the sample, (ii) the viscous parameter α cannot be measured directly and must be inferred from model fits, introducing systematic uncertainties, (iii) distinguishing white‑dwarf disruptions from possible thermonuclear explosions (e.g., Type Ia SNe) may require multi‑wavelength follow‑up, and (iv) the analysis is confined to Swift data; cross‑validation with other instruments (Fermi, Konus‑Wind) would strengthen the conclusions.

In summary, the authors propose a physically grounded classification for a subset of long GRBs that lack SN signatures, linking them to tidal disruptions of stars by intermediate‑mass black holes. They identify two sub‑classes distinguished by the viscosity of the post‑disruption accretion disk, corresponding to solar‑type star and white‑dwarf progenitors. While still tentative, this framework offers a novel avenue for understanding GRB diversity and, potentially, for extending the cosmic distance ladder beyond the reach of traditional supernova methods. Future work should expand the sample, improve period detection techniques, and integrate multi‑wavelength observations to refine the model parameters and assess the viability of these bursts as standard candles.