A Precessing, Magnetically Dominated, Structured Jet Powering the Hour-scale Quasiperiodic GRB 250702B

TL;DR

GRB 250702B presents hour-scale quasi-periodic prompt activity with a hard MeV spectrum and a highly energetic, off-nuclear host. The authors develop a progenitor-agnostic framework in which a magnetically dominated jet, launched by a misaligned BH–disk system, is collimated by a thick inner flow and precesses due to Lense–Thirring effects, providing a clock with $P_{\rm LT} \approx 2825\,\mathrm{s}$. The jet is structured (spine–sheath) and viewed slightly off-axis, so geometric gating explains the bright pulses and missing cycles while reconciling the large $E_{\gamma,\mathrm{iso}}$ with a standard beaming-corrected energy budget; magnetic reconnection drives the radiative output yielding $E_{p,\mathrm{rest}} \sim 1$–$3$ MeV. Late-time afterglow consistent with an off-axis Gaussian jet and the host properties (dusty, star-forming, off-nuclear) support a jet-dominated scenario, with phase-resolved polarization, a Fourier comb at $f_0=1/P_{\rm LT}$, and specific host diagnostics providing falsifiable tests to distinguish among micro-TDE and He-star accretion channels. Overall, the work links prompt timing, spectral hardness, and afterglow within a coherent magnetized precessing-jet picture that can be tested by polarization measurements, high-energy timing, and late-time calorimetry.

Abstract

GRB 250702B shows ultra-long, episodic prompt activity (three hard gamma-ray episodes over ~ 3.2 h with quasi-regular spacing P~ 2825 s preceded by a soft X-ray flare about one day earlier. We interpret these phenomena with a unified scenario in which a stellar-mass black hole accretes from a massive, misaligned debris disk and launches a magnetically dominated, precessing, structured (spine-sheath) jet. The engine "clock" arises from Lense-Thirring precession of the outer annulus of a geometrically thick inner torus at r ~ 250-300 rg, while the hard spectra reflect magnetic-reconnection dissipation in the spine. A slightly off-axis viewing geometry resolves the apparent opening-angle tension without invoking late energy injection. "Missing" pulses in the second/third cycles occur naturally when low-amplitude nutation causes the beaming cone to miss the line of sight. The model yields concrete, falsifiable predictions, providing a self-consistent explanation of GRB 250702B's radiative and outflow anomalies.

Figures (3)

• Figure 1: Geometry of the precessing, magnetically dominated structured jet model proposed for GRB 250702B. A misaligned, geometrically thick torus/winds collimates the jet and defines the precessing axis. The structured jet consists of a narrow spine (orange-color) surrounded by a wider sheath (blue-color), with a small kink at $(2-4) \times 10^2 r_g$ indicating where the jet is forced into precession by the disk wind. The observer’s line of sight (LOS) lies just outside the sheath at a small inclination to the jet axis, enabling lighthouse-like modulation and occasional "missing" pulses. An outer thin disk at $r \gtrsim 250\, r_{g}$ is also indicated.
• Figure 2: Lense–Thirring "mass–meter" for the observed periodicity $P=2825$ s. Curves show the black hole mass $M_\bullet$ implied by a precession radius $\tilde{r}\equiv r/r_g$ for spins $a_*=\{0.3,0.5,0.9\}$. Horizontal dashed lines mark the IMBH band (10$^{4}$–10$^{6}$$M_\odot$); dotted lines mark the stellar-mass band (3–30 $M_\odot$); the thin solid line highlights a GRB-scale $M_\bullet=6\,M_\odot$. For $\tilde{r} \sim 10 - 50\,r_g$, the period selects the IMBH regime; achieving the same period with a $\sim$6 $M_\odot$ engine requires $\tilde{r}\gtrsim 200- 400\,r_g$, or a different precession driver.
• Figure 3: Qualitative placement of GRB 250702B (shaded rectangle) in $(K, q)$ space, where $K \equiv r/r_\mathrm{ISCO}(a_*)$ and $q \equiv M_\mathrm{T}/M_\mathrm{BH}$ is the torus-to-BH mass ratio 2024ApJ...972L..23V. For the observed period $P \simeq 2825$ s and a fiducial $M_\mathrm{BH} = 6\,M_\odot$, GRB 250702B occupies $K \simeq 59$--$130$ for BH spins $a_* = 0.5$--$0.9$ and $q \sim 10^{-4}$ (shaded band). The adopted $q$ value places the event between canonical long GRBs ($q \sim 10^{-2}$) and SLSNe ($q \sim 10^{-7}$). Rectangular boxes show representative $(K, q)$ regions for GW170817B (the post-merger black-hole spin-down signal associated with GW170817 with estimated $K\sim 3-4$ and $q \sim 10^{-3}$--$10^{-2}$, 2024ApJ...972L..23V) and M 87 with minimal torus fraction $q \sim 10^{-7}$--$10^{-8}$ (jet-launching and collimation occur on scales of a few $r_{\rm ISCO}$, inferred from VLBI constraints and jet kinematics, e.g., 2012Sci...338..355D2016AA...595A..54M2018AA...616A.188K and further supported by the detection of jet precession e.g., 2023Natur.621..711C). Both GW170817B and M 87 therefore anchor the "few--$r_{\rm ISCO}$" end of the diagram, whereas GRB 250702B requires a much larger $K$ where the precession is set by the outer working annulus of a thick inner flow.